Download Glaciers caused zooplankton mortality?

Journal of Plankton Research Vol.20 no.7 pp.1233-1240, 1998
Glaciers caused zooplankton mortality?
J.M.Weslawski and J.Legezyriska
Institute of Oceanology Polish Academy of Sciences, Sopot 81-912 Powstancdw
Warszawy 55, Poland
Abstract Dead zooplankton were found in grab samples collected in Kongsfjorden (79°N, Svalbard
archipelago) in summer of 199S and 1996. The concentration of dead plankters reached over 500 individuals of copepods and 130 individuals of Themisto per square metre. The biomass of dead zooplankton reached 0.17 g C m~2 locally, and was not correlated with the depth of the locality, nor with the
surface salinity. The bulk of sedimented plankton was, however, found behind the sill of the inner
fjord basin. We assume that massive discharge of glacial waters in summer causes mass mortality of
surface zooplankton, which drift away and sink in the inner fjord pool, thus supplying benthic
communities with organic carbon.
Introduction
The sinking zooplankton are regarded by sedimentologists as 'swimmers' into
sediment traps, and most often treated as an artefact or disturbance in carbon flux
measurements (Michaels et aL, 1990; Wakeham et aL, 1993). There are some
published accounts on sedimenting zooplankton, but most of these papers deal
with open ocean and great depths (Seiler and Brandt, 1997). There are no quantitative data on the plankton sink from Arctic fjords; however, fjords have often
been supposed to be the sites of mass plankton mortality in freshwater frontal
zones (Hartley and Fisher, 1936; Dunbar, 1951; Bousfield et aL, 1975; Kaartvedt
and Aksnes, 1992). In the Spitsbergen fjords, stranded belts of dead zooplankton
(mostly copepods and chaetognaths) have commonly been observed during low
tide in the inner fjord basins in the peak of summer (authors' personal observations). Dead plankters were also seen on the surface of the water close to the
glacier cliffs. These phenomena have not, however, been measured quantitatively.
Rakusa-Suszczewski (1992) observed 'krill, salpae and seaweeds' stranded on the
shore of a glaciated Antarctic fjord (Admiralty Bay). German observations have
revealed numerous plankters on the top of sediment in grab samples in the Central
Arctic (LKronecke, personal communication). We believe that intensive freshwater mixing following the ice melt in the Arctic acts as an important factor in
plankton mortality. The aim of this study is to present a quantitative estimation of
zooplankton sedimentation to the seabed in a glacier-fed Arctic fjord.
Method
Samples were collected from R/V 'Oceania' during a summer cruise to Kongsfjorden, Svalbard, in July 1995 and 1996. Triplicate Van Venn grab (0.35 X 0.4 m
opening) samples were collected from 20 to 320 m depth, throughout the fjord
area. Samples were washed on a 0.5 mm screen. The water for washing samples
was pumped through double sets of 0.5 mm metal sieves, to exclude contamination with larger organisms from the sea. Washed samples were preserved in 4%
© Oxford University Press
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Fig. 1. Sampling stations and dead plankton on the sea bed.
Segmented
looplankton
mgC/m2
26-60
0-10
Glaciers caused zooplankton mortality?
buffered formaldehyde and analysed 4 months later in the laboratory. Zooplankton were separated from benthos, identified, counted and weighed wet with 1 mg
accuracy. Plankton were sampled at seven stations (Figure 1) with a vertically
hauled (bottom to surface) WP-2 net with 200 \un mesh. The sample from station
5 was analysed according to standard mesozooplankton procedure. Samples from
other stations were sieved through 0.5 mm mesh to sort out minor plankton, only
Calanus and Themisto specimens remaining on the screen were counted.
Zooplankton data are presented as the number of individuals or biomass in the
water column from the surface to the bottom under 1 m2. Dry weight of plankton was taken as 0.2 (20%) of wet weight and organic carbon as 0.06 (6%) of the
wet weight. Hydrological data are from Beszczyriska et al. (1997). As surface
salinity gradient, we assume the difference between surface and near-bottom
salinity. Surface sediment plume indicating the extension of glacial meltwater was
arbitrarily classified into three categories (1, <10 mg mineral matter dm"3; 2,
between 11 and 25 mg dm"3; 3, >25 mg mineral matter dm"3), data on suspensions
were taken from M.Zajaczkowski and L.Kotwicki (personal communication).
