Secchi depth in the Baltic Sea – an
indicator of eutrophication
Vivi Fleming-Lehtinen
ACADEMIC DISSERTATION
To be presented, with the permission of the Faculty of Biological and Environmental
Sciences of the University of Helsinki, for public examination in lecture room 1 (B116),
Metsätalo, on 18 November 2016, at 12 noon.
© Fleming-Lehtinen V (synopsis)
© Elsevier (Paper I, IV)
© Fleming-Lehtinen V and Simis S (Paper III)
© Springer (Paper II)
Author’s address:
Vivi Fleming-Lehtinen
Marine Research Centre
Finnish Environment Institute SYKE
PO Box 140
FI 00251 Helsinki
Finland
Supervised by:
Adjunct Professor Harri Kuosa
Marine Research Centre
Finnish Environment Institute SYKE
Adjunct Professor Hermanni Kaartokallio
Marine Research Centre
Finnish Environment Institute SYKE
Reviewed by:
Professor Agneta Andersson
Department of Ecology and Environmental Science
EMG, Umeå University
Professor Lauri Arvola
Lammi biological station
University of Helsinki
Opponent:
Professor Mike Elliott
Institute of Estuarine and Coastal Studies
School of Biological, Biomedical & Environmental Sciences
University of Hull
ISBN 978-951-51-2704-4 (paperback)
ISBN 978-951-51-2705-1 (PDF)
KWWSHWKHVLVKHOVLQNL¿
8QLJUD¿D2\
Helsinki 2016
Fleming-Lehtinen, V. 2016. Secchi depth in the Baltic Sea – an indicator of eutrophication.
University of Helsinki, Faculty of Biological and Environmental Sciences, Helsinki. 42 pages.
ABSTRACT
Secchi depth, a proxy of water clarity, is widely applied as an indicator of eutrophication or water quality both in open-sea- and coastal areas. In optically complex waters, such as the Baltic
Sea, Secchi depth is known to respond to several components – yet its performance, or possible
restrictions, have not been explored. In this study, I investigated long-term changes in Secchi
GHSWK,DOVRH[SORUHGWKHVWUXFWXUHVFLHQWL¿FEDVLVDQGXVHRI6HFFKLGHSWKDVDQLQGLFDWRURI
eutrophication in the Baltic Sea.
Secchi depth decreased in the open Baltic Sea during the last century (Paper I). The decrease
was especially intense in the northern areas, amounting to 3.3 – 4.0 m (averaging 0.033 – 0.040 m
y-1), when comparing summer time averages in 2005 – 2009 to those observed one hundred years
earlier. The decrease was proposed to be strongly linked with documented simultaneous increase
in chlorophyll-a concentration (Papers I, III).
A closer look at the Finnish coastal areas, where a national monitoring program has taken place
since 1970, revealed clear decreasing trends only in the Archipelago Sea – accompanied by opposing trends in chlorophyll-a (Paper II). Contradictory to this, and to the development in adjacent
open sea areas, Secchi depth was observed to increase in the coastal Bothnian Sea, Quark and
Bothnian Bay. I suggest the increase was at least partly a consequence of decreased concentrations
of dissolved iron in the surface waters near the coast. The relationship between Secchi depth and
WRWDORUJDQLFFDUERQ72&ZDVWHVWHGEXWDVLJQL¿FDQWUHODWLRQVKLSZDVQRWIRXQG±LQGLUHFWO\
indicating that a large part of organic carbon was colorless. Unfortunately, the long-term coastal
dataset did not allow comparison to suspended inorganic matter, leaving the possible effect of
potentially important coastal constituent unrevealed.
The effect of the main optical constituents on light attenuation in the open sea were investigated
through a bio-optical model setup, in order to resolve how the Secchi depth indicator should be
applied in different parts of the Baltic Sea (Paper III). Secchi depth was shown to be highly sensitive to variation in both phytoplankton (by chlorophyll-a as proxy) and colorful dissolved organic
matter (CDOM). As expected, based on the high spatial gradients in both optical constituents, the
evaluation against monitoring data called for sub-basin-wise adjustments to the model outcome.
Secchi depth is often applied together with other indicators, including chlorophyll-a. The modelling exercise revealed, that the environmental targets for Secchi depth, set by the Baltic Sea coastal
states via their collaboration through the Baltic Marine Environment Protection Commission
(HELCOM), were stricter than those set for chlorophyll-a.
To facilitate future management use of the Secchi depth indicator, I made an effort to characterize
it in relation to indicators in general. Secchi depth is a commonly applied and well established
indicator of eutrophication and water quality in the Baltic Sea. It is technically relatively advanced:
quantitative, regularly monitored, and includes ecological targets as well as documented methodology. It is also easily understood by the public. On the other hand, though simple to associate, it
LVDFRPSRVLWHLQGLFDWRUZKLFKUHTXLUHVFDVHVSHFL¿FDQDO\VLVEHIRUHLWVUROHLQWKHHXWURSKLFDWLRQ
SURFHVVFDQEHDFFXUDWHO\GH¿QHG
Finally, Secchi depth was applied in the Baltic Sea eutrophication status assessment (Paper IV),
and alternative ways to apply the indicator were explored. According to the assessment 20072011, all open-sea areas of the Baltic Sea were severely affected by eutrophication. Due to the
3
deteriorated status of all indicators, variation in the construction of the assessment did not affect
the general outcome. Secchi depth on its own expressed deteriorated status in most areas, meeting
its environmental target only in the Bothnian Bay.
The strong relationship between Secchi depth and chlorophyll-a motivates the use of Secchi
depth as a eutrophication indicator throughout the open Baltic Sea. The strong association to
CDOM, however, presents a combination of possible additional autochthonous as well as allochthonous signals. The sensitivity of Secchi depth to the main optical constituents varies between
open-sea areas, and furthermore, needs to be addressed separately in the coastal zone, where
LQRUJDQLFFRQVWLWXHQWVDUHH[SHFWHGWREHVLJQL¿FDQW%HLQJDFRPSRVLWHLQGLFDWRU6HFFKLGHSWK
was found suitable for expressing eutrophication together with other indicators; relying on Secchi
depth alone would introduce a risk of misinterpretations, especially when the role of water clarity
LQWKHHFRV\VWHPLVQRWVROYHGDUHDVSHFL¿FDOO\2QWKHRWKHUKDQG6HFFKLGHSWKPD\WXUQWREH
YDOXDEOHLQUHÀHFWLQJVLJQDOVQRWFXUUHQWO\FDSWXUHGE\RWKHULQGLFDWRUV
Keywords: Secchi depth, water clarity, eutrophication, indicator, assessment, good environmental
status, target.
4
TIIVISTELMÄ
Näkösyvyys kertoo veden kirkkaudesta. Sitä on käytetty laajasti rehevöitymisen tilan ja vedenlaadun osoittimena (indikaattorina) sekä avomerellä että rannikonläheisillä merialueilla. Itämeren
tyyppisissä, optisesti monimuotoisissa vesissä se reagoi useisiin veden ominaisuuksiin. Silti sen
suorituskykyä osoittimena, tai käyttöön liittyviä rajoituksia, ei ole liiemmin selvitetty. Tässä työssä
tarkastelen näkösyvyyden pitkäaikaismuutoksia Itämerellä. Tutkin myös näkösyvyys-osoittimen
ominaisuuksia ja käyttömahdollisuuksia.
Kuluneen vuosisadan aikana Itämeren näkösyvyys laski (Julkaisu I). Voimakkainta lasku oli
pohjoisilla alueilla, yltäen 3.3 – 4.0 m sadassa vuodessa (keskiarvona 0.033 – 0.040 m v-1). Esitän,
että näkösyvyyden lasku liittyy vahvasti samaan aikaan tapahtuneeseen levämäärän (a-klorofyllipitoisuus, lehtivihreän määrä) lisääntymiseen pintavedessä (Julkaisut I, III).
Vuodesta 1970 alkaen jatkunut seuranta mahdollisti Suomen rannikkoalueiden näkösyvyysmuutosten lähemmän tarkastelun. Saaristomerellä todettiin selkeä laskeva suuntaus – ja samanaikainen levämäärän lisääntyminen (Julkaisu II). Selkämeren, Merenkurkun ja Perämeren rannikoilla
suuntaus oli päinvastainen: näkösyvyyden todettiin kasvaneen, mikä oli ristiriidassa myös näitä
rannikkokaistaleita ympäröivien avomerialueiden kehityksen kanssa. Esitän että veden kirkastuminen kyseisissä vesissä on ainakin osittain seurausta liuenneiden rautayhdisteiden määrän vähenemisestä. Näkösyvyyden ja kokonaishiilen (TOC) määrän muutoksia testattiin myös suhteessa
toisiinsa, ilman näyttöä merkitsevästä riippuvuudesta – minkä tulkitsin johtuvan siitä, että ainakin
osa veteen liuenneesta hiilestä on väritöntä. Kiintoaineen suhteen vertailua ei ikävä kyllä ollut
mahdollista tehdä, joten sen merkitystä näkösyvyyden muutoksiin rannikolla ei pystytty tutkimaan.
Pohdin Itämeren avomerialueiden tärkeimpien valon vaimenemiseen vaikuttavien ainesosien
vaikutusta näkösyvyyteen bio-optisen mallijärjestelyn avulla (Julkaisu III). Tämä auttoi selvittämään kuinka näkösyvyysosoitinta tulisi soveltaa Itämeren eri osissa. Näkösyvyys osoittautui
olevan herkkä sekä levämäärän (lehtivihreän kautta tulkittuna) että humusaineiden (CDOM)
vaihtelulle. Herkkyys vaihteli alueellisesti siinä määrin, että mallin tuloksia jouduttiin sovittamaan
merialuekohtaisesti.
Näkösyvyyttä hyödynnetään tilanarvioissa usein yhdessä muiden osoittimien, kuten levämäärän, kanssa. Mallinnuksen seurauksena päädyin esittämään, että näkösyvyydelle kansainvälisesti,
Itämeren Suojelukomission (HELCOM) toimesta asetetut ympäristön hyvän tilan tavoitetasot ovat
levämäärälle asetettuja tavoitteita kunnianhimoisemmat.
Tukeakseni näkösyvyysosoittimen tulevaa käyttöä, tein arvion sen ominaisuuksista suhteessa
osoittimiin yleensä. Näkösyvyys on jo laajasti käyttöönotettu rehevöitymisen ja vedenlaadun
osoitin Itämerellä. Se on teknisesti kehittynyt: määrällinen (kvantitatiivinen), säännöllisesti
seurattu (monitoroitu), menetelmiltään todennettu osoitin, jolle on kyetty määrittämään hyvän tilan
tavoitetasot. Se on myös helposti ymmärrettävä ja käytännönläheinen. Vaikka se on toiminnallisesti
yksinkertainen, on se rehevöitymiseen liittyvien syy-seuraussuhteiden osalta monimutkainen, ja
edellyttää siltä osin aluekohtaisen analyysin ennen käyttöönottoa.
Tutkin lopuksi näkösyvyyden käyttöä Itämerenlaajuisessa rehevöitymisen tilanarviossa (Julkaisu
IV), kokeillen vaihtoehtoisia tapoja yhdistellä sitä muiden osoittimien kanssa. Vuosille 2007-2011
määritetyn Itämeren tilanarvion perusteella kaikki avomerialueet olivat rehevöityneitä. Erikseen
jokaisen rehevöitymisen tilan osoittimen kautta tulkittuna tulos oli useimmilla alueilla sama, joten
niiden uudelleenryhmittelyllä ei ollut vaikutusta kokonaistilanarvioon. Yksin näkösyvyyden kautta
arvioituna rehevöitymisen tila oli huono useimmilla avomerialueilla, Perämerta lukuun ottamatta.
Näkösyvyyden voimakas riippuvuus levämäärään kannustaa hyödyntämään sitä rehevöitymisen tilan osoittimena kautta Itämeren. Humusaineilla, joista merkittävä osa on maalta peräisin,
on lisäksi vaikutusta vedenkirkkauteen – tämä tekee osoittimesta herkän myös rehevöitymisen
5
ulkopuolisille muutoksille. Tämä herkkyys vaihtelee alueellisesti, ja se tulee ottaa huomioon ja
suhteuttaa olosuhteisiin niin rannikoilla kuin avomerellä. Näkösyvyys on parhaimmillaan ympäristön tilanarvioissa yhdessä muiden osoittimien kanssa. Luottaminen yksinomaan tämän syyseuraussuhteiltaan monimuotoisen osoittimen viestiin altistaa virhetulkinnoille, erityisesti mikäli
vedenkirkkauden syitä ja riippuvuuksia ei ole selvitetty aluekohtaisesti. Toisaalta, yhdessä muiden
osoittimien kanssa näkösyys saattaa tunnistaa signaaleja jotka eivät vaikuta muihin osoittimiin.
