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Presentandpastdepthdistributionof
bladderwrack(Fucusvesiculosus)intheBaltic
Sea
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ImpactFactor:1.61·DOI:10.1016/j.aquabot.2005.07.011
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KaireTorn
DorteKrause-Jensen
UniversityofTartu
AarhusUniversity
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Aquatic Botany 84 (2006) 53–62
www.elsevier.com/locate/aquabot
Present and past depth distribution of bladderwrack
(Fucus vesiculosus) in the Baltic Sea
Kaire Torn a,b,*, Dorte Krause-Jensen c, Georg Martin a
a
Estonian Marine Institute, University of Tartu, Marja 4d, 10617 Tallinn, Estonia
Institute of Botany and Ecology, University of Tartu, Lai 40, 51005 Tartu, Estonia
c
National Environmental Research Institute, Vejlsøvej 25, DK-8600 Silkeborg, Denmark
b
Received 23 October 2004; received in revised form 10 June 2005; accepted 5 July 2005
Abstract
This study aimed to (1) assess the present depth distribution of Fucus vesiculosus in the Baltic Sea and evaluate differences between districts and
(2) assess long-term and recent changes in depth distribution and evaluate reasons for such changes. This was done through compilation and
analysis of existing data (3356 obs.). Depth limits were shallowest in the Kattegat, the Danish Belts and the Øresund (1.5 m on average), located
at the entrance of the Baltic Sea and markedly deeper in the central and inner parts of the Baltic (up to 4.5 m on average). This increase in depth
limits to some extent matched the decline in salinity and may in part be explained by reduced competition when species diversity decreases
successively along the Baltic salinity gradient. In the central and inner Baltic Sea, Secchi depths explained part of the variation (16%) in depth
limits and the majority (85%) of the variation in maximum attainable depth limits whereas at the entrance of the Baltic Secchi depths explained a
negligible part of the variation (1%). In most districts, depth limits moved upwards during the 20th century. In many cases this happened during or
shortly after the 1960s/1970s, and was most likely due to eutrophication.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Baltic Sea; Fucus vesiculosus; Depth distribution; Salinity; Water clarity; Long-term change
1. Introduction
Bladder wrack, Fucus vesiculosus L., is widely distributed in
the Arctic and cold temperate region (Lüning, 1990). In the
Baltic Sea F. vesiculosus is extremely wide spread on hard
substratum and often dominates shallow macroalgal communities. As the only fucoid species, it penetrates all the way into
the Gulf of Bothnia in the north and to the Gulf of Finland in the
east where it lives close to its tolerance limit of salinity. This
limit lies around salinities of 4 psu (Rosemarin and Notini,
1996), with occasional reports of isolated and sparse
populations at salinities down to 2 psu (Waern, 1952). Though
F. vesiculosus’ habitat requirements with respect to salinity are
fulfilled almost everywhere in the Baltic Sea, the alga also
requires firm substrate and low-moderate exposure to ice and
waves in order to form stable and healthy communities
(Rosemarin and Notini, 1996).
* Corresponding author at: Estonian Marine Institute, Department of Marine
Biology, Mäealuse 10a, 12618 Tallinn, Estonia. Tel.: +372 5283885;
fax: +372 6718936.
E-mail address: [email protected] (K. Torn).
0304-3770/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.aquabot.2005.07.011
Light is the main factor regulating depth distribution of
seagrass and macroalgal communities (Duarte, 1991; Nielsen
et al., 2002) and may also regulate the depth limit of individual
algal species such as F. vesiculosus. For example, depth limits
of F. vesiculosus correlate with light attenuation in the water
column in the Askö area of the Baltic proper (Kautsky, 1999)
and in the Gulf of Finland (Bäck and Ruuskanen, 2000).
Epiphytic algae and mats of ephemeral macroalgae often
become abundant under eutrophic conditions and may further
reduce light levels and force depth limits upwards (Duarte,
1995; Lehvo and Bäck, 2001).
Other factors more or less directly related to human pressure
can also affect depth limits of F. vesiculosus in the Baltic Sea.
Organic sedimentation may cover the available substrate and
thereby reduce the settlement of F. vesiculosus (Eriksson and
Johansson, 2003), and this factor has increased substantially
due to large-scale eutrophication (Wassmann, 1990; Elmgren,
1989; Jonsson and Carman, 1994). Blue mussel may further
reduce the potential habitat available for F. vesiculosus and has
also increased in abundance in some areas of the Baltic Sea
(Vogt and Schramm, 1991; Kautsky et al., 1986 and references
therein). Grazing by adult Idotea is another regulating factor,
54
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
which also seems to have increased along with increased
eutrophication possibly because of increased abundance of
ephemeral macroalgae which serve as food for juvenile Idotea
(Kangas et al., 1982). Recently, it has also been suggested that
high nutrient levels may directly inhibit spore settlement and
initial development of F. vesiculosus (Bergström et al., 2003).
