Nodularin concentrations in Baltic Sea zooplankton and fish during a cyanobacterial bloom

Toxic cyanobacterial blooms, dominated by Nodularia spumigena, are a recurrent phenomenon in the Baltic Sea during late summer. Nodularin, a potent hepatotoxin, has been previously observed to accumulate on different trophic levels, in zooplankton, mysid shrimps, fish as well as benthic organisms, even in waterfowl. While the largest concentrations of nodularin have been measured from the benthic organisms and the food web originating from them, the concentrations in the pelagic organisms are not negligible. The observations on concentrations in zooplankton and planktivorous fish are sporadic, however. A field study in the Gulf of Finland, northern Baltic Sea, was conducted during cyanobacterial bloom season where zooplankton (copepod Eurytemora affinis, cladoceran Pleopsis polyphemoides) and fish (herring, sprat, three-spined stickleback) samples for toxin analyses were collected from the same sampling areas, concurrently with phytoplankton community samples. N. spumigena was most abundant in the eastern Gulf of Finland. In this same sampling area, cladoceran P. polyphemoides contained more nodularin than in the other areas, suggesting that this species has a low capacity to avoid cyanobacterial exposure when the abundance of cyanobacterial filaments is high. In copepod E. affinis nodularin concentrations were high in all of the sampling areas, irrespective of the N. spumigena cell numbers. Furthermore, nodularin concentrations in herring samples were highest in the eastern Gulf of Finland. Three-spined stickleback contained the highest concentrations of nodularin of all the three fish species included in this study, probably because it prefers upper water layers where also the risk of nodularin accumulation in zooplankton is the highest. No linear relationship was found between N. spumigena abundance and nodularin concentration in zooplankton and fish, but in the eastern area where the most dense surface-floating bloom was observed, the nodularin concentrations in zooplankton were high. The maximum concentrations in zooplankton and fish samples in this study were higher than measured before, suggesting that the temporal variation of nodularin concentrations in pelagic communities can be large, and vary from negligible to potentially harmful.     

Deposit-feeding amphipods (Monoporeia affinis) reduce the recruitment of copepod nauplii from benthic resting eggs in the northern Baltic Sea

We experimentally investigated the effect of different densities of the burrowing, deposit-feeding amphipod Monoporeia affinis on the recruitment of zooplankton from benthic resting eggs. Intact sediment cores with in situ density and species composition of zooplankton resting eggs and benthic fauna were collected in the northern Bothnian Sea, part of the Baltic Sea. We removed as many M. affinis as possible from the cores and then added different numbers of M. affinis to the cores to generate a range of densities. The cores were exposed to different densities of M. affinis for either 3 or 40 days, after which the hatched zooplankton was registered. One subset of the cores were initially incubated under low temperature (2–3 °C, to prevent hatching) for 37 days (the resting phase), to allow for effects of M. affinis on unhatched resting eggs. These cores were then incubated under higher temperature (13 °C) for 3 days (the hatching phase), to induce hatching and allow for effects on hatching or hatched specimens. In a second subset of cores with the same time and temperature schedule, the M. affinis density was experimentally reduced at the start of the hatching phase, to evaluate the effect of M. affinis during the hatching phase. To a third subset of cores, we immediately initiated the hatching phase, without an experimental resting phase, to evaluate the effects induced during the resting phase. The most common zooplankton species that hatched was Eurytemora affinis (Copepoda), followed by Bosmina longispina maritima (Cladocera). In all cores that were subjected to a resting phase, the numbers of hatched E. affinis were log-linearly negatively related to density of M. affinis. An increase of M. affinis density from 1,000 to 5,000 individuals m⁻², normal field densities, reduced the hatching by 60–70%. The negative impact was mainly exerted during the hatching phase, suggesting predation on, burial of or physical injury of hatching nauplii or eggs in a late development stage as likely mechanisms. Also, the number of B. longispina maritima that hatched was reduced by M. affinis during the hatching phase, but no clear relation to density of M. affinis could be identified. The results show that M. affinis can reduce recruitment to zooplankton from benthic resting eggs. Such impact by the benthos on resting stages of zooplankton is therefore a potentially significant link between the benthic and pelagic systems. Received: 10 August 2000 / Accepted: 13 November 2000     

Impact of a burrowing deposit-feeder, Monoporeia affinis, on viable zooplankton resting eggs in the northern Baltic Sea