Avainsanat: näkösyvyys, vedenkirkkaus, rehevöityminen, osoitin, indikaattori, tilanarvio,
ympäristön hyvä tila, tavoitetaso.
6
ACKNOWLEDGEMENTS
This thesis is a product of several years of pondering and debating, on the subjects of eutrophication,
indicators, assessments and good status, involving many scientists and projects. I am not presenting
the whole story, merely a chronicle of one parameter. Early on, I liked to think of us as detectives
solving a mystery story, chasing for evidence buried long ago, or disappeared into the wind. Later,
the mystery was transformed into a puzzle or a game; fitting together missing pieces, as if we
were trying to fix a broken toy. I love the Secchi disk: its elegant design, its history and the humorous simplicity. First, I want to thank Padre Pietro Angelo Secchi (29 June 1818 – 26 February
1878) for giving us the toy, the mystery, the puzzle.
I want to thank my supervisors Harri Kuosa and Hermanni Kaartokallio; our conceptual discussions were some of the most enjoyable moments of this adventure. I am deeply grateful to my
steering group, Maiju Lehtiniemi and Kai Myrberg – my friends; at times, in order to steer, you
had to give the engine a jump start. Additionally, I want to thank Juha-Markku Leppänen and
Anna-Stiina Heiskanen for encouraging me, giving me the opportunity and showing an example,
everyone should have bosses like you!
I am grateful to the crew of the Department of Biological and Environmental Sciences for helping me through the academic jungle; not only Professors Hannu Lehtonen and Jukka Horppila,
but also university lecturer Elina Leskinen – I would not be here without your persistence. My
pre-examiners, Professor Agneta Anderson and Professor Lauri Arvola, I am proud and thankful
for the interest you took in my work.
I want to thank my co-authors Jesper Andersen, Jacob Carstensen, Pirkko Kauppila, Pirkko
Kortelainen, Maria Laamanen, Elżbieta Łysiak-Pastuszak, Ciarán Murray, Minna Pyhälä, Antti
Räike, Stefan Simis and David Thomas; it has been fun, and I hope this was not the end of it! I am
also thankful to Riitta Olsonen for digging up the early data, as well as to the ICES team Else Juul
Green and Hjalte Parner for helping with later data requests, and to Ritva Koskinen for the editing
work. I want to express appreciation to my colleagues at SYKE and in the HELCOM community,
you are invaluable, I have learned a great deal from you.
I want to acknowledge my employer, the Finnish Environment Centre (SYKE), for supporting
this work. The thesis synopsis is also a contribution to the Bonus project ‘Nutrient Coctails in the
Coastal Zone of the Baltic Sea (COCOA)’.
7
Contents
ABSTRACT.............................................................................................................. 3
TIIVISTELMÄ .......................................................................................................... 5
ACKNOWLEDGEMENTS ........................................................................................ 7
List of original papers, and author’s contribution ............................................. 9
Abbreviations ....................................................................................................... 10
1 Introduction ....................................................................................................... 11
'H¿QLQJDQGLGHQWLI\LQJPDULQHHXWURSKLFDWLRQ........................................................ 12
1.3 Assessing eutrophication status with ecological indicators ..................................... 14
1.4 Secchi depth in eutrophication assessments .......................................................... 15
1.5 Aims of the study ...................................................................................................... 15
2 Materials and methods ..................................................................................... 16
2.2 The Secchi depth method ........................................................................................ 17
2.3 Data and sources ..................................................................................................... 18
2.4 Approaches for interpreting information................................................................... 18
3 Results ............................................................................................................... 22
3.2 Temporal changes of Secchi depth along the Finnish coast (Paper II) ...................23
3.3 Changes in Secchi depth in relation to other parameters (Papers I, II) ...................25
3.4 The Secchi depth indicator (Paper III) .....................................................................28
3.5 Secchi depth in the eutrophication assessment ......................................................29
4 Discussion ........................................................................................................ 32
4.2 The relationship of Secchi depth and eutrophication...............................................32
'H¿QLQJWKHLQGLFDWRU ...............................................................................................35
4.4 Assessing eutrophication with the Secchi depth indicator.......................................36
5 Conclusions ....................................................................................................... 38
References ............................................................................................................ 40
8
List of original papers, and author’s contribution
I
Fleming-Lehtinen V & Laamanen M, 2012. Long-term changes in Secchi depth and the
role of phytoplankton in explaining light attenuation in the Baltic Sea. Estuarine, Coastal
and Shelf Science 102-103:1-10.
,,
)OHPLQJ/HKWLQHQ95lLNH$.RUWHODLQHQ3.DXSSLOD37KRPDV''H¿QLQJ
the role of Organic carbon concentration in the northern coastal Baltic Sea between 1975
and 2011. Estuaries and coast 38:466-481.
III
Fleming-Lehtinen V & Simis S. Interpreting the Baltic Sea Secchi depth eutrophication
indicator by means of bio-optical modelling. Manuscript.
,9
)OHPLQJ/HKWLQHQ9$QGHUVHQ-+&DUVWHQVHQ-à\VLDN3DVWXV]DN(0XUUD\&3\KlOl
M & Laamanen M, 2015. Recent developments in assessment methodology reveal that
the Baltic Sea eutrophication problem is expanding. Ecological Indicators 48:380-388.
Paper I
Paper II
Paper III
Paper IV
Original idea
VF, ML
DT
VF, SS
JA, VF, ML
Study design
VF, ML
VF, AR, PKo, DT
VF, SS
VF, JA, ML
Field work and
laboratory analyses
–
–
SS
–
Data compilation and
handling
VF
VF, AR
VF
VF, JC
Statistical analyses
VF
VF, AR
VF
VF, JC, CM
Model parametrization
and simulations
–
–
SS
–
Manuscript
preparation
VF
VF
VF
VF, JA
Manuscript
contribution
ML
AR, PKo, PKa, DT SS
JC, ML, EL, CM, MP
JA = Jesper H. Andersen, JC = Jacob Carstensen, VF = Vivi Fleming-Lehtinen, ML = Maria LaamaQHQ(/ (OĪELHWDà\VLDN3DVWXV]DN&0 &LDUiQ0XUUD\03 0LQQD3\KlOl$5 $QWWL5lLNH
PKo = Pirkko Kortelainen, PKa = Pirkko Kauppila, SS = Stefan Simis, DT = David N. Thomas
9
Abbreviations
aCDOM
Absorption of light caused by CDOM
CDOM
Colourful / chromophoric dissolved organic matter
DIN
Dissolved inorganic nitrogen concentration
DIP
Dissolved inorganic phosphorus concentration
(6
(VWLPDWLRQRILQGLFDWRUOHYHODWDSUHGH¿QHGDVVHVVPHQWSHULRG
ER
Eutrophication ratio, produced from ES and the target, indicating status of
indicator, aggregation group or overall eutrophication
GES
Good Environmental Status (Anonymous 2008)
HEAT
HELCOM Eutrophication Assessment Tool (Andersen et al. 2011)
HELCOM
Baltic Marine Environment Protection Commission
LOESS
Locally weighted scatterplot smoothing
7DUJHW
,QGLFDWRUVSHFL¿FHQYLURQPHQWDOWDUJHWHJWKHERXQGDU\H[SUHVVLQJ*(6
TOC
Total organic carbon concentration
10
1 Introduction
1.1 Water clarity and Secchi depth
Water clarity, or water transparency, is the ability of water to allow the transfer of light. It has a
theoretical maximum of 80 m, and is decreased
by light attenuation caused by the presence of
particulate and dissolved matter. In the marine
environment, attenuation is caused mainly by
planktonic organisms, especially phytoplankton, suspended particulate matter, colored dissolved organic matter (CDOM) and inorganic
compounds (Preisendorfer 1986, Lund-Hansen
2004). Phytoplankton and other suspended particles scatter as well as absorb light, whereas
non-particular substances contribute only to
absorption. Autotrophic phytoplankton is the
dominating optical constituent in most oceanic
waters. In coastal and inland seas, such as the
Baltic Sea, a considerable share of the attenuation constituents is allochthonous (Sandberg et
al. 2004, Alling et al. 2008, Kulinski and Pempkowiak 2011). In these waters the attenuation is
complex, and for example CDOM may play an
important role (Kratzer 2000, Babin et al 2003).
The depth at which a submerged white disk
no longer is visible from the surface, the Secchi depth, has commonly been used as a proxy
for water clarity. It has often been applied as a
measure of the underwater light climate (Kirk
2000, Preisendorfer 1986), or even water quality (Carlson 1977, Lewis et al. 1988, Karydis
2009, Chen et al. 2010). In oceanic waters, it
has also been used to quantify phytoplankton
pigment (chlorophyll-a) concentrations (Boyce
et al. 2012).
Being a visual measure, Secchi depth is subject to apparent properties that do not affect
water clarity, such as: the height of the sun,
VXUIDFHUHÀHFWDQFHDQGYHVVHOVKDGRZ3UHLVHQdorfer 1986). Due to these apparent properties,
and the fact that the measurements are made by
a human eye, it has been argued that they are
subjective and variable in comparison to instrumental measurements. It has been showed, that
variation in Secchi depth in response to changes
in aspects affecting the visibility of the Secchi
disk (such as altering disk size or observer)
increases along with increasing Secchi depth
(Steel and Neuhausser 2002, Aas et al. 2014),
and would thus be of more concern in clear than
murky waters. Although, these differences are
not expected encompass long term bias, they do
emphasize the need for abundant monitoring.
Developed by Father Pietro Angelo Secchi in
1865 (Secchi 1866), the Secchi disk is one of
the oldest hydrological apparats still operated.
The method has been used for monitoring and
research of oceanic, coastal and inland waters,
contributing to time-series covering periods
over 100 years (Lewis et al. 1988, Aarup 2002,
Naumenko 2007, Boyce 2012, Gallegos et al.
2011). During this time, measurements have
been made by bare eye or through a water viewer, mainly using a completely white or white
and black sectored disk, usually 30 cm in diameter (Figure 1-1).
In the Baltic Sea, Secchi depth observations
have been made since 1903 (Aarup 2002). The
method is included as part of the monitoring
program of the Baltic Marine Environment
Protection Commission (HELCOM 2015a),
and observations are reported regularly to the
HELCOM COMBINE database hosted by the
International Council of the Exploration of the
Sea (ICES). At present, measurements are made
from research vessels by bare eye, using a white
or black-and-white sectored disk. The disk diameter is usually 30 cm, but smaller disks may
be used in murky coastal waters (Papers I and
II).
11
Figure 1-1. Observing Secchi depth on board a research vessel in the Baltic Sea (A) in the beginning of the 20th
century with the water viewer and (B) in 2005 by bare eye, using a plain white Secchi disk. From Paper I.
'H¿QLQJDQGLGHQWLI\LQJPDULQH
eutrophication
The term ‘eutrophication’ originates from the
Greek word for ‘well fed’. Broadly speaking,
it refers to increase in the production, organic
supply or nutritional state of an ecosystem, usually caused by nutrient enrichment (Larsson et
al. 1985, Nixon 1995). The opposite phenomenon is oligotrophication (Nixon 2009), decrease
in the primary production in an ecosystem,
generally due to decreasing nutrient inputs or
concentrations. Though especially the former
has been in the focus of environmental management and policy for decades, neither process
is necessarily valuated ‘good’ or ‘bad’ per se.
The development is considered alarming only
when it is of anthropogenic origin and causes
substantial negative effects on the society or
ecology in the area.
The direct negative effect of eutrophication
in aquatic environments is seen to be the increase of primary production, causing increase
12
in the biomass of phytoplankton and opportunistic macrovegetation. The former in turn
has further effects, such as decrease in water
transparency, increase in detrital organic matter,
increase in bottom oxygen consumption and
shifts in pelagic and benthic communities, even
to the extent of harmful algal blooms, extinction of species or formation of inhabitable areas
(Larsson et al. 1985).
However obvious the changes have seemed
DW QXPHURXV VLWHV WKH H[DFW VFLHQWL¿F GH¿QLtion of the term ‘eutrophication’ remains vague.
