Journal of Plankton Research Vol.20 no.6 pp.lO9£-l 117, 1998
Phytoplankton spring blooms in the southern Baltic
Sea—spatio-temporal development and long-term trends
N.Wasmund, G.Nausch and W.Matthaus
Baltic Sea Research Institute Warnemunde, PO Box 3010 38, D-18111 Rostock,
Germany
Abstract. The seasonal and long-term development of the phytoplankton spring bloom in different
regions of the southern Baltic Sea was investigated on the basis of monitoring data. The development
of a spring bloom starts when the upper mixed layer becomes shallower than the euphotic zone, as
proved also by a mesocosm experiment. This already happens in March in Mecklenburg Bight and
the western part of the Arkona Sea, leading to a diatom bloom, but only in April in the Bomholm
Sea, increasingly giving rise to a dinoflagellate bloom. The new production of the spring phytoplankton may be calculated from the decrease in nutrients during spring. In comparison with the
Redfield ratio, phosphorus is taken up in excess (N:P = 9.2-10.2). The consumption of silicate in spring
has been reduced in the southern Baltic proper since 1989, pointing to a decline in diatoms. The
increase in chlorophyll a in the Bornholm and the southern Gotland Seas is related to eutrophication,
whereas the decrease in diatoms in favour of the dinoflagellates is related to mild winters. The lack
of deep-reaching circulation after mild winters may be one reason for the suppression of the nonmotile diatoms.
Introduction
Eutrophication is one of the severe problems in the Baltic Sea. To improve
knowledge on the long-term changes in the factors related to eutrophication and
pollution, the Baltic Monitoring Programme (BMP) was established by the
Baltic Marine Environment Protection Commission (Helsinki Commission,
HELCOM). Consistent data sets, produced by the joint effort of all riparian
countries of the Baltic Sea, have been available since 1979. As the Baltic Sea
Research Institute Warnemunde (IOW) contributed most to the data pool of the
southern Baltic Sea, it felt especially responsible for the comprehensive evaluation of the data of that special area.
Eutrophication is mainly based on increased nutrient input (HELCOM, 1996).
Nutrients that are accumulated during winter are consumed in spring by the
quickly growing algae. The reduction in nutrients in the water during the bloom
is related to new production. The term 'new production' (= nitrate-based primary
production) was created by Dugdale and Goering (1967) in contrast to 'regenerated production' (= ammonium-based primary production). In this paper, we
have extended this term because we have calculated 'new production' also on the
basis of reduction in phosphate during the bloom. The calculation of biomass
growth on the basis of nutrient decrease in the water is especially useful for spring
blooms as they thrive mainly on new production. The ratio between new production and total spring production was estimated to be >75% in Kiel Bight
(Smetacek et al, 1984) and 80% in the northern Baltic proper (Lignell et ai,
1993).
In the different regions of the southern Baltic proper, three large biomass
peaks occur per year, namely in spring, summer and autumn. We concentrated
© Oxford University Press
1099
N.Wasmund, CNausch and W.Matthaus
on the spring bloom and compared the phytoplankton biomass at representative
stations in the eastern Mecklenburg Bight and the southern Baltic proper.
We paid much attention to analysis of the triggering of the spring bloom and
checked the importance of the depth of vertical mixing for spring bloom initiation, as claimed by Sverdrup (1953). The critical-depth model of Sverdrup was
criticized by Smetacek and Passow (1990). They turned their attention to the
upper 10-20 m, where irradiance levels permit maximal division rates, rather than
to the lower layers of the euphotic zone, where respiration becomes more important. Not the 'critical depth', but the euphotic depth, is decisive. The ratio of the
euphotic zone to the mixed zone is probably a major determinant of plankton
primary production (Cole etai, 1992). Algae may quickly grow to bloom concentrations if they are retained in that upper mixed layer. However, von Bodungen
et al. (1981) held the opinion that the bloom had already commenced before the
stratification in the upper layers of the water was formed. We had to examine
these contrasting statements.
Also, the delay in the spring bloom development in the deeper eastern areas
of the southern Baltic proper may be caused by differences in the timing of
thermocline formation (Kaiser and Schulz, 1978). This has to be checked, especially because Kaiser and Schulz (1978) were not able to determine the exact start
of the formation of the thermocline and bloom because of a limited data basis.
The decrease in a spring bloom in the Bornholm Sea has been described in
detail by von Bodungen et al. (1981): sinking out of the diatoms after nitrate
deficiency, combined with low turbulence (ultimately the establishment of the
thermocline); nutrient uptake by the cells on their way down even in the dark;
high sedimentation speed of up to 50 m day 1 , right through the halocline; low
impact of the zooplankton on the spring bloom. The same factors were responsible for the termination of the spring bloom in the Arkona Sea (Schulz et al,
1984). Patchiness and sedimentation of a spring bloom in the southern Gotland
Sea have been investigated by Passow (1990). As the seasonal development of
zooplankton starts only in May in the southern Baltic proper (Witek, 1986), its
impact on the spring bloom is generally low. Also in the northern Baltic proper,
the importance of metazooplankton grazing in controlling the phytoplankton
spring bloom was small, whereas sedimentation accounted for 72% of the bloom's
primary production (Lignell et al, 1993).
