Copepod Limnocalanus Macrurus Research Articles

Fatty acid profiles reveal dietary variability of a large calanoid copepod Limnocalanus macrurus in the northern Baltic Sea

Eutrophication, climate-induced warming, and salinity fluctuations are altering the fatty acid profiles and the availability of essential polyunsaturated fatty acids (PUFAs) in marine zooplankton communities. Limnocalanus macrurus Sars G.O., 1863 is a large calanoid copepod inhabiting the low-salinity areas in the Baltic Sea, where it is a major source of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) to commercially important fish. L. macrurus is sensitive to warming, eutrophication and hypoxia. As an opportunistic feeder, it is capable of dietary shifts, which affects its fatty acid profiles. Although much studied in boreal lakes, there are only a few studies on the fatty acid profiles of the Baltic Sea populations. This study aimed to compare the fatty acid profiles of L. macrurus in three basins of the Baltic Sea, in relation to the community fatty acids and environmental variables. We collected samples of L. macrurus and filtered plankton community for gas chromatographic fatty acid analyses in August 2021 on R/V Aranda. The nutritional quality of L. macrurus to consumers was lower in the Gulf of Finland (GoF) compared to the Gulf of Bothnia, indicated by the low levels of DHA and EPA, as well as the low n-3/n-6 ratio of PUFAs. The lower ratio of 18:1n-7 to 18:1n-9 implied higher degree of omnivory in GoF. In contrast, a diatom marker 16:1n-7 had high proportion in the Bothnian Bay. High temperatures in GoF may have restricted feeding in the upper water column, possibly forcing a shift towards cyanobacteria or seston-based diet, as interpreted from a high proportion of 18:2n-6 and 18:3n-3. We conclude that the ability of L. macrurus to utilize multiple food sources increases its resilience to environmental change, while the consequences on the nutritional quality may have further cascading effects on the food webs.

Oxygen consumption rates and respiratory carbon losses in three species of copepods(Acartia clausi, Calanus helgolandicus and Limnocalanus macrurus) during starvation

Calanoid copepods with different life cycle strategies demonstrate varied velocity in the utilization of energy resources during starvation. This study analyzes the influence of multiday starvation on the rate of total, basal and active energy metabolism in adult females of two Marmara Sea and one Baltic Sea species of copepods; epiplanktonic Acartia clausi unable to accumulate lipid reserves and mesoplanktonic Calanus helgolandicus with minor lipid reserves in the oil sac (approximately 7% of body volume) and the Baltic Sea copepod Limnocalanus macrurus in quasi-diapausing state with large lipid reserves in the oil sac (approximately 19% of body volume). Initially, total weight specific respiration rates were about 0.12 μg C per μg C body weight per day (d$^{-1}$) for both Marmara Sea species at 20 °C and 0.079 d$^{-1}$ in the Baltic L. macrurus at 14 °C, decreasing significantly by 1.6-2.5 times with starvation, while basal metabolic rates, measured in the same anesthetized individuals, was nearly constant during starvation. In this regard, the active metabolic rates, defined as the difference between the respiration rate of active and anesthetized individuals, decreased during starvation by 1.7 times in C. helgolandicus and by 4.7 and 6 times in A. clausi and L. macrurus, respectively. In A. clausi and C. helgolandicus with minimal lipid reserves, total daily energy expenditure (5.1% of initial body carbon content) was twice as high as in L. macrurus (2.2% of initial body carbon content daily), which has a large oil sac. Overall, the trend demonstrates that despite different energy reserves, a decrease in the total energy metabolism in copepods occurs due to a reduction in energy expenditure for locomotor activity.