Study area
Kongsfjorden is situated at 79°N and opens to the west to the North
Atlantic-Greenland Sea (Figure 1). The fjord is divided into two parts: an inner
basin of 50-80 m depth, separated by a 40 m deep sill from the outer basin of
300-400 m depth. Warm (+5°C) Atlantic and cooler (+2°C) coastal Spitsbergen
waters enter the fjord in surface and intermediate layers, while cold (-1.2°C)
near-bottom waters are of local origin. The hydrology of Kongsfjorden is
presented in detail by Weslawski et al. (1991) and Beszczyriska et al. (1997).
Large, extensively melting glaciers (Kongsbreen, Kongsvegen) are situated at the
end of the fjord and discharge ~1 km3 of fresh water annually with a seasonal
peak in July (Lefauconnier et al., 1994; Weslawski et al, 1995; Beszczyriska et al,
1997).
Results
The amount of dead plankton in benthic grab samples is presented in Figure 1.
The biomass ranged from 0.01 to 2.8 g wet weight m~2 (Table I). The highest
values were found in the southern fjord basin close to the Kongsbreen, while dead
plankton were less abundant in samples collected at the northern branch of the
fjord, close to the Kongsvegen glacier. Dead plankton were scarce in the outer
fjord part (Figure 1). The amount of dead plankton on the bottom was not correlated with depth, nor with the amount of plankton in the water column (Figure
2) or local surface salinity gradient (Table I).
Five species of Copepoda and two pelagic Amphipoda species were present in
the benthic samples (Table II). All copepods were in the older stages of development (from copepodite 3 onward); amphipods were represented by subadult
specimens of 5-8 mm length (Table II). Species on the fjord bottom, as well as
their relative abundance, were similar to those in the water column above (Table
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Glaciers caused zooplankton mortality?
Fig. 2. Amount of Calanus in water column below m2 along the sampling profile (plankton net) with
an indication of the amount of dead specimens found on 1 m2 of the sea bed (grab samples). Station
numbers refer to Figure 1.
II). Exceptions were Pseudocalanus spp., which dominated in the plankton
samples; but were absent in the benthic material, and Themisto libellula which
was more frequent (6%) in grab samples than in the water column (0.4%).
Discussion
The first question is to what extent our samples represent actually dead zooplankton ('sinkers') or are biased by the method. When lowered down to the bottom,
a Van Venn grab may collect some zooplankton on its way. In such a case, the
amount of plankton in the sample would be proportional to the amount of plankton in the water column, which was not the case (Figure 2). The contamination
of sample with water during washing was excluded. Final verification may come
from the sediment traps only. Sediment traps of 0.01 m2 area were deployed at
40 m depth for 6-12 h (M.Zajaczkowski and L.Kotwicki, personal communication) and no zooplankton were found in traps.
The thickness of the surface water layer with sharp salinity gradient (below 25
p.p.t.) measured <3 m close to the glacier in Kongsfjorden (Beszczynska et al,
1997). According to Weslawski et al.. (1991) and our present observations, the
average number of plankters larger than 2 mm inhabiting 1 m3 at the surface
ranges from 100 to 200 individuals (making around 450 specimens in the 3-mthick surface brackish layer). Considering the sinking rate of Arctic copepods as
120 m in 24 h (Poltermann, 1997), 50% of mortality in the turbid layer and 450
copepods as standing stock, one trap may collect 1 copepod during at least 12 h
exposure. Hence, it is unlikely to collect zooplankton during 6 h exposure.
The cause of plankton death in Kongsfjorden is uncertain. Lowered salinity in
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J-M.Westawski and J.Legezynska
Table IL Share of zooplankton species in the water column (alive) and on the bottom (dead) in
Kongsfjorden, summer 1996. Only specimens over 2 mm long are considered
Taxon
Calanus finmarchicus
Calanus glaciate
Calanus hyperboreus
Paraeuchaeta norvegica
Neoscolecitrix farrhanx
Rhincalanus nasutus
Metridia longa
Pseudocalanus spp. CVI
Themisto libellula
Thysanoessa inermis
Bradyidius sp.
