Download Highlights of zooplankton dynamics in Estonian waters (Baltic Sea)

ICES Journal of Marine Science, 55: 748–755. 1998
Article No. jm980393
Highlights of zooplankton dynamics in Estonian waters
(Baltic Sea)
Evald Ojaveer, Alide Lumberg, and Henn Ojaveer
Ojaveer, E., Lumberg, A., and Ojaveer, H. 1998. Highlights of zooplankton
dynamics in Estonian waters (Baltic Sea). – ICES Journal of Marine Science, 55:
748–755.
The ecological subsystems in Estonian waters differ in mesozooplankton structure.
Euryhaline, eurytherm marine boreal species dominate in the NE Baltic Proper.
Species adapted to lower salinities are abundant in the upper layers of the Gulf of
Finland and Gulf of Riga, whereas glacial relicts and species of marine origin inhabit
the deepwater layers. Freshwater species are more important in the easternmost parts.
Large fluctuations in zooplankton composition, abundance, biomass, and spatial
distribution are connected with the seasonal cycle. On the coastal slope, largest
zooplankton biomasses and planktivorous fish concentrations were found during
summer in the zones of intense vertical mixing with high nutrient supply. Available
data suggest that predation by planktivorous fish may locally affect zooplankton
abundance. Long-term dynamics of copepod species are mainly triggered by changes in
salinity and temperature conditions, especially in marginal parts of their distribution
areas. In coastal bays, there are some indications of local deviations in copepod
abundance which may be induced by pollution. Cercopagis pengoi, a newcomer from
the Ponto-Caspian region, has occupied an important niche in the pelagic food web in
shallower areas of the Gulf of Riga and supports a significant component in the diet of
planktivorous fish species during summer.
1998 International Council for the Exploration of the Sea
Key words: brackish-water species, Cercopagis pengoi, freshwater species, long-term
changes, marine, pollution, seasonal fluctuations.
E. Ojaveer, A. Lumberg, and H. Ojaveer: Estonian Marine Institute, Viljandi Road 18b,
11216 Tallinn, Estonia. Correspondence to E. Ojaveer: tel: +372 6 281 568; fax: +372 6
281 563
Introduction
The ecosystem of the brackish Baltic Sea consists of a
number of subsystems with differing abiotic conditions
and biocoenoses (Ojaveer and Elken, 1997). Biological
production and functioning of the ecological subsystems
depend strongly on the restricted vertical water exchange,
caused by the existence of a halocline and seasonal thermocline, which limits transport of nutrients from deep
layers into the photic zone. Also, many groups of
organisms populating the sea (e.g. boreal marine and
freshwater species, glacial relicts) live continuously under
conditions of salinity and/or temperature stress. Therefore, annual cycles and species responses to climatic
deviations and anthropogenic impacts have a marked
influence on the functioning of the ecological subsystems
and their productivity, especially near the margins of
distribution areas of individual species.
Because of the relatively limited number of species and
a variety of unoccupied ecological niches in the Baltic
1054–3139/98/040748+08 $30.00/0
Sea (Elmgren, 1984), zooplankton plays a decisive role
in determining the fraction of primary production which
is converted into a form accessible to top predators or
harvestable by man. Regular zooplankton studies in
Estonian waters were started in 1924 (Frisch and
Riikoja, 1925) and resumed after World War II. The
same methods have been used from the 1960s onwards up
to the early 1990s. This mesozooplankton time series has
been used to clarify variations in production processes in
time and space and these studies have contributed considerably to the understanding of the functioning and
dynamics of the ecological subsystems in the area. This
paper gives an overview of the results obtained.
Material and methods
Extensive zooplankton samples have been collected
since World War II at stations along transects in the
Gulf of Finland and Gulf of Riga (Fig. 1). For monitoring the state of the ecosystem and estimating feeding
1998 International Council for the Exploration of the Sea
Highlights of zooplankton dynamics in Estonian waters
22
°N
24
26
749
28
°E
60
1(5)
2(4)
3(3)
4
5
59
6
58
8(6)
7(3)
7(9)
57
Figure 1. Location of zooplankton sampling transects: 1–3 – eastern, central, and western transects in the Gulf of Finland;
4 – Kolga Bay; 5 – Muuga Bay; 6 – Pärnu Bay; 7 – northern part of the Gulf of Riga; 8 – western part of Irbe Sound. Numbers
in parentheses denote number of sampling stations in a transect.
conditions of pelagic fish, material has been routinely
collected with a Juday net (mesh size 170 ìm) in May,
August, and October/November during the period
1963–1992 along the eastern, central, and western
transects (1, 2, and 3) situated at the southern coast of
the Gulf of Finland. For studies on vertical distribution,
monthly samples have been taken at a station of the
central transect (2) from March to October 1974 at the
following depth intervals: 0–10, 10–25, 25–50, and from
50 m to the bottom.