Eventually, toxic substances may prevent F. vesiculosus from
colonising otherwise suitable habitats (Kautsky et al., 1992).
Competition among macroalgae is also important in
determining the depth penetration of F. vesiculosus. While F.
vesiculosus is an intertidal species with a narrow distribution
belt in the North Sea region where competition is strong, it
becomes sublittoral and obtains wider distribution belts in the
Baltic Sea partly as a consequence of decreased competition
from other large algal species at lower salinities (Waern, 1952;
von Wachenfeldt, 1975; Snoeijs, 1999; Pedersen and Snoeijs,
2001). This tendency was identified already in the late 19th
century (Reinke, 1889) and was described as the ‘‘downward
process’’ by Waern (1952).
Several studies indicate that F. vesiculosus in the Baltic Sea
is threatened by pollution both locally close to point sources
and on a larger scale due to a general eutrophication of the
Baltic Sea (e.g. Cederwall and Elmgren, 1990; Kautsky et al.,
1992; Schramm, 1996). Decreases in distribution and
abundance or even disappearances of F. vesiculosus have been
reported from locally polluted areas such as the inner
Stockholm Archipelago (Pekkari, 1973), the Tallinn Bay (Trei,
1985; Kukk, 1985), the Gulf of Gdansk (Plinski, 1987), the
Helsinki Archipelago and the Gulf of Riga (von Wachenfeldt
et al., 1986). Decreases have also been reported from areas with
little local pollution, e.g. the Lågskär skerries (Rönnberg and
Mathiesen, 1998), the Tvärminne area (Bäck and Ruuskanen,
2000) and the Öregrund Archipelagos (Kautsky et al., 1986)
and it has been suggested that large-scale hydrographical
changes may have contributed to the negative development in
Fucus communities (Kangas et al., 1982; Hällfors et al., 1984;
Rönnberg et al., 1985).
F. vesiculosus is often characterised as the most important of
all phytobenthic species in the Baltic coastal zone. This is due
to its wide distribution and high biomass and productivity along
rocky and stony coasts where Fucus belts play an important
structuring role and have a positive effect on biodiversity,
being habitats for species-rich epiphytic and epibenthic
communities (e.g. Haage, 1975, 1976; Kautsky and Kautsky,
1989; Wallentinus, 1991). As a consequence, changes in the
distribution and abundance of F. vesiculosus are likely to
markedly influence the coastal Baltic ecosystem including
coastal fish catches (Aneer et al., 1983; Haahtela, 1984).
Despite the importance of F. vesiculosus, the information on
present and past depth distribution is still relatively scattered.
Major changes along the Baltic gradient have been summarised
(e.g. Wallentinus, 1991; Schramm, 1996), but data have not
been compiled for the Baltic Sea as a whole. Based on a
compilation of existing information on F. vesiculosus in the
Baltic Sea this work aims to (1) assess the present depth
distribution across the Baltic Sea and evaluate differences
between various districts and (2) assess long- and short-term
changes in the depth distribution and evaluate reasons for such
changes.
2. Materials and methods
2.1. Data compilation
We compiled data on the depth distribution of F. vesiculosus
from published articles and reports as well as from previously
unpublished national databases. We searched for present and
past data sets on three parameters: (1) the maximum depth limit
of F. vesiculosus specimen (i.e. the deepest occurring specimen), (2) the maximum depth limit of F. vesiculosus belts (i.e.
continuous Fucus stands) and (3) the depth of maximum
abundance (typically based on measurements of cover) of F.
vesiculosus. For selected districts at the entrance and in inner
parts of the Baltic Sea, we supplemented with data on cover of
F. vesiculosus at specific depths along depth gradients. All
compiled data sets were organised according to the district of
the Baltic Sea they represented (Table 1). The Baltic Sea and its
districts were defined according to Nielsen et al. (1995) who
operates with 22 different districts of the Baltic Sea. The
compiled data represented the period from 1900 up to present.
Field methods used for assessing depth distribution showed
some differences between countries and time periods, but most
recent literature data and data from national databases were
based on diver observations gathered in a comparable manner.
Diver observations typically encompassed visual assessments
of cover either using percent estimates or a cover scale in
various points or depth intervals along a depth gradient. The
depth of maximum abundance was subsequently identified as
the deepest point or the deepest end of the depth interval at
which F. vesiculosus had maximum cover. Most diver
investigations moreover included direct estimates of the depth
limit of specimen and/or the depth limit of the algal belt. In
cases when direct estimates of the depth limit lacked, we
identified the deepest interval where cover was assessed and
used the deepest end of this interval as a maximum estimate of
the depth limit of specimen. Though hard substratum is
scattered at the Baltic entrance, lack of substratum did not set
the depth limit of F. vesiculosus, as other attached algae
occurred deeper than the deepest F. vesiculosus specimen along
the depth gradients. Most of the early information on depth
limits was obtained based on sampling from a boat (e.g. Reinke,
1889; Lakowitz, 1907; Ravanko, 1972) and could be connected
with some error. Apart from these distinctions, we identified no
obvious or systematic differences between data sets in the
methods used to assess depth limits and depths of maximum
abundance.