Exposure to leachates from post-consumer plastic and recycled rubber causes stress responses and mortality in a copepod Limnocalanus macrurus

Effects of household post-consumer plastics and tyre rubber on a Baltic Sea copepod Limnocalanus macrurus were assessed. Fragments of commercial recycled low-density polyethylene vegetable bags and rubber originating from recycled car tyres were incubated in seawater, and the copepods were exposed to the filtrate of the water. L. macrurus experienced erratic swimming behaviour and increased mortality in the filtrate of unwashed vegetable bags, containing elevated concentrations of alcohols, organic acids and copper. Responses of the antioxidant defence system (ADS) were recorded in copepods exposed to rubber treatments containing high concentrations of zinc. Significant responses in the ADS enzymes indicate that reactive oxygen species (ROS) formation was exceeding the detoxification capacity of the ADS which may further lead to prolonged state of oxidative stress. Observed effects of exposure on the biochemical level coincide with impaired swimming activity of the copepods, indicating possible irreversible cellular responses leading to behavioural changes and mortality.

Immunochemical methods for identification of prey in freshwater zooplankton

AbstractLarge, predatory, non‐indigenous species such as the cladoceran, Bythotrephes longimanus, can have a profound effect on aquatic food webs, but determining the prey consumed by such species is problematic because they shred prey externally and consume only soft body tissues. We assessed the usefulness of an immunochemical approach, in which putative zooplankton prey were homogenized and polyclonal antibodies against total proteins were raised in rabbits and purified against predator proteins. Using either dot‐blot or enzyme‐linked immunosorbent assay‐plate formats, assays easily detected prey proteins in single B. longimanus. Issues encountered included some degree of cross‐reaction with predator proteins and among prey species for some antisera and questions concerning determination of a correct blank value for assays, however, there are feasible approaches to resolving these issues. Immunochemical assays were readily applied in to a total of 185 individual B. longimanus in the Lake Michigan pelagic ecosystem in summer of 2008 and 2009 and indicated ingestion of cladoceran and copepod species, typically described in the literature, but also dreissenid veliger larvae and the large copepod Limnocalanus macrurus, not usually considered prey for Bythotrephes. Assays results showed some degree of correlation with numerical abundances of zooplankton prey in the water column, suggesting generalist feeding. Protein immunochemical assays offer a quick, sensitive and practical approach to examining planktonic food‐web questions that are compatible with, but have certain advantages over, alternatives such as nucleic acid techniques.

Genetic Diversity of Copepod Limnocalanus macrurus from Russian Arctic Seas

The diversity, phylogenetic relationships, and demographic history of glacial relict copepod Limnocalanus macrurus from estuaries of large Siberian Arctic rivers (the Ob, Khatanga, Lena, Indigirka, and Kolyma) are studied using mitochondrial cytochrome oxidase subunit I gene (COI mtDNA). It is shown that Siberian populations of Limnocalanus macrurus, together with those from the Canadian Arctic and Baltic Sea, belong to a single Palearctic phylogenetic lineage, which probably survived in one refugium during the Last Glacial Maximum and then rapidly expanded within the Arctic about 21 000–7500 years ago. The presence of common haplotypes in L. macrurus from three Arctic seas and the lack of differences in haplotype frequencies may be associated with recent origin of populations or current gene flow between them.

Multidecadal dynamics of the Arctic copepod Limnocalanus macrurus in relation to environmental variability in the Baltic Sea

Abstract The Arctic Limnocalanus macrurus is a prominent representative of large copepods which performs several essential functions in freshwater and marine ecosystems. Being a cold stenotherm species, its distribution is primarily confined to deeper water layers. Based on the long-term observations from one of the largest spatially confined natural populations of this species in the Baltic Sea, we detected profound long-term variability of L. macrurus during 1958–2016: high abundances before the 1980s, then nearly disappearance in the 1990s and recovery in the 2000s. The main environmental parameters explaining the interannual variability of L. macrurus in spring were herring spawning stock biomass in preceding year, winter severity, and bottom water temperature in preceding summer. The effect of winter severity and water temperature was also non-linear. The sliding window correlation analysis pointed to a non-stationary relationship between the abundance of L. macrurus and the key variables. Given the observed pronounced seasonality in the population structure of L. macrurus (young stages dominated in the beginning of the year and only adults were left in the population in summer and autumn) we identified the dynamics of key environmental variables to understand this species under different ecosystem configurations and different combinations of drivers of change.

