Diel vertical migration and feeding patterns of Mysis mixta (Crustacea,Mysidacea) in the Baltic Sea

The extent of the nocturnal vertical migration of Mysis mixta Lilljeborg varied between early July and late October (of 1985 and 1986) in a coastal area of the Baltic Sea. Migration was more restricted in early July and late October. Seasonal changes in surface light levels and transparency were sufficient to explain the observed differences. Mysids avoided light levels above 10^-4 lux throughout the study period. Smaller juveniles migrated higher up than larger juveniles and adults. A two-layered distribution with part of the population close to the bottom was observed at night. Zooplankton were more abundant in water layers above the main concentration of mysids. M. mixta fed on phytoplankton, detritus, copepods, cladocerans, rotifers and tintinnids. Diel changes in gut fluorescence indicated a higher intake of phytoplankton at night, but levels were low compared to primarily herbivorous zooplankton. Comparisons of stomach contents of mysids caught at the bottom in the evening and in the water column at night showed a higher ingestion of zooplankton at night and of detritus during the day. Mysids caught at the bottom at night had an intermediate diet. Copepods and cladocerans constituted between 90 and 100% of ingested material by weight in all mysid groups.     

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     

Winter food utilisation by sympatric mysids in the Baltic Sea,studied by combined gut content and stable isotope analyses

We studied the winter dietary characteristics of two sympatric mysid species, Mysis mixta and M. relicta, which exploit both benthic and pelagic habitats during diel vertical migrations. Samples collected before and after the ice-covered period in the northern Baltic Sea were investigated using both stomach content analyses and stable isotope analyses of carbon and nitrogen. Both of the mysid species were omnivorous during winter and utilised both benthic and pelagic food sources. The main food source before the ice period was calanoid copepods (40 and 36% for M. mixta and M. relicta, respectively), and after ice-out calanoid copepods (23%) and zooplankton resting eggs (23%) for M. mixta and diatoms (44%) and calanoids (25%) for M. relicta. Their patterns of food utilisation broadly followed seasonal fluctuations in the abundance of the main prey groups. Although pelagic food availability is low in winter both mysid species utilised pelagic prey widely. We also show that when combining these different diet analysis methods it is important to take into account the time lag in isotopic signatures, otherwise the obtained results do not correspond but instead show the feeding history at different times.     

Larval development in the Tasmanian mysids Anisomysis mixta australis, Tenagomysis tasmaniae and Paramesopodopsis rufa (Crustacea: Mysidacea)

We describe the brood duration and marsupial development of three temperate coastal mysid species, Anisomysis mixta australis (Zimmer), Tenagomysis tasmaniae Fenton and Paramesopodopsis rufa Fenton, found commonly in Tasmanian waters. Larvae cultured in vitro had brood durations at 13 °C (17 °C) of 22 (15), 23 (15), and 28 (20) d, respectively. Development through seven larval stages, and brood durations for these three species are similar to those reported for coastal mysids from other temperate areas throughout the world. Received: 10 June 1997 / Accepted: 1 July 1997     

Geographical variability in the bioenergetic characteristics of Monoporeia/Pontoporeia spp. populations from the northern Baltic Sea,and their potential contribution to benthic nitrogen mineralization

The physiological condition, determined as the ammonia excretion rate (V NH ₄ ⁺ ), total lipid level and lipid class composition, of two deposit-feeding benthic amphipods, Monoporeia (=Pontoporeia) affinis and Pontoporeia femorata, was studied from 12 opensea stations in the northern Baltic Sea between 24 May and 11 June 1993. The M. affinis populations can be geographically grouped according to their physiological condition: (1) eastern Gulf of Finland, with moderate lipid level (mean 24.4% of dry wt) and high V NH ₄ ⁺ (45.2 mol NH ₄ ⁺ g^-1 dry wt d^-1); (2) Bothnian Sea, wigh high lipid level (34.5%) and low V NH ₄ ⁺ (24.6 mol NH ₄ ⁺ g^-1 dry wt d^-1); and (3) Bothnian Bay, with low lipid level (15.2%) and high V NH ₄ ⁺ (44.3 mol NH ₄ ⁺ dry wt d^-1). A similar pattern could be observed also in the level of triacylglycerols and the neutral-to-polar lipid ratio. P. femorata, the dominating species in the western Gulf of Finland, showed variable station-specific excretion rates (22.3 to 43.0 mol NH ₄ ⁺ g^-1 dry wt d^-1) and lipid levels (23.4 to 30.4%). The spatial variability in the weight-specific V NH ₄ ⁺ of M. affinis could not be explained by the differences in the size of individuals, lipid level or lipid class composition; this emphasizes the significance of the effects of spatially differing nutritional conditions, which manifest themselves as different modes of metabolic energy production and different intensities of energy storage. In addition, the potential contribution of the amphipod populations to benthic nitrogen mineralization was estimated; in May to June, the NH ₄ ⁺ release of different populations ranged from 12 to 237 mol NH ₄ ⁺ m^-2d^-1. In general, populations with high abundance and/or biomass release the greatest amounts of NH ₄ ⁺ , but the values are modified by the physiological condition of the individuals.     