In addition to data on depth distribution of F. vesiculosus, we
searched for connected information on Secchi depth and
salinity. We used data from national databases on Secchi depth
measured at water chemistry sites neighbouring the vegetation
sites and representing the period May–September of the same
year as the vegetation data. Old Estonian data on Secchi depths
(1967–1969, Trei, 1973) were measured at the same site and
time as vegetation data were collected. Data on salinity levels in
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
55
Table 1
Overview of compiled data
District
Country
Year
Fucus obs. (no.)
Method D/B
Secchi depth
Source
Kattegat W
Danish belts
Kattegat E
Denmark
Denmark
Sweden
1989–2001
1989–2001
1940s
1965–1972
279
2179
1
1
D
D
x
x
Øresund
Denmark
1988–1990
1989–2001
1965–1972
1
66
1
D
D
x
Zealand and Scania
Gulf of Kiel
Denmark
Germany
D
x
Arkona Sea S
Hanö Bay
Germany
Sweden
28
1
1
1
12
1
1
26
1
1
1
1
1
1
3
208
19
1
3
2
1
24
14
23
14
1
6
D
D
x
x
D
D
B
x
6
4
11
1
D
D
D
NERIs marine database (MADS)
NERIs marine database (MADS)
Kylin (1947)
Pedersen and Snoeijs (2001)
(von Wachenfeldt, 1975)
Pedersen and Snoeijs (2001)
NERIs marine database (MADS)
Pedersen and Snoeijs (2001)
(von Wachenfeldt, 1975)
NERIs marine database (MADS)
Reinke (1889)
Black (1978)
Vogt and Schramm (1991)
EMAUG
EMAUG
Sjöstedt (1920)
SUSE
Malm and Kautsky (2003)
Malm et al. (2001)
Lakowitz (1907)
Ciszewski et al. (1962)
Ciszewski et al. (1992)
Kruk-Dowgiallo (1996)
Kovaltshouk (1996)
SUSE
NERIs marine database (MADS)
Labanauskas (2000)
EMI
IAE
Trei (1973)
EMI
EMI monitoring database
Trei (1973)
EMI monitoring database
Kautsky (1999)
Kautsky et al. (1992)
(Jansson and Kautsky, 1977)
Kautsky et al. (1992)
Kautsky (1999)
EMI monitoring database
Bäck and Ruuskanen (2000)
(Purasjoki, 1936)
Bäck and Ruuskanen (2000)
(Strandström, 1980)
FEI, UH, Alleco
Bäck and Ruuskanen (2000)
(Ruuskanen et al., 1999)
Haahtela and Lehto (1982) (Ravanko, 1972)
Haahtela and Lehto (1982)
Mäkinen et al. (1984)
Kautsky et al. (1986) (Waern, 1952)
Eriksson et al. (1998) (Waern, 1952)
Rönnberg and Mathiesen (1998)
(Mathiesen, formerly Andersson, 1974)
Kautsky et al. (1986)
Eriksson et al. (1998) (Kautsky et al., 1986)
Rönnberg and Mathiesen (1998)
Eriksson et al. (1998)
SUSE
Gulf of Gdansk
Baltic proper W
Bornholm Sea
Baltic proper E
Gulf of Riga
Poland
Sweden
Denmark
Lithuania
Latvia
Estonia
Estonian Archipelago
Baltic proper NW
Gulf of Finland
Estonia
Estonia
Sweden
Estonia
Finland
1989–2001
1880s
1970s
1987/1988
1996–2000
1998
1920
1990–1994
1995
2000
1900s
1957
1984
1996*
1939, 1947, 1993*
1984–2001
1989–1999
1998
1996
1999
1959
1984, 1987
1995–2001
1962–1969
1995–2001
1974
1974/1975
1988, 1990
1993–1996
1997–2001
1930s
1970s
Finnish Archipelago Sea
Åland Sea
Bothnian Sea
Finland
Sweden
Sweden
D
D
D
D
D
D
D
D
D
D
D
D
D
D
1
1991–2001
1994
348
1
1968–1969
1975–1980
1981–1982*
1943–1944
1943–1944
1956
1
1
1
11
5
1
1984
1984
1993
1996
1992
11
5
1
5
19
x
D
D
B/D
D
D
D
D
D
D
D
x
x
x
When a source includes already published data, the supplementary source is mentioned in parenthesis. Field methods are indicated by ‘D’: diver’s observations and
‘B’: sampling from a boat. Observations of lack of F. vesiculosus are indicated by ‘*’. Abbreviations: ‘EMAUG’: Ernst–Moritz–Arndt University of Greifswald,
Germany; ‘EMI’: Estonian Marine Institute, Estonia; ‘FEI’: Finnish Environment Institute, Finland; ‘IAE’: Institute of Aquatic Ecology, Latvia; ‘NERI’: National
Environmental Research Institute, Denmark; ‘UH’: University of Helsinki, Finland; ‘SUSE’: Stockholm University, Department of System Ecology, Sweden.
