Oxygen and salinity characteristics of predator–prey distributional overlaps shown by predatory Baltic cod during spawning

In the distributional overlap volume of Baltic cod Gadus morhua and its prey, studied in the Bornholm Basin in the southern Baltic Sea, only a fraction of the sprat Sprattus sprattus population vertically overlapped with the Baltic cod population. Sprat occurred in the intermediate water, in the halocline and in the bottom water, while herring Clupea harengus and Baltic cod occurred exclusively in the halocline and in the bottom water. Only parts of the sprat population were hence accessible for Baltic cod, and only a fraction of the sprat had access to the Baltic cod eggs below the halocline. Baltic cod–clupeid overlap volumes appeared to be determined by salinity stratification and oxygenation of the bottom water. Hydrography time series were used to estimate average habitat volumes and overlap from July to September in 1958–1999. In the 1999 survey spawning Baltic cod had greater ratios of empty stomachs and lower average rations than non-spawning Baltic cod. The average ration for Baltic cod caught within 11· 4 m from the bottom (demersal) did not differ from the average ration of Baltic cod caught in shallower waters (pelagic), because spawning and non-spawning Baltic cod in both strata were caught at equal rates. The diet of the Baltic cod caught demersally contained more benthic invertebrates, especially Saduria entomon, but Baltic cod caught pelagically also had fresh benthic food in their stomachs, indicating vertical migration of individual fish.     

The growth of pikeperch (Sander lucioperca L.) and perch (Perca fluviatilis L.) under different water temperature and salinity conditions in the Curonian Lagoon and Lithuanian coastal waters of the Baltic Sea

Pikeperch and perch perform seasonal migrations between the Curonian Lagoon and the coastal waters of the Baltic Sea. The Curonian Lagoon is a freshwater basin, while salinity in the coastal waters varies between of 4.9–6.8 psu. In the Curonian Lagoon water temperature is generally higher than in the coastal waters. Field studies of growth and condition characteristics of pikeperch and perch were carried out in these water bodies with the aim to estimate growth differences of the two fish species under different salinity and temperature conditions. Additionally, an experimental study of the impact of salinity on the growth of perch young-of-the-year (YOY) was performed to test the hypothesis that a brackish environment positively influences percids. Field observations revealed that body length, condition factor, fatness coefficient and fat content in muscles were significantly higher in individuals inhabiting the cooler, brackish waters of the Baltic Sea than in individuals inhabiting the Curonian Lagoon. A positive effect of low salinity on growth was also established in the experimental study. Hence, the study results suggest that under certain temperature conditions, brackish waters beneficially affect the growth of pikeperch and perch.     

Changes of the macrozoobenthos at 3 monitoring stations in the western Baltic Sea and the Sound

The soft bottom fauna of the western Baltic Sea and the Sound has been sampled and analysed every year since 1979 under the Baltic Monitoring Programme. Furthermore, benthos studies have been carried out in the area at intervals from as far back as 1871. In the area a distinct halocline exists between the overlying low saline Baltic water and the high saline North Sea water.The variation in the species richness, abundance and biomass of the soft bottom fauna is mainly related to 3 abiotic factors.First, many species live at the limit of their distribution. The low salinity of the Baltic Sea prevents their penetration into the Baltic proper. However, the marine species may be able to survive and grow but not to reproduce. Consequently, the population will depend on an influx of larvae for it's survival.Second, the distinct halocline prevents the transport of oxygen to the deeper parts of the Baltic Sea. Oxygen will be supplied under special weather conditions where inflow of high-saline oxygen rich North Sea water occur. The incidences of salt water inflow have increases in the last four decades.Third, an increasing load of the Baltic Sea with nutrients and organic matter has influenced the fauna. The result have been an increased biomass of the benthos above the halocline. Below the halocline the result has been a decrease in the biomass and a change in the species composition.     