We investigated the effect of different densities of the burrowing deposit-feeding amphipod Monoporeia affinis on the potential for recruitment of zooplankton from benthic resting eggs. Hatching of resting eggs was induced in the laboratory on sliced and resuspended 1-cm depth-sections of sediment cores, collected at six stations in an archipelago area of the Gulf of Bothnia, Baltic Sea. The uppermost 5 cm of the sediment was studied. The most common species that hatched was Eurytemora affinis (Copepoda). Individuals from another copepod genus, Acartia, hatched in significant numbers only in the cores from two stations with low amphipod abundance. Cores from stations with high amphipod densities showed a deeper distribution of emerging E. affinis nauplii compared with stations with few amphipods; the oxidised sediment layer was also deeper at high M. affinis densities than at low. Total (0 to 5 cm strata pooled) number of hatched E. affinis nauplii was independent of amphipod density. This indicates that the effect of M. affinis on E. affinis eggs involves deeper burial due to bioturbation, rather than predation. Decreased benthic recruitment of zooplankton at localities with high M. affinis density is suggested, since more deeply positioned eggs are less likely to hatch. When hatching was induced in intact, non-sliced cores from one station, the number of E. affinis nauplii that hatched was on average 43% of the number that hatched in the upper centimetre of the sliced cores from the same station. This fraction (43%), if applied to the other stations, implied a potential for benthic recruitment of up to 80 000 ind m⁻² for E. affinis. Due to its high abundance, M. affinis is likely to greatly reduce benthic recruitment of zooplankton in this system. Received: 17 September 1999 / Accepted: 23 February 2000     

Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta, in natural prey assemblages

Predation rates and prey selection of the pelagic mysid shrimp, Mysis mixta, were studied experimentally in the northern Baltic Sea in 1998 during their most intensive growth period, from June to October. Functional responses during 5 months were determined by providing the mysids with a natural zooplankton assemblage, diluted to several different concentrations. The results show that ingestion rate increased, along with mysid growth, from early summer to autumn and that saturation level was reached between 400 and 500 μg C l⁻¹. Ingestion rates increased with increasing prey concentration, and sigmoidal curves explained mostly the variation in ingestion rates (explanatory levels of 86–97%). Prey selection was evident in June, July and August, though weaker during the latter 2 months. Selection differed between the studied months but, generally, copepods were more positively selected than cladocerans. Rotifers were the main prey during June and July, when mysids were small, while larger mysids fed on copepods and cladocerans. Of the copepods, Eurytemora affinis was a truly selected species. This study shows that mysids feed on many zooplankton taxa and that they undergo ontogenetic diet shifts. Received: 19 July 2000 / Accepted: 19 October 2000     

Dopamine release by the green alga <Emphasis Type="Italic">Ulvaria obscura</Emphasis> after simulated immersion by incoming tides

Ulvaria obscura, a prominent component of green tide blooms in Washington, is unique among macroalgae because it contains dopamine. To examine dopamine release by U. obscura following simulated low tides, we conducted 6 field experiments in which algae were emersed for 75 min and then immersed in filtered seawater (FSW). Dopamine was measured in algal tissues prior to emersion and 3 h after immersion and in seawater for 3 h following immersion. In our experiments, algae released 7–100% of their tissue dopamine, resulting in average seawater concentrations of 3–563 μM. In 5 of 6 experiments, seawater dopamine concentrations were highest immediately after immersion, and then decreased over time. The percentages of dopamine released were not correlated with tissue dopamine concentrations, but were positively correlated with solar radiation during emersion. The release of dopamine, which is both cytotoxic and genotoxic, may explain the negative effects of U. obscura exudates on marine organisms.     

Effects of dissolved cyanobacterial toxins on the survival and egg hatching of estuarine calanoid copepods

Toxic cyanobacteria occur in fresh, brackish, and marine waters worldwide. Harmful effects of intact cyanobacterial cells on zooplankton are well known, but only a few studies have focused on the effects of dissolved toxins. We investigated experimentally how the cyanobacterial toxins anatoxin-a, microcystin-LR, and nodularin affect estuarine calanoid copepods, by monitoring effects on egg hatching and survival in Eurytemora affinis. We also investigated effects of nodularin on the survival of Acartia bifilosa. All exposures were carried out using purified dissolved toxins. We found that the egg hatching was unaffected by these toxins, whereas survival was marginally affected by high concentrations of anatoxin-a and nodularin. For microcystin-LR, we found reduced survival at concentrations that were lower than those known to affect other meso-zooplankton.     