56
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
Fig. 1. Map showing the location of compiled recent data (1989–2001) on Fucus vesiculosus.
the various districts were based on Schramm (1996, Table 5.1),
and represent the average of the minimum and maximum values
reported for each district.
2.2. Data analyses
The first set of analyses concerned spatial differences across
the entire Baltic Sea regarding the depth limit of F. vesiculosus
specimen, the depth limit of belts and the depth of maximum
abundance. For each of the Baltic districts, we calculated means
and frequency distribution of the three depth distribution
parameters and tested differences between districts using onefactor ANOVA supplemented by the non-parametric Kruskal–
Wallis test because the requirement of normal distribution and
homogeneity of variances were not always fulfilled.
Differences in depth distribution (median values) between
districts were related to differences in salinity by linear
regression analysis. Similarly, differences in depth distribution
between sites were related to differences in Secchi depth by
linear regression analysis. As salinity data were available only
at the level of districts, it was not possible to apply multiple
regression analysis including Secchi depth as well as salinity
and consequently the analyses do not evaluate the combined
effect of the two variables. In order to describe the maximum
attainable depth limit of F. vesiculosus at a given Secchi depth,
we analysed the limit to the variation, i.e. the boundary of the
depth limit–Secchi depth relation. This was done by sorting the
data set according to Secchi depth, then splitting it into groups
of similar size (18 data points in each) and for each group of
data points calculating the 90% percentile as a measure of the
boundary (see Krause-Jensen et al., 2000). The linear
regression line of this ‘‘boundary data set’’ describes the
maximum attainable depth limit at a given Secchi depth.
The second set of analyses concerned temporal changes in
the depth distribution of F. vesiculosus. We used the available
time series, in some cases dating as far back as 1900. Data
were grouped according to district and plotted as functions of
time. Long-term trends were analysed visually as the relatively
few and heterogeneous early data sets did not allow statistical
analysis. Recent changes (after 1989) were analysed using
linear regression analysis when data series were available and
using one-way ANOVA when data represented only two time
periods.
All linear regression analyses were performed using Type I
regression which demands that the independent variable is
determined with higher accuracy than the dependent variable.
This requirement was fulfilled as the independent variables
salinity, Secchi depth, and sampling year were determined
more accurately than Fucus depth limits.
3. Results
The compiled database included a total of 3356 observations
of depth distribution of F. vesiculosus from 19 districts of the
Baltic Sea. Most data sets were recent (1989–2001), but some
districts had data dating back to the 1920–1970s and in few
cases even to around 1900 (Table 1: all data, Fig. 1: recent data).
3.1. Spatial differences in the present depth distribution of
F. vesiculosus in the Baltic Sea
The recent data showed that F. vesiculosus was generally
most abundant, in terms of % cover, at intermediate water
depths and least abundant in shallow and deep waters. Data
from the Øresund at the entrance of the Baltic Sea and from the
Gulf of Riga in the central Baltic exemplify this pattern (Fig. 2).
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
57
Fig. 2. Cover (%) of F. vesiculosus (means and standard errors) as a function of
depth at the entrance of the Baltic Sea (exemplified by the Øresund) and in the
central Baltic (exemplified by the Gulf of Riga). Data from 1989 to 2001.
Number of observations in each depth interval: 2–37.
The depth distribution range of F. vesiculosus extended from
close to the coastline and down to maximum depths of
specimen at 1.5–5.5 m on average, depending on district
(Fig. 3A). The shallowest depth limits of F. vesiculosus
specimen (1.5 m on average) were recorded in the Kattegat,
the Danish Belts and the Øresund, located at the entrance of the
Baltic Sea while depth limits became markedly deeper in the
central and inner parts of the Baltic. In the Baltic between
Sealand and Scania the depth limit of F. vesiculosus specimen
thus averaged 2.5 m and from the Gulf of Kiel to the Bothnian
Sea depth limits were as deep as 4–5.5 m on average. In
extreme cases, depth limits exceeded 10 m in the central Baltic
Sea. Depth limits of F. vesiculosus specimen differed
significantly between areas (ANOVA and Kruskal–Wallis test
p < 0.001) and differences were largest between the entrance
and the central/inner Baltic (Fig. 3A).