Change of Limnocalanus macrurus (Copepoda, Calanoida) Population State under High Summer Temperature

The many-year data on state of population of the relict copepod Limnocalanus macrurus Sars, 1863 in the Sita Lake (Belarus) were analyzed. Numbers of the copepod sharply decreased after water temperature in the upper water layer exceeded 26°C. Analysis of the vertical distribution of Limnocalanus showed that it avoids water layers with the oxygen deficit and elevated temperature. Growth of the surface temperature and decrease of the oxygen content in the hypolimnion resulted in reducing of the spatial niche and drastic decrease of the Limnocalanus numbers, and thus increased risk of its extinction. Similar mechanism of the numbers decrease and further extinction of the relict planktonic crustaceans probably acts in other lakes with similar morphometric and trophic characteristics.

Fatty acid composition and lipid content in the copepod Limnocalanus macrurus during summer in the southern Bothnian Sea

The lipid reserves and occurrence of the cold-stenothermic, omnivorous copepod Limnocalanus macrurus were studied in the Bothnian Sea (northern Baltic Sea) during spring and summer 2013–2014 with a special emphasis on the fatty acid composition of adults and their potential food. The individual total wax ester (WE) content, determined from the size of oil sacs in the prosoma, ranged on average from 1.3 to 2.6 µg, and showed a decreasing trend towards September. Lipids were dominated by fatty acids 16:0, 18:1(n-9), 18:2(n-6), 20:5(n-3) and 22:6(n-6), forming 56–61% of total fatty acids in June–September. Decreasing abundance of adults and reduction of the lipid storage implied that during summer adults suffered from starvation and, as a result, became eliminated from the population. The lipid content and dietary fatty acid markers suggested that in May, adult L. macrurus utilized the phytoplankton bloom, consisting mainly of diatoms and dinoflagellates, but later, during July–September, consumed either algae or heterotrophic organisms sinking from upper water layers or crustaceans inhabiting the same deeper water layers as L. macrurus. In the face of the climate change, the rising temperatures may force L. macrurus permanently to deeper water levels. If also the food resources are limited, we conclude that the summer season may act as a bottleneck limiting the propagation of L. macrurus and having implications further along the food web.

Oxidative stress biomarkers in the copepod Limnocalanus macrurus from the northern Baltic Sea: effects of hydrographic factors and chemical contamination

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 538:131-144 (2015) - DOI: https://doi.org/10.3354/meps11471 Oxidative stress biomarkers in the copepod Limnocalanus macrurus from the northern Baltic Sea: effects of hydrographic factors and chemical contamination K. A. Vuori1,*, K. K. Lehtonen2, M. Kanerva1, H. Peltonen2, M. Nikinmaa1, N. A. Berezina3, E. Boikova4 1Laboratory of Animal Physiology, Department of Biology, University of Turku, Turku 20014, Finland 2Marine Research Centre, Finnish Environment Institute, Hakuninmaantie 6, Helsinki 00430, Finland 3Zoological Institute of the Russian Academy of Sciences, St. Petersburg 199034, Russia 4Institute of Biology, University of Latvia, Salaspils 2169, Latvia *Corresponding author: kranvu@utu.fi ABSTRACT: Zooplankton channels energy and various inorganic and organic substances from primary production to consumers at higher trophic levels; thus, its nutritional value as well as content of harmful substances may have profound effects on marine food webs. Indicators of environmental stressors such as oxidative stress biomarkers are highly useful in estimating how environmental factors affect lower levels of the food web. These biomarkers were determined in field-collected brackish water calanoid copepods Limnocalanus macrurus to analyze possible spatial variations in specimens collected from open-sea areas of the northern Baltic Sea. L. macrurus from the Bothnian Sea showed elevated levels of glutathione metabolism-associated enzyme activities and total glutathione (totGSH), whereas samples from the Gulf of Finland showed higher levels of superoxide dismutase activity (SOD) and lipid peroxidation (LPX), and a lower ratio of reduced to oxidized glutathione (GSH/GSSG). Hydrographic factors and selected indirect proxies describing general contaminant loads in the study areas partly explained the observed differences in biomarker values. The higher levels of SOD, LPX and lower GSH/GSSG together with high concentrations of polychlorinated dibenzodioxins/furans (PCDD/Fs) and polychlorinated biphenyls (PCBs) measured in the L. macrurus samples suggest the presence of multiple environmental stressors in the Gulf of Finland compared to the Bothnian Sea. The novel miniature biomarker measurement methods used and results obtained can be further applied in studies on the effects of environmental stressors in zooplankton species and communities in conjunction with environmental quality assessments. KEY WORDS: Limnocalanus macrurus · Zooplankton · Oxidative stress biomarkers · Baltic Sea Full text in pdf format PreviousNextCite this article as: Vuori KA, Lehtonen KK, Kanerva M, Peltonen H, Nikinmaa M, Berezina NA, Boikova E (2015) Oxidative stress biomarkers in the copepod Limnocalanus macrurus from the northern Baltic Sea: effects of hydrographic factors and chemical contamination. Mar Ecol Prog Ser 538:131-144. https://doi.org/10.3354/meps11471 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 538. Online publication date: October 28, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Stuck between a rock and a hard place: zooplankton vertical distribution and hypoxia in the Gulf of Finland, Baltic Sea

