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Spatio-temporal distribution of Oithona similis in the Bornholm Basin (Central Baltic Sea) ¨ LLMANN1,2, ULRIKE SCHU ¨ TZ AND HANS-HARALD HINRICHSEN2 FRANK C. HANSEN*, CHRISTIAN MO 1
¨ NDE, SEESTRASSE BALTIC SEA RESEARCH INSTITUTE WARNEMU
15, D-18119 ROSTOCK, GERMANY, DANISH INSTITUTE FOR FISHERIES RESEARCH, 2 ¨ STERNBROOKER WEG 20, CHARLOTTENLUND CASTLE, DK-2920 CHARLOTTENLUND, DENMARK AND LEIBNIZ INSTITUTE OF MARINE SCIENCE, DU D-24105 KIEL, GERMANY *CORRESPONDING AUTHOR:
[email protected]
Received on August 21, 2003; accepted on February 27, 2004; published online on March 30, 2004
Oithona similis is an abundant but poorly studied cyclopoid copepod in the brackish Central Baltic Sea. We describe the spatio-temporal distribution of O. similis in a Central Baltic deep basin (Bornholm Basin) during spring and summer 1999. Using vertically resolving sampling in parallel with hydrographic measurements, we found the copepod to dwell in the permanent halocline characteristic of a Central Baltic deep basin. The habitat of O. similis is thus limited from above by low salinity and from below by low oxygen conditions, both characteristic for the area. Horizontally resolving sampling yielded abundance surfaces which were compared by analysis of variance showing similar patterns among sampling dates. Comparison with flow fields from a threedimensional hydrodynamic model suggests that the horizontal distribution is primarily the result of circulation in the dwelling depth. The study shows how the physical environment in the area determines the spatial distribution which might affect abundance and production of this copepod.
INTRODUCTION Copepods are among the most numerous and widely distributed marine animals on the globe. Feeding on small particles and being major food for micronekton (e.g. fish larvae), they form an important link in the food chain, coupling protists (e.g. microalgae, protozoans) to higher trophic levels. The circumglobally occurring cyclopoid Oithona spp. has been described as a eurythermal, euryhalin, omnivorous species and thus is adapted to a wide range of habitats [(Fransz et al., 1991) and references cited therein]. Despite its importance, Oithona is much less studied than calanoid copepods; for review see Paffenho¨fer (Paffenho¨fer, 1993) and Sabatini and Kiørboe (Sabatini and Kiørboe, 1994). Historically, the abundance of cyclopoid copepods has probably been considerably underestimated as a result of the use of nets with inappropriate mesh-size for quantitative sampling (Turner and Dagg, 1983; Gallienne and Robins, 2001). Hence, it has been suggested that Oithona is probably the most abundant copepod in the world (Gallienne and Robins, 2001). Additionally, recent studies have documented the significance of Oithona spp. with respect to biomass and trophodynamics in temperate seas [e.g. (Kiørboe
and Nielsen, 1994; Nielsen and Sabatini, 1996; Uye and Sano, 1998)]. The small egg-carrying cyclopoid clearly differs from free-spawning calanoids in its behaviour, reproduction and life-cycle strategy, i.e. being less motile, depending on moving food, having lower fecundity and feeding rates, longer egg-hatching times and higher mortality of ovigerous females [(Kiørboe and Sabatini, 1994) and references cited therein]. Spatio-temporal distribution patterns decribed for Oithona spp. are variable and depend on the species and area studied (Tiselius et al., 1994; Fernandez de Puelles et al., 1996). Nevertheless, several studies concordantly point towards a distribution maximum occurring late in the season (Sidrevics, 1984; Fransz and Gonzalez, 1995; Hansen et al., 1999) and below the euphotic zone (Sameoto, 1984; Tiselius et al., 1994). To what degree Oithona performs diel vertical migration remains unclear (Furuhashi, 1976; Turner and Dagg, 1983; Shaw and Robinson, 1998). In the Baltic Sea, O. similis is by far the most abundant cyclopoid copepod. However, knowledge of its seasonal life-cycle and production patterns, as well as its spatial distribution in the Central Baltic Sea, is scarce. Hernroth
doi: 10.1093/plankt/fbh061, available online at www.plankt.oupjournals.org Journal of Plankton Research Vol. 26 No. 6 Ó Oxford University Press 2004; all rights reserved