Effects of cadmium on Pontoporeia affinis (Crustacea: Amphipoda) in laboratory soft-bottom microcosms

Laboratory soft bottom microcosms, containing sediment with natural populations of meio-and microfauna and the amphipod Pontoporeia affinis Lindström, were used to investigate chronic effects of cadmium on a typical Baltic Sea benthic community. Temperature, salinity, organic content of the sediment and the life cycles of the amphipods in laboratory control microcosms were in good agreement with conditions in the Baltic Sea. In Experiment 1, P. affinis was exposed to four assayed cadmium concentrations, 0 (control), 6.3, 41 and 127 g Cd·1^-1, for 265 d. At the lowest cadmium concentration (6.3 g Cd·1^-1), the sediment contained about 700 times, and P. affinis about 1 700 times, the water concentration at the end of the experiments. In microcosms without P. affinis, the sediments accumulated less cadmium. Effects on P. affinis embryogenesis were detected after 105 d of exposure in all tested cadmium concentrations. Mortality of juveniles was significantly increased after 265 d of exposure in mean assayed concentrations of 41 and 127 g Cd·1^-1. In Experiment 2, P. affinis was exposed to cadmium in assayed concentrations, 0, 6.5 and 46 g Cd·1^-1 for 460 d. At the lowest water concentration (6.5 g Cd·1^-1), the prolonged exposure did not increase the sediment concentration, which indicates a steady state, while concentrations in P. affinis reached about 3 500 times the lowest water concentration after 460 d. Juvenile P. affinis matured sexually and reproduced normally in all microcosms, but their offspring died in all microcosms with added Cd. This occurred at water concentrations not uncommon in polluted marine areas. The effects are serious in comparison to those found in conventional short-term laboratory experiments with the same concentrations and demonstrate the importance of long-term experiments in ecotoxicology.     

Water-soluble vitamins in natural plankton (copepods) during two consecutive spring blooms compared to vitamins in Artemia franciscana nauplii and metanauplii

Weight-specific contents of asorbic acid, thiamin, riboflavin, niacin, pantothenic acid, vitamin B₆, biotin, folate and vitamin B₁₂ were analysed in natural zooplankton collected from a sea water lagoon in a halibut fry production plant. The samples of natural zooplankton were collected from April/May to the end of June during two consecutive spring blooms. For comparison, the same vitamins were measured in nauplii of Artemia franciscana, just after hatching and after enrichment for 24 and 48 h with a commercial diet. The changes in vitamin content in zooplankton, which consisted mainly of Temora longicornis, during the two spring blooms were <2.5-fold for each specific vitamin. The large number of high quality halibut fry produced using the plankton as a dietary source suggests that the plankton contained sufficient amounts of all water-soluble vitamins. With the exception of thiamin and ascorbic acid, Artemia– both newly hatched and enriched for 24 and 48 h – contained higher levels of water-soluble vitamins than natural zooplankton. Enrichment feeding of Artemia for 48 h stabilised or increased the level of the water-soluble vitamins. Received: 8 March 1999 / Accepted: 21 February 2000     

Seasonal variation of membrane fluidity of the naturally acclimatized Baltic Sea amphipods Gammarus spp. and Monoporeia affinis

The membrane fluidity of the Baltic Sea crustaceans Gammarus spp. and Monoporeia affinis was studied in different seasons. Gammarus spp. were collected at a location with stable salinity and with temperature fluctuations from about 0 to 20 °C, and M. affinis at a deep location with stable salinity and a constant low temperature of about 1.5 to 4.5 °C. The membrane fluidity was measured from preparations enriched with plasma and mitochondrial membranes employing a fluorescence polarization technique using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. The measurements were carried out without any preceding acclimation of the animals. In Gammarus spp., the membrane fluidity increased in the order: summer animals < autumn animals < spring animals. The differences between seasons were significant, with pronounced homeoviscous adaptation, and correlated well with the water temperature. It is suggested that in Gammarus spp., temperature is the determining factor for membrane fluidity. In M. affinis, the membranes of summer animals were the most fluid, and there was a significant difference only between summer and spring animals. In M. affinis the difference was not connected to the water temperature and no homeoviscous adaptation was noticed. The deep-water M. affinis experience a long period of fresh food deficiency, which probably affects the membrane fluidity. Although there are seasonal differences in fluidities between Gammarus spp. and M. affinis, it is interesting to note that all data obtained from M. affinis settled between the polarization lines of spring and summer Gammarus spp., regardless of the prevailing temperatures. Received: 30 June 1999 / Accepted: 7 June 2000     