On spawning grounds of spring spawning herring in
the north-east part of the Gulf of Riga, sampling was
aimed primarily at determining forage reserves for
larval and young herring. Material was collected
weekly at 11 stations using a Juday net (mesh size
90 ìm) from May to July 1957–1994. Shorter time
series have been collected in the northern part of the
Gulf of Riga (transect 7; mainly 1994–1996), and west
of the Irbe Sound in the North-east Baltic Proper
(transect 8; 1970s).
The material has been treated by routine counting
methods (Lumberg, 1976; Lumberg and Ojaveer,
1991; Simm, 1995; Ojaveer, 1997). Species composition,
abundance (ind m 3), and biomass (mg m 3) of mesozooplankton were determined by depth intervals and
stations.
Species composition
There are notable differences in species composition
between the North-east Baltic Proper, Gulf of Finland,
and Gulf of Riga, which are closely connected with
marked differences in salinity, temperature regime, and
vertical structure of the water masses in the different areas. Differences occur in seasonal and long-term
abundance dynamics and spatial distribution of
zooplankton. The Gulf waters are characterized by
relatively higher temperatures during summer and lower
temperatures during winter than the waters of the open
Baltic. Zooplankton abundance and biomass reflect
these differences in temperature regime, with higher
values during summer and lower ones during winter (see
also Nikolaev, 1961).
In the North-east Baltic Proper (surface salinity of
7–8), the marine euryhaline and eurytherm species
Temora longicornis, Centropages hamatus, Acartia
longiremis, Evadne nordmanni, and mollusc larvae
dominate in the layers in and above the thermocline.
In contrast, adult Pseudocalanus minutus elongatus is
abundant in the deeper and colder layers, while
Fritillaria borealis is also frequently observed.
Persistent hydrological fronts exist in the western
part of the Gulf of Finland, which effectively separate
750
Evald Ojaveer et al.
(a)
Mar May June
Aug
July
Sep
Oct
0–10 m
10–25 m
25–50 m
> 50 m
–3
–10 000 ind m
(b)
Mar May June
July
Aug
Sep
Oct
0–10 m
10–25 m
25–50 m
–3
> 50 m
–0.10 g m
Copepoda
Cladocera
Rotatoria
Varia
Figure 2. Vertical distribution of zooplankton abundance (a) and biomass (b) in the central station (depth 73 m) of the central
transect in the Gulf of Finland in March–October 1974.
the fresh eastern part (salinity variation at surface from
2–3 to 5) from the open sea. Thus, freshwater zooplankton, mainly Keratella quadrata, K. cochlearis,
Daphnia cuccullata, Chydorus sphaericus, and Ceriodaphnia quadrangula dominate in the eastern part and in
small coastal bights. In cold bottom layers, the glacial
relict Limnocalanus grimaldii occurs. The importance of
brackish-water species (e.g., Acartia bifilosa, Eurytemora
hirundoides, Synchaeta baltica, S. monopus, Bosmina
coregoni maritima) increases westwards. In the western
part and at the mouth of the Gulf, T. longicornis,
E. nordmanni, Podon leuckarti (in upper layers), and
P. m. elongatus (in bottom layers) are abundant together
with Balanus improvisus and F. borealis.
In the Gulf of Riga (salinity of 3–6), brackishwater eurytherm, euryhaline species dominate, e.g.