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and Ackefors (Hernroth and Ackefors, 1979) studied the zooplankton of the Baltic Proper (Arkona Sea, Bornholm Sea, Gotland Sea) and provided a comprehensive overview on a larger spatio-temporal scale, with low horizontal (one station in the Bornholm Sea) and vertical resolution (25–50 m intervals). Most of their samples were collected with a Nansen net of 160 mm mesh size, which probably does not quantitatively retain Oithona (Gallienne and Robins, 2001). Apart from this study, data on O. similis distribution in the Central Baltic are sporadic or from coastal areas [e.g. (Chojnacki et al., 1975; Chojnacki and Drzycimski, 1976)]. Environmental conditions and animal behaviour mainly determine the spatial pattern of zooplankton communities. The vertical distribution and migration of zooplankton interplay with the general circulation to produce horizontal patterns of distribution. These may be invasions of shallower coastal regions and aggregation in fjords and channel heads as demonstrated for Calanus sp. (Falkenhaug et al., 1995; Miller et al., 1998; Heath et al., 1999; Plourde et al., 2002). The vertical positioning of plankton organisms and their production is often controlled by vertical hydrographic structures. Characteristically in estuaries strong vertical salinity gradients (haloclines) play a key role in shaping the vertical distribution of animals, although this has only been demonstrated for a restricted number of taxa (Lance, 1962; Harder, 1968; Bjørnsen and Nielsen, 1991; Vazquez and Young, 1996). Recently conducted laboratory experiments revealed that among others O. davisae from San Francisco Bay accumulate in or below the halocline (Lougee et al., 2002). Such a permanent halocline also restricts the water exchange between the bottom water and the surface layer in the deep basins of the Central Baltic Sea. Below the halocline a lowoxygen, deep-water layer prevents a deeper distribution of organisms. In this paper, we describe the spatio-temporal distribution of O. similis in the Central Baltic deep basin (Bornholm Basin) during spring and summer 1999. By doing so, we show how the physical environment in the area determines the spatial distribution which might affect abundance and production figures of this primarily marine copepod.
METHOD Sampling Sampling of zooplankton from discrete depths was performed with a 1 m2 BIOMOC multiple opening/ closing net, a modified MOCNESS system (Wiebe et al., 1976) in May and August 1999 in the centre of the Bornholm Basin (Figure 1; Table I). For sampling of small zooplankton (e.g. Oithona), 50 mm liners were
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Sweden
56°00´
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4 80 m
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6 17
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11 10
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40 m 60 m
20
23
25
29
32
80 m
27
35
55°00´
40 m
Poland
Bornholm 42
38
40 43
45 60 m
54°30' 20 m
15°00´
16°00´
17°00´
E
Fig. 1. Map of the Central Baltic Sea with sampling stations in the Bornholm Basin.
Table I: Summary of sampling types, periods and gears for the present study Type
Period
Gear
Horizontal
April 14–17, 1999
Babybongo
Horizontal
May 18–21, 1999
Babybongo
Horizontal
June 1–5, 1999
Babybongo
Horizontal
August 9–12, 1999
Babybongo
Vertical
May 25–26, 1999
BIOMOC
Vertical
August 10–11, 1999
BIOMOC
mounted inside the 335 mm nets of the BIOMOC. The gear was towed horizontally at 3 knots for 3 min per 10 m depth stratum, resolving the water column down to a maximum of 10 m above the sea floor. In parallel with the zooplankton sampling, vertical profiles of temperature, salinity and oxygen were recorded using a CTD-probe with a calibrated oxygen sensor mounted on a water-rosette sampler [Meerestechnik Elektronik (ME), Kiel, Germany]. Horizontally resolving sampling of zooplankton was conducted during five cruises between April and August 1999 (Table I) on a station grid in the Bornholm Basin of the Central Baltic Sea (for location of stations see Figure 1). Sampling was performed using a Babybongo net (mouth-opening 0.2 m, 150 mm mesh size) equipped with a calibrated flowmeter. In addition, a 50 mm liner
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Depth-integrating grid sampling yielded horizontal surfaces of abundance for the different developmental stages of O. similis. Significant abundances were not observed in April 1999, thus spatial analyses have only been performed for data derived in May, June and August 1999. As a result of their low occurrence, adult copepods have not been considered in the analysis. After log-transformation of the data, we tested for statistically significant differences among successive surfaces according to Legendre and McArdle (Legendre and McArdle, 1997). We used their technique for the case of systematic sampling at fixed locations without replication, applying the following analysis of variance (ANOVA) model:
2500
Abundance (n m–3)
2000
1500
1000
500
0 April
May
June
August
Month
Fig. 2. Comparison of catch efficiencies for copepodites of Oithona similis among the 150 mm Babybongo (white bars) and the 50 mm liner (black bars) in the different months of 1999; error bars represent the standard error of the mean.