When the marine eutrophication problem was
observed half a century ago, experts of the
¿HOG SURGXFHG D FRPPRQ HXWURSKLFDWLRQ UHYLHZDWWHPSWLQJWREHJLQE\GH¿QLQJWKHWHUP
(Hutchinson 1969). The disputation has continued since.
2QHRIWKHPRVWVLWHGGH¿QLWLRQVRIHXWURSKication was made by Scott W. Nixon (1995,
2009), as the increase in the rate of supply of
organic matter to an ecosystem. Importantly,
Nixon emphasizes eutrophication to be a pro-
cess, restricting from valuating it as good or
EDG+LVGH¿QLWLRQIRFXVHVRQWKHDPRXQWRIHQergy or organic matter available to support the
system, separating strictly the causes and consequences from the phenomenon itself. On the
other hand, he disregards the relevance of the
source of organic matter: all supply of organic
matter is included in the process, regardless of
whether the matter is produced in the sea (autochthonously) or introduced from elsewhere
(allochthonously). All forms of organic matter
are considered equal, regardless of their role
and mobility in the system.
A second approach, focusing on nutrient-driven increase of phytoplankton, algae and
plant growth, has been introduced and widely
used (e.g. Larsson et al. 1985). This approach
emphasizes the role of autochthonous organic
PDWWHU DOVR LGHQWL¿HG E\ 1L[RQ DV JHQHUDOO\
being the main, while not the only, concern of
eutrophication. In environments with a food
web supported by dissolved organic matter,
the autochthonous approach might lead to an
underestimation of the trophic state, when com-
EutrophicaƟon
according to Nixon
(1995)
SDUHG WR RQH EDVHG RQ 1L[RQ¶V GH¿QLWLRQ ,Q
some situations, eutrophication has been de¿QHG VWULFWO\ DV LQFUHDVH RI PLQHUDO QXWULHQWV
(Richardson and Jørgensen 1996). Here the
focus has shifted away from carbon supply, to
what according to Nixon (1995, 2009) would be
categorized as a cause instead of the phenomenon itself. In other uses, the causes as well as
indirect consequences, such as oxygen depletion or imbalanced ecosystem functioning, are
LQFOXGHG LQWR WKH GH¿QLWLRQ RI HXWURSKLFDWLRQ
(eg. Gray 1992).
,QRUGHUWRDYRLGH[FOXGLQJDQ\RIWKHGH¿QLtions presented earlier, eutrophication was sequenced in this thesis into four process-related
categories, which may be referred to separately
or together (Figure 1-2): Causative eutrophicationLVGH¿QHGDVLQFUHDVHLQWKHUDWHRIVXSSO\
of mineral nutrients from outside to an ecosystem (suggested by Richardson and Jørgensen
1996). Allochthonous eutrophicationLVGH¿QHG
as increase in the rate of supply of organic matter from outside to an ecosystem (Nixon 1995).
Autochthonous eutrophicationLVGH¿QHGDVLQ-
Allochthonous
eutrophicaƟon
Autochthonous
eutrophicaƟon
EutrophicaƟon according
to Gray (1992) and others
Indirect
eutrophicaƟon
CausaƟve
eutrophicaƟon
EutrophicaƟon according to
Richardson & Jörgensen (1996)
Naturally originated
Anthropogenically originated
Figure 1-2. A schematic figure expressing the eutrophication categories applied in this thesis. The arrows indicate
causative relationships. Naturally occurring processes are marked dark gray, and processes directly affected by
human activities are marked gray. The process categories are grouped by gray dashed lines according to three
optional definitions of eutrophication (see text for further explanations).
13
crease in the amount of organic matter produced
in the ecosystem, as a direct response to added
supply of mineral nutrients (Larsson et al. 1985,
Nixon 1995). Indirect eutrophication LVGH¿QHG
as changes in structure, function or stability of
organisms or habitats, as a result of changes in
autochthonous or allochthonous eutrophication
elements (Gray 1992).
The role of water clarity in the eutrophication
process may differ, depending on the water area
in question. The effect of nutrient enrichment
on water clarity is relatively straight forward
in optically simple waters, where phytoplankton alone dominates the attenuation of light:
in such cases it has even been used as a proxy
for chlorophyll-a (Boyce et al. 2012), indicating autochthonous eutrophication. In optically
complex waters, the causes of change in water
clarity are numerous, and the link to eutrophication may be blurred. Light attenuation is strongly affected by changes in the concentration of
CDOM, which in turn may be allochthonous as
well as autochnonous (Chrost and Faust 1983,
Hoikkala et al 2015). Being a descriptor of the
underwater light climate, water clarity can also
be regarded as an element of indirect eutrophication, caused by absorption and scattering of
autochthonous and allochthonous eutrophication constituents, having even further indirect
eutrophication effects e.g. on macrophyte distribution (Kautsky et al. 1986).
1.3 Assessing eutrophication
status with ecological indicators
Assessing the eutrophication status for management purposes is not an easy task, even when
the progress and cause-effect relationships are
well understood (Karydis 2009). The scientifLFDOO\GH¿QHGQRQYDOXDWHGSURFHVVGHVFULEHG
earlier must be interpreted in a new way, with
the aim of distinguishing ‘desirable’ from ‘undesirable’ status (Andersen et al. 2006, Tett et
al. 2007, Ferreira et al. 2011). A natural trophic
level in one environment might be detrimental
for the sustainability of another. The case-speFL¿F NH\ SDUDPHWHUV PXVW EH LGHQWL¿HG DQG
measured – and their status must be related to
ecosystem health (Constanza et al 1992).
14
A reliable assessment collectively involves
all the key features of eutrophication, in agreePHQWZLWKWKHGH¿QLWLRQIRXQGPRVWDSSURSULDWH
for the purpose. The set of features should be
optimized, in the sense that irrelevant parameWHUVQRPDWWHUKRZVLJQL¿FDQWDWSRWHQWLDORWKHU
sites, are not included along. In a quantitative
assessment, numerical indicators, representing
the eutrophication features, are integrated into
an overall evaluation (Ferreira et al. 2011, Andersen et al 2015).
Ecological indicators are thus used to communicate the status of the key eutrophication
features to the public and decision makers.
They are applied as building blocks of environmental assessments, in an attempt to simplify
and restrict the information assembled when
evaluating the eutrophication status of an ecosystem. As opposed to regular metrics, they are
supposed to tell us something more than what
they actually measure (Daan 2005). Ecological
indicators may in practice vary from measurable quantitative parameters, applicable only
together with other indicators, to broad non-numeric compilations covering several ecosystem
HOHPHQWV6SHFL¿FGH¿QLWLRQVDQGUHTXLUHPHQWV
for ecological indicators have been listed, but
concise and commonly agreed guidelines do
not exist. Below are features recognized here as
essential in quantitative ecological indicators:
–
–
The most important task of an indicator
LV WR UHÀHFW WR WKH IHDWXUH RI FRQFHUQ LQ
other words, show ¿GHOLW\ (James 1978).
An indicator may be robust and insensitive
to slight variation in state, but it should
respond in an expected way to substantial
changes, within a reasonable time-span.
For example, the status of an eutrophication indicator should change predictably
along with increased nutrient supply and
primary production, with a delay short
enough to initiate management responses.
An indicator must be able to distinguish
desirable from undesirable state, preferably through an environmental target, reference point or boundary valueUHÀHFWLQJ
the threshold between desirable and undesirable (Constanza et al. 1992). Combined
–
–
–
–
–
with the estimation of present level, this
value may be applied to evaluate quantitative indicator status.
When applied to facilitate environmental
management, the indicators shall be linked
to one or several environmental pressures,
and react to changes in these. The link may
be either direct or indirect. In optimal cases, the relationship can be demonstrated
from monitoring data, but often the complexity of the system prohibits this, and the
link may be established only conceptually.
An indicator should be applicable in all geographical areas of interest, and throughout the timespan of interest. In practice,
WKLV UHTXLUHV FRQ¿GHQFH LQ WKH LQGLFDWRU
describing the same aspect of the process,
regardless of time and space. The relevant
spatial and temporal scale/resolution for
WKHLQGLFDWRUPXVWEHGH¿QHG
Ecological indicators are foremost management tools, and must thus be commonly
understandable, also by others than exSHUWVRIWKH¿HOG7KHFRQFHSWDVZHOODV
results should be easy to communicate.
Good assessments are not one-offs, but
used to follow change in environmental
status in a longer term. Indicators need
to be updatable at a regular interval, and
continuous monitoring activity must be in
place, or at least realistically possible, to
allow this.
The approach and methodology applied for
updating the indicator must be well documented. Optimally this involves reporting
all the above mentioned aspects together
with guidelines for indicator update.
1.4 Secchi depth in eutrophication
assessments
Secchi depth has been recommended to be applied as an indicator of eutrophication in phytoplankton-dominant clear waters, where the
relationship between water clarity and chlorophyll-a is strong (Anonymous 2000, Tett et al.
2007, Anonymous 2008, Ferreira et al. 2011). In
this kind of use, it is determined as an indicator
of autochthonous eutrophication, and Secchi
depth has been used as a second alternative
for expressing autotrophic biomass. The main
advantage of the indicator is the potential for
VRXQG WDUJHWVHWWLQJ SUR¿WLQJ IURP KLVWRULFDO
monitoring activities beyond the time when
observation methods for more direct proxies
were in place.
The use of Secchi depth has however not
been restricted to optically simple waters, but
it has frequently been applied as an indicator
of autotrophic biomass also in waters strongly affected by CDOM or suspended inorganic
matter (HELCOM 2009, Dobiesz et al. 2008).
In these waters, demonstrating the relationship
between Secchi depth and optical constituents
is essential. The indicator may capture non-autotrophic yet eutrophication-related signals
from increased autochtonous CDOM, resulting
from autotrophic or heterotrophic production.
Or, water clarity might have a strong organic or
inorganic allochthonous component.
Theoretically speaking, Secchi depth could
DOVREHFODVVL¿HGDVDQLQGLFDWRURIindirect eutrophication. This interpretation could be used
if decrease in water clarity is regarded rather as
a subsequence than a proxy of phytoplankton
increase, causing further consequences in the
structure of the system. Secchi depth might for
example be distinguished as a proxy for something more complicated to measure, such as
macrophyte depth distribution, in a case when
macrophyte growth is restricted by light availability as a result of eutrophication (Kautsky
et al. 1986).
1.5 Aims of the study
The main objective of this study was to explore
WKHVWUXFWXUHVFLHQWL¿FEDVLVDQGXVHRI6HFFKL
depth, a proxy of water clarity, as an indicator
of eutrophication in the Baltic Sea. It is already
applied widely to indicate eutrophication or water quality both in open-sea and coastal areas,
but how does it perform as an indicator? Are
there restrictions to its use, for example in that it
indicates change not related to eutrophication?
Is it similarly applicable in different geographical areas?
15
In the study, I examined the long-term development of Secchi depth in the Baltic Sea, taking
advantage of observations made in the course
of a century of monitoring. I mainly investigated the development in the different open-sea
sub-basins, but took also a closer look at the
coastal areas surrounding Finland, where the
national monitoring program had taken place
since 1970.
Furthermore, I made an attempt of linking
the long-term development of Secchi depth to
changes in other parameters potentially affecting attenuation of light: mainly chlorophyll-a,
but in the coastal areas, also total organic carbon (TOC) and dissolved iron concentrations.
Unfortunately the long-term coastal dataset did
not allow comparison to suspended inorganic
matter.
Importantly, I studied the effect of the main
optical constituents on light attenuation in the
open sea through a bio-optical model setup, in
order to resolve how the Secchi depth indicator
should be applied in different parts of the Baltic
Sea. When investigating the use of Secchi depth
as an indicator of eutrophication, the application of the term eutrophication needed to be
GH¿QHG,GLVFXVVHGGLIIHUHQWLQWHUSUHWDWLRQVRI
eutrophication, and categorized them in attempt
to link theory to practice.
To facilitate future management, I characterized the Secchi depth indicator. I listed requirements and recommendations relevant for indicators in general, and inspected Secchi depth
in light of these.
Finally, I applied Secchi depth in the Baltic Sea eutrophication status assessment, and
discussed its role in light of the overall result.
I also explored alternative ways to apply the
indicator, to see how those would affect the
evaluation. Linking these results to the model
simulation, I was able to evaluate how well the
environmental target-setting of Secchi depth
and chlorophyll-a are aligned.