Fucus belts were recorded along the rocky coastlines of
Estonia, Finland and western Sweden. Depth limits of the
belts increased gradually from Hanö Bay (1.5 m on
average) over the Baltic proper, the Gulf of Riga and the
Estonian Archipelago to the Gulf of Finland (3 m on
average) and the Bothnian Sea (5.5 m on average, 7 m at
maximum), and thus followed the same trend as depth limits
of specimen (Fig. 3B). Depth limits of Fucus belts differed
significantly between districts (Kruskal–Wallis test, ANOVA,
p < 0.001).
Depth of maximum cover of F. vesiculosus averaged 1–2 m
in most areas. Only the Bornholm Sea, the Gulf of Riga and the
Bothnian Sea had slightly deeper values (2.5 m on average,
Fig. 3C). Though depths of maximum cover differed
significantly between districts (ANOVA, p < 0.001), they
showed no clear geographical trend and thus no tight coupling
to depth limits.
Fig. 3. Depth limits of F. vesiculosus specimen (A), depth limits of F.
vesiculosus belts (B) and depths of max abundance of F. vesiculosus (C) in
various districts of the Baltic Sea. Squares represent mean values, lines
represent medians, boxes delimit the 25 and 75% percentiles, whiskers 5–
95% percentiles and crosses 1–99% percentiles. Data from 1989 to 2001.
Fig. 4. Depth limit (medians) of F. vesiculosus specimen in various districts of
the Baltic Sea as a function of the average salinity of the districts. Linear
regression: y = 0.14x + 4.7, R2 = 0.53, p = 0.011, n = 11. Data from 1989 to
2001.
58
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
3.2. Relations between F. vesiculosus depth distribution
and selected abiotic parameters
Depth limits of F. vesiculosus specimen declined significantly as salinity increased (Fig. 4, lin. reg.: R2 = 0.53,
p = 0.011, n = 11). Data on depth limits of F. vesiculosus belts
were available only for the salinity range 5–8 psu and though
they tended to be deepest at the lowest salinity, the trend was
not significant (lin. reg.: R2 = 0.38, p = 0.19, n = 6).
Coupled recent data sets on depth limits of F. vesiculosus
specimen and Secchi depths were available from 10 districts.
Depth limits at the entrance of the Baltic Sea were very
almost independent of Secchi depths (lin. reg.: R2 = 0.014,
slope = 0.06, p < 0.001, n = 804). Depth limits in the rest of the
Baltic Sea showed a closer coupling to Secchi depth (lin. reg.:
R2 = 0.16, slope = 0.48, p < 0.001, n = 180) though depth
limits varied markedly for given Secchi depths. There was a
boundary (B) to this variation, however, which was strongly
related to Secchi depth (S) (lin. reg.: B = 0.94S + 0.74,
R2 = 0.85, p < 0.001, Fig. 5).
3.3. Long-term changes in the depth distribution of
F. vesiculosus
Data on long-term changes in depth distribution of
F. vesiculosus were available for 11 districts of the Baltic
Sea (Fig. 6, Table 1). All these districts had data dating back at
least to 1960s/1970s, five had data as far back as 1920s/1940s
and two had data from around 1900. In all areas except Kattegat
E and possibly the Gulf of Riga depth limits declined over the
investigation period. Early declines seem to have occurred in
the Gulf of Gdansk (between 1900 and 1937) and the Åland Sea
(between 1943/1944 and 1957). Most districts (the Øresund, the
Gulf of Kiel, the Gulf of Gdansk, Askö in the Baltic proper, the
Estonian Archipelago, Tvärminne in the Gulf of Finland, Seili
in the Finish Archipelago Sea) showed declines in the 1960s/
Fig. 6. Long-term changes in the depth limit of F. vesiculosus in various
districts of the Baltic Sea. Data represent depth limits of Fucus belts in the
Eastern Scania, the Finnish Archipelago Sea and the Gulf of Finland but of
Fucus specimen in other areas. Data points connected by lines represent the
same locations, while unconnected data points represent different locations
within the district. ‘’ indicates that Fucus was absent. Origin of data: see
Table 1.
1970s or shortly thereafter. In eastern Scania, Hanö Bay it is
unclear when the decline occurred, as data lack between 1920s
and the 1990s.
The deepest early observations of depth limit (10 m) were
found in the Øresund (1965–1972), the Gulf of Kiel (1880s and
1970s), the Gulf of Gdansk (1900s), the Gulf of Finland,
Tvärminne, (1930s, 1970s) and the Åland Sea (1943–1944).