Method
Field data from 1979 to 1993 were extracted from the common data bank of the
Helsinki Commission (HELCOM) and supplemented by current data of the
IOW from 1994 to 1996. Standard methods have been used (Table I). Samples
were taken by rosette samplers in combination with CTD probes from 0,2.5,5,
7.5 and 10 m depth. From these samples, 'surface' means were calculated by
trapezium integration. Only for phytoplankton analyses were the samples of the
five depths mixed to produce an integrated sample. The phytoplankton were
counted in an inverted microscope while assigned to species and size classes. The
cell volume was calculated from the size measurements by using the appropriate
1100
Phytoplankton spring blooms in the southern Baltic Sea
Table I. Table of methods used. References: 1 = HELCOM (1988), 2 = Rohde and Nehring (1979), 3
= Grasshoffefo/. (1983)
Parameter
Units
Principle of the method
Reference
Water temperature
Quantum irradiance
Salinity
Reactive phosphate
Ammonium
Nitrite
Nitrate
°C
uE m-2 s-1
PSU
mmol n r '
mmol nr-11
mmol nrmmol m~-'
1
Silicate
mmol m~'
Chlorophyll a
mg nr-1
Phytoplankton
biomass
mg wet weight nr-1
CTD probe and thermometer
Ll-COR data logger, flat sensor
CTD probe (conductivity)
molybdene blue method (ascorbic acid)
indophenol blue method
reaction to azo dye
reduction to nitrite (by Cd),
followed by nitrite
determination
molybdene blue method
(oxalic acid)
acetone extraction.
spectrophotometric
reading, correction for
phaeopigments
counting by inverted microscope
1
2,3
2,3
2,3
2,3
2,3
1
1
stereometric formula. It was converted to wet weight assuming that the density
of the plasma is equal to that of water (-1 mg mm"3). Further transformations
were unnecessary because the biomass data served only as a rough estimate. We
denned the beginning of the bloom by a doubling of the biomass relative to the
winter level.
As the data are based on discrete samplings of relatively low frequency (for
sampling frequency, see Figures 5 and 6), the spring bloom might not have been
recorded completely in some years. Also, maxima and minima of nutrients and
temperature may not have been met exactly. Tofillsome gaps, Swedish data from
1994, Station K2 (see Figure 1), were included. Furthermore, the Finnish data
from the Algaline programme (Rantajarvi and Leppanen, 1994) were consulted.
Since 1992, continuous records of temperature and salinity, registered by the
automatic measuring station of the IOW at Darss Sill, have been available
additionally.
To check a hypothesis on bloom initiation and triggering, a special mesocosm
experiment was performed from 18 January to 23 February 1996. Two plastic
tanks were filled with 1000 1 of surface water by means of a bucket: tank 1 at
Station K5 in the Arkona Sea (salinity 7.7 PSU), tank 2 at Station 165 in the
Pomeranian Bight (salinity 6.1 PSU). The tanks were kept on deck of the ship at
in situ water temperature, but with continuous (24 h per day) artificial light (500
W halogen lamp, quantum irradiance above water surface of 200 uE m~2 s"1,
which was similar to the average light intensity in January at noon). The mesocosms were stirred before sampling. Phytoplankton biomass, chlorophyll a
concentration and nutrient concentrations were determined according to the
routine procedures of the monitoring programme (Table I). Polycarbonate
bottles (280 ml) were incubated for primary production measurements directly in
1101
N.Wasmund, G-Nausch and W.Matthaus
56
•Kl
Southern
Gotland Sea
55
55
13
14
15
16
Fig. 1. Investigation area and its location in the Baltic Sea.
the mesocosms, 0.5 m below the water surface, for 6 h (I4C method, liquid scintillation counting).
Hydrographic characteristics of the area
The Baltic Sea is among the world's largest brackish sea areas, with a total
surface, including the Kattegat, of 412 500 km2. It is subdivided into a number of
different regions. The southern Baltic proper (96 600 km2) consists of the following regions (Figure 1): Arkona Sea (18 700 km2; Stations K4, K5, K7, all -47 m
depth, and Station K8, 22 m depth), Bornholm Sea (39 000 km2; Station K2, 91
m depth) and the southern part of the eastern Gotland Sea, called southern
Gotland Sea (Station Kl, 90 m depth). The Darss Sill (sill depth 18 m) is the topographical and oceanographic borderline between the Baltic proper and the Mecklenburg Bight (Stations Ml and M2, -25 m depth). For the exact geographical
position of the stations, see HELCOM (1988).
The Baltic proper is permanently stratified and the Mecklenburg Bight mostly
stratified. A pycnocline separates the lower saline surface water (6-8 PSU) from
the more saline deep water (15-20 PSU) and excludes the deep water from vertical mixing. The depth of the pycnocline varies in general between 15 and 25 m in
the Mecklenburg Bight, between 30 and 40 m in the Arkona Sea, between 50 and
60 m in the Bornholm Sea, and between 60 and 80 m in the southern Gotland
Sea.