16
2 Materials and methods
2.1 The Baltic Sea
The study was conducted in the Baltic Sea, a
semi-enclosed brackish water basin situated
in north-eastern Europe (Figure 2-1). It has a
mean depth of 54 meters, and is separated from
the North Sea only by the narrow and shallow
Danish sounds. It expresses typically decreasing
salinity and temperature gradients (Leppäranta
and Myrberg 2009), and increasing amounts of
land-based dissolved organic matter (Hoikkala
et al. 2015), when moving from the south-western sound-area towards the north-eastern parts
of the sea. Both biotic and abiotic features vary
seasonally, as the sea is characterized by a dark
and cold winter and frequent ice-cover, followed
by the onset of a phytoplankton spring bloom
and subsequent a short summer with potential
cyanobacterial blooms. The sea is shallow, and
subject to considerable nutrient and carbon loads
from land, which together with the slow water
exchange make it vulnerable to eutrophication.
$FFRUGLQJWRWKHGH¿QLWLRQDJUHHGE\+(/COM, the Baltic Sea consists of 17 open-sea
sub-basins (HELCOM Monitoring and AsVHVVPHQW 6WUDWHJ\ KWWSZZZKHOFRP¿DFtion-areas/monitoring-and-assessment/monitoring-and-assessment-strategy, Papers I, III,
IV). Nine of these, characterized as the large
open-sea sub-basins within the Danish Sounds,
were included in this study: 1) the Arkona and
2) Bornholm Seas are southern sub-basins, separated from the Baltic Proper and each other by
an underwater sill. The Baltic Proper contains
the main volume of the sea, and is divided into
three smaller units: 3) the Western and 4) Eastern Gotland Basins and the 5) Northern Baltic
Proper. The 6) Gulf of Riga is a bay separated
from the Eastern Gotland Basin by islands and
an underwater sill. The 7) Gulf of Finland is a
direct north-eastern extension of the Northern
Baltic Proper. 8) The Bothnian Sea is separated
from the Northern Baltic Proper by the Archipelago Sea as well as a sill and a narrow but
deep sound. 9) The Bothnian Bay is a further
extension of the Bothnian Sea, separated from it
by the shallow Quark-area (Papers I, III and IV).
Figure 2-1. The Baltic Sea and its sub-basins. The countries sharing the Baltic Sea shoreline are also indicated (EST
= Estonia, DEN = Denmark, FIN = Finland, GER = Germany, LAT = Latvia, LIT = Lithuania, POL = Poland, RUS = Russia
and SWE = Sweden). Produced using shapefiles received from HELCOM
The coastal study was restricted to the Finnish waters, adjacent to the northern open-sea
sub-basins: the Gulf of Finland, Northern Baltic
Proper, Archipelago Sea, Bothnian Sea, Quark
and Bothnian Bay (Paper II). The areas differ
from each other geologically. The coast along
the Gulf of Finland is buffered from the open
sea by a narrow strip of islands. The Archipelago
Sea consists of mosaic archipelago between the
western Gulf of Finland and the southern Bothnian Sea. The coastlines of the Bothnian Sea
and Bothnian Bay are mainly open to the sea,
while the Quark consists of a shallow coastal
archipelago. All the coastal areas are strongly
LQÀXHQFHGE\IUHVKZDWHULQSXW3DSHU,9
2.2 The Secchi depth method
Secchi depth is described as the depth at which
a submerged white disk disappears from sight,
when viewed from above the surface. The
measurements are generally made by lowering
a 30 cm diameter white disk, attached to a line,
from a vessel. Both the color and the diame17
ter of the disk may however vary: It is usually
completely white, but also disks with white and
black sectors have been applied (though mostly in lakes). In the Finnish coastal areas, a 20
cm diameter white disc or top lid of a Limnos
water sampler is most common (paper II). The
observations during the bio-optical sampling
were made using a white disk of 30 cm diameter
(Paper III). This was apparently also the case
for the long-term open sea data from the 1940’s
RQZDUGWKRXJKLWFRXOGQRWEHFRQ¿UPHGVLQFH
the size or type of the disk was not always informed by the data providers. Aas et al. (2014)
estimated, that a considerable decrease of disk
size would reduce Secchi depth at least at levels
above 6 m, but in turbid or murky waters, such
as the Baltic Sea, the reported slight changes
in disk diameter were not expected to have had
a substantial effect in the measurement result
(Holmes 1970).
In the early 20th century, Secchi depth was
usually measured using a disk of 60 cm in diameter, observed by a water viewer (Figure 1).
7KLVZDVDVLJQL¿FDQWGLIIHUHQFHWRWKHPHWKodology applied later on, and was considered
to cause potential bias in the results through
overestimating long-term change. The measurements made prior to 1944 were therefor
corrected according to empirical testing results
from the Baltic Sea (Paper I and references
within).
Secchi depth measurements are known to
be affected by sun glitter, sea roughness and
lack of light (Preisendorfer 1986, Aas et al.
2014). According to the HELCOM COMBINE monitoring program for the Baltic Sea
(HELCOM 2015a), measurements are to be
restricted to circumstances with good visibility
and light conditions, when the sea is relatively
calm. Although not mentioned in the manual,
measurements are recommended to be taken
on the sunny side of the vessel (Tyler 1968).
The above instructions were applied during the
bio-optical cruises (Paper III). Information on
light and weather conditions was not available
from the long-term monitoring data (Papers I,
II, III), and could thus not be corrected for. They
were, however, assumed not to have temporal
18
or spatial patterns, and were taken to be of minor consequence to the long-term investigation.
2.3 Data and sources
The study relied mainly on Secchi depth monitoring observations, in some cases accompanied
by chlorophyll-a monitoring measurements at
the surface (Papers I-IV, Table 2-1). The monitoring had been conducted by national authorities of the Baltic Sea coastal states, and results
had been reported to national databases and/or
the HELCOM COMBINE database hosted by
the International Council for the Exploration
of the Sea (ICES). Some of the national data
were reached via the Baltic Environmental Database (BED), which is a distributed database
hub hosted by the Baltic Nest Institute (BNI).
The data used in the bio-optical modelling
exercise (Paper III, Table 2-1) included in-water optical measurements, chlorophyll-a water
samples and Secchi depth measurements. The
observations were collected on research cruises.
2.4 Approaches for interpreting
information
Various statistical and visualization approaches,
from data interpolation to modeling or use of
D VSHFL¿F DJJUHJDWLRQ WRRO ZHUH DSSOLHG IRU
interpreting the information (Table 2-2). Each
approach was chosen to suit the question and
dataset at hand.
Spatial aggregation
The HELCOM sub-basin -division (HELCOM 2013a) was applied when investigating
the open-sea areas of the Baltic Sea (Papers
I, III, IV). The division is based on information on the physico-chemical properties of the
DUHDVDLPLQJDWVXI¿FLHQWKDUPRQ\IRUDVVHVVment purposes within each sub-basin, and has
been commonly agreed by the Baltic coastal
states via HELCOM. During the last decades,
the sub-basin-division had been borne in mind
when planning the common Baltic Sea monitoring programme (HELCOM 2015a).
Table 2-1. The datasets used in the thesis, as well as the included parameters, spatial scale, time-period, temporal or
vertical cropping applied, the sources of data and the paper(s) the data was used in. The details are fully presented in
the papers.
Dataset
Parameters
Spatial scale
Time-period
Cropping
Sources
Presented
Oceanographic data
Secchi depth
Open Baltic
Sea
1903-2012
Months:
Jun-Sep
Dec-Feb
ICES - OCEAN
database SMHI SHARK database
IMWM - oceanographic database
LIAE Centre of
Marine Research
in Lithuania
Papers I, III
Introduction
Finnish
national
monitoring
data
Secchi depth
Finnish coast
1975-2011
SYKE
Paper II
HELCOM
eutrophication assessment data
Secchi depth
Open Baltic
Sea
2007-2011
Months:
Jun-Sep
ICES – HELCOM
COMBINE database BNI – BED
database
Paper IV
Surface data
Secchi depth
chlorophyll-a
Open Baltic
Sea
1972-2012
Depth (of
chlorophyll-a):
0–2m
SYKE
Papers I, III
In-water
optical data
Secchi depth
chlorophyll-a
aCDOM(ʄ) aʔ(ʄ)
b(ʄ) bb(ʄ)
aCDOM(ʄ)
aCDOM
Open and
coastal Baltic
Sea
2008-2011
Stefan Simis
(Finnish Environment Institute
SYKE / Plymouth
Marine Laboratory) and Tiit
Kutser (Estonian
Marine Institute)
Paper III
Table 2-2. Methods used for data analysis in papers I-IV.
Paper
I
II
Spatial
interpolation
Time
series
X
X
X
Trend
analysis
Comparison
of timeperiods
Correlation
analysis
X
X
III
IV
As the borders between the open-sea and
FRDVWDO ]RQH ZHUH ¿UVW GH¿QHG E\ +(/&20
only in 2013 (J. Kaitaranta, Baltic Marine Environment Protection Commission, personal
communication; HELCOM 2009; HELCOM
2014), they were not exactly identical in Papers
III, IV (submitted, 2015, respectively) to those
applied in Paper I (2012), where the division
between the coastal and open sea was estimated
by the authors. The difference was considered to
KDYHDVLJQL¿FDQWHIIHFWRQO\WKHVSDWLDOFRQWH[W
of the Gulf of Riga, where the shallow northern
SDUWZDVQRWGH¿QHGFRDVWDOEHIRUH$VD
precaution, the long-term time-series of Paper I
Regression
analysis
Modelling
Assessment
tool
X
X
X
X
X
X
were updated with the new sub-basin division,
including additional data from 2011-2015. Also less important changes, such as renaming
the Bornholm Basin as Bornholm Sea, were
introduced in 2013 (J. Kaitaranta, Baltic Marine
Environment Protection Commission, personal
communication; HELCOM 2009; HELCOM
2014).
The HELCOM sub-basin division, originally
designed for assessment purposes, was considered to be useful in a long-term study, where
monitoring was not restricted to constant stations. The division of the Baltic Sea into smaller units, which takes into account most of the
19
spatial gradients affecting eutrophication-related parameters, and relies on extensive expert
ZRUN ZDV WDNHQ WR EH VXI¿FLHQWO\ KDUPRQLous. Some exceptions, with chlorophyll-a and
CDOM gradients within sub-basins potentially
causing gradients in Secchi depth, were identi¿HG,QWKHVHDUHDVVSDWLDOO\ELDVHGPRQLWRULQJ
would have the potential to result in a biased
outcome. Interpolation maps were used to examine the possible spatial gradients within the
open-sea assessment units (Paper I).
In the study on Finnish coastal zone (Paper
II), where monitoring was restricted to consistent stations, a station-wise analysis was applied.
In these areas, the varying land-sea interplay
and bottom topography affect hydrographical
as well as biochemical conditions to the extent
that interpolating data between stations was not
found to be an ideal solution. In order to make
generalizations and interpret the results, each
station was additionally assigned to a HELCOM sub-basin.
er, accompanying the box-and-whiskers -plots,
the method was useful.
The century-long open-sea dataset was extensive, although the number of observations varied and periods with missing data occurred. In
this type of a situation, temporal changes could
EHLGHQWL¿HGWKURXJK¿WWLQJDORFDOO\ZHLJKWHG
scatterplot smoothing (LOESS) curve (Paper
I). The approach was able to detect non-linear
FKDQJHEXWGLGQRWUHTXLUHWKHVSHFL¿FDWLRQRID
function, and as a consequence, did not provide
a quantitative estimate of change. It however
enabled overcoming gaps in data, and through
WKH FRQ¿GHQFH LQWHUYDO DOVR LQFRUSRUDWHG WKH
subsequent uncertainty caused into the result.
A clear quantitative signal of change in opensea Secchi depth was achieved by comparing
time-periods (Student’s t-test). This approach
relies on the assumption of linear change between the compared periods. The periods were
therefore chosen to represent levels of minimum and maximum Secchi depth, determined
through visual inspection of the LOESS-curves.
Temporal change
The application of annual box-and-whisker
plots, showing the yearly median, percentiles
and outliers, was a simple approach for visualizing change and variation in Secchi depth
(Paper II). The method made no expectations
on normality, and presented explicitly the variation as well as the possible gaps in the data.
It also facilitated the detection of “breaking
points”. It was found to be especially suitable
for interpreting the Finnish coastal data, which
was regularly monitored, yet scarce at certain
stations. No generalizations on the development
of Secchi depth were however provided by this
approach.