(mouth-opening 0.04 m) was placed inside the Bongo-ring. Double-oblique hauls were conducted from the surface to 10 m above the sea bottom at 3 knots. Comparing the abundances of O. similis copepodites caught in the 150 mm Babybongo with the respective abundances in the 50 mm liner showed that the Babybongo had a better catch efficiency for the copepodite stages (Figure 2). Consequently for further calculations the abundances from the 150 mm net of the Babybongo were used, except for nauplii which were taken from the 50 mm liners.
Laboratory analysis Samples were immediately fixed in borax-buffered formaldehyde–seawater solution (4% concentration) and analysed in the laboratory. Several subsamples were taken with a 1 mL wide-bore pipette for subsequent microscope identification (magnification: 50) until at least 500 individuals (ind.) in total had been counted. Oithona similis from the Babybongo nets and the inserted 50 mm liners were identified to developmental stage (grouped into nauplii N, copepodites C1 to C5, adult males C6-m, and females C6-f ). Cephalothorax length was measured on a subsample of about 20 animals per group and species. Abundances (m2) of O. similis were calculated based on volume filtered through the net and water depth at stations.
Numerical analyses As an index of the vertical orientation of the copepods, weighted mean depths (WMD); (Bollens and Frost, 1989) per season and year were computed: X X WMD = ni di = ni ð1Þ where ni is the abundance (ind. m3) in each depth stratum with the midpoint di.
Yijk = m þ si þ ljðiÞ þ tk þ stik þ eijk
ð2Þ
where Yijk is the value of the jth location (l ) from the ith stratum (s) on the kth occasion (t ), m is the overall mean; lj(i ) measures the location-within-stratum effect. Strata (stra) were derived by pairing up the following neighbouring stations (sta): stra 1–sta 2, 4; stra 2–sta 8, 11; stra 3–sta 6, 10; stra 4–sta 16, 17; stra 5–sta 13, 15; stra 6–sta 20, 32; stra 7–sta 23, 29; stra 8–sta 25, 27; stra 9–sta 35, 38; stra 10–sta 42, 43; stra 11–sta 40; 45 (Figure 1). The null hypothesis tested is the composite one, (tk + stik) = 0, detecting any change that occurred either in individual strata or averaged over the whole surface (Legendre and McArdle, 1997). For visualisation of the spatial structure, data were contoured with the SURFER software package using kriging as the interpolation routine.
Production Biomass was estimated from abundances and individual weights, derived from size measurements applying the length–weight relationship WW = 8 ð0:021 CLÞ3
ð3Þ
where WW is the wet weight (mg) and CL is the cephalothorax length (mm) after Krylov (Krylov, 1968). Wet weight was transferred to carbon content (C) using C = 0:29 WW
ð4Þ
according to Mauchline (Mauchline, 1998). In cases where insufficient numbers of animals had been measured, standard weights given by Hernroth (Hernroth, 1985) were applied. Production (P ) was calculated as P = Bg
ð5Þ
where B is the biomass and g is the weight-specific growth rate. Following Sabatini and Kiørboe (Sabatini and Kiørboe, 1994), who measured constant growth
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rates in O. similis of 0.20 day1 for N2–C4 and 0.074 day1 for C5–C6, and applying a Q 10 = 3.0, we assumed weight-specific growth rates of 0.07 day1 for all nauplii and copepodites C1–C4, and 0.03 day1 for copepodites C5 and adults at 6 C (the average temperature encountered in the halocline). Net production (PN) was calculated from change in biomass (B1, B2) between two sampling dates t1, t2 as PN ¼ ðB2 B1 Þðt2 t1 Þ1
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-40000
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CI-V
Similarly, net growth rates ( gN) were calculated from changes in biomass over a period of time (t), assuming exponential growth
Depth (m)
g N ¼ ½lnðBt Þ lnðB0 Þ t 1
20
ð7Þ
40 60 80 100
RESULTS
-80000
Results from vertically resolving sampling indicated a very stable distribution of O. similis throughout the water column. Maximum abundances of all stages were observed between 60 and 70 m, independent of daytime and season (Figure 3). Comparing WMDs of the different stages with the vertical hydrographic structure showed O. similis to concentrate centrally in the permanent halocline (Figure 4). Thus, the copepod clearly headed for high salinities, thereby avoiding oxygen concentrations 1 106 ind. m2 during the FLEX-studies at the Fladen Ground (Krause and Thrams, 1983; Krause and Radach, 1989). These values appeared to be higher compared to the mean abundance in the Bornholm Sea (this study) of 75 000 copepodites m2. The reason behind this may be that O. similis is an oceanic, halophilic copepod and its
