Food web components, and physical and chemical properties of Baltic Sea ice

Abstract

Fast ice and underlying water of the northern Baltic Sea were sampled at 1 wk intervals during winter 2000 in a coastal location in SW Finland. To investigate seasonal succession and interactions inside the food web and between the food web and physical and chemical properties of the ice, several biological (chlorophyll a, abundance of bacteria, flagellates and ciliates, bacterial cell volume and leucine incorporation, POC), chemical (total and dissolved nutrients) and physical (salinity, temperature) parameters were measured. The first ice-algae bloom with chlorophyll a (chl a) concentrations of up to 18.5 μg l-1 occurred in mid-January, followed by the main ice-algae bloom in March and a heterotrophic postbloom situation shortly before the ice deteriorated in April. The ice-organism assemblage was autotrophy-dominated; during the entire ice-covered period, ice algae formed on average 82 ± 10 % (mean ± SD) of the organism biomass. The largest heterotrophic groups by biomass were bacteria (16.7 ± 9.4%), ciliates and metazoans (4.4 ± 2.7 and 4.4 ± 4.8%, respectively). The ice food-web was characterized by the importance of ice bacteria and the presence of a potential microbial loop (bacterial secondary production accounting for a mean of 27.1 ± 19.3 μmol C m-2 d-1), grazing control on bacteria and flagellates, and also possible 'shortcuts' such as ciliate grazing on bacteria and metazoan herbivory. An ice-sheet warming event in February clearly affected the physical and chemical properties of the ice as well as the structure and function of ice-organism assemblages. Ice organisms probably controlled dissolved nutrient concentrations inside the ice by uptake and regeneration.