AME 27:301-311 (2002)  -  doi:10.3354/ame027301

ABSTRACT: The present study compared O₂ exchange measurements, fluorometry and ¹⁴C incubations for measuring productivity of sea-ice algae. It is demonstrated that brine leaking out of ice during 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 ¹⁴C incubation, co-varied as a function of biomass and irradiance. The ratio between the net O₂ efflux and the gross C fixation rate (the photosynthetic 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.

KEY WORDS: Sea-ice algae · Primary production · Microelectrodes · Oxygen · Pulse amplitude modulated fluorometry · Fluorometer