Markedly shallower (4.5–7.5 m) early observations were found
in eastern Scania in Hanö Bay (1920), the Askö area of the
Baltic proper (1974), the Gulf of Riga (1959), and the Estonian
Archipelago Sea (1962–1969). The shallowest early observations were found in the Kattegat E (1940s: 1 m) and at Seili in
the Finnish Archipelago Sea (1968–1969: 3 m).
An evaluation of long-term changes in the depth of maximum
cover was possible only for the Estonian Archipelago where
maximum cover occurred at 1–6 m depth and relatively high
cover down to 10 m depth in 1962–1969 (Trei, 1973) while in
1995–2001 high cover only occurred to 3 m depth.
3.4. Recent changes in the depth distribution of
F. vesiculosus
Fig. 5. Relations between depth limit of F. vesiculosus specimen and Secchi
depth from various districts of the Baltic Sea. Data from the entrance of the
Baltic (left panel) cover the period 1989–2001 and include the Kattegat W
(n = 30), the Danish Belts (n = 759), the Øresund (n = 4) and the Baltic
between Zealand and Scania (n = 11). Linear regression of entrance data:
y = 1.06 + 0.06x (R2 = 0.014, p < 0.001). Data from the central/inner Baltic
(right panel) cover the period 1991–2001 and include the Gulf of Kiel (n = 8),
the Hanö Bay (n = 21), the Arkona Sea (n = 1), the Baltic proper W (n = 134),
the Gulf of Riga (n = 6) and the Gulf of Finland (n = 10). Linear regression of
data from the central/inner Baltic: all data (*, dashed line): y = 0.84 + 0.48x
(R2 = 0.16, p < 0.001); depth limit boundary (*, solid line): y = 0.94x + 0.74
(R2 = 0.85, p < 0.001).
Recent changes in depth limits differed between districts and
only few trends were significant (Table 2). Depth limits of F.
vesiculosus specimen increased significantly in the Danish
Belts from 1989 to 2000 and at Tvärminne in the Gulf of
Finland from 1999 to 2001, while significantly negative trends
were observed in the Kattegat W from 1989 to 2000 and in the
Baltic proper W from 1989 to 2001. Depth limits of Fucus belts
declined significantly in the Baltic proper W, from 1989 to
2001. No other significant trends were identified.
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
59
Table 2
Recent (1989–2001) changes in depth limits of F. vesiculosus specimen (Zs), depth limits of F. vesiculosus belts (Zb) and depths of max F. vesiculosus abundance (Zab)
District
Site
No. of sites per year
Years
Trend
Zs
Kattegat W
Danish Belts
Øresund
Gulf of Riga
Estonian Archipelago
Gulf of Finland
Hanö Bay
Baltic proper W
Zb
Zab
+
0
+
0
0
+
+
***
+ ***
0
0
Kõiguste
Küdema
3–43
39–182
3–8
1
1
1989–2000
1989–2000
1989–1999
1995–2001
1995–2001
Over the Gulf 1
Over the Gulf 2
Tvärminne 1
Tvärminne 2
Tvärminne 3
Porkkala
Helsinki
Eru
17
3–9
13
16
6
11
3–18
1
1991, 2001
1993–2001
1999, 2001
1994, 2001
1994, 2001
1994, 2001
1993–2001
1997–2001
+
+
0
+
0
1990, 1991, 1994
1989–2001
0
**
0
***
0
Blekinge
Kalmar
6–11
3–31
+ **
0
Where site are mentioned, data from the exact same locations were analysed; otherwise sampling sites differ between years; ‘+’ indicates a positive trend, ‘’ a
negative trend, and ‘0’ no observed trend. Trends were analysed using linear regression analysis when data series were available and using one-way ANOVA when
data represented only two time periods.
**
p < 0.01.
***
p < 0.001.
4. Discussion
4.1. Present depth distribution—differences between
districts
Our study demonstrates that F. vesiculosus generally grows
deeper in the central and inner Baltic Sea as compared to the
entrance of the Baltic. The most marked change in depth limit
between neighbouring districts is the increase from the Danish
districts to the Gulf of Kiel. Our study combines earlier
published and previously unpublished data into one large data
set and the results therefore support and supplement earlier
findings of Fucus’ depth distribution in the Baltic Sea that have
been based on more limited data sets.
The increase in depth limits across the Baltic Sea matches
the decline in salinity and differences in salinity explains about
50% of the variation in median depth limits between districts.
The coupling between depth limit and salinity is not due to a
direct physiological effect of salinity but to the fact that salinity
affects species diversity and, thereby, the competitive pressure.