Zooplankton often migrate vertically to deeper dark water during the day to avoid visual predators such as fish, a process which can strengthen benthic–pelagic coupling. In the Gulf of Finland, Baltic Sea, a pronounced hypoxic layer develops when there is an inflow of anoxic bottom water from the Central Baltic Sea, which could be a barrier for vertical migrants. Here, we report an acoustic study of the distributions of crustacean zooplankton (mysid shrimp and the copepod Limnocalanus macrurus), gelatinous zooplankton (Aurelia aurita) and fish. Zooplankton trawl nets were used to ground-truth acoustic data. Vertical profiles of oxygen concentration were taken, and the physiological impact of hypoxia on mysids was investigated using biochemical assays. We hypothesised that the vertical distribution of zooplankton and fish would be significantly affected by vertical heterogeneity of oxygen concentrations because anoxia and hypoxia are known to affect physiology and swimming behaviour. In addition, we hypothesised that mysids present in areas with hypoxia would exhibit a preparatory antioxidant response, protecting them from oxidative damage during migrations. The acoustic data showed peaks of crustacean zooplankton biomass in hypoxic ( 75 m), whereas fish shoals and A. aurita medusae were found in normoxic (5–6 mL L−1) upper water layers (<40 m), with individual fish in deeper water excepting that rule. Mysid shrimp from areas with hypoxia had significantly enhanced antioxidant potential compared with conspecifics from areas with no hypoxia and had no significant indications of oxidative damage. We conclude that mysids can protect themselves from oxidative damage, enabling them to inhabit hypoxic water. Our data suggest that hypoxic and low oxygen zones (up to 4 mL L−1) may provide some zooplankton species with a refuge from visual predators such as fish.

Zooplankton trophic structure in Lake Michigan as revealed by stable carbon and nitrogen isotopes

Abstract Within the last few decades Lake Michigan's plankton community has undergone substantial changes. Oligotrophication of the pelagic zone has led to a decrease in zooplankton biomass, while species composition has shifted, with copepod species becoming more dominant. Although these observations have been relatively well documented, their implications for the food web have not been well addressed. To define feeding relationships within the zooplankton community, we measured the stable carbon and nitrogen isotope composition and biomass of major zooplankton species, three seston size classes, and seston collected from three depths in southeastern Lake Michigan. Nitrogen stable isotope ratios suggest a complex plankton food web with zooplankton appearing to occupy at least three distinct trophic positions. The large calanoid copepod Limnocalanus macrurus occupied a tertiary consumer trophic position, more than one trophic position above all other zooplankton species sampled. Carbon stable isotopes revealed significant differences among zooplankton species. Though the mechanisms responsible for these differences are not clear, these results suggest that there is significant partitioning of resources at the base of the food web.