The gulf of Bothnia: The northernmost part of the Baltic Sea

Summary The Baltic Sea, one of the largest brackish water areas in the world, can be characterized as a young, cold sea containing an impoverished ecosystem due to salinity stress. The present Baltic Sea was formed as late as 2000 to 2500 years ago when the Danish sounds became more narrow and shallow. The inflow of freshwater from the surrounding land areas caused the Baltic to gradually attain its brackish character. Today the Baltic covers an area of some 366,000 km² as a series of basins separated by shallower areas and filled with about 22,000 km³ of brackish water. These basins are, from north to south, the Gulf of Bothnia, the Gulf of Finland, the Gotland Sea and the Bornholm Sea. The climate gradient ranges from almost arctic conditions in the extreme north to a more maritime climate in the southern parts. The North Sea salt water is connected to the Baltic through the shallow Kattegat and the sills in the Danish sounds. The inflow of salt water occurs in two different ways,viz. as a continuous flow along the bottom due to the salinity gradient and as pulses of salt water generated by the distribution of air pressure and the direction of the wind. The freshwater input (500 km³) from mainly the large rivers equals roughly the net outflow and stresses the south-bound current along the Swedish coast that also compensates for the salt water inflow. Tidal movements can be seen in the southern Baltic, but are of minor importance for the system. The residence time of the total water mass is 25 years and the hydrographical conditions within the different basins are stable and dominated by a permanent halocline, and a thermocline developing every spring. The salinity ranges from about 1–2 per mille in the innermost part of the Gulf of Bothnia to 10–15 per mille in the Bornholm Sea. Total vertical mixing takes place during winter in at least the northern parts of the sea. Due to the climate-gradient, the ice condition differs from about four months of total ice-cover in the inner parts of the Gulf of Bothnia to one month or less of coastal ice in the southern part of the Baltic. Thus, the seasonal effect is more pronounced in the northern parts.The living systems of the Baltic are reduced and adapted to these varying conditions. When comparing the deeper soft bottoms of the Gulf of Bothnia to the rest of the Baltic, the following pattern can be seen. The pelagic primary productivity increases by a factor 6 from north to south. The southern parts of the sea show a pronounced spring peak, while in the north the spring development is delayed or replaced by a summer maximum. The total increase of the macrofauna biomass is striking, from about 1 g.m^–2 (w.wt) in the north to 100 g.m^–2 (w.wt) or more in the south. The meiofauna and the zooplankton biomasses show less variability. The meiofauna increases by a factor of 2–4, giving a biomass of about twice that of the macrofauna in the northernmost part. The extremely low salinity of this area causes the exclusion of bivalves (filter-feeders) from the fauna. Available data, pooled with the high metabolic rate of the meiofauna, roughly follow the changes in primary productivity within the Baltic Sea. The changing ratio of macro- to meiofauna, as well as results from intensive studies of the macrobenthic amphipodPontoporeia affinis (Lindström), suggest that the macrofauna is regulated mainly by food limitation and that the benthic and pelagic systems are closely coupled.     

Distribution, sex ratio and growth of Carassius gibelio (Bloch) in coastal and inland waters of Estonia (north-eastern Baltic Sea)

Gibel carp Carassius gibelio (Bloch) was first introduced into fish ponds and small lakes of Estonia in 1948–49, and first detected in Estonian brackish waters (Gulf of Riga) in 1985. Since the mid‐1990s, the species has spread along the entire Estonian Baltic coastline. Growth rate in the brackish water population does not differ much from freshwater populations, but the freshwater populations are gynogenetic (or show high dominance of females) in contrast to the Baltic Sea population, which presents a normal sex ratio. The recent explosion of this species in the Baltic Sea could be explained by unusually warm summers during the 1990s and by the low abundance of predatory fish.     

The ability of Baltic cod eggs to maintain neutral buoyancy and the opportunity for survival in fluctuating conditions in the Baltic Sea

In the brackish water of the Baltic Sea successful spawning of Baltic cod Gadus morhua is restricted to the Bornholm, Gdansk and Gotland basins below the halocline, occurring at 50–80 m depth. Due to irregular mixing of the deep water, stagnant conditions occur regularly accompanied with unfavourable oxygen conditions. In avoiding stressful oxygen conditions maintenance of egg buoyancy is considered a major limiting factor for successful spawning of Baltic cod. Batches of eggs were incubated experimentally in a density gradient column. Egg specific gravity changed during development, decreasing from the time of gastrulation, then increasing prior to hatching. The changes in specific gravity varied among egg batches from different females and were related to egg quality, egg size and ambient salinity. Eggs achieve different specific gravity depending on incubation salinity. Initial egg specific gravity together with the ability of eggs to gain and maintain buoyancy up to hatching, determine larval specific gravity and the depth where hatching will occur, and thus opportunities for larval survival, avoiding stressful oxygen conditions and developing at favourable feeding conditions.     