Community-level and species-specific responses of coastal phytoplankton to the presence of two mesozooplanktonic grazers

The response of phytoplankton to the presence of two mesozooplanktonic grazers was studied in a 44-h laboratory experiment from the phytoplankton perspective. The <45-μm-filtered plankton community, as well as females of the copepod Eurytemora affinis and nauplii of the barnacle Balanus improvisus originated from the northern Baltic Sea. The phytoplankton community was dominated by the dinoflagellate Heterocapsa triquetra, which was preferred as food by the mesozooplanktonic grazers, especially E. affinis. Cryptophytes were eaten by the B. improvisus nauplii, while heterotrophic nanoflagellates increased in the presence of B. improvisus nauplii, and colonial cyanobacteria increased in the presence of E. affinis. Because of the strong selective feeding on the dominant, large-sized phytoplankton species, the negative effect of the mesozooplanktonic grazers on total chlorophyll a was stronger than any cascade effect releasing phytoplankton from protozoan grazing.     

Genetic heterogeneity among <Emphasis Type="Italic">Eurytemora affinis</Emphasis> populations in Western Europe

Evolutionary diversification of the broadly distributed copepod sibling species complex Eurytemora affinis has been documented in the northern hemisphere. However, the fine scale geographic distribution, levels of genetic subdivision, evolutionary, and demographic histories of European populations have been less explored. To gain information on genetic subdivision and to evaluate heterogeneity among European populations, we analyzed samples from 8 locations from 58° to 45°N and 0° to 23°E, using 549 base pairs of the mitochondrial cytochrome oxidase subunit I (COI) gene. We discovered three distinct lineages of E. affinis in Western Europe, namely the East Atlantic lineage, the North Sea/English Channel (NSEC) lineage, and the Baltic lineage. These geographically separated lineages showed sequences divergence of 1.7–2.1%, dating back 1.9 million years (CI: 0.9–3.0 My) with no indication of isolation by distance. Genetic divergence in Europe was much lower than among North American lineages. Interestingly, genetic structure varied distinctively among the three lineages: the East Atlantic lineage was divided between the Gironde and the Loire populations, the NSEC lineage comprised one single population unit spanning the Seine, Scheldt and Elbe rivers and the third lineage was restricted to the Baltic Proper (Sweden). We revealed high haplotype diversity in the East Atlantic and the Baltic lineages, whereas in the NSEC lineage haplotype diversity was comparatively low. All three lineages showed signs of at least one demographic expansion event during Pleistocene glaciations that marked their genetic structure. These results provide a preliminary overview of the genetic structure of E. affinis in Europe.     

Risk to humans of consuming metals in anchovy (<Emphasis Type="Italic">Coilia</Emphasis> sp.) from the Yangtze River Delta

Concentrations of metals were determined in four species of anchovy (Coilia sp.) from the Yangtze River, Taihu Lake, and Hongze Lake in Jiangsu Province, China. Concentrations of Cr in anchovy fish muscle ranged from 2.6 × 10⁻² to 5.0 mg/kg ww, and Coilia nasus taihuensis in Jiaoshan, Taihu Lake contained the highest concentrations of Cr, which was almost 111-fold higher than the mean value at other locations. Concentrations of Pb ranged from 1.5 × 10⁻² to 1.3 × 10⁻¹ mg/kg ww. Comparisons of concentrations of lead (Pb) among the four species indicated that anadromous species contained higher concentrations of Pb than did freshwater species. However, concentrations of Pb in C. nasus from the Nanjing and Haimen locations in the Yangtze River were not significant higher than those of two freshwater species: C. nasus taihuensis from Taihu Lake and C. brachygnathus from Hongze Lake (Duncan’s test, α = 0.05). While concentrations of Cd and Zn ranged from 7.0 × 10⁻⁴ to 3.6 × 10⁻³ mg/kg ww and 3.4 to 4.8 mg/kg ww, respectively, there were no significant differences in concentrations among the eight locations. The only concentration of the metals studied that exceeded the Chinese National Standard was Cr in Coilia from Jiaoshan, Taihu Lake, which was 2.5-fold higher than the standard. These results indicate that people who consume the genus Coilia are not at risk due to concentrations of metals, except Cr in C. nasus taihuensis from Jiaoshan in Taihu Lake. Concentrations of all of the metals studied except for Cr were similar to or less than those of metals in most other areas in the world.     

