Drift-ice and under-ice water communities in the Gulf of Bothnia (Baltic Sea)

The algal, protozoan and metazoan communities within different drift-ice types (newly formed, pancake and rafted ice) and in under-ice water were studied in the Gulf of Bothnia in March 2006. In ice, diatoms together with unidentified flagellates dominated the algal biomass (226 +/- A 154 mu g ww l(-1)) and rotifers the metazoan and protozoan biomass (32 +/- A 25 mu g ww l(-1)). The under-ice water communities were dominated by flagellates and ciliates, which resulted in lower biomasses (97 +/- A 25 and 21 +/- A 14 mu g ww l(-1), respectively). The under-ice water and newly formed ice separated from all other samples to their own cluster in hierarchical cluster analysis. The most important discriminating factors, according to discriminant analysis, were chlorophyll-a, phosphate and silicate. The under-ice water/newly formed ice cluster was characterized by high nutrient and low chlorophyll-a values, while the opposite held true for the ice cluster. Increasing trends in chlorophyll-a concentration and biomass were observed with increasing ice thickness. Within the thick ice columns (> 40 cm), the highest chlorophyll-a concentrations (6.6-22.2 mu g l(-1)) were in the bottom layers indicating photoacclimation of the sympagic community. The ice algal biomass showed additional peaks in the centric diatom-dominated surface layers coinciding with the highest photosynthetic efficiencies [0.019-0.032 mu g C (mu g Chl-a (-1) h(-1)) (mu E m(-2) s(-1))(-1)] and maximum photosynthetic capacities [0.43-1.29 mu g C (mu g Chl-a (-1) h(-1))]. Rafting and snow-ice formation, determined from thin sections and stable oxygen isotopic composition, strongly influenced the physical, chemical and biological properties of the ice. Snow-ice formation provided the surface layers with nutrients and possibly habitable space, which seemed to have favored centric diatoms in our study.