Seasonal and long-term variations in the body size of planktonic copepods in the northern Baltic Sea

Seasonal and long-term variations in the body size of planktonic copepods were studied in the northern Baltic Sea. The influence of temperature, salinity and phytoplankton concentration to the body size of Eurytemora affinis, Acartia bifilosa and Limnocalanus macrurus (Calanoida) was examined at three sea areas, differing in their hydrographical and trophic conditions (an archipelago area and an enclosed bay on the SW coast of Finland in 1992 and at an open sea station in the Gulf of Bothnia in 1991). There was an inverse relationship between copepod body length and temperature, while there was a direct effect of phytoplankton concentration. According to multiple regression analysis, the relative importance of these factors varied according to species, developmental stage and study area. In the archipelago area, copepod body size was mainly determined by temperature, while in the open Gulf of Bothnia, phytoplankton concentration was usually more important. Interannual variation in the summer body length of E. affinis and A. bifilosa was examined using samples collected over 18 years (1967 to 1984) in the archipelago area. In this analysis, the inverse relationship between water temperature and copepod body size disappeared; in E. affinis the average female length correlated positively with summer temperature. We suggest that the length and timing of the study period greatly influence the investigator's view of the factors regulating copepod body size.     

Sexuality and cyst formation of the spring-bloom dinoflagellate Scrippsiella hangoei in the coastal northern Baltic Sea

The temporal sequence and the magnitude of the sexual reproduction and subsequent cyst deposition of the common spring-bloom dinoflagellate Scrippsiella hangoei (Schiller) Larsen was studied during spring 1996 on the SW coast of Finland, Baltic Sea. The abundances of the different size of fractions of S. hangoei (14 to 18 μm, 18 to 22 μm and >22 μm) were monitored in the water column, and the deposition of resting cysts was measured using moored sediment traps. Cyst sedimentation rates were measured throughout the seasonal cycle in order to estimate cyst resuspension rates for the quantitative assessment of the fraction of population undergoing encystment. The onset of sexual reproduction, indicated by a significant increase of the small cells (14 to 18 μm) representing gametes, occurred in a nutrient replete environment well before the exponential growth phase and peak abundances of vegetative cells. Gamete formation was followed by high abundances of large cells (>22 μm) representing planozygotes, and subsequent sedimentation of resting cysts. Approximately 60% of the asexually growing bloom population was estimated to form planozygotes, suggesting that encystment was an important factor in bloom termination and possibly plays a role in the regulation of the magnitude of the bloom. Finally encystment accounted for 40% of the entire S. hangoei population, resulting in a considerable loss of the bloom population and an input of the vernal phytoplankton biomass to the benthos. Received: 11 December 1998 / Accepted: 8 April 1999     

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     

Correlation between the seasonal distribution of harbour porpoises and their prey in the Sound,Baltic Sea

Low densities of harbour porpoises in winter (November–March) and high densities in summer (April–October) were found in the Sound, connecting the Baltic Sea and Kattegat. Due to their high energy requirements, it is hypothesized that the density of harbour porpoises is related to local prey abundance. This was tested by examining the stomach content of 53 harbour porpoises collected between 1987 and 2010 in the Sound (high season, 34 porpoises; low season, 19 porpoises). A total of 1,442 individual fish specimens from thirteen species were identified. Twelve of these were present in the high–porpoise density season and seven in the low-density season. The distribution of occurrence and the distribution of number of fish species were different between seasons, indicating a shift in prey intake between seasons. Furthermore, during the high-density season, the mean and total prey weight per stomach as well as the prey species diversity was higher. However, no difference was found in the number of prey species between the two seasons, indicating a higher quality of prey in the high-density season. Atlantic cod was found to be the main prey species in terms of weight in the high-density season while Atlantic herring and Atlantic cod were equally important during the low-density season. Prey availability and predictability are suggested as the main drivers for harbour porpoise distribution, and this could be caused by the formation of frontal zones in spring in the northern part of the Sound, leading to prey concentrations in predictable areas.     