The effects of temperature and salinity on reproductive success of <Emphasis Type="Italic">Temora longicornis</Emphasis> in the Baltic Sea: a copepod coping with a tough situation

At specific locations within the Baltic Sea, thermoclines and haloclines can create rapid spatial and temporal changes in temperature (T) and salinity (S) exceeding 10°C and 9 psu with seasonal ranges in temperature exceeding 20°C. These wide ranges in abiotic factors affect the distribution and abundance of Baltic Sea copepods via species-specific, physiological-based impacts on vital rates. In this laboratory study, we characterized the influence of T and S on aspects of reproductive success and naupliar survival of a southwestern Baltic population of Temora longicornis (Copepoda: Calanoida). First, using ad libitum feeding conditions, we measured egg production (EP, no. of eggs female⁻¹ day⁻¹) at 12 different temperatures between 2.5 and 24°C, observing the highest mean EP at 16.9°C (12 eggs female⁻¹ day⁻¹). Next, the effect of S on EP and hatching success (HS, %) was quantified at 12°C for cohorts that had been acclimated to either 8, 14, 20 or 26 psu and tested at each of five salinities (8, 14, 20, 26 and 32 psu). The mean EP was highest for (and maximum EP similar among) 14, 20 and 26 psu cohorts when tested at their acclimation salinity whereas EP was lower at other salinities. For adults reared at 8 psu, a commonly encountered salinity in Baltic surface waters, EP was relatively low at all test salinities—a pattern indicative of osmotic stress. When incubated at 12°C and 15 different salinities between 0 and 34 psu, HS increased asymptotically with increasing S and was maximal (82.6–84.3%) between 24 and 26 psu. However, HS did depend upon the adult acclimation salinity. Finally, the 48-h survival of nauplii hatched and reared at 14 psu at one of six different temperatures (10, 12, 14, 16, 18 and 20°C) was measured after exposure to a novel salinity (either 7 or 20 psu). Upon exposure to 7 psu, 48-h naupliar mortality increased with increasing temperature, ranging from 26.7% at 10°C to 63.2% at 20°C. In contrast, after exposure to 20 psu, mortality was relatively low at all temperatures (1.7% at 10°C and ≤26.7% for all other temperatures). An intra-specific comparison of EP for three different T. longicornis populations revealed markedly different temperature optima and clearly demonstrated the negative impact of brackish (Baltic) salinities. Our results provide estimates of reproductive success and early survival of T. longicornis to the wide ranges of temperatures and salinities that will aid ongoing biophysical modeling examining climate impacts on this species within the Baltic Sea.     

Silica uptake kinetics of Halichondria panicea in Kiel Bight

Besides diatoms Demospongiae are the most important consumers of dissolved silica in the sea. They can play an important role for the silica budget especially in the shallow water areas of the Baltic Sea. The dependence of the silica uptake rate on the silica concentration of the seawater was measured for the sponge Halichondria panicea (Pallas, 1766). The sponges were collected in Kiel Bight. The uptake conformed to Michaelis–Menten kinetics with a half-saturation constant of 46.41 μM and a saturated uptake rate of 19.33 μmol h⁻¹ g⁻¹ ( p < 0.01). In the red algae zone of Kiel Bight the sponges depend on silica supply from the surrounding waters and may be silica-limited rather than food-limited in growth. Because of the much faster uptake of silica by diatoms and their lower saturation point, as well as the difference in spatial distribution of the two main silica consumers, a competition for silica between sponges and diatoms seems unlikely. Received: 21 June 1997 / Accepted: 15 July 1997     

A comparison of transport-related gill enzyme activities and tissue-specific free amino acid concentrations of Baltic Sea (brackish water) and freshwater threespine sticklebacks, Gasterosteus aculeatus, after salinity and temperature acclimation