An increase in the depth limit of F. vesiculosus from the North
Sea region towards the Baltic Sea was first identified by Reinke
(1889) and specified in more detail by, e.g. Waern (1952), von
Wachenfeldt et al. (1986) and Snoeijs (1999). A similar trend or
‘‘downward process’’ has also been described for species like
Corallina officinalis and Fucus serratus when moving from the
less saline Kattegat to the more saline Skagerrak and the
process is most conspicuous for the large belt and canopy
forming algae (Pedersen and Snoeijs, 2001). The phenomenon
can be explained by a reduced competition pressure from other
large perennial species, particularly euhaline and polyhaline
red and brown algal species, as salinity declines (Waern, 1952;
Snoeijs, 1999; Pedersen and Snoeijs, 2001). In the southern
Baltic Sea two other canopy-forming species, F. serratus and
Laminaria saccharina are still found in addition to F.
vesiculosus, but these are ‘pressed down’ to greater depths,
probably by competition from F. vesiculosus which is better
adapted to low-salinity conditions (Snoeijs, 1999). It is likely
that F. vesiculosus is also pressed down to deeper, more saline
waters as it approaches its inner distribution boundary in the
Gulf of Bothnia.
Though competition is strongest at the entrance of the Baltic,
F. vesiculosus is also subject to competition in the central
Baltic. Filamentous algae are important competitors and their
importance tends to increase with increasing eutrophication
(Plinski and Florczyk, 1984; Breuer and Schramm, 1988;
Baden et al., 1990; Råberg, 2004). A few perennial species also
tolerate the low salinities of the Baltic Sea. F. serratus is one of
the most important competitors at salinities down to 7 psu
(e.g. Malm et al., 2001). On the southern Baltic coasts F.
serratus generally replaces F. vesiculosus at about 2–3 m depth
while F. vesiculosus may penetrate down to 5–7 m depth in
areas without F. serratus (von Wachenfeldt et al., 1986). F.
serratus is less tolerant to physical stress like ice scouring and
exposure since it apparently cannot produce new fronds from
the hold fasts (Malm et al., 2001; Malm and Kautsky, 2003). A
varying competition pressure by, e.g. F. serratus may therefore
contribute to explain the large range in depth limits within
specific districts of the Baltic Sea (Fig. 3A).
While depth limits were almost unrelated to light levels at
the entrance of the Baltic, Secchi depths explained part (16%)
of the variation in depth in the central and inner Baltic Sea, and
85% of the variation in the deepest attainable depth limit, i.e.
the depth limit boundary (Fig. 5). At a Secchi depth of, e.g. 3 m,
60
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
this boundary was 3.6 m. The light level (I) available at a depth
limit boundary (z) of 3.6 m can be predicted to 6% of surface
irradiance (I0) by applying the law of Lambert Beer and
assuming that 10% of surface irradiance is available at the
Secchi depth (S):
Iz ¼ I0 e ln 10=Sz
This light level is higher than light levels reported for the
deepest growing kelps (0.6–1.2% of I0) but approaches the level
reported for closed kelp canopies (4% of I0, Lüning, 1990, p.
286). If Secchi depth were the only factor regulating the depth
limit boundary, we would therefore expect the boundary to be
even deeper. Additional shading from, e.g. epiphytes may
contribute to reduce the light level available at the boundary.
The large scatter in the relation between depth limit and
Secchi depth illustrates that F. vesiculosus in the Baltic Sea
rarely penetrates to the light defined depth limit boundary.
Competition for space is one hindrance, and sedimentation of
organic material is another. Recent studies have demonstrated
that artificial removal of sediments from the hard surfaces
below the F. vesiculosus belt can promote a deeper settlement of
F. vesiculosus (Eriksson et al., 2002; Eriksson and Johansson,
2003). Low levels of eutrophication and increased wave
exposure both tend to reduce sedimentation, and increased
wave exposure has the supplementary positive effect of keeping
the substrate free of sediment and helping control epiphytes
through the whiplash effect (Kiirikki, 1996), so sedimentation
should affect depth limits most markedly in protected,
eutrophic regions.
While the depth limit of F. vesiculosus specimen and belts
showed marked geographical trends, the depth of maximum
abundance of F. vesiculosus was more similar among districts.
It was typically located at 1–2 m depth and showed no obvious
geographical pattern. This result suggests that the best
conditions for F. vesiculosus growth in the Baltic Sea is at
intermediate water depths where exposure levels are moderate
and light levels still sufficient to allow maximum growth rates.
4.2. Long-term and recent changes in the depth limits of F.
vesiculosus
Most of the compiled long-term data sets showed marked
declines in depth limits of F. vesiculosus in the Baltic Sea.