An extremely generalizing method, applied
together with the box-and-whisker plots on Secchi depth at the Finnish coastal stations, was
trend analysis (Paper II). The method was found
appropriate when consistent long-term change
was expected and searched for. It was however
not able to detect a trend in the case of scarce
data. Furthermore, it assumed linearity, and was
thus not optimal at stations where reverse or
non-linear trends might have occurred. Howev20
Estimating relationship with other
parameters
The dependence between Secchi depth and the
variables expected to affect light attenuation
in the coastal zone was investigated from the
Finnish long-term data (Paper II). Consistent
long-term monitoring had been conducted for
chlorophyll-a, TOC and dissolved iron, and
their correlation to Secchi depth was tested.
An additional relevant parameter in the coastal zone, total suspended inorganic matter, had
WR EH OHIW RXW RI WKH VWXG\ GXH WR VLJQL¿FDQW
changes in sampling methodology. The Partial
Pearson’s method was chosen for investigations
on correlation, in order to discard the possible
effect of multiple correlations between more
than one parameter. The disadvantage of using this approach was its ability to detect only
linear relationships. Examinations of correlation matrices did not lead to suspect non-linear
dependences of Secchi depth to TOC or iron,
though non-linear relationships seemed to occur between Secchi depth and chlorophyll-a.
The relationship of Secchi depth and chlorophyll-a in the open sea areas were described
through non-linear regression of maximum
values (Paper I). The use of maximum values
DOORZHG¿OWHULQJRXWWKHVLWXDWLRQVZKHUH6HFFKL
depth was affected by multiple parameters, and
helped to distinguish the situations with clear
dependence only between the two parameters.
The bio-optical model
A bio-optical model setup was constructed,
in order to describe the relationship of Secchi depth and the main constituents affecting
light attenuation in the open sea: chlorophyll-a
and CDOM (Paper III; Figure 2-2). The setup consisted of several steps. In-water optical
measurements were used as input parameters to
a generic optical model, which produced outcome suitable for the radiative transfer model.
The latter model was able to describe the path
of radiation from above the surface to the depth
where the white disk disappears from sight,
WDNLQJLQWRDFFRXQWUHÀHFWLRQDEVRUSWLRQDQG
scattering properties of the water at different
depths. In the third step, the in-water optical
measurements were applied to calibrate the outcome of the radiative transfer model. The three
steps were performed separately for the spring
(April) and summer (July-August) periods.
BIO-OPTICAL MODEL
spring
In-water
opƟcal
measurements
2008-2011
summer
Generalizing
Generalizing
model
opƟcal properƟes
Step 1
spring
summer RadiaƟve
transfer model
RadiaƟve transfer
Step 2
spring
Model
calibraƟon
Model calibraƟon
summer
Step 3
Modelled Secchi depth
...
Monitoring
data
1972-2011
spring, basin 2
summer, basin 2
spring, basin 1
ValidaƟon of
ValidaƟon of
model output
ValidaƟon
of
model output
ValidaƟon
of
summer,
basin 1
model output
AdjusƟng
model
output
model output
Historically adjusted modelled Secchi depth
Figure 2-2. The bio-optical model setup which was followed by adjustment based on historical monitoring data.
Adapted from Paper III.
21
7KH¿QDORXWFRPHRIWKHWKUHHPRGHOVWHSV
the so-called modelled Secchi depth, was evaluated against monitoring data from 1972-2012,
containing simultaneous observations of Secchi
depth and chlorophyll-a near the surface. In
the absence of corresponding measurements of
&'20FRQVWDQWEDVLQVSHFL¿F&'20DEVRUStion values were used. These values were adopted for the southern areas from a study conducted by Stedmon et al. (2000), and for the central
and northern areas from a study conducted by
Ylöstalo et al. (P. Ylöstalo, J. Seppälä and S.
Kaitala, Finnish Environment Institute, pers.
FRPP6XEEDVLQVSHFL¿FUHJUHVVLRQPRGHOV
based on the evaluation were used to adjust the
model outcome, in order to inline it with the
historical monitoring data.
The HELCOM Eutrophication
Assessment Tool (HEAT)
The Secchi depth indicator results were calculated and subsequently combined to results of
other indicators, in order to perform an overall
eutrophication assessment (Paper IV). This was
done using a hierarchical framework, the HELCOM Eutrophication Assessment Tool (HEAT;
Andersen et al. 2011), that allowed the implementation of the EU Marine Framework Directive (Anonymous 2008, 2010) requirements
into a quantitative multi-parametric status assessment. The tool was based on weighted averaging and determination through worst status
(e.g. the one-out-all-out approach).
6XEEDVLQ VSHFL¿F LQGLFDWRU UHVXOWV 6WHS
1 in Figure 2-3) included the estimated level
DW D SUHGH¿QHG DVVHVVPHQW SHULRG (6 UHSresented in the study by the average summer
(June-September) value in 2007-2011. The
actual indicator status was expressed through
the eutrophication ratio (ER). For indicators
responding negatively to eutrophication pressures, as Secchi depth does, ER was calculated
DVDUDWLREHWZHHQWKHEDVLQVSHFL¿FHQYLURQmental target and ES. An indicator reaching it’s
environmental target resulted with a ER equal
to or above one.
To perform the assessment, Secchi depth
was combined with four other eutrophication
22
indicators, namely dissolved inorganic nitrogen
(DIN), dissolved inorganic phosphorus (DIP),
chlorophyll-a and oxygen debt (Paper IV). The
indicators were aggregated into groups (Step 2
in Figure 2-3). The groups were constructed to
H[SUHVVWKHVHJPHQWHGGH¿QLWLRQRIHXWURSKLFDtion (Figure 1-2), excluding allochthonous eutrophication in lack of suitable indicators. In the
HELCOM eutrophication status assessment,
Secchi depth was included to autochthonous
eutrophication, together with chlorophyll-a
(Figure 2-3a). An alternative interpretation of
including Secchi depth to indirect eutrophication, along with oxygen debt, was tested as well
(Figure 2-3B). At the lower level of hierarchy,
weighted averaging was applied, to allow indicators in the same aggregation group to outweigh each other when their outcomes differed.
The outcome of each aggregation group could
EHFODVVL¿HGWREHLQ*RRG(QYLURQPHQWDO6WDWXV*(6ZKHQWKHDYHUDJH(5”
Finally, the worst outcome of the three aggregation groups determined the overall eutrophication status (Step 3 in Figure 2-3). This
so-called one-out-all-out principle was applied
here at the highest hierarchical level of the assessment. The overall eutrophication status was
derived separately for each sub-basin, and clasVL¿HGWRH[SUHVV*(6ZKHQ(5”
3 Results
3.1 Development of Secchi depth in
the open sea (Paper I)
In the beginning of the 20th century (1905 –
1909), the average summer-time Secchi depth
was 7.9 – 9.4 m in all open-sea sub-basins except the Gulf of Riga (4.3 m). No clear spatial
trend could be distinguished. The sub-basins
with highest levels were the Northern Baltic Proper (9.4 m) and Bothnian Sea (9.1 m);
though also the southern areas, the Arkona Sea
(8.5 m) and Bornholm Sea (8.8 m) expressed
high levels.
In the open sub-basins, summer-time Secchi
depth decreased 14 – 44 % during the last 100
years. Between the early 1900’s (1905 – 1909)
A
STEP 1:
Indicators
DIN
DIP
Chlorophyll-a
Causal
eutrophicaƟon
STEP 2:
AggregaƟon
STEP 3:
Assessment
Secchi depth
Autochthonous
eutrophicaƟon
Oxygen debt
Indirect
eutrophicaƟon
OVERALL EUTROPHICATION STATUS
B
STEP 1:
Indicators
STEP 2:
AggregaƟon
STEP 3:
Assessment
DIN
Causal
eutrophicaƟon
DIP
Chlorophyll-a
Secchi depth
Autochthonous
eutrophicaƟon
Oxygen dept
Indirect
eutrophicaƟon
OVERALL EUTROPHICATION STATUS
Figure 2-3. Two optional ways of applying the Secchi depth indicator in an aggregation theme, depending on how
it is interpreted to express eutrophication. The indicators: DIN = dissolved inorganic nitrogen concentration (winter, surface), DIP = dissolved inorganic phosphorus concentration (winter, surface), chlorophyll-a concentration
(summer, surface), Secchi depth (summer), oxygen debt (annual). Adapted from Paper IV.
and the early 2000’s (2005 – 2009), a decrease
of 1.2 – 4.0 m, averaging 0.012 – 0.040 m y-1,
was observed in most open-sea sub-basins. The
decrease was most pronounced in the northern
areas: the Northern Baltic Proper, Bothnian
Bay, Gulf of Finland and Bothnian Sea; but
clear also in the Western Gotland Basin, Eastern Gotland Basin, Bornholm Sea and Arkona
Sea (Table 1 in Paper I; Figure 3-1).
'XHWRVLJQL¿FDQWGDWDJDSVLQWKHHDUO\DQG
mid-1900’s, identifying the full development
of Secchi depth throughout the century was not
possible in any of the open-sea areas. Continuous monitoring took place since the 1960’s –
1970’s in most sub-basins, and revealed periods
of intense Secchi depth decrease (Paper I; Figure 3-1). This late decrease was most distinct
in the Arkona Sea, Bornholm Sea and Eastern
Gotland Basin in 1960 – 1980, and in the more
northern areas after 1980. Observations from
WKH¿UVWKDOIRIWKHFHQWXU\GLVFORVHGKRZHYHU
that substantial decrease had taken place already earlier, especially in the northern areas.
In the early 21st century (2005 – 2009), the
average summer-time Secchi depth was 3.1 –
7.3 m, with a slight decreasing north-eastward
gradient. The lowest summer averages were
observed in the Gulf of Riga and the Gulf of
Finland, 3.1 m and 4.4 m respectively, and the
highest values in the Arkona Sea (7.3 m) and
Bornholm Sea (6.6 m) (Paper I; Figures 3-1
and 3-2).
3.2 Temporal changes of Secchi
depth along the Finnish coast
(Paper II)
The yearly average Secchi depth varied between 2.2 and 5.9 meters during 1975 – 2011
in the Finnish coastal stations. The highest average values were observed in the western coasts
of the Gulf of Finland and in the southwestern
23
Bothnian Bay
20
n =715
Bothnian Sea
20
n =939
Gulf of Finland
20
15
15
15
10
10
10
5
5
5
0
0
0
1900 1920 1940 1960 1980 2000
1900 1920 1940 1960 1980 2000
n =514
1900 1920 1940 1960 1980 2000
Northern Baltic Proper
Western Gotland Basin
20
20
n =802
Gulf of Riga
20
15
15
15
10
10
10
5
5
5
0
0
0
1900 1920 1940 1960 1980 2000
1900 1920 1940 1960 1980 2000
n =861
20
n =1535
Eastern Gotland Basin
20
15
15
15
10
10
10
5
5
5
0
0
0
1900 1920 1940 1960 1980 2000
1900 1920 1940 1960 1980 2000
n =741
1900 1920 1940 1960 1980 2000
Bornholm Sea
Arkona Sea
20
n =593
n =1139
1900 1920 1940 1960 1980 2000
Figure 3-1. Long-term development of summer-time (June to September) Secchi depth in the Baltic sub-basins
between 1903 and 2015. A LOESS curve with 95% confidence intervals (solid black lines) is fitted to the data. The
number of observations (n) is given in the upper left corner of each plot. From Paper I, updated to 2015.
Archipelago Sea. The lowest values were observed in the inner parts of the Archipelago Sea
and in the Bothnian Sea (Paper II).
Between 1985 and 2011, trends could not be
detected in more than half of the coastal staWLRQV:KHUHFOHDUDQGVLJQL¿FDQWWUHQGVZHUH
found, however, they were consistent within
the basin: in the Archipelago Sea, Secchi depth
24
decreased, whereas in the coastal Bothnian Sea,
Quark and the Bothnian Bay, it increased. The
strongest increasing trends were found in the
northern stations of the Bothnian Sea (Paper II).