Distinctive lipid composition of the copepod Limnocalanus macrurus with a high abundance of polyunsaturated fatty acids.

We studied the copepod Limnocalanus macrurus for seasonal variation in the composition of fatty acids, wax esters and sterols in large boreal lakes, where it occurs as a glacial-relict. Vast wax ester reserves of Limnocalanus were accumulated in a period of only two months, and comprised mono- and polyunsaturated fatty acids (PUFA) and saturated fatty alcohols. In winter, the mobilization of wax esters was selective, and the proportion of long-chain polyunsaturated wax esters declined first. PUFA accounted for >50% of all fatty acids throughout the year reaching up to ca. 65% during late summer and fall. Long-chain PUFA 20:5n-3 and 22:6n-3 together comprised 17-40% of all fatty acids. The rarely reported C24 and C26 very-long-chain PUFA (VLC-PUFA) comprised 6.2 ± 3.4 % of all fatty acids in August and 2.1 ± 1.7% in September. The VLC-PUFA are presumably synthesized by Limnocalanus from shorter chain-length precursors because they were not found in the potential food sources. We hypothesize that these VLC-PUFA help Limnocalanus to maximize lipid reserves when food is abundant. Sterol content of Limnocalanus, consisting ca. 90% of cholesterol, did not show great seasonal variation. As a lipid-rich copepod with high abundance of PUFA, Limnocalanus is excellent quality food for fish. The VLC-PUFA were also detected in planktivorous fish, suggesting that these compounds can be used as a trophic marker indicating feeding on Limnocalanus.

Temporal and Spatial Dynamics of Ice-Covered Upper Dumbell Lake (Ellesmere Island, Arctic Canada) during the Summer of 1959

We report on a limnological study of ice-covered Upper Dumbell Lake (Ellesmere Island, Canada) conducted during the summer of 1959. The lake was vertically profiled for physical (temperature, light), chemical (alkalinity, pH, oxygen, nutrients), and biological (chlorophyll a, gross and net primary productivity) variables on 21 dates spanning from early July to early September. Zooplankton density and age structure were also determined on four dates. Factors such as temperature, alkalinity, pH, and oxygen varied little with depth or over time, whereas nutrients (nitrate, dissolved silica, soluble reactive phosphorus), light, chlorophyll a, and gross and net photosynthesis varied substantially. Comparing July to August, nitrate and light intensity decreased while dissolved silica, chlorophyll a, and gross and net primary production increased, with two distinct peaks in algal biomass occurring over the month of August. Chlorophyll a in this lake was negatively correlated with nitrate concentrations, suggesting uptake of nitrogen as algal biomass increased. The copepod Limnocalanus macrurus was the dominant zooplankton taxon present; the age structure of the population advanced over the summer. This study reveals the dynamic nature of vertical habitat gradients even under the ice of Arctic lakes and provides important baseline data for conditions in an Arctic lake during the mid-20th century.

The Freshwater Copepod &lt;i&gt;Limnocalanus macrurus&lt;/i&gt; in the Canadian Arctic Archipelago: Numbers, Weights, and Respiration Observed from September 1961 – July 1962

Numbers, weights, and oxygen consumption of the copepod Limnocalanus macrurus were measured through the winter 1961 – 62 under the ice of Immerk Lake on Devon Island, Arctic Canada. Maximum abundance was 2361 animals per m3 found under ice in mid June, average adult wet weight was 65 μg, and hourly oxygen consumption per adult ranged from 0.26 μg in late summer to 0.03 μg in early winter. The results are compared with results from Char Lake and Resolute Lake on Cornwallis Island, Arctic Canada. The Immerk population was more stable than those at the other lakes, and weights and oxygen consumption appear to be comparable. The seasonal breeding cycle at Immerk Lake was different from that at Char Lake and similar to that at Resolute Lake. A comparison of Immerk Lake data from 1961 – 62 and 1972 – 73 showed almost identical levels of total oxygen metabolism. Immerk Lake copepod oxygen consumption was 6.5% of the total lake metabolism, while that at Char Lake was 6%. These data may assist in future assessment of climate or anthropogenic changes.