E. hirundoides, A. bifilosa, B. c. maritima, Podon
polyphemoides, S. baltica, S. monopus in upper layers
and L. grimaldii in bottom water. Towards the coast,
mesozooplankton abundance and biomass increase and
freshwater species (e.g., K. quadrata, K. cochlearis,
Daphnia cristata, D. cucullata, Sida crystallina) as well as
larvae of molluscs (Line and Sidrevics, 1995) become
gradually more important. Marine euryhaline and
eurytherm species occur chiefly in areas of higher
salinity (6–7) close to the Irbe Sound: T. longicornis,
E. nordmanni, P. m. elongatus, and C. hamatus.
Seasonal changes
In March, the zooplankton community consists mainly
of copepods which are distributed usually in deeper
water (Fig. 2). Rotifers and cladocerans overwinter as
resting eggs. In spring, the warm-water species rise to the
thermocline or the waters above. Abundances of all
groups increase, especially of rotifers and cladocerans.
The biomass is influenced mainly by copepods and
cladocerans, the small rotifers contributing only in the
case of extremely high abundance. The importance of
cladocerans and rotifers, although highly variable
from year to year, is limited to a short period: high
abundance and biomass values occur usually in
June–July, sometimes also in August (Figs 2 and 3).
Depending on species composition, the character of
seasonal variations in zooplankton distribution, abundance, and biomass differs by area. In areas dominated
by copepods (open Baltic, western part of the Gulf of
Finland), seasonal fluctuations are markedly more
moderate than in the eastern part of the Gulf of Finland
Highlights of zooplankton dynamics in Estonian waters
751
5
(a)
4
3
2
1
0
50
(b)
Abundance
40
30
20
10
0
12
(c)
8
4
0
63 64 66 67 68 69 70 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19
Year
3
3
Figure 3. Dynamics of zooplankton abundance (10 ind m ) in the Gulf of Finland, 1963–1992 (averaged data for the three
transects). (a) May, (b) August, (c) October–November. Varia ( ); Rotifers ( ); Cladocerans (); Copepods ().
and in the Gulf of Riga. Seasonal fluctuations also
depend on the dimensions of changes in environmental
parameters (e.g. temperature), which makes them highly
variable from year to year (Fig. 3; Lumberg and
Ojaveer, 1991).
Spatial differences
Zooplankton vertical distribution shows clear effects of
stratification. During the warm season, cold-water
species occur in the near-bottom layer, whereas warmwater plankton is richest in the upper layers, above or in
the thermocline (e.g., Nikolaev, 1961; Kostrichkina
et al., 1980; Fig. 4).
It has long been known that zooplankton production
in particular regions of the Baltic Sea is consistently
higher than in adjacent areas. According to Nikolaev
(1961), the Irbe Sound, an area of constant mixing
of different water masses, is one of these localities. The
dynamics of copepod abundance and biomass depend
on reproduction rate (determined primarily by the
availability of suitable phytoplanktonic food) and
mortality rate (Kirboe and Nielsen, 1994). However,
in the Baltic Sea, with the constant advection of deep
water below the halocline and strong currents in surface
layers, quantification of these processes is extremely
complicated.
The dependence of zooplankton composition, biomass, and distribution on hydrological variables and
nutrient conditions was studied on the eastern slope of
the Gotland Basin, west of the Irbe Sound (Ojaveer and
Kalejs, 1974; Fig. 1). The highest biomass values of
cold-water species (especially Pseudocalanus) were found
in the mixing zone at the halocline (Fig. 4). This term
refers to the highly dynamic, turbulent zone around the
intersection of the thermocline or halocline with the
752
Evald Ojaveer et al.
4.0
8.0
3.0
10.0
30.0
7.0
3.5
40.0
1.0
50.0 mg*m–3
Figure 4. Zooplankton biomass on different parts of the coastal slope. (——) isolines of P-PO4 concentration (8, 10, 30, and
40 mg m 3); (– – –) isolines of oxygen concentration (1 and 7 ml l 1); (· · ·) isoline of 3.5C; ( ) thermocline; ()
halocline; (:::) intense mixing zone; circles=zooplankton biomass (radius is proportional to biomass); () Pseudocalanus;
(/0) Centropages+Temora; (///) Eurytemora+Acartia; () varia; (
) herring; (
) sprat.
bottom profile (Ojaveer and Kalejs, 1974). Intense
upward supply of nutrients through this zone promotes
richer biological production than in areas where vertical
mixing is hindered by a sharp stratification (see isolines
of P-PO4 concentration and zooplankton biomasses).