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56.00
0 30
00 14 00 10 0 20
500
700
500 300
10
600
500
55.00
0
00 0 0 10 60 20
100
700
55.50
70 0
N
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0
54.50 56.00
300
450
0
300 150 50
100
350
30
55.00
500
100
0 25
12
40
0
2000
0 55 50 4 0 25
55.50
00 40 0
54.50 56.00
200
2200
00
00 0 4
100
10
500
500
00
14
0 00 503000100
00
00
0 150 00 10
55.00
18
22
55.50
54.50 15.00 15.50 16.00 1 6.50 17.00
1 5.00 15.50 16.00 16.50 17.00
15.00 15.50 16.00 16.5 0 17 .00
E
Fig. 6. Horizontal surfaces of Oithona similis abundance (n m–3) of nauplii (upper panels), copepodites C1–C3 (middle panels) and copepodites C4–C5 (lower panels) in May (left column), June (middle column) and August (right column) 1999; note different scales used per panel.
4
Log abundance
3
2
1
0 1
2
3
4
5
6 Stratum
7
8
9
10
11
Fig. 7. Mean spatial variability of Oithona similis during the sampling in May, June and August 1999 indicated by log-abundance in different strata for nauplii (black dots), copepodites C1–C3 (white dots) and copepodites C4–C5 (black triangles); error bars indicate standard error of the mean.
occurrence in the Bornholm Basin is close to its distribution limit (Ackefors, 1981). As we demonstrated here, O. similis is vertically restricted to a layer of higher salinity and sufficient oxygen. Salinity and oxygen in this layer have decreased since the 1980s, and thus the size of suitable habitat for this copepod, as a result of a reduction in pulses of water intrusions from the North Sea (Ha¨nninen et al., 2000). The low salinity/oxygen conditions encountered might have impaired the population of O. similis. Furthermore, food availability in the halocline region is largely unknown, but has the potential to influence living conditions of the copepod. In most of the studies, mesh sizes 102 mm have been used for sampling. Although differences in the mesh sizes deployed (30–102 mm) are likely to influence the catchability of small copepodites and nauplii significantly (Galienne and Robins, 2001), seasonal and area-specific differences seem to have a larger influence on the abundance patterns. We compared the catchability of two different nets with 50 mm and 150 mm mesh sizes.
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Table III: Seasonal development in production of Oithona similis: biomass and gross production (mean SE) as well as net production and net growth rates Date
Biomass
Gross production
Net production
P/B
Net growth rate
(mg C m2)
(mg C m2 day1)
(mg C m2 day1)
(% C day1)
(% C day1)
April 15
16 6
2.2 0.7
4.0
13.6
6.6
May 19
154 43
27.0 7.6
1.0
17.6
0.7
June 4
169 49
26.8 7.6
2.1
15.9
1.1
August 7
305 115
53.5 20.4
N 56.00
17.6
Surprisingly, the 150 mm Babybongo better retained all developmental stages of O. similis, except for nauplii. Since the width of younger copepodites is less than 150 mm net properties other than mesh size must have played a significant role in determining catch efficiencies. The small mesh size of the liner in conjunction with the towing speed (3 knots) may have created a dynamic pressure in front of the net opening. In addition, the bridles connecting the liner with the frame of the Babybongo may have decreased the liner’s catchability. Therefore, we caution against comparing abundances on basis of mesh sizes alone if different nets have been applied. Comparing the abundance of O. similis with the standing stocks of dominant calanoid copepods sampled in parallel in the Bornholm Basin (Hansen et al., submitted for publication) revealed a low relative importance (Table IV) of O. similis. On average, abundances of O. similis were only half of the standing stock of Pseudocalanus sp. and Acartia spp. Abundances of the presently dominating Temora longicornis were on average four times the value found for O. similis. These comparisons further indicate that O. similis as a marine copepod is surviving in non-optimal living conditions, and is, in contrast to other areas, not the most abundant copepod species in the Bornholm Basin.