The osmoregulatory abilities of one freshwater and two brackish water (Baltic Sea) populations of the euryhaline teleost fish Gasterosteus aculeatus were studied with respect to evolutionary physiology. Plasma osmolality, activities of Na⁺K⁺-ATPase, citrate synthase, creatine kinase in the gill and free amino acids in liver, axial muscle and pectoral fin muscle were measured. After transfer from 10 to 35 ppt at 15 °C, time-course changes of plasma osmolality and gill Na⁺K⁺-ATPase showed no significant fundamental differences between the freshwater and one of the Baltic Sea populations. In a multi-factorial experiment, each population was exposed to four different abiotic regimes. Both brackish water populations had high mortality in freshwater at 4 °C, which is discussed as a failure of osmotic regulation (reduced taurine concentrations). Freshwater specimens had higher levels of glycine in the axial and pectoral fin muscles compared to the brackish water populations. This is interpreted as a genetically based effect. In brackish (20 ppt) water of 15 °C, the freshwater population had high activities of Na⁺K⁺-ATPase, but low activities of creatine kinase, whereas both brackish water populations behaved in the opposite way. A fundamental difference between the freshwater and brackish water populations on the level of the osmoregulatory machinery was not observed. Received: 10 December 1998 / Accepted: 22 September 1999     

Temporary cyst formation of<Emphasis Type="Italic"> Heterocapsa triquetra</Emphasis> (Dinophyceae) in natural populations

Heterocapsa triquetra (Ehrenberg) Stein is a phototrophic marine dinoflagellate with wide coastal distribution. It is known to be capable of mixotrophy and diel vertical migration. The species was particularly abundant in the Gulf of Finland (the Baltic Sea) during the summers of 1996 and 1998, leading to discolouration of water on the south-west coast of Finland. Large-scale (50 m³) coastal mesocosm experiments in the north-west Gulf of Finland (the Baltic Sea) in the summers of 1996 and 1998 with daily mineral nutrient additions provoked a biomass increase of phytoplankton dominated by H. triquetra. From the first days of the experiment temporary cysts of H. triquetra were found in the bottom sediment water of the mesocosms. Maximum temporary cyst production rates reached values up to 20×10⁶ cysts m^–2 day^–1, accounting for <1% of the depth-integrated motile population size. The environmental features favouring temporary cyst production remain uncertain; zooplankton grazing and nutrient stress are potential factors. Temporary cysts of H. triquetra were observed in a unialgal culture (f/2 medium) isolated in summer 1999 from Eel Pond (Woods Hole, Mass., USA).Communicated by M. Kühl, Helsingør     

Moderate seawater acidification does not elicit long-term metabolic depression in the blue mussel Mytilus edulis

Marine organisms are exposed to increasingly acidic oceans, as a result of equilibration of surface ocean water with rising atmospheric CO₂ concentrations. In this study, we examined the physiological response of Mytilus edulis from the Baltic Sea, grown for 2 months at 4 seawater pCO₂ levels (39, 113, 243 and 405 Pa/385, 1,120, 2,400 and 4,000 μatm). Shell and somatic growth, calcification, oxygen consumption and \textNH₄ ⁺ {\text{NH}}_{4}^{ + } excretion rates were measured in order to test the hypothesis whether exposure to elevated seawater pCO₂ is causally related to metabolic depression. During the experimental period, mussel shell mass and shell-free dry mass (SFDM) increased at least by a factor of two and three, respectively. However, shell length and shell mass growth decreased linearly with increasing pCO₂ by 6–20 and 10–34%, while SFDM growth was not significantly affected by hypercapnia. We observed a parabolic change in routine metabolic rates with increasing pCO₂ and the highest rates (+60%) at 243 Pa. \textNH₄ ⁺ {\text{NH}}_{4}^{ + } excretion rose linearly with increasing pCO₂. Decreased O:N ratios at the highest seawater pCO₂ indicate enhanced protein metabolism which may contribute to intracellular pH regulation. We suggest that reduced shell growth under severe acidification is not caused by (global) metabolic depression but is potentially due to synergistic effects of increased cellular energy demand and nitrogen loss.     