Seasonal variation in the mode of reproduction of Ulva intestinalis in a brackish water environment

We explored the reproductive modes of Ulva intestinalis in the inner part of the Baltic Sea during three consecutive years by using five microsatellite loci to estimate the relative abundance of diploid sporophytes and haploid gametophytes. Our results suggest that both diploid sporophytes and haploid gametophytes occur regularly in the Baltic Sea. The ratio of haploid to diploid individuals changes with seasons. Sporophytes are more abundant than gametophytes throughout the year, but the proportion of haploids increases from 10% in early summer to 35% in September. The over-wintering takes primarily place as diploid spores released by sporophytes. The sporophytes appear to reproduce both sexually and asexually in the Baltic Sea, since clones were found for this life phase. The fraction of individuals which belonged to an apparent diploid clone was higher in spring (62%) than in autumn (33%). We also found evidence for asexual clones in haploid gametophytes. The presence of both diploid and haploid individuals and the pattern of genetic and genotypic diversity provide evidence of sexual reproduction in the Baltic Sea. Thus the sporophytes and gametophytes do not function as two reproductively separate units. Compared with many other algal species with a reduced reproductive cycle in low salinity, U. intestinalis differs by having a multitude of reproductive modes also in the brackish water Baltic Sea, which can in part explain the dynamic propagation and high adaptability of the species.     

Diversity of Aphanizomenon flos-aquae (cyanobacterium) populations along a Baltic Sea salinity gradient

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.     

Seasonal variation in the mode of reproduction of Ulva intestinalis in a brackish water environment

We explored the reproductive modes of Ulva intestinalis in the inner part of the Baltic Sea during three consecutive years by using five microsatellite loci to estimate the relative abundance of diploid sporophytes and haploid gametophytes. Our results suggest that both diploid sporophytes and haploid gametophytes occur regularly in the Baltic Sea. The ratio of haploid to diploid individuals changes with seasons. Sporophytes are more abundant than gametophytes throughout the year, but the proportion of haploids increases from 10% in early summer to 35% in September. The over-wintering takes primarily place as diploid spores released by sporophytes. The sporophytes appear to reproduce both sexually and asexually in the Baltic Sea, since clones were found for this life phase. The fraction of individuals which belonged to an apparent diploid clone was higher in spring (62%) than in autumn (33%). We also found evidence for asexual clones in haploid gametophytes. The presence of both diploid and haploid individuals and the pattern of genetic and genotypic diversity provide evidence of sexual reproduction in the Baltic Sea. Thus the sporophytes and gametophytes do not function as two reproductively separate units. Compared with many other algal species with a reduced reproductive cycle in low salinity, U. intestinalis differs by having a multitude of reproductive modes also in the brackish water Baltic Sea, which can in part explain the dynamic propagation and high adaptability of the species.     

Effects of low salinity on settlement and strobilation of scyphozoa (Cnidaria): Is the lion’s mane <Emphasis Type="Italic">Cyanea capillata</Emphasis> (L.) able to reproduce in the brackish Baltic Sea?

Several species of scyphozoan medusae occur in river estuaries and other brackish waters but it is often unknown if the planulae settle and the scyphopolyps reproduce in those low-salinity waters. In the present study, scyphozoan species from the German Bight (North Sea) were tested in laboratory experiments to investigate their tolerance of low salinity. Planula larvae released from medusae in salinity 32 were still active after the salinity was reduced to 10 (Cyanea capillata, Cyanea lamarckii) and to 7 (Chrysaora hysoscella) in laboratory treatments. Planulae did not settle on the undersides of floating substrates when salinity was reduced to <20. By contrast, planulae released from C. capillata medusae in Kiel Bight (western Baltic Sea) in salinity 15 developed into polyps in laboratory cultures. Polyps reared from planulae in salinity 36 survived a reduction to 12 (C. capillata, C. lamarckii) and to 8 (Aurelia aurita). Polyps of all tested species strobilated and released young medusae (ephyrae) in salinity 12. These results show a high tolerance of planulae and polyps to low salinity, indicating their possible occurrence in estuaries and brackish waters. In addition to laboratory observations, young C. capillata ephyrae were collected in the western Baltic Sea (Kiel Bight) in salinity 15, which indicates that they were probably released by a local polyp population. We suggest that the polyps of the painfully stinging lion’s mane, C. capillata, may be more widespread in the Baltic Sea than previously assumed and that the occurrence of the medusae may not only depend on inflow of water masses from the North Sea.     