Although the abundance maximum of warm-water zooplankton was found on the coastal slope, above the
intense mixing zone (Fig. 4), the largest concentration of
(planktivorous) herring and sprat occurred in the area of
the zooplankton-rich, mixing zone. However, the distribution of these fish appeared also to be limited by low
temperatures (<3.5C) in the cold water layer between
the thermocline and halocline. The fact that the warmwater zooplankton biomass just above the largest fish
aggregations was clearly smaller than in neighbouring
areas hints at the possibility that plankton abundance
had been reduced by intense predation during the
active phase of diurnal migration. The Baltic Sea is
characterized by large biomasses of pelagic fish (e.g.
herring, sprat, sticklebacks), and high grazing pressure
on copepods in particular (e.g. Arrhenius and Hansson,
1993; Lankov and Raid, 1997) could result in high
mortality rates of planktonic prey organisms on feeding
grounds of pelagic fish schools.
Long-term dynamics
Characteristic of brackish-water ecosystems, the subsystems vary substantially. Also, species of different origin
(marine, brackish-water, freshwater, and glacial relicts)
react differently to short- and long-term fluctuations in
environmental factors such as salinity, temperature,
feeding conditions, which are often triggered by climatic
changes. High temperatures in summer increase the
abundance of warm-water species, whereas the numbers
of glacial relicts decrease after mild winters. In general,
the abundance of zooplankton correlates well with the
amount of nutrients above the pycnocline (e.g.,
Nikolaev, 1961; Kostrichkina and Yurkovskis, 1982;
Lumberg and Ojaveer, 1991; Sidrevics et al., 1993;
Viitasalo, 1993, 1994; HELCOM, 1997).
The rather high zooplankton abundance in the Gulf
of Finland in 1963–1967 was followed by a moderate to
low level in 1968–1973 and a new period of generally
high values in 1974–1982. During the period of
stagnation starting in 1983, the importance of marine
species in the plankton in this area decreased dramatically and total zooplankton abundance and biomass
were low. This situation lasted at least until 1991 (Figs 3
and 5).
The material collected (usually three seasonal samples
per station per year) does not allow conclusions to be
drawn concerning factors governing the dynamics for
most species, especially those exhibiting considerable
short-term variations in abundance, such as rotifers and
cladocerans. Only the response of copepods to longterm, climate-induced environmental variations is considered here. The main variables connected with climate
Highlights of zooplankton dynamics in Estonian waters
4
753
7.2
3.5
7
3
2
6.6
Salinity
Abundance
6.8
2.5
1.5
6.4
1
6.2
0.5
0
6
74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19
Year
Figure 5. Abundance of copepods in May and Pseudocalanus minutus elongatus in August (103 ind m 3), and salinity of 0–60 m
water layer in the Gulf of Finland, 1974–1992. Pseudocalanus (); Copepoda (); Salinity (——).
– salinity and winter temperature – fluctuate periodically
in the Baltic Sea (Kalejs and Ojaveer, 1989). Based on
plankton samples collected in the Gulf of Finland, the
abundance dynamics of P. m. elongatus were followed
during 1974–1992 (Fig. 5). This time-span covers the end
of a period of low river discharge and fairly high salinity
(1974–1977) and a period of high river inflow and the
associated decrease in salinity (1977–1992). This species
is of marine origin and the conditions in the area are
close to its lower salinity limit. The marked decrease in
abundance over the period correlated significantly with
the salinity of a 0–60 m water layer (r2 =0.73, p<0.01).
More generally, a dependence of the total copepod
abundance on salinity (Fig. 5) was observed in the area
during the same period (r2 =0.32, p<0.05), which
appears to be connected with a westward shift of the
main distribution area of some copepod species during
the period of freshwater inflow (Lumberg and Ojaveer,
1991).
In the Gulf of Riga, zooplankton abundance and
biomass increased gradually from the 1950s to the 1980s
(Line and Sidrevics, 1995). The increase was most pronounced during summer, but was also observed during
spring and autumn. Furthermore, the increase was faster
in coastal regions than offshore. While some warmwater species like Eurytemora (up to the 1980s),
Keratella (until the mid-1970s), Bosmina, Podon, and
Acartia increased in abundance, the cold-water species
L. grimaldii declined after 1965 and reached its lowest
level during the period of mild winters at the end of
the 1980s and the first half of the 1990s (Sidrevics et al.,
1993). The causes of these changes in abundance, species
composition, and distribution have been related to
deviations in abiotic conditions triggered by both
climate changes and eutrophication (Line and Sidrevics,
1995; HELCOM, 1997). Zooplankton dynamics in the
north-east part of the Gulf of Riga differ from those in
the other parts: abundance of rotifers and cladocerans
was highest towards the end of the 1950s, numbers of
copepods were highest in the 1970s and those of meroplankton in the 1960s. During the 1957–1990 period, the
abundance of E. nordmannii and P. polyphemoides
increased (Simm, 1995).
