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.