No clear non-linear patterns in the development of Secchi depth were detected in the Gulf
of Finland (Figure 3-3). An intense period of
decrease was observed in stations from the Ar-
65°00’
65°00’
64°00’
64°00’
63°00’
63°00’
62°00’
62°00’
61°00’
61°00’
60°00’
60°00’
59°00’
59°00’
58°00’
58°00’
57°00’
57°00’
56°00’
56°00’
55°00’
55°00’
54°00’
54°00’
12°00’ 14°00’ 16°00’ 18°00’ 20°00’ 22°00’ 24°00’ 26°00’ 28°00’ 30°00’
12°00’ 14°00’ 16°00’ 18°00’ 20°00’ 22°00’ 24°00’ 26°00’ 28°00’ 30°00’
Secchi
(m)
65°00’
65°00’
64°00’
64°00’
16
63°00’
63°00’
14
62°00’
62°00’
12
61°00’
61°00’
60°00’
60°00’
59°00’
59°00’
58°00’
58°00’
57°00’
57°00’
56°00’
56°00’
55°00’
55°00’
10
8
6
4
2
0
54°00’
54°00’
12°00’ 14°00’ 16°00’ 18°00’ 20°00’ 22°00’ 24°00’ 26°00’ 28°00’ 30°00’
12°00’ 14°00’ 16°00’ 18°00’ 20°00’ 22°00’ 24°00’ 26°00’ 28°00’ 30°00’
Figure 3-2. Spatial variation of summer-time (June to September) Secchi depth (meters, m) in the open Baltic Sea
in (A) 1905-09, (B) 1930-34, (C) 1971-75 and (D) 2005-09. The colors used to refer to different Secchi depths are
indicated in the scale bar. The sites of the observations are marked with crosses and large areas with missing data
are left blank. From paper I
chipelago Sea, from the early 1980’s to the late
1990’s. In the northern Bothnian Sea and the
southern Quark, the increasing trend in 1985
– 2010 was actually lead by a clear period of
decrease between 1976 – 1985, but this was
not observed in any of the other Bothnian Sea,
Quark or Bothnian Bay stations.
3.3 Changes in Secchi depth
in relation to other parameters
(Papers I, II)
$VLJQL¿FDQWQHJDWLYHFRUUHODWLRQEHWZHHQ6HFchi depth and chlorophyll-a was found at 12 of
the 20 coastal stations. The correlation varied
between -0.22 and -0.69, and strong correlations (exceeding -0.40) were found in all basins
H[FHSWWKH4XDUN3DSHU,,$VLJQL¿FDQWWUHQG
in chlorophyll-aZDVREVHUYHGRQO\DW¿YHRI
the 20 stations, but in three of these it was ac25
Secchi depth (m)
GOF-1
GOF-2
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
Secchi depth (m)
1975 1979 1983 1987 1991 1995 1999 2003 2007
GOF-5
GOF-6
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
ARC-1
Secchi depth (m)
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
GOF-4
1975 1979 1983 1987 1991 1995 1999 2003 2007
ARC-2
ARC-3
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
ARC-4
Secchi depth (m)
GOF-3
10
1975 1979 1983 1987 1991 1995 1999 2003 2007
ARC-5
ARC-6
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
26
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
1975 1979 1983 1987 1991 1995 1999 2003 2007
Secchi depth (m)
BS-1
BS-2
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
Secchi depth (m)
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
QU-1
1975 1979 1983 1987 1991 1995 1999 2003 2007
BB-1
QU-2
10
10
10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
1975 1979 1983 1987 1991 1995 1999 2003 2007
BB-2
Secchi depth (m)
BS-3
10
10
10
8
8
6
6
4
4
2
2
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
BB-3
0
1975 1979 1983 1987 1991 1995 1999 2003 2007
1975 1979 1983 1987 1991 1995 1999 2003 2007
Figure 3-3. Secchi depth (m) in the Finnish coastal stations, measured during the months June-September. The
locations of the stations are presented in Paper II (GOF = Gulf of Finland, ARC = Archipelago Sea, BS = Bothnian
Sea, QU = Quark, BB = Bothnian Bay). The line is set at the median, box includes 50% of the observations, whiskers include 75% of the observations and circles express the observation falling outside the 75% range. Note that
early years prior to 1985 were not included in the trend analysis presented in Paper II.
27
FRPSDQLHGE\DVLJQL¿FDQWRSSRVLQJWUHQGLQ
Secchi depth (Paper II).
In the open Baltic Sea, chlorophyll-a concentration was regularly low at high levels of
Secchi depth, but varied substantially when the
levels of Secchi depth were low. When applying
QRQOLQHDUUHJUHVVLRQDPRGHO¿WFRXOGQRWEH
obtained for the entire data pool. Yet a very
JRRG¿WZDVDFKLHYHGZKHQUHODWLQJWKHPD[Lmum chlorophyll-a value to Secchi depth values at 0.5 m intervals (Paper I). This leads to the
VXJJHVWLRQWKDWXQGHUVSHFL¿FFLUFXPVWDQFHV
Secchi depth is strongly connected to chlorophyll-a, although overall, such a connection is
KDUGWR¿QG
6HFFKL GHSWK FRUUHODWHG VLJQL¿FDQWO\ QHJDtively with iron in 12 of the 20 coastal stations.
Strong correlations (exceeding -0.40) were
found in all basins, but mostly in the Bothnian
Bay, Quark and Bothnian Sea. In the latter two,
strong negative trends in iron were accompanied with positive trends in Secchi depth. A
relationship between Secchi depth and TOC
could not be found along the Finnish coast; a
VWURQJVLJQL¿FDQWQHJDWLYHFRUUHODWLRQH[FHHGing -0.40) was found only at one station. This
implies, that the connection between Secchi
depth and TOC is not nearly as strong as the
relationship between Secchi depth and chlorophyll-a.
3.4 The Secchi depth indicator
(Paper III)
The bio-optical modelling exercise revealed,
that Secchi depth is sensitive to changes in both
chlorophyll-a and CDOM absorption (aCDOM),
at the ranges typical to the Baltic Sea (Paper
III). This is particularly the case when either of
the parameters is at a low level. The effect of
chlorophyll-a was stronger in the spring than
during summer.
The Secchi depth targets agreed by HELCOM, when modelled against the ones for
chlorophyll-a, were not aligned (Paper III,
Figure 3-4). In all open-sea basins, the Secchi
depth target was more ambitious than the chlorophyll-a target. This difference exceeded 1 m
Target
8
Simulated target
Secchi depth (m)
6
4
2
0
BB
BS
GoF
GoR
NBP
EGB
WGB
Bor
Ark
Figure 3-4. Comparison of the environmental targets of Secchi depth and chlorophyll-a (Paper IV): The actual
target for Secchi depth (“Target”, dark bars) and the simulated Secchi depth at the target level of chlorophyll-a
(“Simulated target”, light bars), in the open Baltic Sea sub-basins (BB = Bothnian Bay, BS = Bothnian Sea, GoF =
Gulf of Finland, GoR = Gulf of Riga, NBP = Northern Baltic Proper, EGB = Eastern Gotland Basin, WGB = Western Gotland Basin, Bor = Bornholm Sea, Ark = Arkona Sea). The simulation could not be done for EGB, due to
lacking information on CDOM absorption (aCDOM) in the area. From Paper III.
28
(in Secchi depth) in the Bothnian Bay, Bothnian Sea, Gulf of Riga, Western Gotland Basin,
Bornholm Sea and Arkona Sea.
The sensitivity of Secchi depth to natural variation in aCDOM (approximated as the variation
between the 24.5 to the 97.5 percentiles) was
estimated using the optical model. It was highest in the Bothnian Bay, resulting in a variation
1 m in Secchi depth, when modelled at close
to the environmental target level. Secchi depth
responded with a variation of 0.5 m to natural
variation in aCDOM in the Bothnian Sea and Gulf
of Finland. In the central and southern sub-basins, no clear effect could be seen (Paper III).
3.5 Secchi depth in the
eutrophication assessment
According to the HELCOM eutrophication status assessment 2007 – 2011, Secchi depth ES
was below the environmental target in most of
the open sub-basins (Paper IV; Table 3-1). In
the Bothnian Bay, Secchi depth ES was exactly
at the target level. The highest divergences between the Secchi depth ES and the target were
observed in the main Baltic proper: the Western Gotland Basin, Eastern Gotland Basin and
Northern Baltic Proper. Also ER was highest in
these sub-basins.
The ES’s of the other eutrophication indicators, namely DIN, DIP, chlorophyll-a and
oxygen debt, were mostly estimated to be below the targets as well (Table 3-2). The ES’s
of Chlorophyll-a and DIN were slightly above
the target in the Gulf of Riga, and DIP ES was
clearly above the target in the Bothnian Bay.
2[\JHQGHEW(6GHULYHGRQO\LQ¿YHVXEEDsins, was not above the target in any of these. Of
the indicators applied, the ER of Secchi depth
was highest in the Gulf of Finland and Bothnian
Sea; it was not lowest in any of the sub-basins
(Paper IV; Table 3-2). Averaging over all the
sub-basins, Secchi depth ER was second lowest, when comparing to the ER’s of the other
indicators.
All nine open Baltic Sea sub-basins were assessed as eutrophied by the HELCOM eutrophication status assessment (Paper IV). In the assessment, Secchi depth was applied as a direct
effect of eutrophication (or in other words as
an indicator of autochthonous eutrophication,
Figure 3-6A). As also other indicators failed
to reach the target in all the sub-basins, Secchi
depth was never the indicator determining overall eutrophication status. On the other hand, due
to the grouping together with chlorophyll-a,
which frequently expressed even worse ER,
Secchi depth ended up improving the overall
eutrophication in eight sub-basins, though never to the extent of reaching GES.
The use of Secchi depth as an indicator of
indirect eutrophication was also tested (Figure
3-6B). Alterating the aggregation approach did
not affect the overall estimate on eutrophication
status in terms of reaching GES. When removed
from autochthonous effects, Secchi depth was
no longer averaged with chlorophyll-a, which
decreased the status of autochthonous eutrophication in all sub-basins except the Gulf of Riga.
The ER of Secchi depth and oxygen debt were
at similar levels, and subsequently the introduction of Secchi depth did not markedly affect the
status of indirect eutrophication in the sub-basins where oxygen debt was applied. Introducing Secchi depth to indirect eutrophication had
WKHSRWHQWLDORILQFUHDVLQJWKHFRQ¿GHQFHLQWKH
assessment of the Bothnian Bay, Bothnian Sea,
Gulf of Riga and Bornholm Basin, where no
other indicator of indirect eutrophication was
available.
29
Table 3-1. The results of the Secchi depth indicator for 2007-2011: estimation of Secchi depth (ES), standard deviation
between the assessment years, the commonly agreed environmental target (or GES-boundary), the difference between
the target and ES and the eutrophication ratio (ER; ratio between GES boundary and ES). From data used in Paper IV.
Secchi depth
estimation
2007-2011 (m)
Standard deviation
between years
2007-2011 (m)
Target
(ET, m)
Difference betw.
estimation and
target (m)
Secchi depth
eutrophication
ratio (ER)
Arkona Sea
5.8
0.9
7.2
-1.4
1.24
Bornholm Sea
5.6
0.5
7.1
-1.5
1.29
Eastern Gotland Basin
6.0
0.7
7.6
-1.6
1.27
Western Gotland Basin
5.7
0.8
8.4
-2.7
1.47
Northern Baltic Proper
5.5
0.3
7.1
-1.6
1.29
Gulf of Riga
4.0
0.6
5.0
-1.0
1.25
Gulf of Finland
4.8
0.5
5.5
-0.7
1.15
Bothnian Sea
6.5
0.6
6.8
-0.3
1.05
Bothnian Bay
5.8
0.4
5.8
0.0
1.00
Sub-basin
Table 3-2. The eutrophication ratios (ER) for the indicators applied in the eutrophication status assessment 2007 –
2011. ER ≤ 1 (marked in bold) indicates that the target has been met and that the indicator is in good status. From data
used in Paper IV.
Sub-basin
Secchi depth
Chlorophyll-a
DIN
DIP
Arkona Sea
1.24
1.48
1.29
1.72
Bornholm Sea
1.29
2.07
1.19
2.02
1.12
Eastern Gotland Basin
1.27
1.72
1.32
1.86
1.22
Western Gotland Basin
1.47
2.35
1.41
1.76
1.22
1.22
Northern Baltic Proper
1.29
1.69
1.29
1.99
Gulf of Riga
1.25
0.91
0.93
1.54
Gulf of Finland
1.15
1.52
2.07
1.44
Oxygen debt
1.22
Bothnian Sea
1.05
1.66
1.31
1.37
Bothnian Bay
1.00
1.17
1.31
0.64
Average
1.22
1.62
1.35
1.59
1.20
Minimum
1.00
0.91
0.93
0.64
1.12
Maximum
1.47
2.35
2.07
2.02
1.22
30
2.5
A
Causal
Autochthonous
Indirect
Secchi depth (m)
2.0
1.5
1.0
0.5
BB
2.5
BS
GOF
GOR
NBP
WGB
EGB
Bor
GOF
GOR
NBP
WGB
EGB
Bor
Ark
B
Secchi depth (m)
2.0
1.5
1.0
0.5
BB
BS
Ark
Figure 3-5. Status for causal, autochthonous and indirect eutrophication, expressed as eutrophication ratio (ER).