In spring, when the surface water warms up, a thermocline develops in 10-30
m depth and lasts until the autumn overturn (deep convective mixing). In winter,
mixing down to the permanent pycnocline takes place if the water cools down to
the temperature of maximum density, e.g. 2.5°C at a salinity of 7 PSU (cf. Figure
2). A further decrease in temperature leads to a new stabilization of the water
1102
Phytoplankton spring blooms in the southern Baltic Sea
4•
Temperature of
maximum density
at 7 PSU and 11 PS
Mecklenburg Bight
—•— ArkonaSea
- * - Bomholm Sea
— « - South. Gotland Sea
79 80 81
82
83 84 85
86 87 88 89
90 91 92 93
94 95
96
Fig. 2. Trend of minimum temperatures in the winters from 1978/79 to 1995/96 in four regions of the
Baltic Sea. Temperatures of maximum density of water at typical salinities of the southern Gotland
Sea (7 PSU) and Mecklenburg Bight (11 PSU), calculated after Keala (1965), are indicated by straight
lines.
column which lasts until the spring overturn. If the water temperature does not
decrease to the temperature of maximum density, the warmest water within the
upper layer is still found at the surface. When the water warms up in spring, stabilization of the water column becomes greater, without preceding spring overturn.
For our investigation, we termed winter seasons as 'mild' if the water temperature of the upper layer stayed higher than the temperature of maximum density.
Defined in such a way, mild winters were recorded in the area investigated in
1982/83, from 1988/89 to 1991/92 and in 1994/95. In the winter of 1987/88, the
temperatures of density maximum were reached in the Arkona Sea and almost
reached in the other areas. In 1992/93, the mentioned criteria for a mild winter
were fulfilled only in the Bornholm and the southern Gotland Seas.
We considered a water column as 'mixed' if the vertical density gradient is
<0.008 nr 1 for the water column above this layer. A temperature gradient of
0.01°C within a depth interval of 1 m already indicated a stabilization of the water.
This weak stratification may, however, easily be disturbed by stronger wave
action.
We assumed that the depth of the.euphotic zone is twice the Secchi depth
(Parsons etal, 1977).
Results
Spring bloom development
We compared the growth of spring phytoplankton biomass at representative
stations in the eastern Mecklenburg Bight, the Arkona, the Bornholm and the
1103
N.Wasmund, GJNausch and WJVIatthaus
southern Gotland Seas. As examples, the years 1994 and 1995, which differed in
the severity of the preceding winter, will be discussed.
In 1994, after a cold winter, the spring bloom started between 10 and 23 March
in the Mecklenburg Bight (Station Ml; Figure 3a). The whole upper mixed layer
2000
(a)
(d)
Station M 1
1994
10.2.22.2.1.3.
3000
10.3.23.3.
11.4.21.4. 5.5.
(b)
1500
Station M1
1995
16.1. 18.2. 23.3. 2.4. 4.4. 11.5. 23.5.
r—
Station K5
1994
H
^
IP00"
V15O0
miooo
500
0
I I111I
q^) u w fulfilfigg^ j ^ ^
12.2. 22.2. 2.3. 3.3. 9.3. 26.3. 1.4. 12.4. 15.4. 6.5.
ffl20OO
D
26.5.
16.1. 11.2. 23.3. 2.4. 4.4. 11.5. 22.5.
25O0
19.2. 24J.
12.5. 2X5.
Date
16.2. 21.2. 4 J . 5 J . 7 J . 27J. 31.3.
14.4. 7.5. 10.5. 25J.
• DinofUgellates BDUtoms
BEuglenophycaeOothera
Fig. 3. Seasonal development of phytoplankton wet weight in 1994 (a-c) and in 1995 (d-1) in different regions: (a) and (d) Mecklenburg Bight; (e) western Arkona Sea; (b) and (f) central Arkona Sea;
(c) Bomholm Sea.
1104
Phytoplankton spring blooms in the southern Baltic Sea
(above the halocline, 8-12 m depth) was situated within the euphotic zone. The
bloom may have been triggered by the generally increasing global radiation in
March (up to 30 E nr 2 day 1 ). On 23 March, there were still nutrients available
for further growth (0.42 mmol nr 3 PO 4 ,9.5 mmol nr 3 NO3,10.9 mmol nr 3 SiO4
in surface water).
In the central part of the Arkona Sea (Station K5; Figure 3b), the bloom was
already growing on 26 March 1994, but was in an earlier developmental stage than
in the Mecklenburg Bight. The mixed layer reached down to 12-13 m depth.
Secchi depth was 9 and 10 m on 3 and 26 March, respectively. The bloom reached
its peak in the middle of April. At the more easterly Station K4, the bloom started
only at the beginning of April; at this station, the mixed layer extended to -17 m,
which was still within the euphotic zone.