Though early depth limits are connected with some error
because they were typically based on samplings from boat and
the observations were few, the general decline across the Baltic
Sea suggests that the trend is real. Lack of continuous studies
makes it impossible to identify precisely when declines
occurred, and whether declines had already occurred before
the time series started. However, relatively early declines have
been documented in the Gulf of Gdansk (between 1900 and
1937) and the Åland Sea (between 1943/1944 and 1957) and
most districts showed declines in or shortly after the 1960s/
1970s. These declines have typically been interpreted as effects
of eutrophication caused primarily by shading (Kautsky et al.,
1986, 1992; Kautsky, 1999; Eriksson et al., 1998; Bäck and
Ruuskanen, 2000), but also by increased sedimentation
(Eriksson and Johansson, 2003), increased grazing pressure
(Kangas et al., 1982) and increased abundance of blue mussel
that competes for space (Vogt and Schramm, 1991). Largescale eutrophication is indeed the most obvious change in
environmental conditions of the Baltic Sea. It is estimated that
phosphorus inputs to the Baltic Sea have increased eight-fold
and nitrogen inputs four-fold from 1900s to 1980s (Larsson
et al., 1985), and Secchi depths have declined by 0.05 m y1
during 1919–1939 and 1969–1991, corresponding to a 3.6 m
decline over the entire period 1919–1991 (Sandén and
Håkansson, 1996). As Secchi depths control the maximum
attainable depth limit by approximately a 1:1 relation, we can
assume that the reductions in Secchi depth have reduced the
maximum attainable depth limit by about 3.6 m. Reductions in
Secchi depth therefore could explain the observed long-term
reductions in depth limits of F. vesiculosus. Increased inputs of
toxic substances may have caused additional negative effects at
least on a local scale (Kautsky et al., 1992). Depth limits of
Zostera marina have also declined in regions of the Baltic Sea
during the 20th century most likely in response to reduced
water clarity (Boström et al., 2003).
Another large-scale change that has taken place in the Baltic
Sea during the 20th century is an increase in surface salinity by
0.5–0.9 psu (Matthäus, 1977). As discussed earlier salinity is an
important indirect factor regulating depth distribution of F.
vesiculosus. An increase in salinity could imply increased
competitive pressure and a reduced depth limit. However, the
relation between depth limit and salinity across the Baltic
gradient show a decrease in depth limit of only 0.14 m when
salinity rises 1 psu (Fig. 4), so it is unlikely that the large-scale
increase in salinity has had any major effect on the depth
distribution of F. vesiculosus. Eutrophication effects remain the
most plausible factor explaining the observed decline in depth
limit of F. vesiculosus over the 20th century.
The change in the location of F. vesiculosus belts from
deeper towards shallower water can be harmful to the stability
of the whole community as physical stress in the form of wave
exposure and ice scouring are stronger in shallow water
(Eriksson et al., 1998; Kiirikki, 1996). Moreover, as the width
of the F. vesiculosus belt declines, the population becomes
more prone to local extinction as the basis for both vegetative
and sexual reproduction diminishes. Declines in the distribution
area of F. vesiculosus may thereby become a self-accelerating
process.
During the last two decades, trends in F. vesiculosus depth
distribution have differed between districts and do not seem to
clearly follow reported changes in nutrient or light levels. At the
entrance of the Baltic Sea, depth limits have declined in
Kattegat W and increased in the Danish Belts over the period
1989–2000 in spite of a general increase in transparency from
the early 1980s to 1993s (Rasmussen et al., 2003). In the Gulf of
Finland, F. vesiculosus has shown no overall change in
distribution limit over the period 1991–2001 in spite of
ameliorated light levels (Nappu et al., 2002). In the eastern Gulf
of Finland this may be because Cladophora rupestris occupies
the space below the F. vesiculosus belt while in the western Gulf
the blue mussel has settled below the F. vesiculosus belt and
K. Torn et al. / Aquatic Botany 84 (2006) 53–62
may delay recolonisation (Ari Ruuskanen, pers. commun.). An
occupation of former F. vesiculosus areas by other species or by
sediment layers (Eriksson and Johansson, 2003) may thus
represent a barrier to deeper penetration of F. vesiculosus and
hinder a direct linear response of F. vesiculosus depth limits to
ameliorated light conditions. There are several examples from
the literature of how shifts in ecosystems following a
perturbation may hinder or delay a return to former conditions
even after the perturbation has stopped (Scheffer et al., 2001).
Moreover, in areas of low salinity like the Gulf of Finland, rates
of sexual reproduction are limited (Serrão et al., 1996) and may
also delay recolonisation. The present knowledge on time
scales of recolonisation and possible impediments to recolonisation of F. vesiculosus and other key species is very limited
and these aspects deserve more attention in future studies.
Acknowledgements
This research was supported by the EU projects CREAM
(HPMT-CT-2001-00265) and CHARM (EVK3-CT-200100065) and by the Estonian Governmental Programme No.
0182578s03. We wish to thank Ari Ruuskanen, Sergej Olenin,
Anda Ikauniece, Sif Johansson, Jan Warzocha and Sigrid Sagert
for providing data. We are also grateful to Dr. Jonne Kotta for
valuable comments on the manuscript.
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