Differences in stress tolerance and brood size between a non-indigenous and an indigenous gammarid in the northern Baltic Sea

Differences in stress tolerance and reproductive traits may drive the competitive hierarchy between non-indigenous and indigenous species and turn the former ones into successful invaders. In the northern Baltic Sea, the non-indigenous Gammarus tigrinus is a recent invader of littoral ecosystems and now occupies comparable ecological niches as the indigenous G. zaddachi. In laboratory experiments on specimens collected between June and August 2009 around Tvärminne in southern Finland (59°50′N/23°15′E), the tolerances towards heat stress and hypoxia were determined for the two species using lethal time, LT₅₀, as response variable. The brood size of the two species was also studied and some observations were made on maturation of juveniles. Gammarus tigrinus was more resistant to hypoxia and survived at higher temperatures than G. zaddachi. Brood size was also greater in G. tigrinus than in G. zaddachi and G. tigrinus matured at a smaller size and earlier than G. zaddachi. Hence, there are clear competitive advantages for the non-indigenous G. tigrinus compared to the indigenous G. zaddachi, and these may be further strengthened through ongoing environmental changes related to increased eutrophication and a warming climate in the Baltic Sea region.     

Ecology of the northern Red Sea crinoids and their Epi- and Endozoic fauna

Fourteen crinoid species are found along the coral reefs of the northern Red Sea that form aggregations of different species at various depths. The shallowest aggregation consists partly of Lamprometra klunzingeri and Capillaster multiradiatus, occurring subtidal to a depth of 2 to 5 m. Of this group, Heterometra savignii inhabits depths down to 12–15 m. Further down, another group of 10-armed crinoids occurs, dominated by Decametra chadwicki and Oligometra serripinna. The deepest aggregation of crinoids occurs at 45 m; its most common members are Colobometra arabica and Comaster distinctus. Feeding on micro- and nanoplankton, shallow-water crinoids show a circadian activity rhythm; in deeper aggregations (10 to 12 m), this behaviour changes with decreasing illumination, to a diurnal activity rhythm. The symbiotic animals living on crinoids comprise 27 taxons, among them Copepoda (6 species), Mollusca (2 species), Polychaeta (11 species, especially Myzostomida); Ophiurida (1 species), Crustacea, Decapoda (6 species); and the clingfish Lepadichthys lineatus. These symbiotic animals form food webs at various depths, according to the distribution of their crinoid host. The occurrence of Indo-West Pacific and Mediterranean commensals on the same crinoid in the Gulf of Aqaba is of special interest.     

On the distribution of particulate metals,chlorophyll and seston in the Baltic Sea

During the end of the phytoplankton spring bloom, about 100 water samples were taken at 23 stations in the Baltic Sea. The samples were analyzed for particulate Cu, Fe, Zn, Cd, and Pb by atomic absorption as well as for chlorophyll and seston. Further subsamples were counted with an inverted microscope, and phytoplankton carbon content was calculated. The hydrographical-biological state of the Baltic Sea during spring is described. Two different water masses could be differentiated and these were separated by the halocline. The concentrations of particulate metals are in accordance with those published by other authors. No significant differences could be calculated between mean values of particulate metal concentrations in the two water bodies. Correlations between particulate metal concentrations and chlorophyll, seston, phytoplankton carbon and cell-count were calculated for both photic layer and deeper waters. Significant correlations were found only in the photic layer. The importance of standing crop for metal accumulation in seston is discussed.     

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     

Increased seawater temperature and light during early springs accelerate receptacle growth of Fucus vesiculosus in the northern Baltic proper

In many temperate estuarine areas, ongoing climatic changes are expected to lead to higher seawater temperatures, to increased inflow of freshwater and nutrients (due to increased rainfall) and to altered light conditions. During the last two decades, several winters have been unusually mild in southern Finland, and the subsequent springs have been early. This may have consequences for the seasonality of many species, including the bladder-wrack, Fucus vesiculosus, whose reproductive cycle may be especially sensitive. In the present study, F. vesiculosus receptacles from the field were weighed regularly during three warm (early) springs and compared with receptacles from three cold (late) springs. On average, the first sign of receptacle growth in the field occurred 5–6?weeks earlier and receptacles probably matured 2–3?weeks earlier during the warm springs than during the cold springs. This may be due to differences in seawater temperatures among years, but significantly heavier receptacles at 0.8?m compared to at 3.1?m further suggests the importance of light for receptacle growth as a measure of maturation, since there were no depth-related differences in seawater temperatures. Also, when plotting receptacle wet weights against seawater temperatures, the importance of light can be seen as different regression relationships (slopes and intercepts). In an outdoor aquarium experiment, the effects of temperature and light on receptacle growth were also tested, and there were significant differences between shallow and deep specimens, but temperature and light had no effects, except on initial receptacle growth. Since oogonian development, and thereby the time for zygote release, followed the receptacle growth curve, this could mean a mismatch between the timing of F. vesiculosus zygote settlement and the cover of competing filamentous algae, but such species interactions require further investigations in order to better understand their possible consequences.     