PCR-DHPLC assay for the identification of predator-prey interactions

Denaturing high-performance liquid chromatography (DHPLC) is a relatively new method for separating amplicons in a mixture, and was recently developed for parasite detection in the blue crab Callinectes sapidus. That assay used a peptide nucleic acid (PNA) PCR hybridization blocking probe (PNA–PCR–DHPLC) to decrease the generic PCR bias of dominant templates (the host) in the mixture prior to separation on the DHPLC column, thus enhancing the less abundant parasite DNA. The same assay and rational can be used to investigate predator–prey interactions. However, in ecosystem studies with many predator–prey relationships, development of specific PNA-blocking probes for each predator would be too laborious. Here, we have developed a PCR–DHPLC assay excluding the dominant predator amplicons in a first DHPLC run, followed by re-amplification of the non-predator retention volumes and further separation and characterization in a second DHPLC run. This assay generated data on the specific trophic interactions between the calanoid copepod Limnocalanus macrurus and its prey from a seasonal sampling programme. The assay provides an efficient way for an unbiased screening of predator–prey relationships, and although developed for L. macrurus in this study, the approach has wide applicability for any predator–prey interaction.

Structure of the zooplankton communities in the region of the Ob River’s estuarine frontal zone

The studies were carried out on September 27–30, 2007, in the area of the Ob estuarine frontal zone and over the adjacent inner Kara Sea shelf. Based upon the latitudinal changes in the salinity, the 100 nautical mile wide estuarine frontal zone was marked out. The frontal zone was inhabited by a specific zooplankton community dominated by species that occurred outside the frontal zone in only minor amounts. The biomass of the mesozooplankton averaging 984 mg/m3 in the frontal zone exceeded by 1.5 and 6 times the corresponding values in the inner desalinated area of the estuary and the adjacent areas of the Kara Sea shelf. At the inner southern periphery of the frontal zone, at maximal latitudinal salinity gradients (>2 psu per mile), the maximal development of the mesoplankton with the mean biomass for the water column of 3.1 g/m3 (37 g/m2) and up to 5.8 g/m3 in the subpycnocline layer was observed. The latitudinal extension of the biomass in the maximum zone did not exceed 10 miles. More than 90% of the maximum was composed of herbivorous zooplankton with the strong domination of the copepod Limnocalanus macrurus. The daily consumption within the zooplankton maximum area was estimated at 820 mgC/m2 per day. This value exceeds by two orders of magnitude the local primary production. At that level of consumption, the available phytoplankton biomass was consumed by grazers in less than 8 hours (!). A zooplankton aggregation at the southern periphery of the estuarine front exists due to the advection of phytoplankton from the adjacent river zone. The aggregation forms a natural pelagic biofilter where new allochthonous organic matter delivered by the river flow is accumulated and high secondary production is formed on its basis. An anomalously high concentration of planktic predatory Parasagitta elegans with biomass of over 1 g/m3 (46% of the total zooplankton biomass) was associated with the outer northern periphery of the estuarine frontal zone.

The Status of Limnocalanus macrurus (Copepoda: Calanoida: Centropagidae) in Lake Erie

The calanoid copepod Limnocalanus macrurus showed large declines in abundance and a narrowing of spatial distribution with the onset of cultural eutrophication and increases in rainbow smelt (Osmerus mordax) abundances in Lake Erie in the mid 20th century. Since 1995, however, Limnocalanus macrurus appears to have repopulated in western Lake Erie to levels of abundance that have not been observed since the late 1930s. We hypothesize that phosphorus abatement and the subsequent decrease in low dissolved oxygen events have assisted this resurgence. However, Limnocalanus macrurus abundances have not increased in the central and eastern basins, even though water quality has improved there too. High densities of rainbow smelt and associated smelt predation pressure in the central and eastern basins may be responsible for the low numbers in these basins.

