In situ net primary productivity of an Antarctic fast ice bottom algal community

Net primary production was measured in situ in an Antarctic fast ice bottom algal mat at Cape Evans, McMurdo Sound, Antarctica. Under-ice PAR irradian ces between 18 November and 4 December 1997 were between 3 and 55 mu mol ph otons m(-2) s(-1). This produced a net oxygen export between 0.0084 and 0.0 440 nmol O-2 cm(-2) s(-1). P-max was 0.034 nmol O-2 cm(-2) S-1, E-k was 14 mu mol photons m(-2) s(-1) and the compensation point was approximately 2 m u mol photons m(-2) s(-1). These values are equivalent to a carbon-based pr oduction of 3.50 to 18.46 mg C m(-2) h(-1) and assimilation numbers of betw een 0.294 and 2.01 mg C mg(-1) chl a h(-1). Production levels on sunny days were so high that oxygen bubbles formed at the ice water interface and pre sumably contributed to the demise of the algal mat. Grazing by amphipods wa s also observed. While increasing net oxygen export was found to be strongl y correlated with increasing irradiance, increasing under-ice current veloc ity was also found to increase production. The reduction in diffusive bound ary layer thickness caused by increasing current velocity would have allowe d both a more efficient transport of nutrients into the mat and a more effi cient transport of oxygen away from the mat. Accumulation of sea ice algal biomass is not just a function of light but is also influenced by under-ice current velocity and possibly by oxygen build-up and grazing by amphipods and other invertebrates. In spite of the high under-ice irradiances reporte d from Cape Evans, loss mechanisms such as grazing and possibly oxygen toxi city were able to prevent the buildup of additional biomass.