Population dynamics and ecological impacts of the cirriped Balanus improvisus, the polychaete Marenzelleria neglecta and the cladoceran Cercopagis pengoi were investigated in the north-eastern Baltic Sea. After an increase during the first decade of invasion, the density of M. neglecta and C. pengoi declined afterwards. The studied abiotic environmental variables did not explain the interannual variability in the seasonal cycles of M. neglecta and C. pengoi indicating that the species are at their initial phase of invasion. The population dynamics of B. improvisus was best described by water temperature. B. improvisus promoted the growth of the green alga Enteromorpha intestinalis. M. neglecta enhanced the content of sediment chlorophyll a and reduced growth and survival of the polychaete Hediste diversicolor and growth of the amphipod Monoporeia affinis. Concurrent with the invasion of C. pengoi the abundance of small-sized cladocerans declined, especially above the thermocline. C. pengoi had become an important food for nine-spined stickleback, bleak, herring and smelt.

Studies on the effects of increasing acidification on marine communities have been previously mostly carried out in truly marine areas whereas brackish-water ecosystems such as the Baltic Sea have been less studied. The current study analyses how acidification induced by elevated atmospheric carbon dioxide affects the photosynthetic net production of different macroalgal species in the brackish Baltic Sea. Research methods include sets of laboratory and field experiments carried out in shallow coastal brackish waters. The aim of the laboratory experiments was to develop the necessary techniques and experience for the mesocosm experiments. Laboratory experiments were carried out using specimens of the red alga Furcellaria lumbricalis collected from Kakumäe Bay. The mesocosm experiments were conducted in Kôiguste Bay during the field season of 2011. Separate mesocosms were operated in each set with different CO2 concentrations and a control treatment in natural conditions. Field experiments were carried out with three species representing three different morphological and ecological groups: Ulva intestinalis, a fast-growing green alga; Fucus vesiculosus, a perennial brown alga with a slow metabolism; and Furcellaria lumbricalis, a perermial red alga. Photosynthetic activity was used as the response variable. In the laboratory decreasing pH increased the net primary production of F. lumbricalis with the lowest net primary production values measured at pH 8.0 and the highest at pH 6.5. Results of the field experiments indicated that increased CO? levels in seawater favoured photosynthetic activity of the macroalgae U. intestinalis and F. lumbricalis, but F. vesiculosus showed no response to elevated CO2. Elevated CO2 levels are suggested to favour the production of fast-growing filamentous species, which thus may indirectly enhance the effect of eutrophication in the shallow coastal brackish waters.

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An analysis of plankton seasonal succession in large shallow eutrophic lake Vdrtsjlrv (270 km', mean depth 2.8 m, max. depth 6 m) is presented. Weekly samples for 1995 have been analysed using the PEG model approach. In winter, light was the main factor controlling phytoplankton growth. In early spring phytoplankton was mainly resource-controlled, competition for phosphorus being the main driving force. Ciliates (Parudileptus sp., Strobilidiiim sp. and Vorricella sp.) were the first herbivores which started to increase in April causing a twofold decline of phytoplankton biomass. The annual maximum of primary production (PP) in early May was probably caused by soluble reactive phosphorus (SRP), regenerated by herbivores, and stirred up from bottom sediments as a result of strong wind stress. This primary production peak provided substrate for further increase of bacterial biom food supply supported the development of the second spring peak of herbivores nile copepods) which was followed by the second modest "clear water phase" in late May. Silicon was depleted by the end of May causing a strong decrease in primary production of the diatom-dominated community, whereas the biomass of the cyanophytes increased under the improved nutrient conditions. Some weeks later, inorganic N was depleted and the period of N limitation with the appearance of Nfixing cyanophytes began. The ciliate collapse at the beginning of June coincided with the start of the cladoceran development and with the increase of other metazooplankton groups. This explains the further decrease of the biomass of phytoplankton and bacteria in spite of their high production. Beginning from late June, silicon appeared again and SRP started to occur periodically, while inorganic N remained close to zero until November. During this period, phytoplankton development relied to a great extent on the N-fixation and N-regeneration potential. The collapse of the ciliate community in September removed the top-down control from bacteria and their biomass increased, while the development of cladocerans still suppressed phytoplankton biomass in spite of a quite high PP. In October phytoplankton biomass and chlorophyll o (Chlu) increased, SRP was completely depleted by the middle of October reflecting a slow regeneration due to the declined activity of zooplankton in cold water. In November nitrates appeared again, and silicon reached the same level as in spring. The biomass of N-fixing Aphcrnizornenon skitjae decreased while Limnorhrix redekei and L. plunctonica were quite abundant together with diatoms.