The Native Bacterioplankton Community in the Central Baltic Sea Is Influenced by Freshwater Bacterial Species

The Baltic Sea is one of the largest brackish environments on Earth. Despite extensive knowledge about food web interactions and pelagic ecosystem functioning, information about the bacterial community composition in the Baltic Sea is scarce. We hypothesized that due to the eutrophic low-salinity environment and the long water residence time (>5 years), the bacterioplankton community from the Baltic proper shows a native “brackish” composition influenced by both freshwater and marine phylotypes. The bacterial community composition in surface water (3-m depth) was examined at a single station throughout a full year. Denaturing gradient gel electrophoresis (DGGE) showed that the community composition changed over the year. Further, it indicated that at the four extensive samplings (16S rRNA gene clone libraries and bacterial isolates from low- and high-nutrient agar plates and seawater cultures), different bacterial assemblages associated with different environmental conditions were present. Overall, the sequencing of 26 DGGE bands, 160 clones, 209 plate isolates, and 9 dilution culture isolates showed that the bacterial assemblage in surface waters of the central Baltic Sea was dominated by Bacteroidetes but exhibited a pronounced influence of typical freshwater phylogenetic groups within Actinobacteria, Verrucomicrobia, and Betaproteobacteria and a lack of typical marine taxa. This first comprehensive analysis of bacterial community composition in the central Baltic Sea points to the existence of an autochthonous estuarine community uniquely adapted to the environmental conditions prevailing in this brackish environment.     

Current Trends in Hydrographic and Chemical Parameters and Eutrophication in the Baltic Sea

Analysis of phosphate and nitrate data recorded in the winter surface layer of the Baltic Sea from 1958 to 1989 indicate positive overall trends, although the rate of accumulation has decreased since 1978. The long term accumulation of these algal nutrients in the oxic deep water below the halocline has also decreased since 1977. Under anoxic conditions, however, the trend shown by the phosphate concentration is distinctly positive in recent times owing to phosphate remobilization from the sediment. The salinity has decreased in the surface water and even more rapidly below the halocline in the central Baltic basins during the period marked by the absence of major salt water inflows since 1977. The oxygen conditions have also deteriorated in the near-bottom water layer during this time as shown by the considerable rise in the hydrogen sulphide concentrations. These changes, including the recent phosphate accumulation under anoxic conditions, can thus be considered natural rather than anthropogenic. The impact of long-term variations in nutrients and hydrographic parameters is discussed with respect to the development of Baltic fish stocks.     

Specific gravity and vertical distribution of sprat eggs in the Baltic Sea

During peak spawning of sprat Sprattus sprattus in the Baltic Sea in May–June egg specific gravity averaged ± s . d . 1·00858 ± 0·00116 g cm⁻³ but was significantly higher in the beginning and significantly lower towards the end of the spawning season. A close relationship was found between egg diameter and egg specific gravity ( r ² = 0·71). This relationship, however, changed during the spawning season indicating that some other factor was involved causing the decrease in specific gravity during the spawning period. The vertical egg distribution changed during the spawning season: eggs were distributed mainly in the deep layers early in the season, occurred in and above the permanent halocline during peak spawning, and above the halocline towards the end of the spawning season. Consequently, poor oxygen conditions in the deep layers and low temperatures in layers between the halocline and the developing thermocline may affect egg development. Thus, opportunities for egg development vary over the spawning season and among spawning areas, and depending on frequency of saline water inflows into the Baltic Sea and severity of winters, between years.     

Sedimentation of Nodularia spumigena and distribution of nodularin in the food web during transport of a cyanobacterial bloom from the Baltic Sea to the Kattegat

Nodularia spumigena is a toxic cyanobacteria that blooms in the Baltic Sea every year. In the brackish water of the Baltic Sea, its toxin, nodularin, mainly affects the biota in the surface water due to the natural buoyancy of this species. However, the fate of the toxin is unknown, once the cyanobacteria bloom enters the more saline waters of the Kattegat. In order to investigate this knowledge gap, a bloom of N. spumigena was followed during its passage, carried by surface currents, from the Baltic Sea into the Kattegat area, through the Öresund strait. N. spumigena cells showed an increased cell concentration through the water column during the passage of the bloom (up to 130 10³ cells ml⁻¹), and cells (4.2 10³ cells ml⁻¹) could be found down to 20 m depth, below a pycnocline. Sedimentation trap samples from below the pycnocline (10–12 m depth) also showed an increased sedimentation of N. spumigena filaments during the passage of the bloom. The toxin nodularin was detected both in water samples (0.3–6.0 μg l⁻¹), samples of sedimenting material (a toxin accumulation rate of 20 μg m^-2 day⁻¹), zooplankton (up to 0.1 ng ind.⁻¹ in copepods), blue mussels (70–230 μg kg⁻¹ DW), pelagic and benthic fish (herring (1.0–3.4 μg kg⁻¹ DW in herring muscle or liver) and flounder (1.3-6.2 μg kg⁻¹ DW in muscle, and 11.7-26.3 μg kg⁻¹ DW in liver). A laboratory experiment showed that N. spumigena filaments developed a decreased buoyancy at increased salinities and that they were even sinking with a rate of up to 1,7 m day⁻¹ at the highest salinity (32 PSU). This has implications for the fate of brackish water cyanobacterial blooms, when these reach more saline waters. It can be speculated that a significant part of the blooms content of nodularin will reach benthic organisms in this situation, compared to blooms decaying in brackish water, where most of the bloom is considered to be decomposed in the surface waters.     

