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Resumen:
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The vernal phytoplankton development and its regulation by meteorological, hydrographical and chemical factors were investigated by means of frequent sampling in March-May. The total phytoplankton community, measured as chlorophyll a and biomass, and the dominant taxa were monitored, together with the hydrographical and nutrient conditions. Besides a flagellate bloom below the ice in March, the vernal high production stage included seven different peaks of diatoms and dinoflagellates, the first in connection with the break-up of the ice and strong turbulence in mid April, and the last ones in late May, caused by outflow of oligohaline surface water rich in nutrients from Pojo Bay. The study area in the outer archipelago has a strongly fluctuating environment, in which upwelling, mixing and outflow of surface water from Pojo Bay contributed the nutrients enabling several successive phytoplankton peaks, composed of a mixture of autochthonous and allochthonous assemblages at different successional stages. After the break-up of the ice, the microphytoplankton chiefly consisted of Arctic, marine diatoms and Gonyaulax catenata. The outflow of oligohaline surface water in May was associated with the dominance of Diatoma elongatum, euryhaline marine diatoms, and in addition several freshwater diatoms. Nanoflagellates and picoplankton (Nannochloropsis sp.) were abundant during the whole study period, although their contribution to the total biomass was small during the diatom peaks. The proportion of colourless nanoflagellates increased markedly when the phytoplankton assemblage declined in late May. In comparison with the conditions in 1969 and 1970, the nutrient level of the Baltic surface water had increased markedly in 1984. The silicate level, which in 1969/70 decreased in May to ca. 150 µg SiO₄-Si∙l¯¹ now decreased to the detection limit. This points to raised nitrate and phosphate levels, and increased diatom production
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