Anthropogenic pollution
In general, zooplankton may not be the best indicator of
pollution impact on the marine environment, because
zooplankton is easily transported by water movements. In coastal areas, however, persistent and severe
pollution may exert a clear influence on plankton
communities (e.g. Simm, 1982). We followed the effect
of pollution on zooplankton composition and biomass
in two small bights on the south coast of the Gulf
of Finland (Fig. 1). Compared with the relatively
unpolluted Kolga Bay, the coastal zone of the neighbouring Muuga Bay suffered heavy industrial pollution
(Fe, Cu, Mn, Mo, Mg, Zn, Ni, Th, Ti, U, As, SO42 ,
and phenols) caused by effluents from a nearby mining
area up to the late 1980s (Pihlak et al., 1988; Lumberg
and Ojaveer, 1997). Copepod biomass in Muuga Bay
(Fig. 6) was remarkably lower in two shallow stations
(up to 10 m depth) during the years of heavy pollution
754
Evald Ojaveer et al.
0
*
–10
Depth
–20
*
–30
*
–40
–50
*
–60
–70
0.10
*
0.15
0.20
0.25
Biomass
0.30
0.35
Figure 6. Average biomass (g m 3) of copepods in Kolga (K)
and Muuga (M) bays by stations of different depth (m) in
August for 1975–1987 and 1975–1992 (data from Lumberg and
Ojaveer, 1997). M, 1975–1992 (——); M, 1975–1987 (—*—);
K, 1975–1992 (– –– –); K, 1975–1987 (– –– –).
(1975–1987) compared with the mean for the entire
period (1975–1992), which incorporates the period
when pollution load had been reduced considerably. No
obvious differences were observed in the mean biomass
values at deeper stations (d20 m) for the two periods. In
the data for Kolga Bay, no clear differences in abundance or vertical distribution are apparent between the
two sets. Based on this comparison, the differences in the
average biomass values of copepods (dominating taxa
E. hirundoides and Acartia spp.) in near-coastal areas in
Muuga Bay could be due to changes in the pollution
load of the basin.
Invasion of a new cladoceran species
The Baltic Sea is a relatively young sea and the system is
still in a phase of transformation. Colonization of the
Baltic by new species has recently intensified, supposedly
due to human impact (e.g. immigration of nonindigenous species with ballast water). An important
zooplankton species which recently invaded the Baltic
area is Cercopagis pengoi (Ostroumov). The original
area of distribution of this cladoceran species is in the
Ponto-Caspian region (Mordukhai–Boltovskoi and
Rivier, 1987) and it was first found in Estonian coastal
waters in July 1992 in the North-east Gulf of Riga and
Pärnu Bay. Since then, the species has appeared in
increasing numbers in zooplankton samples during
summer. The highest concentrations (795 ind m 3,
24 mg m 3) were found at 12 m depth in the north-east
part of the Gulf of Riga in September 1995, when its
biomass made up to about 25% of the total zooplankton
biomass.
Cercopagis constitutes locally an important portion of
the diet of herring, sticklebacks, bleak, and smelt
(Ojaveer and Lumberg, 1995). Herring and sticklebacks
appear to prefer this cladoceran species as food,
possibly because of its relatively large dimensions. C.
pengoi made up nearly 50% of herring food in the
North-east Gulf of Riga and Pärnu Bay in September
1996 and almost 100% on some stations in August 1994.
Thus, this newcomer is already fully incorporated within
the pelagic food web of the basin and appears to
facilitate the energy flow from the lower to the higher
trophic levels. From a purely economic point of view,
this species is a welcome addition to the limited forage
reserve for pelagic species in this area, where periods of
low growth rates have been connected with lack of food
(Lankov and Raid, 1997).
Acknowledgements
We thank Dr G. Behrends and Dr O. S. Astthorsson,
whose comments considerably improved the quality of
the paper. This study was partly financed by EC projects
CIPA CT93-0146 and BASYS/INCO IC20 CT96-0080.
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