Jonna Piiparinen

Role of Sea-ice Biota in Nutrient and Organic Material Cycles in the Northern Baltic Sea

Abstract This paper compiles biological and chemical sea-ice data from three areas of the Baltic Sea: the Bothnian Bay (Hailuoto, Finland), the Bothnian Sea (Norrby, Sweden), and the Gulf of Finland (Tvärminne, Finland). The data consist mainly of field measurements and experiments conducted during the BIREME project from 2003 to 2006, supplemented with relevant published data. Our main focus was to analyze whether the biological activity in Baltic Sea sea-ice shows clear regional variability. Sea-ice in the Bothnian Bay has low chlorophyll a concentrations, and the bacterial turnover rates are low. However, we have sampled mainly land-fast level first-year sea-ice and apparently missed the most active biological system, which may reside in deformed ice (such as ice ridges). Our limited data set shows high concentrations of algae in keel blocks and keel block interstitial water under the consolidated layer of the pressure ridges in the northernmost part of the Baltic Sea. In land-fast level sea-ice in the Bothnian Sea and the Gulf of Finland, the lowermost layer appears to be the center of biological activity, though elevated biomasses can also be found occasionally in the top and interior parts of the ice. Ice algae are light limited during periods of snow cover, and phosphate is generally the limiting nutrient for ice bottom algae. Bacterial growth is evidently controlled by the production of labile dissolved organic matter by algae because low growth rates were recorded in the Bothnian Bay with high concentrations of allochthonous dissolved organic matter. Bacterial communities in the Bothnian Sea and the Gulf of Finland show high turnover rates, and activities comparable with those of open water communities during plankton blooms, which implies that sea-ice bacterial communities have high capacity to process matter during the winter period.

Life associated with Baltic Sea ice

1. The formation of sea ice impacts directly on the physical dynamics of water masses (e.g. wind stress at the sea surface) and air-sea exchange processes (e.g. vertical heat fluxes). 2. The annual cycle of formation, consolidation and melting of sea ice has a major influence on the ecology of both the benthic and pelagic components of the Baltic Sea ecosystem. 3. There is considerable inter-annual variation in the extent of sea ice in the Baltic Sea and thus in the size of the habitat for sympagic (ice-associated) microbial and metazoan communities as well as for larger organisms living on the ice, notably the ringed seal. 4. There is a pronounced gradient in ice characteristics, from more saline ice in the south of the Baltic Sea to freshwater ice in the north. The former is more porous and supports more ice-associated biology than the latter. 5. The Baltic sympagic communities consist mainly of prokaryotic and eukaryotic microbes (bacteria, diatoms, dinoflagellates, flagellates), ciliates and rotifers. These communities are recruited from the plankton when the ice forms, followed by an ice-adapted successional pattern with an expansion of substrate-bound pennate diatoms, which does not occur in the seawater beneath the ice. 6. The sea-ice food webs inside the ice are truncated compared to the open-water food webs because organisms larger than the upper size limit of the brine channels are lacking in the internal sympagic communities. 7. Global climate change decreases the extension and thickness of the sea ice as well as the length of the ice season, and therefore the seasonal effects that sea ice has on the Baltic Sea winter-spring ecosystem dynamics.

Bacterial communities in Arctic first-year drift ice during the winter/spring transition.

Horizontal and vertical variability of first-year drift-ice bacterial communities was investigated along a North-South transect in the Fram Strait during the winter/spring transition. Two different developmental stages were captured along the transect based on the prevailing environmental conditions and the differences in bacterial community composition. The differences in the bacterial communities were likely driven by the changes in sea-ice algal biomass (2.6-5.6 fold differences in chl-a concentrations). Copiotrophic genera common in late spring/summer sea ice, such as Polaribacter, Octadecabacter and Glaciecola, dominated the bacterial communities, supporting the conclusion that the increase in the sea-ice algal biomass was possibly reflected in the sea-ice bacterial communities. Of the dominating bacterial genera, Polaribacter seemed to benefit the most from the increase in algal biomass, since they covered approximately 39% of the total community at the southernmost stations with higher (>6 μg l(-1) ) chl-a concentrations and only 9% at the northernmost station with lower chl-a concentrations (<6 μg l(-1) ). The sea-ice bacterial communities also varied between the ice horizons at all three stations and thus we recommend that for future studies multiple ice horizons be sampled to cover the variability in sea-ice bacterial communities in spring.