In the Bornholm Sea (Station K2; Figure 3c), the pycnocline was situated in
-50 m depth on 31 March (Secchi depth 13 m). Assuming that the depth of the
euphotic zone is twice the Secchi depth, the upper mixed layer would extend to
the aphotic zone. Algae circulating in this layer would not get enough light for
growth to bloom concentrations. On 14 April, the highest temperature gradient
of the upper layer was found between 12 and 13 m with 0.067°C nr 1 , which is an
indication for the stabilization of the water column. Algae circulating in this
upper layer may grow in optimal light conditions to bloom concentrations. The
nitrate especially decreased significantly from 31 March to 13 April and was only
0.1 mmol nr 3 on 7 May, when the bloom culminated (cf. Figure 4). In the southern Gotland Sea, the bloom develops even later.
The early spring bloom in the Mecklenburg Bight consisted almost exclusively
of diatoms (mainly Thalassiosira baltica), whereas in the late spring blooms in the
15
1994
Station K2
.t
/jC
X~\
/\\
I
I
-»-PO4
-»-NO3
—Chl.a
-^SKX
a
January
=Chl.a
382
12
V
\
55
*** P
February
March
April
May
Fig. 4. Changes in chlorophyll a and nutrient concentrations during the spring bloom at Station K2
in 1994.
1105
N.Wasmund, G-Nausch and W.Matthaus
Bornholm and Gotland Seas dinoflagellates (mainly Peridiniella catenata) were
dominant. In the Arkona Sea, there was a clear succession within the spring
bloom from diatoms (mainly Thalassiosira levanderi and T.baltica) to dinoflagellates (mainly P.catenata and some Protoperidinium spp.) and, to a lesser degree,
Skeletonema costatum. Euglenophyceae (Eutreptiella sp.) developed from the
middle of April to the middle of May.
Also in 1995, after a mild winter, the spring bloom started in near-coast regions
of the Mecklenburg Bight in the middle of March (with Euglenophyceae and the
diatoms Leptocylindrus danicus and T.baltica). In these areas, the nutrients
(namely phosphate) were already exhausted at the end of March. In the eastern
part of the Mecklenburg Bight (Station Ml) and the western and central part of
the Arkona Sea (Stations K8 and K5), the bloom developed almost simultaneously until the end of March (Figure 3d-f). In both regions, a temperature
gradient of 0.2°C nr 1 had established in 12-14 m depth on 23 and 24 March, indicating a stabilization of the water column. As Secchi depth was 8 m at that time,
the light conditions were sufficient for bloom growth in this surface layer. For
instance, the phosphate concentrations at Station K8 declined from 0.27 to 0
mmol nr 3 in the period from 23 March to 2 April 1995. The bloom consisted
mainly of Rhizosolenia setigera and T.baltica at Station Ml, and of S.costatum at
Stations K8 and K5. At Station K4, the water column was still homogeneous
down to the halocline (35 m depth) on 24 March; phytoplankton biomass was still
low (176 mg rrr3) and no nutrient decline was noticed in comparison with winter
concentrations. On 11-12 May, dinoflagellates (Gymnodinium lohmannii) were
already dominant in the Mecklenburg Bight and the Arkona Sea.
In the Bornholm and southern Gotland Seas, a slight thermal gradient built up
in 25-35 m depth on 31 March/1 April, indicating the boundary of the upper
mixed layer. Secchi depth was high (Station K2: 10 m. Station Kl: 16 m). The
algae which were trapped in this upper layer supposedly got just enough light for
bloom growth. Nutrients had not declined yet. The phytoplankton biomass
(mainly P.catenata, some diatoms: Thalassiosira spp., Chaetoceros spp., S.costatum) had doubled compared to the winter level, indicating the start of the spring
bloom. By the end of April, the nutrients, especially nitrate, had been almost
completely exhausted (e.g. at Station Kl from 5.46 mmol nr 3 on 31 March to 0.04
mmol m~3 on 18 May 1995). This bloom was not composed of diatoms, which was
also indicated by the remaining silicate pool (e.g. at Station Kl: 12.2 mmol nr 3
on 18 May 1995).
The hypothesis, that the spring bloom is triggered by a reduction in the depth
of the upper mixed layer, was proved by a mesocosm experiment. By filling the
water into the tanks, a shallow water column was created, which made a higher
light intensity available for the algae in the tanks. In all tanks, primary production, chlorophyll a concentration and phytoplankton biomass increased strongly
after day 10, while the nutrient concentrations suddenly decreased (not shown
here). Diatoms especially developed in the tanks, but the species were different
due to different salinities. Mainly Chaetoceros subtilis and S.costatum occurred in
Tank 1, and Chaetoceros calcitrans and Skeletonema subsabum in Tank 2. No
phytoplankton development occurred in the field in January and February 1996.
1106
Phytoplankton spring blooms in the southern Baltic Sea
Only in March, a bloom of T.levanderi developed in the Arkona Sea and a bloom
of S.subsalsum in the Pomeranian Bight (around Station 165). The development
of diatoms in the tanks showed that a seed population was present in the surface
water in winter. There was no need to import a seed population from shallow
coastal water or from the sediment by water currents or active migration.
Long-term trends
Phytoplankton biomass. The long-term variation of spring phytoplankton
biomass from 1979 to 1996 in different regions is shown in Figure 5. The differences between years should be considered with a certain reservation as the data
sets are not homogeneous. Blooms may have been sampled at different stages of
their development and therefore may be represented to a higher or lesser degree.