Polymorphism of mannose phosphate isomerase in North Sea and Baltic Sea populations of the amphipods Gammarus zaddachi and G. salinus

Genetic variation at the mannosephosphate isomerase (MPI) locus was examined in the euryhaline sibling species Gammarus zaddachi Sexton and G. salinus Spooner. Both crustacean amphipods share identical enzyme mobilities, following electrophoresis on vertical starch gels. The MPI locus turned out to be highly polymorphic; it is encoded by 6 alleles in G. zaddachi and 7 alleles in G. salinus. Geographic variation of allelic diversity was studied in samples from 9 G. zaddachi and 10 G. salinus populations, primarily obtained from Baltic Sea and North Sea sites. Patterns of inter- and intraspecific heterogeneity are described. Differences in allelic composition exist between Baltic and North Sea samples of G. salinus. In G. zaddachi, levels of polymorphism are higher in North Sea populations than in those from Baltic Sea areas. The significance of these findings is discussed in the light of previous biochemical genetic investigations on the population structure of the two amphipods considered.     

Ecophysiology of the benthic amphipod Monoporeia affinis in an open-sea area of the northern Baltic Sea: seasonal variations in oxygen consumption and ammonia excretion

Seasonal variations in the oxygen uptake rate ( ) and ammonia excretion rate ( ) of the benthic deposit-feeding amphipod Monoporeia affinis vere studied in an open-sea population of the northern Baltic Sea during three consecutive years (1991 to 1993). In the early summer an increase in the weightspecific by 22% (standard 1-mg individual) was registered; an even more striking change was noted in the , which showed an increase in the early summer of ca. six times the extremely low winter/spring valnes. The metabolic changes observed could not be attributed to temperature, which remained at steadystate throughout the year, nor could seasonal variations in the biochemical composition of the amphipods explain the results. Therefor, it is evident that quality-food abundance following the sedimentation of the spring phytoplankton bloom induces the observed early-summer elevations in and . In turn, the marked decrease in the towards autumn reflects the gradual depletion of quality-food in the benthic environment. Atomic O: N ratios indicated that at the sampling location, M. affinis uses lipid almost exclusively for metabolic energy throughout the year, except in early summer when the use of lipid and protein was observed to be almost equal. Results also indicated a time-lag between sedimentation of the phytoplankton bloom and metabolic response in the amphipod population.     

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.     

In-situ observations of Baltic herring (Clupea harengus membras) spawning behaviour in the Askö-Landsort area,northern Baltic proper

In a Baltic herring (Clupea harengus membras L.) spawning ground study, carried out in summer 1982 in the northern Baltic proper, spawning herring were encountered on two line transects on June 14. The spawning behaviour of minor groups as well as of a large school was studied in situ by four divers for approximately three hours. No behavioural differences or systematic coordinated behaviour between the sexes could be detected. Most of the spawning took place in a limited depth interval, 0.5 to 4 m, and Ceramium tenuicorne was the substrate most used.     

Submarine experiments on benthic colonization of sediments in the western Baltic Sea. II. Meiofauna

From July, 1972 to February, 1973, meiofauna samples were taken from submerged platforms. Substrates were gravel, fine sand, and clayey mud. The individual samples consisted of 20 cm² 100 cm³. A diver collected the samples by hand. Some days after submersion of the platforms, colonization by meiofauna began. The submerged substrates were colonized almost exclusively by nematodes and harpacticoids; gastrotrichs were never present. In general, the harpacticoid fauna can be classified into two groups: the first, confined to the mesopsammon, did not colonize the platforms; the second, not limited to the interstitial habitat, did colonize. Two harpacticoid species, Ameira parvula and Danielssenia typica, were markedly dominant. The main migration route to the submerged platform was up the securing lines.Contribution no. 35 from the Sonderforschungsbereich 95, Kiel.     

Inorganic skeleton of the demosponge Halichondria panicea. Seasonality in spicule production in the Baltic Sea

Seasonality in spicule production of the demosponge Halichondria panicea (Pallas, 1766) was studied in the Western Baltic from July 1992 to July 1993. Spicule production increased in parallel with increasing temperatures in spring, being most pronounced during summer after release of larvae. Large proportions of up to 30% immature spicules occurred in the population during skeleton production peaks, and this resulted in reduction of mean spicule dimensions by ≃20%. Effects on spicule width were more obvious than on spicule length. In winter, when wave forces were largest, higher percentages of mineral skeleton were observed: 50% of dry weight compared to 40% in summer. Enhanced spicule densities in the tissues were not attained by marked spicule production but through a reduction in organic content by shrinking. In spring, growth of body tissue was faster than the build-up of skeleton, so that the relative amount of the latter was reduced. The indirect, growth-related developments of the skeleton were inversely proportional to temperature and directly to salinity, silicate concentrations and wave energy. Spicule size and the number of immature spicules were correlated with temperature and only slightly with salinity. No correlation with oxygen concentration or pH could be detected. Received: 3 April 1997 / Accepted: 8 August 1997