| Summary: | Physical, chemical and biological properties of sea ice and underlying brackish water (0-10 m water depth) were investigated in the Bothnian Bay and at a coastal station at the entrance of the Gulf of Finland in March 2000. Sea-ice texture was analysed and vertical profiles for delta(18)O, inorganic nutrient concentrations [NO3-, NO2-, NH4+, PO43+, Si(OH)(4)] and chlorophyll a and pheopigment concentrations were obtained with a vertical resolution of 1-10 cm. Biomass estimates of bacteria, protists and metazoans were based on microscopical counts. Due to low ice bulk salinities, sea-ice brine volumes were small with a median value of only 3%. Ice thickness varied between 22 and 27 cm. Refrozen snow and granular ice dominated in the surface parts of the sea ice; columnar ice formed the bottom layers. The total biomass in the sea ice and under-ice water ranged from 2.7 mg C m(-2) to 45.1 mg C m(-2) and 36.8 mg C m(-2) to 3,874.3 mg C m(-2), respectively. Mean integrated ice values were dominated by pennate diatoms (33%), followed by centric diatoms (29%), autotrophic flagellates (23%), bacteria (8%) and heterotrophic flagellates (7%). The low integrated biomass of ice-associated metazoans (1%) consumed on average only 1.5% of the algal standing stock per day. The combined data suggest that abiotic factors (particularly nutrient availability as reflected by small brine volumes), rather than the heterotrophic component of the sympagic community, controlled autotrophic biomass in Baltic sea ice during March 2000.
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