Therefore, Figure 5 attaches importance more to the relative composition of the
blooms (see below) than to the absolute biomasses.
The measured chlorophyll a concentrations are plotted in Figure 6. In comparison with the biomass data, they show a smaller variation, making them more suitable for discussing trends. The regression analysis reveals a significant trend
(P = 0.05) only in the Bornholm (Figure 6c) and the southern Gotland Seas
(Figure 6d). Rahm et al (1995) suggested that non-parametric tests, like that by
Hirsch and Slack (1984), should be favoured instead of linear regression. Using
that test, the same trends were found as described from the linear regression
analysis (Wasmund et al, 1998).
Nutrients and new production. The above-mentioned problems of inadequate
samplings of all stages of the bloom are excluded by the calculation of new
production because only samples before and after the bloom are needed. An
example for the calculation of new production from the decrease in nutrients
during the bloom is depicted in Figure 4. That bloom consumed 0.61 mmol m~3
phosphorus and 4.59 mmol m~3 nitrogen. Assuming that these nutrients are incorporated by phytoplankton according to the ratio C:N:P (by atoms) = 106:16:1
(Redfield et al, 1963), a new production during the spring bloom of 365 mg C m~3
per 3 months and 776 mg C nr 3 per 3 months can be calculated on the basis of
nitrogen and phosphorus, respectively. The mean new production in the spring
blooms from 1980-1993 is compiled in Table II. It becomes obvious that the
uptake of phosphate (PO43"), relative to dissolved inorganic nitrogen (= NH4+ +
NC>2~ + NO3"), was much higher than would have been expected from the
Redfield ratio. That means, on the basis of P consumption, a biomass nearly twice
as high as on the basis of dissolved inorganic nitrogen could have been produced.
The new production in the spring bloom is highly variable and without any
trend in the period from 1981 to 1993 (Figure 7). Also, the primary production in
the spring period (March-May) measured by the 14C method shows no long-term
trend (Wasmund et al, 1998).
It is worth mentioning that the phytoplankton biomass, chlorophyll a concentration and primary production of the spring period are higher in the
1107
N.Wasmund, CNausch and W.Matthaus
(a)
6000
Station M2
S- 5000
OOOmn
• a 4000
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~ 3000
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.22
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Station K4, K5 and K7
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94
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88
153
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£
2500
,§, 2000
• 1500
J 1000
a
500
it
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8 1 8 2 8 3 8 4 a S 8 6 8 7 S 8
90
91
92
93
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98
Year
(C)
u] mui
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o
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Station K2
1000
acxtm*
800
5
600
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1 B
ca , _
7
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O
500
0
7 9 8 0
8 1 6 2 8 3 8 4 6 6 8 8
87
Jl
2
8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5 9 6
Fig. 5. Mean biomass (wet weight) of spring phytoplankton (March-May) in surface water (mean of
0-10 m depth) from 1979 to 1996 in (a) Mecklenburg Bight, (b) Arkona Sea, (c) Bornholm Sea and
(d) southern Gotland Sea. The figure above the bar indicates the number of samples.
1108
Phytoplankton spring blooms in the sontheni Baltic Sea
7 9 8 0
8 1 8 2 8 3 8 4 8 5 8 8
87
8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5 9 6
Station K4, KS and K7
•
t »ftr = ft(f • • * •
•
7 9 8 0
8 1 8 2 8 3 6 4 8 5 8 8 8 7 8 8 8 9 9 0
9 1 9 2 9 3 9 4 9 5
Year
In
(c)
38.2
Station K2
t
P 8
"•?
t
6
5«
2
= 89
1—
-*-
fcfei
79
80
81
\
82
84
85
66
67
88
88
90
91
= 0.215
•
•
4
9 2 9 3 9 4 9 S 9 S
Year
7 9 8 0
8 1 8 2 8 3 8 4 8 5 8 8 8 7 8 8 8 9 9 0 9 1 8 2 9 3 9 4 9 5 9 8
Fig. 6. Chlorophyll a concentration in surface waters (mean of 0-10 m depth) in spring (March-May)
from 1979 to 1996 in (a) Mecklenburg Bight, (b) Arkona Sea, (c) Bomholm Sea and (d) southern
Gotland Sea.
1109
N.Wasmnnd, G-Nausch and W.Matthaus
Table II. Mean phytoplankton wet weight and chlorophyll a (March-May, 1979-1996) as well as mean
new production of phytoplankton in spring 1981-1993, calculated from the decrease in dissolved
inorganic nitrogen and phosphate, respectively, from February to May, and ratio of this N and P
decrease
Wet
Region
weight
(mg nr 3 )
Mecklenburg Bight
Arkona Sea
Bornholm Sea
Southern Gotland Sea
^-600-,
2778
1057
412
606
Chlorophyll a
(mgm-')
New production
calculated from
nitrogen (mg C nr 3 )
New production
calculated from
phosporus (mg C m ° )
N decrease/
P decrease
(mol/mol)
3.41
2.43
2.46
2.97
639
415
374
322
1011
10.2
710
678
632
9.6
9.4
9.2
(a)
(b)
800
o
o>400-
600-
"""300400|
200-
1001980
1985
' 1990
Year
1995
1*980
1985
1990
1995
Year
Fig. 7. New production of phytoplankton in the Bornholm Sea from 1981 to 1993, calculated on the
basis of the decrease in (a) dissolved inorganic nitrogen and (b) phosphate in the water during the
spring bloom.
