Observed changes in pelagic and hydrographic conditions of the Baltic Sea.
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In this study, we measured depth-dependent benthic microalgal primary production in a Bothnian Bay estuary to estimate the benthic contribution to total primary production. In addition, we compiled data on benthic microalgal primary production in the entire Baltic Sea. In the estuary, the benthic habitat contributed 17 % to the total annual primary production, and when upscaling our data to the entire Bothnian Bay, the corresponding value was 31 %. This estimated benthic share (31 %) is three times higher compared to past estimates of 10 %. The main reason for this discrepancy is the lack of data regarding benthic primary production in the northern Baltic Sea, but also that past studies overestimated the importance of pelagic primary production by not correcting for system-specific bathymetric variation. Our study thus highlights the importance of benthic communities for the northern Baltic Sea ecosystem in general and for future management strategies and ecosystem studies in particular.
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This paper focuses on dynamics and changes in coastal shallow water ecosystem in the eastern Baltic Sea that is main habitat for representatives of malacostracan crustaceans and the most sensitive ecotone subjected to eutrophication and climate changes. Global warming has facilitated the rapid dispersal of everythermic crustaceans from Ponto-Caspian and Mediterranean basins to the Baltic Sea and further their high ecological significance in coastal communities. Climatic changes influence primarily on water temperature, hydrology and nutrient balance that can be environmental limits leading to deceleration of species number and abundance and change in trophic links within community. It analyses history, current distribution of malacostracan crustaceans and shifts in recipient communities in the north-eastern part (primarily Gulf of Finland) and compares with similar patterns in south-eastern part of the Baltic Sea (mainly Curonian and Vistula lagoons). Species and community responses to climate influence differ strongly between cold and warm years.
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Biogeochemical Cycle
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The most seriously threatened European seas are the North Sea, the Baltic and the Black Sea. Here the rapid degradation of these marine environments is examined comparatively to identify similarities and differences in the driving forces and responses of the marine systems. Although it is difficult to distinguish between anthropogenic changes and those due to natural climate fluctuations, an attempt is made here to define possible effects based on a multidisciplinary mix of recent observations and modelling. The systems studied range from almost totally enclosed domains to marginal seas adjoining a large ocean body, and from shallow, dissipative and tidally dominated shelf regions to deep, relatively stagnant basins with adjoining energetic shelf regions, and from completely anoxic to oxygen-saturated chemical environments. Climate control appears as a background theme, while the most significant human effect is eutrophication, which is leading to rapid changes in these systems.
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Eutrophication of natural systems is one of the most relevant problems confronting the human society today. Compared to terrestrial and limnological systems, however. the marine environment is affected to a minor degree only, except for coastal waters. Based on the GDR data covering the period of 1976-1988, trend analyses for chlorophyll, primary production, zooplankton biomass, and water transparency have been carried out for the Mecklenburg Bight and different areas of the Baltic Proper. As expected from the increase in nutrient levels, observed over several years, increasing trends in some pelagic biological variables were noticeable. The trends in chlorophyll content were significant (95% probability level) for all the areas investigated. Primary production, too, showed a tendency to increase, the trend, however, not being significant in each subarea. Almost no changes could be observed in the zooplankton-related variables. The results confirm earlier findings and substantiate an assumption that the process of eutrophication in the Baltic is continuing. All the data sets show a high interannual variability which can be partly explained by meteorological and oceanological conditions and by variability inherent in any ecosystem. Although the nutrient levels in winter have ceased to increase, the nutrient input to the system from land via rivers and from the air has not decreased. This impact continues all year round and is particularly relevant in early summer, a larger input of fresh water in that particularly productive season making it still more important. It seems also very likely that the ammonia input via the atmosphere is still highly underestimated. Some other features of Baltic, apart from the nutrient input, support the notion of the continuing eutrophication, e.g., the summer stable three-layer stratification enhancing stagnation and disturbing the phosphorus-nitrogen relationship. The surplus of phosphates gives rise to summer blooms of nitrogen-fixing cyanobacteria and thus to an additional input of nitrogen to the ecosystem. It seems also very likely that the current stagnation observed in the Baltic for thirteen years, by virtue of decreased salinity and weakened permanent halocline, is enhancing the productivity of the ecosystem.
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Ocean Acidification
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In marine and brackish waters, the acidity of the water is mainly controlled by the inorganic carbon system (see Fact Box 1).Anthropogenic CO 2 emissions will -unless reduced -gradually move the Baltic Sea towards a state where acidification becomes harmful for some organisms.The effect is caused by the uptake of CO 2 in the water, but can be further enhanced by other climate effects, such as increased water temperature and a possible freshening of the sea water.This is expected to lead to changes in species composition, both directly (competitive advantages/ disadvantages) and indirectly (altered food availability), potentially influencing ecosystem functioning.Coastal seas, such as the Baltic Sea, are highly influenced by their catchment areas, which means that pH dynamics is generally more complex than in the open ocean.The reason is that pH, in addition to the response to increasing CO 2 , is also influenced by changes in hydrology and changes in the supply of carbon and nutrients.High-productive waters typically experience larger seasonal pH variations than low-productive waters, with higher pH peaks in spring/summer and also a more pronounced pH decline in winter.The comparatively weak long-term acidification trend can be masked behind much larger short-term variations.Furthermore, since acidification is a slow process, organisms can to varying degrees adapt to the changes. Model simulations performed as a part of the OMAI (Operational MarineAcidification Indicator) project indicate that the expected acidification in the Baltic Sea generally follows the same trajectory as the open oceans, with a pH decline of almost 0.4 by year 2100 and a further decline of 0.3 by year 2300 in the worst-case scenario.Due to large regional differences in the area, the annual mean pH in the Bothnian Bay might decline from present-day 7.8 to 7.4 by year 2100, whereas in the Gotland Sea and Southern Kattegat mean pH could decline from present-day 8.1 to 7.7.The degree of eutrophication has a comparatively small effect on the annual mean pH, but on the other hand a considerable impact on the seasonal amplitude and thus minimum and maximum values.The complex situation in the Baltic Sea gives a strong incentive to improve the temporal and spatial coverage of acidification monitoring.This would broaden the
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