Total
Station 5,:zooplankton
in water column
Vicinity of station 5,
dead zooplankton
nnr3
%
nnr 2
%
ISO
80
16
1
1
1
4
200
2
1
0
456
32.9
17.5
3.5
0.2
0.2
0.2
0.9
43.9
0.4
0.2
0
100
99
54
7
0
2
0
0
0
10
0
2
174
57
31
4
0
1
0
0
0
6
0
1
100
frontal zones is commonly reported as a main cause of marine organism mortality, and a range between 5 and 8 PSU is regarded as the critical salinity for both
marine and freshwater species (Khlebovitsch, 1990). On the other hand, numerous neritic species withstand a salinity decrease from 30 to 1 PSU especially well
in low temperatures (Thomas and White, 1969).
Marine organisms exposed to low salinity receive an osmotic influx of water to
compensate for the density difference (Aarset and Aunaas, 1987). This may lead
to an increase in weight, disintegration of body organs, and eventual sinking of
the plankter. A number of Arctic invertebrates exposed to salinity gradients in
pack ice are known to be osmoregulators; however, Themisto libellula exposed
to 6 PSU died instantly (Aarset and Aunaas, 1987). Another reason for death
might be the ingestion of fine mineral particles, known among plankters exposed
to turbid waters close to Arctic glaciers (Lewis and Syvitsky, 1983), which disturbs
nutrition and breeding. Marine plankton entering brackish and turbid waters in
a North Sea estuary show high mortality (Soetaert and Herman, 1994). The
mechanism which may force marine plankton to mix with low-salinity waters in
a glaciated fjord is the local upwelling or turbulence caused by catabatic winds
forcing estuarine circulation. Such a phenomenon was commonly reported by
ornithologists observing seabirds feeding (Hartley and Fisher, 1936; Dunbar,
1951; Mehlum and Gabrielsen, 1993).
The percentage frequency of zooplankton species found in grabs was generally
similar to those from the water column (Table II). Exceptional is the high share
of Themisto spp. in grabs, compared to the amount in the water column, and the
absence in benthic samples of Pseudocalanus spp. which is numerous in plankton. The first species was described to be a stenohaline osmoconformer (Aarset
and Aunaas, 1987), with poor survival abilities in low salinity; the second species
is known to inhabit brackish waters with salinities down to 6 PSU (Koszteyn and
Kwasniewski, 1989); however, it is also possible that Pseudocalanus was sieved
away on the 0.5 mm screen.
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Glaciers caused zooplankton mortality?
When compared to organic sedimentation in Kongsfjorden, ranging from 50 to
300 mg C nr 2 day 1 (Zaj^czkowski and Kotwicki, in preparation), the dead plankton biomass seems to contribute 6-8% of total organic sedimentation. Compared
to the mean biomass of benthic fauna (600-9000 mg C nr 2 , our own data;
Weslawski et al, 1995), the dead plankton biomass contributes to some 10% of
the benthic biomass in the inner fjord basin.
The biomass of zooplankton in Kongsfjorden pelagial ranges from 0.3 to 6 g C
m~2 (Weslawski et al, 1991; our own data from 1996) so the biomass of dead
plankters constitutes 1-10% of the biomass in the water column above. The
overall biomass of dead plankton in Kongsfjorden might be roughly estimated to
40 tonnes of fresh weight (based on 170 km2 of the fjord area, and distribution of
the biomass as shown in Figure 1). Necrophagic (carrion feeder) animals are most
likely to be those to benefit first from the sinking zooplankton. Necrophages are
known to be abundant in polar waters; however, the disadvantage of this mode
of feeding is the random occurrence of carrion on the sea bed (Presler, 1990).
However, dead euphausids were found to be the main food item for abyssal
Antarctic ophiurids (Sokolova, 1994). Necrophages in Spitsbergen fjords are
represented by lysianassid amphipods inhabiting glacier bays in abundance
(Weslawski, 1992). Strong turbidity, surface salinity gradient and resulting plankton mortality observed in glacier bays may create the predictable feeding ground
for necrophages in the summer.
Acknowledgements
This project was partly funded by a grant from the Committee on Scientific
Research no. PO4-95-09. We would like to thank Ms Kasia Dmoch for plankton
analysis, and the crew and research team of R/V 'Oceania' for their assistance
during two field seasons.
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Received on May 6, 1997; accepted on January 14,1998
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