Mecklenburg Bight than in the southern Baltic proper (Table II). This agrees with
the winter concentrations in nutrients (HELCOM, 1996).
Phytoplankton groups, silicate and temperature. The diatoms are, now as before,
the dominating part in the spring phytoplankton of the Mecklenburg Bight and
the Arkona Sea (Figure 5a and b). In the Bornholm and southern Gotland Seas,
the share of diatoms decreased in favour of the dinoflagellates (Figure 5c and d).
The composition of the diatom spring populations did not change drastically.
Chaetoceros spp., Thalassiosira spp. and S.costatum are still the dominating
diatom species in the spring blooms of the southern Baltic proper. The growth in
dinoflagellates is mainly based on the native P.catenata.
In order to confirm this decrease in diatoms, the trends in consumption of silicate were recorded. As diatoms are the only significant organisms that incorporate silicate, their growth results in a decreasing amount of silicate in the water.
Such a decrease from winter to late spring can be observed every year (see the
bars in Figure 8). The consumption has declined, however, since 1989 in the
southern Gotland Sea, the Bornholm Sea and, to a lesser degree, in the Arkona
Sea. This agrees roughly with the phytoplankton results (Figure 5).
1110
Phytoplankton spring blooms in the southern Baltic Sea
Station M 2
90
80 81 S3 33 94 85 86 87 88 89 90 91
92
93
94
95
96
92 93 34 95 96
Fig. 8. Silicate concentration in surface water in late winter (curve) and difference in pre-bloom and
post-bloom silicate concentration (bar) from 1979 to 1996 in (a) Mecklenburg Bight, (b) Arkona Sea,
(c) Bomholm Sea and (d) southern Gotland Sea.
It is of special interest that the winter concentrations of silicate (shown by the
curve in Figure 8) did not decrease to such an extent. The low values from 1989
in the Mecklenburg Bight and the Arkona Sea are related to a particularly strong
1111
N.Wasmund, CNausch and W.Matthaus
diatom bloom in summer/autumn 1988. It can be assumed that the silicate was
still sufficient for diatom blooms in the 1990s.
The decrease in diatoms is related to the mild winters in the period from
1988/89 to 1991/92, also in 1992/93 in the Bornholm and southern Gotland Seas
(Figure 2). Whether the temperature is the only causal factor remains questionable as the diatoms did not completely recover after the cold winters of 1993/94
and 1995/96 and, on the other hand, grew well after the solitary mild winter of
1982/83. Perhaps only a longer period of two or more successive mild winters has
a lasting effect on the diatoms.
Discussion
We made an attempt to use the long-term routine monitoring data available since
1979 in order to extract knowledge on spring bloom development in different
time scales. This is a difficult task since natural systems are very complex and
monitoring programmes do not normally focus on specific problems. Nevertheless, the examination of spring phytoplankton data in relation to physical and
chemical parameters led us to some interesting results.
Spring bloom development
The development of algae needs some preconditions: a seed population, nutrients and light. As enough algae and nutrients are dispersed in the water in winter
(as shown from our mesocosm experiment and from the winter nutrient concentrations, e.g. Figure 4), the light is supposed to be the limiting factor (cf. Townsend
et al, 1994). The light available to an alga is not only dependent on the incident
radiation, but also on light extinction in the water and the position of the alga in
the water column.
The algae trapped in the only 1.2-m-deep tanks of our mesocosm experiment
received much more light than those in the sea, which had to circulate in a 40-mdeep water column. Therefore, a bloom was already triggered in the tanks in
January, whereas the natural bloom was recorded in the Arkona Sea only after
stabilization of the water column in March. Riley (1967) suggested that the critical light intensity triggering the bloom was reached when the depth-averaged,
vertically integrated irradiance within the mixed layer increased to ~40 Ly day 1
(20.9 W nr 2 or -96 uE nr 2 s"1).
We could also prove the importance of a shallow mixing depth for spring bloom
initiation in the sea. We found the earliest blooms in shallow bights. Circulation
of the algae within the euphotic zone is best ensured in shallow coastal waters
(Brunet et al, 1996) or waters with a shallow permanent pycnocline, e.g. in Kiel
Bight (Peinert et al., 1982) or the Gulf of Finland (Kaiser and Schulz, 1978; Kahru
and Nommann, 1990). In the deeper areas of the Bornholm and southern Gotland
Seas, a bloom can develop only when trapped in the euphotic zone. However, the
formation of a stable summer thermocline lasts until May in the Baltic proper
(e.g. Matthaus, 1979; Lignell et al, 1993). This led von Bodungen et al (1981) to
the opinion that the bloom had already commenced before the stratification in
1112
Phytoplankton spring blooms in the southern Baltic Sea
the upper layers of the water was formed. We agree that the spring bloom develops before a stable thermocline has established, but have to stress that the water
column has become relatively stable long before (at least since the middle of
April), indicated by a slight temperature gradient only.