The Secchi depth indicator is applied in alternative ways: A) as direct effect or autochthonous eutrophication and
B) as indirect eutrophication (see Fig. 2-3). The overall eutrophication status was determined by the group with
highest ER, and Good Environmental Status (GES) is reached when ER ≤ 1 (dashed line). Adapted from Paper IV
31
4 Discussion
4.1 Long-term development of
Secchi depth
Summer-time Secchi depth decreased during
the last century throughout the open Baltic Sea
(Paper I). Proportionally this decrease was substantial. The change was generally not linear,
varying from strong decrease to slight increase.
Especially in the northern areas, the previous
three decades was a time of intense decrease.
Also in the adjacent seas, the Kattegat-Skagerrak area and North Sea, this was a time of Secchi depth decrease (Dupont and Aksnes 2013,
Capuzzo et al. 2015).
The timing and intensity of Secchi depth decrease varied in different parts of the Baltic Sea
(Papers I, II; Table 4-1). The open Baltic Sea
was spatially more homogenous in the early
20th century, with average summer time Secchi depth generally between 8 and 9 m, than
at present. The only exception was the Gulf of
Riga, represented by very scarce data, expressing low Secchi depth already in the early years
of monitoring.
In the open Bothnian Bay, and to some extent
also the Bothnian Sea, changes were evident already some decades earlier than in the adjacent
sub-basins, before the the 1970’s. These were
possibly caused by increased input of CDOM,
due changes in the drainage area. A barely detectable increase occurred again between the
1970’s and the 1980’s, followed by a decade of
intense decrease. Most of the adjacent Finnish
coastal sites included in the study showed contradictory simultaneous increase. It is apparent,
that at least at that time, the coast and open sea
were subject to different processes. Dissolved
iron, which was possibly one of the substances
determining Secchi depth at the coast, is often
ÀRFFXODWHGDQGVXEVHTXHQWO\EXULHGQRWHQWHUing the open sea system in large quantities (Paper II). In the open Bothnian Sea, on the other
hand, the increasing chlorophyll-a concentration (Fleming-Lehtinen et al. 2008) probably
initiated the decreases in Secchi depth.
In the Gulf of Finland, Northern Baltic Proper, Gulf of Riga and the Western Gotland Basin,
32
a drastic decrease in Secchi depth started in
the 1980’s. The Archipelago Sea, a coastal area
adjacent to the Northern Baltic Proper and the
Gulf of Finland, showed similar trends. During
this period, the frequency and intensity of cyanobacterial blooms increased as well (Finni et
al. 2001, Suikkanen et al. 2007, Kahru 2014),
probably being the main cause of the change
in water clarity.
Secchi depth ceased to decrease in the southern Baltic sub-basins (Arkona Sea, Bornholm
Basin and to some extent also the Eastern Gotland Basin) in the 1980’s, and even increased
thereafter (Paper I). A similar pattern was observed in the North Sea by Dupont and Aksnes
(2013).
4.2 The relationship of Secchi
depth and eutrophication
An increase in autotrophic biomass (measured
as chlorophyll-a concentration or phytoplankton biomass) coinciding with the long-term decrease in Secchi depth was observed throughout the Baltic Sea (Wasmund and Uhlig 2003,
Fleming-Lehtinen et al. 2008,). This supports
the assumption, that changes in Secchi depth
were a subsequence of phytoplankton increase
in many areas. This was witnessed in the open
sea as dependence on chlorophyll-a concentration, both through regression models (Paper I)
and bio-optical simulation models (Paper III).
In coastal areas, it was shown through correlations in long-term data (Paper II). As a result of
sensitivity to phytoplankton, Secchi depth can
EHPRWLYDWHGWREHVLJQL¿FDQWO\OLQNHGWRDXtochthonous eutrophication, even in the Baltic
Sea (Figure 4-1).
Baltic Sea Secchi depth was shown to be
strongly affected also by concentrations of
CDOM, causing increased absorption of light
(Paper III). Dissolved organic matter may be
partly produced autochthonously by phytoplankton or heterotrophic organisms (Hoikkala
et al. 2015). The magnitude of this eutrophication-related fraction of CDOM should be
resolved, when applying Secchi depth as an
environmental indicator.
33
Gulf of Finland, coastal
Gulf of Finland, open
Northern Baltic Proper,
open
Archipelago Sea
Bothnian Sea and S Quark,
coastal
Bothnian Sea, open
Bothnian Bay and N Quark,
coastal
Bothnian Bay, open
Year
1920
1940
1960
Dissolved iron
.........͘͘͘͘͘ќ͘͘͘͘͘......
…..͙͙͘͘͘͘њ .............
……….͘͘͘͘͘͘͘͘͘͘͘͘͘њ……................
....................͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘ќ͘͘͘͘͘͘͘͘͘͘....................................
..........͘͘͘͘њ͘͘͘͘͘͘͘.....
͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘ќ͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘
PAPER II
PAPER II
Secchi depth
TOC
Fleming-Lehtinen et al. 2008
PAPER I
Fleming-Lehtinen et al. 2008
PAPER I
PAPER II
PAPER II
PAPER II
PAPER II
PAPER II
Fleming-Lehtinen et al. 2008
PAPER I
PAPER II
PAPER II
PAPER II
PAPER I
Reference
Chlorophyll-a
Secchi depth
Chlorophyll-a
Secchi depth
TOC
Secchi depth
........͘͘͘͘͘͘ќ͘͘͘͘͘͘......
……͘͘͘͘͘͘͘͘͘ќ͘͘͘͘͘͘.....
Dissolved iron
.........͘͘͘͘͘ќ͘͘͘͘͘͘͘.....
Chlorophyll-a
Secchi depth
.........͘͘͘͘͘њ͘͘͘͘͘͘......
…..͘͘͘͘͘͘͘͘͘њ͘͘͘.........
Chlorophyll-a
............͘͘њ…..….....
Secchi depth
TOC
Secchi depth
………͙͘͘͘͘њ ...........
Secchi depth
Parameter
........͘͘͘͘͘͘͘њ͘͘͘͘͘͘͘͘͘͘
2000
.........͘͘͘͘͘͘ќ͘͘͘͘͘͘͘͘͘͘
1980
͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘ќ͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘
.........͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘ќ͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘͘
1900
Table 4-1. Reported periods of increase (upward arrow with dots) or decrease (downward arrow with dots) of Secchi depth and parameters expected to contribute to water
clarity, based on monitoring data. Red indicates development that parallel to decreasing water transparency, while green indicates development that is parallel to increasing water
transparency. The coastal and open-sea areas adjacent to each other are grouped together (map in Figure 2-1). TOC = total organic carbon concentration, N = northern, S = southern, open = open sea.
In the Baltic Sea, CDOM is to a large extent derived from land (Sandberg et al. 2004,
Alling et al. 2008, Kulinski & Pempkowiak
2011, Hoikkala et al 2015). The input of organic
matter to the sea is particularly strong (Asmala
2012, Räike et al. 2015), and increase of water
color in the major rivers discharging to the sea
has been documented (Kritzberg and Ekström
2012). It is thus probable, that Secchi depth,
through the CDOM component, has an increasing allochthonous component. Yet it is not a
practical indicator of allochthonous eutrophication, if a large fraction of terrigenous carbon
compounds are colorless, and the relationship
between CDOM and terrigenous organic carbon concentration vary spatially and temporally
(Asmala et al. 2012, Harvey et al. 2015).
Dissolved iron has been shown to have an
LQÀXHQFH RQ 6HFFKL GHSWK .RUWHODLQHQ HW DO
1986; Kritzberg and Ekström 2012). This was
probably the case also in the coastal Bothnian
Bay, Quark and Bothnian Sea, where simultaneRXVFRQWUDGLFWLQJWUHQGVDQGVLJQL¿FDQWVWURQJ
negative correlations were found (Paper II).
Dissolved iron as well as suspended particles
are mainly land-driven, or released from the
bottom through mixing. They are expected to
settle relatively close to their sources – although
some uncertainty on the extent of settling still
remains (Gustafsson et al. 2000, Krachler et
al. 2015). In optical studies, iron is considered
as part of CDOM, even if it is dissolved in an
inorganic form (Paper III). It cannot be considered to be an eutrophication-related constituent
of Secchi depth. Unfortunately, iron could not
be distinguished from the organic fraction of
CDOM in our bio-optical modelling exercise.
Capuzzo et al. (2015) proposed that in the
North Sea, decrease in Secchi depth was mainly
due to increase in suspended matter, resulting
from increased benthic release. This is not a
probable scenario in the open Baltic Sea, which
is known to exhibit a low ratio of suspended inorganic matter to chlorophyll-a (Paper
III, Babin et al. 2003, Håkanson and Eckhéll
2005, Kratzer et al. 2008). The coastal areas
may however be subject to long-term changes
in suspended non-planktonic matter (Harvey
2015), introducing an abiotic, non-eutrophicaWLRQUHODWHGDQG\HWXQTXDQWL¿HGFRPSRQHQWWR
Secchi depth.
In summary, Secchi depth is a complex parameter, potentially expressing autochthonous
and allochthonous eutrophication as well as
abiotic factors (Figure 4-1). Through its role as
the link between phytoplankton increase and
ALLOCHTHONOUS
EUTROPHICATION
BalƟc Sea
EutrophicaƟon
Assessment
(HELCOM 2009,
2013)
SECCHI
DEPTH
AUTOCHTHONOUS
EUTROPHICATION
INDIRECT
EUTROPHICATION
CAUSATIVE
EUTROPHICATION
Figure 4-1. A schematic figure expressing the eutrophication categories presented in Figure 1-2 (introduction),
and Secchi depth in relation to them. The arrows indicate causative relationships. The segments included in the
Baltic Sea eutrophication assessment, discussed in Chapter 2.9, are indicated separately.
34
communities suffering from changes in the light
climate, it may also be regarded to express indirect eutrophication (Chapter 1.2).
3.
'H¿QLQJWKHLQGLFDWRU
The indicator concept, as it is referred to in literature, is vague and manifold, and indicators
DUHGLYHUVH$FDWHJRULFDOGH¿QLWLRQRILQGLFDtor features would be helpful for the experts
applying them, for example when attempting
to distinguish whether or not an indicator is
VXLWDEOHIRUDVSHFL¿FSXUSRVHDQGZKHWKHULW
can be combined with other indicators into an
DVVHVVPHQW %HORZ LV DQ HIIRUW WR GH¿QH DQG
characterize the Secchi depth indicator (Table
4-2). The essential features listed in Chapter 1.3
are included along.
1.
2.
Secchi depth is a quantitative indicator.
The most distinguishable feature from the
user’s point of view is whether the indicator
is quantitative or descriptive. Descriptive
indicators are useful in explaining change,
but they are not practical for assessing status. Quantitative indicators may be set with
precise targets or reference levels and the
LQGLFDWRUVWDWXVIRUDVSHFL¿FWLPHSHULRG
may be estimated (Paper IV).
In addition to being a quantitative indicator, Secchi depth is composite, in the sense
that it combines the effect of several other
measurable parameters in the water column
(Papers I, II, III). Each of these parameters
has their own, sometimes even contradicting role in relation to the eutrophication
process. Secchi depth has been applied as
a proxy of phytoplankton biomass or chlorophyll-a (Boyce et al. 2012), which may
work well in oceanic waters. However, in
optically complex waters such as the Baltic
Sea, other constituents with non-anthropogenic origin affect Secchi depth as well.
The role of the constituents may vary both
in space and time, and cause challenges
in interpreting change in indicator status.
The relationship of Secchi depth to the various optical constituents must be resolved
before applying the indicator in optically
4.
5.
6.
complex waters, in order to ensure indicaWRU¿GHOLW\-DPHV
Though composite, Secchi depth is still
directly measurable (Papers I, II). As
opposed to computable indicators, such
as oxygen debt (Carstensen et al. 2014)
and intensity of spring bloom (Fleming &
Kaitala 2006), or multiparametric indeces,
observations made at sea may, after simSOH¿OWHULQJDQGDYHUDJLQJSURFHGXUHVEH
directly applied to provide the indicator
estimate.
