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The present study compared O2 exchange measurements, fluorometry and 14 C incuba- tions for measuring productivity of sea-ice algae. It is demonstrated that brine leaking out of ice dur- ing freezing conditions is supersaturated with respect to O2, while the melting water released during thawing is undersaturated with O2. This results in a highly variable O2 concentration at the ice-water interface during expanding or shrinking of sea ice. Consequently, great care should be taken when inferring biological activity from O2 exchange rates, whether they are obtained via microprofiles, bulk incubations or direct measurement of O2 concentrations in water below sea ice. Accounting for the O2 dynamics related to changes in sea-ice structure, the photosynthetic activity of a mixed ice algal culture, as measured by net O2 exchange rates and 14 C incubation, co-varied as a function of biomass and irradiance. The ratio between the net O2 efflux and the gross C fixation rate (the photo- synthetic quotient) was similar for frozen and non-frozen ice algal communities and averaged 1.43 ± 0.48. Quantification of the relative electron transport rate (ETR) from photosystem II (PSII) and the minimum fluorescence yield (F0) by a pulse-amplitude-modulated (PAM) fluorometer offers a fast, simple and non-invasive approach for estimating activity and biomass of ice algal communities. The relative ETR measurements correlated with the other 2 measures for primary production. However, the correlation was non-linear, leading to a poor resolution of the fluorometer approach at higher photosynthetic activities. A similar observation was made for the correlation between measurements and the concentration of chlorophyll a.

aquatic microbial ecology

A laboratory study on O2 dynamics and photosynthesis in ice algal communities: quantification by microsensors, O2 exchange rates, 14C incubations and a PAM fluorometer