Similar findings are already known from the literature. It was suggested by
Williams and Robinson (1973) and Colebrook (1979) that very weak or even transient periods of vertical stability are sufficient to initiate and maintain the start of
the spring bloom. Garside and Garside (1993) inferred from nutrient analyses
during the JGOFS North Atlantic bloom study in 1989 that phytoplankton
growth started prior to the development of thermal stratification. Townsend et al.
(1992) provided direct evidence that phytoplankton blooms in the offshore
waters of the Gulf of Maine may commence in a neutrally stable water column
provided that vertical mixing is insufficient to produce light limitation, and that
the growth rates of the phytoplankton population exceed other losses.
Also, the regional differences in the timing of the spring blooms, as already
found by Kaiser and Schulz (1978), are related to the mixing depth. As the investigated stations are located at almost the same latitude, they receive the same
irradiation, if temporary variations of cloud cover are ignored. The turbidity
(Secchi depth) is also similar. The differences in the time of bloom onset in different regions cannot be explained by the incident radiation, but by the different
depths of the upper mixed layer. In the deeper areas, a much larger volume has
to be warmed up, leading to a much later establishment of thermal stratification
after a cold winter. Only the early spring blooms in the Mecklenburg Bight and
the central Arkona Sea were directly triggered by the increasing light intensity in
March.
Schulz et al. (1992) took the influence of the preceding winter into consideration and showed that the time lapse between the various regions of the southern Baltic disappeared after a mild winter. The data basis employed by Schulz et
al. (1992) was, however, rather poor. After carrying out a detailed causal analysis on the basis of a much higher sampling frequency, we were able to confirm
their ideas.
In a model of Townsend et al. (1994), the spring bloom appeared 1 week later
in cold years than in warm years. They defined a 'warm' year on the basis of a
water temperature not lower than 2°C, independently of the mechanisms of
density maximum and circulation.
Despite the differences in timing and species composition of the spring bloom,
the southern Baltic proper is a relatively uniform water body which is different
from the Mecklenburg Bight. Although the Darss Sill is generally accepted as the
borderline between these two regions, our data on the onset of the spring bloom
reveal some evidence that the region of transition between the Mecklenburg
Bight and the southern Baltic proper stretches far into the Arkona Sea. The
western and central parts of the Arkona Sea (Stations K5-K8) were very similar
to the Mecklenburg Bight (Station Ml), especially in 1995 (cf. Figure 3d-f). On
the basis of spring phytoplankton, this borderline turns out to be a broad transition area which normally does not correspond with Darss Sill, but divides,
temporarily, the Arkona Sea into a western and an eastern part.
1113
N.Wasmund, CNausch and W.Matthaus
Long-term trends
While the seasonal development of phytoplankton blooms has been well documented in the literature, there is still a deficiency of long-term data sets. The
results of the Baltic Monitoring Programme are compiled in Periodic Assessments. In the Third Periodic Assessment (HELCOM, 1996), different chlorophyll
a trends were described in the different seasons in the Baltic proper. From
1979/80 to 1988/89 especially, the summer chlorophyll a concentrations decreased, but the autumn chlorophyll a increased in the Bornholm and the southern Gotland Seas. Since 1989/90, the trends have tended to be opposite. As
related spring data were not reported in the Third Periodic Assessment, our
investigations on spring blooms filled this gap.
No increase in chlorophyll a was detected in the summer period from 1969 to
1986 in the open western Gulf of Finland, but in respective coastal areas there
was a remarkable increase in the spring period after 1983 (Gronlund and Leppanen, 1990). In the Gdansk Deep (southern Gotland Sea), the annual upward
trend in primary production and mean chlorophyll a concentration accounted for
-1.5-2% (Nakonieczny et al., 1989). Sanden and Hakansson (1996) calculated
from the declining Secchi depth in the Baltic proper a yearly increase in chlorophyll a of - 1 % , but emphasized that the calculations may be unreliable due to
uncertainty regarding the relationships between Secchi depth and chlorophyll.
Our regression line of the chlorophyll a concentration in the southern Gotland
Sea (Figure 6d) indicates nearly a tripling within 18 years. This seems to be too
high and should not be taken so strictly as the data are still too scattered to allow
for a reliable regression line. We expect that the slope of the line will become
more gentle with the continuity of the data series.