REPRODUCTIVE SUCCESS OF FUCUS VESICULOSUS (PHAEOPHYCEAE) IN THE BALTIC SEA

Marine organisms colonizing brackish habitats such as the Baltic Sea must cope with the negative effects of low salinities on reproductive success because these may reduce gamete viability and/or increase polyspermy. Reproductive characteristics of the marine seaweed Fucus vesiculosus L. were studied in several brackish habitats, particularly in the northern Baltic Sea, to understand its ability to reproduce where few other marine species survive. Polyspermy and fertilization success were variable at the boundary of the continuous distribution of F. vesiculosus in the Baltic Sea, and polyspermy was high (10%–30%) when fertilization was successful. A strong female bias (80%–86%, ca. 5.5:1) was found at the northernmost limit of Baltic F. vesiculosus. Electrophysiological studies showed that many eggs have a high input resistance (519 ± 150 MΩ[mean ± SE, n = 14] at Drivan, 1995), which may be helpful in preventing polyspermy in this brackish habitat. The polyspermy block remains sodium-dependent in the northern Baltic. Sperm bound quickly to northern Baltic eggs in natural water, but fertilization was delayed compared to marine F. vesiculosus. A subset of northern Baltic eggs studied during an optimal reproductive period (7–11 July 1995) had a membrane potential (E_m) of ca. −100 mV and an effective fertilization potential (FP) of ca. 2 min with a plateau of −25 mV, but repolarized too rapidly for the FP to be protective. Pronuclear migration and cell wall secretion occurred more slowly in Baltic than in marine zygotes. The reproductive success of theseboundary populations may be dependent upon windows of opportunity when there are favorable combinations of the levels of salinity, water motion, population density, and sex ratio. These factors and the short duration of the reproductive season in the northern Baltic Sea may result in reproductive failure in some years.     

Diversity of Aphanizomenonflos-aquae (Cyanobacterium) Populations along a Baltic Sea Salinity Gradient

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.     

Distribution patterns of epilithic diatoms along climatic,spatial and physicochemical variables in the Baltic Sea

The species richness and community composition of the diatom communities were studied in the Baltic Sea, Northern Europe, to enhance knowledge about the diversity of these organisms in a brackish water ecosystem. Many organisms in the Baltic Sea have been studied extensively, but studies investigating littoral diatoms are scarce. The goal of this study was to examine the importance of climatic, spatial and water physicochemical variables as drivers of epilithic diatoms in the Gulf of Finland and the Gulf of Bothnia. The variation in species richness was best explained by pH, total phosphorus and total nitrogen. Redundancy Analysis indicated that the most important factors correlating with species composition were air temperature, silicon, total phosphorus, water temperature, salinity and pH. Variation Partitioning showed that the species composition was mostly affected by climatic and spatial variables, whereas physicochemical variables had little impact. However, the strongest factor was the combined influence of climatic, spatial and physicochemical variables. The results suggest that diatom species richness in the northern Baltic Sea is primarily regulated by local factors, while climatic and spatial variables have little impact on richness. Species composition is mostly affected by climatic and spatial variables. We conclude that understanding the distribution patterns of Baltic Sea diatoms requires the inclusion of climatic, spatial and water chemistry variables.     

Baltic Sea eutrophication: area-specific ecological consequences

Eutrophication of coastal waters is a global phenomenon. the amounts of nutrients in the brackish water of the Baltic Sea have increased several times during the last century, with severe ecological effects on the biota. With the increasing environmental problems caused by nutrient over-enrichment, public awareness to the problem has also risen. The Baltic Sea cannot be regarded as a uniform water mass, and area-specific ecological responses can be described. Changes in and detection of eutrophication-related parameters are discussed in relation to a generalized conceptual eutrophication model for the Baltic Sea. The cascading trophic and ecosystem-responses to eutrophication in 9 different sub-regions of the Baltic Sea are illustrated and discussed. The results clearly show the need not only for a common remedy for the Baltic Sea, but primarily show the importance of regional ecological assessment in relation to basin-wide eutrophication.