Calanoid resting eggs in the Baltic Sea: implications for the population dynamics of Acartia bifilosa (Copepoda)

Resting eggs of Acartia spp. (presumably A. bifilosa Giesbr.) at densities between 65 and 125 eggs cm^-2 were found in sediment off the southern coast of Finland and represent the first observations of dormant calanoid eggs in the Baltic Sea. The highest egg densities were found at sediment depths between 4 and 8 cm. The hatching percentage varied between 0 and 90% in the different sediment layers and at different experimental temperatures (13, 16 and 19°C). Some nauplii emerged from the eggs extracted from sediment layers 9 to 10 cm beneath the surface layer and other nauplii emerged from eggs after 82 d of dark storage at 3°C. Light was not needed to trigger hatching; the nauplii were able to hatch in darkness when the eggs were resuspended in filtered seawater. Indirect evidence suggests that the marked spring peak of Acartia spp. nauplii seen in a monitoring data from the years 1973 to 1984 was derived from benthic eggs and not from spawning. The possible mechanisms for induction of hatching and the adaptive benefits of resting egg production for A. bifilosa are discussed.     

Stoichiometry of mesozooplankton in N- and P-limited areas of the Baltic Sea

The Baltic Sea is a very suitable site for stoichiometric studies, since its subbasins differ in their concentration of elemental components, and primary production can therefore be either nitrogen or phosphorus limited. To reveal if the nutrient limitation of mesozooplankton mirrors that of the primary producers, carbon, nitrogen and phosphorus content of both seston and grazers (Acartia sp., Centropages hamatus, Daphnia cristata, Eurytemora affinis, Limnocalanus macrurus, Temora longicornis) were measured in midsummer in the Baltic proper, the Gulf of Finland and the Gulf of Bothnia. The mineral ratios of the different taxa were equal, apart from L. macrurus with notably higher C:P and N:P ratios. Molar C:N ratios were relatively stable (5.1-6.3), whereas C:P and N:P ratios fluctuated more (41-144 and 6.6-24). However, zooplankton elemental composition and limitation did not depend on the limiting nutrient of the phytoplankton, the seston mineral ratio or the sea area. Both the seston-zooplankton elemental imbalance and the food threshold ratio indicated phosphorus limitation of most of the grazers. While L. macrurus may be C or N limited, the possible P deficiency of the other studied taxa suggests that the Baltic Sea zooplankton may act as a potential phosphorus sink, as the freshwater secondary producers do.     

Importance of food quality in determining development and survival of Calanus pacificus (Copepoda: Calanoida)

Mixed zooplankton were collected in June and July of 1985 and 1986 from La Jolla Bay, California, USA, and experiments were conducted to determine how selected dinoflagellates affect development and survival of nauplius larvae of Calanus pacificus. We raised nauplii from eggs on nine species of dinoflagellates at concentrations generally >300 g C l^-1, and compared their development and survival to controls reared using the diatom Thalassiosira weissflogii or filtered seawater. Experiments were conducted for 6 d at 17°C. Development and survival rates of the nauplii fell clearly into one of two groups, depending upon the phytoplankton used as food. The first group was characterized by high development rate (0.46 to 0.84 stage d^-1), and by >27% of the original cohort surviving to at least Nauplius IV or V. The five species producing this result were Gymnodinium simplex, G. splendens, Exuviaella marie-lebourae, Gyrodinium dorsum, and T. weissflogii. The second group was characterized by a development rate similar to that in filtered seawater (0.21 to 0.34 stage d^-1), and by nauplii generally failing to molt past the first feeding stage (Nauplius III), often accompanied by high mortality. The five species producing this result were Gyrodinium resplendens, Ptychodiscus brevis, Glenodinium sp., Amphidinium carterae, and Gonyaulax grindleyi. Development rate and survival were not related to cell size or cell carbon, nor to shape or texture (thecate vs athecate dinoflagellates). Poor growth could be related to the absence of some important, but unidentified, nutritional factors. Alternatively, it could be caused by the presence of plant secondary metabolites which are deleterious to growth, a factor we suspect in P. brevis in particular. Prefeeding nauplii exposed to P. brevis lost neuromuscular control prior to becoming lethargic and dying; nutritional deficiencies may not explain these effects. Methods employed in this study provide useful bioassays for detecting chemical interactions between marine plants and animals. Lethal or sublethal effects of dinoflagellates on their most likely potential predators — copepods — may partially explain why they form significant blooms.