The disadvantage of low sampling frequency may be overcome by integrating
samples (e.g. sediment traps) or integrating parameters (e.g. nutrient consumption for calculation of new production). The biomass production of the spring
bloom was estimated from the nutrient decrease during spring because this bloom
especially is almost exclusively based on new production. The calculated new
production did not show a clear trend (Figure 7). As the winter concentrations of
nutrients are almost completely used up by the spring bloom, they should be
related to the intensity of new production. Indeed, just like the new production
(but unlike chlorophyll), the winter concentrations of phosphate and nitrate in
surface layers showed strong fluctuations, but no trend within our investigation
period (Nehring and Matthaus, 1991; HELCOM, 1996). Only if the whole nutrient data sets from 1969-1993 were considered, significant positive trends were
found. The clear eutrophication trend in the 1970s obviously had a long-lasting
effect on phytoplankton biomass, leading to a continued increase in spring
chlorophyll in some areas. One reason for the discrepancy between increasing
chlorophyll trend and lack of a trend in new production may be the increase in
turbidity, described by Sanden and HSkansson (1996). However, if data of all
seasons were pooled, both chlorophyll and primary production revealed similar
positive trends from 1979 to 1995 in the Bornholm and southern Gotland Seas
(Wasmund etal, 1998).
1114
Phytoplankton spring blooms in the southern Baltic Sea
In the Baltic proper and the Mecklenburg Bight, the winter N:P ratio is much
less than 16. Nevertheless, both nitrogen and phosphorus are used up almost
completely (cf. Figure 4), i.e. not in accordance with the Redfield ratio. Luxury
uptake of the excess phosphorus may be one reason. In the Mecklenburg Bight,
phosphorus is exhausted even faster than nitrogen. The phosphorus would allow
a biomass production nearly twice as high as on the basis of nitrogen (Table II
and Peinert et ai, 1982). Perhaps other nitrogen sources are also used, such as
atmospheric input (HELCOM, 1996) or urea, which occurs in the southern Baltic
proper in concentrations of -0.25-0.50 mmol nr 3 .
The decrease in silicate concentrations during spring could be used as an indicator for diatom growth. The silicate consumption has gone down in the southern Baltic proper since 1989. The winter silicate concentrations, even if
decreasing, are still high enough to support diatom growth. The molar Si:N ratios
in winter have always been higher than one, indicating that silicate should not be
the limiting nutrient.
A cause of the diatom decline may be the relatively high water temperature in
the winters 1988/89-1992/93, whereby not the direct impact of temperature on
algae, but its influence on water stratification, is decisive. In these winters, the
water temperature did not fall below that temperature at which the water has its
greatest density. If the water warms up in spring, a spring overturn does not
appear. This reduces the development of the non-motile diatoms, but stimulates
the dinoflagellates. Consequently, the change in species composition is not a
eutrophication effect.
An interchange between the algal groups of the spring bloom was also reported
from the entrance to the Gulf of Finland (Kononen and Niemi, 1984). From 1968
to 1975, dinoflagellates (P.catenata) were dominant, but were less important in
the late 1970s, when diatoms dominated. The dominance of dinoflagellates was
related to the mild winters of 1971/72-1974/75. After the cold winters of
1968/69-1969/70 and 1977/78-1980/81, when the ice broke up in April, the spring
bloom was mainly formed by marine diatoms together with P.catenata. An explanation was not given.
Also in zooplankton, a long-term shift from copepods to rotifers has been
noticed in spring since 1989 (HELCOM, 1996). As the zooplankton have an insignificant impact on the spring bloom, possible changes in feeding behaviour may
not be a reason for changes in the phytoplankton composition.
Wasmund and Heerkloss (1993) have shown an influence of pH on the species
composition in a lagoon, where cyanobacteria were replaced by green algae at
decreasing pH. The IOW measurements of pH in the open Baltic Sea did not indicate any changes.
Net radiation was above the average in 1989-1991 (Russak, 1994). It is true that
radiation may influence the timing of the bloom, but not the general species
composition. It should be expected that the diatoms can adapt to a slight increase
in natural underwater light intensity. Moreover, the general decrease in Secchi
depth (Sanden and Hakansson, 1996) might compensate for the increase in net
radiation.
We favour the temperature as the causal factor for the decline in diatoms. This
1115
N.Wasmund, CNausch and W.Matthaus
hypothesis may be proved when the diatoms recover after a series of cold winters.
The continuation of the monitoring programme will make it possible to settle this
problem conclusively.
Acknowledgements
We wish to thank Kate Kunert for counting and determining the phytoplankton,
and Birgit Sadkowiak for calculating the relevant nutrient data. We are thankful
to our colleagues Falk Pollehne, Giinter Jost, Eberhard Kerstan and Monika
Nausch, who were involved in the mesocosm experiment, for putting their data
at our disposal. We also acknowledge the Oceanographical Laboratory of the
Swedish Meteorological and Hydrographical Institute (SMHI) in Goteborg for
making hydrographic-chemical data available. The non-parametric trend test
was generously carried out by Riitta Olsonen from the Finnish Institute of Marine
Research. The data of the Finnish Algaline programme, which have been
supplied to the World Wide Web by Eija Rantajarvi and Juha-Markku Leppanen,
have been greatly appreciated for confirming our data and conclusions. The
authors are grateful to the anonymous reviewers for constructive remarks and
suggestions. The biological part of the IOW monitoring programme was
supported by the Bundesministerium fur Umwelt, Reaktorsicherheit und
Naturschutz under grant 102 04 395 and the additional mesocosm experiment by
the Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie
(BMBF) through grant 03F0105B.
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Received on July 30, 1997; accepted on January 19, 1998
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