Secchi depth has a numerically negative
response to increasing eutrophication.
In other words, it is expected to decrease
along with deteriorating eutrophication
status. The response is not necessarily linear, as accelerating decrease is expected
along with increasing primary production
(Papers I, III). When applied in an assessment together with indicators responding
positively to increasing eutrophication,
this should be taken into account.
One special characteristic of Secchi depth
and water clarity in general is that it is
commonly observable by the layman.
This distinguishes it from similar measurable and quantitative indicators such as
dissolved inorganic phosphorus (DIP) or
chlorophyll-a: Secchi depth can be experienced. It is in fact one of the signs of eutrophication most commonly distinguished
among the general public, and has therefor
EHHQDSSOLHGZKHQDQDO\]LQJWKHEHQH¿WV
of nutrient mitigation (Kosenius 2010).
In addition to being easily observable,
Secchi depth can be estimated to be also
commonly understandable, both as an indicator and through its role in the system.
Both causes and consequences of change
are easily associated by the public. Chlorophyll-a, on the other hand, is an example
of an indicator that is not commonly observable, but can be quickly explained in a
way that it is understandable. An opposite
example is oxygen debt, which, with its
complicated methodology and indirect role
in eutrophication, falls in the category of
being understood only by experts.
35
7.
The ambiguous origin of change in indicator status is what complicates the use of the
Secchi depth indicator. Due to the composite nature of the indicator, with different
optical constituents affecting the outcome,
also the causal link from human actions to
change in indicator status varies in space
and time. When chlorophyll-a dominates
the attenuation of light, the anthropogenic
link is stronger than when attenuation is
caused by CDOM, suspended inorganic
matter or dissolved iron (Papers I, III).
For management purposes, the link to human-induced nutrient pressures should be
established in a satisfactory way, before
applying the indicator in an assessment.
8. As a result of its quantitative nature, historical monitoring time-series and relatively
good suitability for use in ecological models (Meier et al. 2012), ecological targets
for Secchi depth have been established in
many areas. In the Baltic Sea, common
environmental targets have been agreed
for open-sea areas by the coastal states
(HELCOM 2014), and national targets
have been set for coastal areas. Together
with estimations of present level (ES), the
targets enable the use of Secchi depth in
quantitative assessments (Paper IV).
9. The geographical applicability of Secchi
depth varies regionally, based on differing
origin of change in indicator status. This
does not restrict the use of Secchi depth in
different geographical areas, but corrections to the indicator calculation or use in
assessment are required (Papers III, IV).
10. In the Baltic Sea, adequate documentation
of the Secchi depth indicator exists, on the
VFLHQWL¿FEDFNJURXQG3DSHUV,,,,DVZHOO
as on the indicator approach and update
procedures (HELCOM 2015b). The documentation is essential for achieving transparent status evaluations, which in turn is a
prerequisite for commitment of the parties
applying the assessment.
11. The practical use of an indicator is dependent on existing monitoring. Secchi
depth is included in all national monitoring programmes in the Baltic Sea, and is
36
frequently monitored and reported (HELCOM 2015a). Besides traditional in-situ
monitoring, there is a strong potential for
WKHXVHRIQRYHOFRVWHI¿FLHQWPRQLWRULQJ
methods (Platt and Sathyendranath 2012).
In summary, Secchi depth is a commonly
used indicator, often supported by long monitoring datasets reaching back to a time unaffected by eutrophication. It is easily understandable and can be experienced – a property that
makes it extremely attractive. Technically, it is
a relatively advanced indicator: quantitative,
commonly monitored, includes ecological targets and a well documented methodology. On
the other hand, although apparently simple to
associate, it composite, and requires substantial
FDVHVSHFL¿FLQYHVWLJDWLRQVEHIRUHLWVH[DFWUROH
LQ WKH HXWURSKLFDWLRQ SURFHVV FDQ EH GH¿QHG
(Table 4-2).
4.4 Assessing eutrophication with
the Secchi depth indicator
The modeling exercise demonstrated, that the
environmental targets set for Secchi depth
were stricter than those set for chlorophyll-a
(Paper III, Chapter 3.4). The reason behind this
discrepancy could not be solved; it may be a
FRQVHTXHQFHRILQVXI¿FLHQWKLVWRULFDOLQIRUPDtion available for the chlorophyll-a target-setting (HELCOM 2013b, 2014), or an effect of
long-term changes in the relationships of the
parameters.
In the eutrophication status assessment for
the Baltic Sea, Secchi depth ES was estimated
to be below the environmental target in eight
of the nine sub-basins; the target was (barely)
reached only in the Bothnian Bay (Paper IV,
Table 3-1). Secchi depth did not differ in this
sense much from the other indicators, their ES
below the target in most of the sub-basins. Secchi depth did however express a lower ER than
chlorophyll-a, DIN and DIP, in average. In all
but two sub-basins, chlorophyll-a was estimated to be in worse status than Secchi depth (when
comparing ER).
The fact that Secchi depth ER showed small
variation compared to some of the other indica-
Table 4-2. Features of the Secchi depth indicator used in the Baltic Sea.
1. Quantifiable / descriptive
XപറQuantitative
Descriptive
2. Complicity
Explicit
XപറComposite
3. Procedure
XപറMeasurable
Computable
4. Numerical response to increasing
eutrophication
XപറNegative
5. Commonly observable
XപറChange can be easily observed in nature by a layman
Positive
Change can be observed in nature only by an expert
Change cannot be seen by the bare eye
6. Commonly understandable
XപThe indicator and it’s role in the system are easily understood by
layman.
The indicator and/or it’s role in the system is not simple, but can be
explained to a layman in an understandable way.
RThe indicator and/or it’s role in the system is complicated and can
be understood only by experts
7. Origin of change
Anthropogenic dominating
XപറBoth anthopogenic and non-anthropogenic
8. Ecological targets / reference levels
XപറData-based with strong certainty (1940 or earlier)
Data-based with weak certainty
XപറHindcast modeling
Process modeling
Expert evaluation
No targets exist
9. Geographical applicability
Same applicability in all geographical areas
XപറApplicable in all geographical areas but role differs
Not applicable in all geographical areas
10. Documentation
XപറScientifically documented approach and well described procedure for
update
Scientifically documented approach, but update procedure is not
documented to ensure repeatability
Approach not scientifically documented
11. Monitoring
XപറMonitored throughout Baltic Sea
Monitored in some areas
No regular monitoring
tors, was partly a methodological consequence:
in cases where an indicator responds negatively
to increased eutrophication pressure, as Secchi
depth does, ER behaves exponentially, instead
of linearly, in relation to increasing pressure
(Andersen et al. 2011). In practice, this leads to
smaller variation in the Secchi depth ER compared to the ER of the indicators responding
positively to eutrophication pressure, when ES
is above or relatively close to the target (ie. ER
is below or relatively close to 1). This property
of the Secchi depth indicator downscaled the
effect of Secchi depth when applying it together with positively responding indicators in the
HEAT 3.0 tool. It subsequently overruled the
disharmony in target-setting, proposed earlier
(Paper III).
In the HELCOM eutrophication status assessment, Secchi depth was aggregated together
with chlorophyll-a, as autochthonous eutrophLFDWLRQVHHFKDSWHUDQG¿JXUH$,QDOO
of the nine sub-basins, autochthonous eutrophication was estimated to be below GES, and
in the Bornholm Sea and the Western Gotland
Basin, it was the aggregation group expressing
worst status. It was not alone, however: with the
exception of causal eutrophication in the Bothnian Bay, the other aggregation groups were
also below GES in all sub-basins. Naturally,
Secchi depth did not end up determining the
overall status in any of the sub-basins.
37
In the case of the Baltic Sea, where eutrophication effects are severe and broadly distributed,
alterations in the assessment grouping had little
effect in the overall result: all sub-basins were
estimated to be below GES, whether Secchi
depth was grouped as an indicator of autochthonous or indirect eutrophication (Figure 3-5A
and 3-5B). Since chlorophyll-a was generally in
worst status (when comparing ER), the overall
eutrophication status tended to move further
below GES when removing Secchi depth from
autochthonous eutrophication group (with the
exception of the Gulf of Riga). As the number
of eutrophication indicators in the assessment
was low, and nearly half of the sub-basins had
no indicator of indirect eutrophication, including Secchi depth to indirect eutrophication had
WKHSRWHQWLDOWRLQFUHDVHDVVHVVPHQWFRQ¿GHQFH
This is however an opportunistic approach: the
choice of indicator use and grouping must be
based on theoretical background. Instead of relocating existing indicators in the assessment
tool, introducing new appropriate indicators
should be considered.
5 Conclusions
Water clarity has decreased in the open Baltic
Sea during the last century. This was demonstrated by a 14 – 44 % decrease, revealed by a
unique historical dataset of Secchi depth (Paper I). The decrease was especially intense in
the northern areas, amounting to 3.3 – 4.0 m
(averaging 0.033 – 0.040 m y-1), when comparing summer time averages in 2005 – 2009 to
those observed one hundred years earlier. The
decrease is proposed to be strongly linked with
documented simultaneous decrease in chlorophyll-a concentration (Papers I, III).
Secchi depth monitoring has been conducted also in the Finnish coastal areas, since the
1970’s (Paper II). During the last 2.5 decades,
clear decreasing trends were detected only in
the Archipelago Sea, together with simultaneous opposing trends in chlorophyll-a. Contradictory to the development in adjacent open-sea
areas, Secchi depth increased in the coasts of
the Bothnian Sea, Quark and Bothnian Bay.
38
This was proposed to be at least partly a consequence of decreased concentrations of dissolved iron in the surface waters near the coast.
6WURQJVLJQL¿FDQWUHODWLRQVKLSVEHWZHHQ6HFFKL
depth and TOC were rarely found– indirectly
indicating that a large part of coastal organic
carbon in the area was colorless.
A bio-optical model setup revealed, that Secchi depth in the Baltic Sea is highly sensitive
to variation in the amount of phytoplankton (by
chlorophyll-a as proxy) and CDOM (Paper III).
The relationship to the former motivates the
use of Secchi depth as a eutrophication indicator. The latter connection, however, implies the
presence both eutrophication- and non-eutrophication-related components.
Secchi depth is a commonly applied and well
established indicator of eutrophication and water quality in the Baltic Sea. It is commonly
understandable and easily experienced, and
EHQH¿WV IURP D PRQLWRULQJ KLVWRU\ UHDFKLQJ
back to pre-eutrophicated times. It is a technically advanced: it is quantitative, commonly
monitored, includes ecological targets and a
well-documented methodology. On the other
hand, although apparently simple to associate,
it is composite, in that it includes signals from
several elements.
It is often applied together with other indicators, including chlorophyll-a. The modelling exercise revealed, that the environmental
targets for Secchi depth, set by the Baltic Sea
coastal states via their collaboration through the
Baltic Marine Environment Protection Commission (HELCOM), were stricter than those
set for chlorophyll-a.
According to the HELCOM Baltic Sea eutrophication status assessment 2007 – 2011, all
open-sea areas of the Baltic Sea were severely affected by eutrophication. This was also
shown by the status of the Secchi depth indicator, meeting the environmental target only in
the Bothnian Bay. The overall eutrophication
assessment was not dominated by the weak
status of Secchi depth, as the other eutrophication indicators were also unable to reach their
environmental targets in most of the sub-basins.
Due to the deteriorated status of all indicators,
alterations in the way Secchi depth was applied
in the assessment did not affect the general outcome of eutrophication status.
Secchi depth was found suitable for expressing eutrophication in optically complex waters
such as the Baltic Sea. It is able to quantify
several eutrophication-related elements into
RQH VLQJOH PHDVXUH UHÀHFWLQJ ERWK DOORFKWonous and autochtonous signals. In this study,
we were able distinguish the shares of phytoplankton and CDOM at a large scale. We were,
however, not yet able to further identify the
elements unrelated to primary production. We
would be especially interested in distinguishing
the autochthonous signals, related to eg. heterotrophic activity, from the allochtonous ones. In
shallow and coastal areas, recognizing also the
inorganic fraction is essential when applying
the indicator. Due to these restrictions, Secchi
depth should not be used alone, as the only
indicator measuring eutrophication. Applied
WRJHWKHU ZLWK RWKHU LQGLFDWRUV ZLWK VXI¿FLHQW
backgroung information, it adds to the certainty
of an assessment.
39
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