Sedimentation of copepod fecal material in the coastal northern Baltic Sea: Where did all the pellets go?

We investigated the sedimentation of copepod fecal pellets in three different sea areas representing a sheltered bay, an archipelago area, and the open sea on the southwestern coast of Finland in the northern Baltic Sea. Fecal carbon sedimentation was always &lt;0.05% of the total sedimentation of particulate organic carbon, whereas the fecal carbon production (estimated from copepod abundance, assuming production rate of 10 pellets copepod−1 d−1) contributed to 4–17% of particulate organic carbon sedimentation. Thus, &gt;99% of copepod fecal material was remineralized within the mixed water layer (0–20 m). However, in the area and season dominated by the large calanoid copepod Limnocalanus macrurus (bay station in spring), fecal carbon sedimentation was an order of magnitude higher than at the other two stations. From June onwards, when the bay station was dominated by cyclopoids, the situation changed: the fecal carbon sedimentation remained 30% lower in the bay than in the archipelago, although the fecal carbon production was estimated to be 2 times higher in the bay. Furthermore, pellet fragmentation (percentage of broken pellets of total fecal carbon sedimentation) was highest in spring and autumn at all areas and increased towards the open sea, being 27%, 45%, and 61% at the bay, archipelago, and open sea stations, respectively. This gradation was probably due to more intense turbulence and water column mixing in the open sea, resulting in more efficient loosening and breakup of pellets. The overall contribution of copepod feces to vertical carbon export in the northern Baltic Sea appears to be small, but seasonal and spatial variations in hydrography and mesozooplankton community structure significantly affect the fecal pellet sedimentation rates.

Prey of Deep-water Hydra in Lake Superior

Specimens of Hydra attached to a soft-substrate bottom were collected from a manned submersible at a 275-m-deep station near the Keweenaw Peninsula in Lake Superior. Near-bottom zoo-plankton were collected to assess Hydra feeding preference. We identified and counted the zooplankton samples, and identified the zooplankton prey found in the Hydra. Of 86 Hydra, 21 contained remains of 23 prey of 4 zooplankton taxa. Compared with expected numbers based on the proportions of the net samples, these Hydra appeared to be feeding selectively, with a preference for larger zooplankton. The large calanoid copepod Limnocalanus macrurus was particularly selected. A very provisional estimate of the rate of Hydra predation on the bottom of Lake Superior is one prey per 4 days. The relatively constant environmental conditions, lack of commensals and predators, and the zooplankton food source suggest that the bottom of Lake Superior is a stable habitat for Hydra.

Habitat Switch and Niche Overlap in Coregonid Fishes: Effects of Zooplankton Abundance

Vendace (Coregortus albula) in Lake Mjøsa lived in the pelagic zone and gillnet catches were correlated with water temperature (r2 = 0.457, p &lt; 0.005). Whitefish (Coregonus lavaretus) exploited both epibenthic and pelagic areas. Whitefish with body lengths between 25 and 35 cm moved from the epibenthic to pelagic zone during summer; other length groups remained in epibenthic habitats. The pelagic fish returned to epibenthic areas in the autumn. Cladocerans were important food items for both fish species in the pelagic zone. The large copepod Limnocalanus macrurus was eaten only by vendace whereas surface insects were more important prey for whitefish. This suggests that vendace is the more specialized zooplanktivore. The pelagic gillnet catch of whitefish and the pelagic habitat overlap between whitefish and vendace increased with increasing zooplankton densities at depths between 0 and 50 m (r2 = 0.609 and 0.494, respectively, p &lt; 0.01). During spring and autumn we observed a time lag between the change in zooplankton abundance and habitat switches of whitefish. The fish moved to the more profitable habitat several days after food availability had changed, indicating an element of learning in choice of habitat.