a)
55.50
55.00
54.50 56.00
b)
55.50
55.00
54.50 56.00
c)
Table IV: Comparison of the abundance (n m3) of Oithona similis with the calanoids Pseudocalanus sp., Temora longicornis and Acartia spp.
55.50
55.00
Date
54.50 15.00 15.50 16.00 16.50 17.00 E Fig. 8. Average flow fields in 60–70 m depth in the Bornholm Basin during the sampling periods; (a) May, (b) June and (c) August.
Oithona
Pseudocalanus
Temora
similis
sp.
longicornis
Acartia spp.
April 15
443
1718
2552
1419
May 19
1599
2972
6677
3041
June 4
1393
1504
4964
3343
August 7
2059
3339
7137
3868
Calanoid abundance data from Hansen et al. (submitted for publication).
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We provided a first rough estimate of potential production for O. similis using observed biomass and growth rates from the literature, a method which assesses gross production and ignores population losses to predation, natural mortality or advection. To calculate potential production at 6 C, we applied daily growth rates of 0.03 for N1–C4 and 0.07 for C5 and adults, based on data given by Sabatini and Kiørboe (Sabatini and Kiørboe, 1994). Growth rates are highly temperature-dependent. In Oithona spp. growth rates range from 0.015 day1 in polar regions (Fransz and Gonzalez, 1995) up to 0.46 day1 in the tropics (Hopcroft and Roff, 1998). In O. davisae from the temperate Inland Sea of Japan, Uye and Sano (Uye and Sano, 1998) determined copepodite growth rates to range between 0.060 day1 at 8.9 C and 0.446 day1 at 28.2 C. Extrapolation of their exponential relationship to 6.0 C would lead to a growth rate of 0.049 day1, which lies between the growth rates we applied for younger and for older copepodites of O. similis. We estimated O. similis net production (all stages) in the Bornholm Sea between April 1 and August 31 to be 4.6 g C m2. Production estimates for the North Sea and Kattegat are in the same order of magnitude, see review by Nielsen and Sabatini (Nielsen and Sabatini, 1996). Depending on the area of study and method used, annual Oithona spp. production ranges from 1.0 g C m2 (McLaren et al., 1989) to 3.7 g C m2 excluding nauplii (Roff et al., 1988). The latter is a 15 year average of annual production, which varies between 0.8 and 5.0 g C m2. Taking into account the lower ambient temperature (6 C) in the Baltic halocline region, our production estimate for the Bornholm Basin is comparatively high. Furthermore, when considering the low abundance of O. similis observed in our study area relative to purely marine areas, this result indicates that extrapolation of growth rates to areas with differing environmental conditions is of limited value. Thus it points towards an urgent need for in situ production studies of O. similis in the Central Baltic deep basins.
Conclusions This study showed the vertical distribution of O. similis to be determined by the salinity stratification in the Bornholm Basin forcing the marine cyclopoid copepod to accumulate in the deep halocline layer, limited from below by low oxygen conditions. Together with the circulation, horizontal distribution patterns were created which were found to be similar during the sampling period. Abundance figures observed in our study area were found to be on average lower than in regions with higher salinities, not explicable from the first rough production estimates provided here.
Our study shows the need for further investigations on the ecology of this copepod species in the area. This is especially important because this animal, as a result of its marine origin, is similar to the calanoid Pseudocalanus sp., in danger of suffering from the presently changing climate which is leading to a freshening of the Baltic Sea (Mo¨llmann et al., 2003a,b).
ACKNOWLEDGEMENTS This study was part of the STORE-project, financed by the EU, contract no. Fair CT 98 3959. We thank the crew of RV ‘Alkor’ and all cruise participants involved in collecting the samples. Heide Sandberg and Antje Burmeister are thanked for their technical assistance and analysis of samples.
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