ALGAL PHOTOSYNTHETIC ACTIVITY AND ITS RESPONSE TO MEROMIXIS IN HYPERSALINE MONO-LAKE, CALIFORNIA

Photosynthetic activity was measured in hypersaline Mono Lake during an 8-yr period (1983-1990) spanning the onset (1983), persistence (1984-1987), and breakdown of meromixis (1988). Algal biomass in spring and autumn decreased following the onset of meromixis and annual photosynthetic production was reduced (269-462 g C m-2 yr-1; 1984-1986) compared to nonmeromictic conditions (499-641 g C m-2 yr-1; 1989 and 1990). A gradual increase in photosynthetic production occurred even before meromixis ended because of increased vertical flux of ammonium due to deeper mixing and the buildup of ammonium in the monimolimnion. Annual production was greatest in 1988 (1,064 g C m-2 yr-1) when the weakening of chemical stratification and eventual breakdown of meromixis in November resulted in large fluxes of ammonium into the euphotic zone. Most of the variation in rates of light-saturated carbon uptake normalized to chlorophyll a (assimilation numbers) was explained by a regression on temperature (60%); measures of the light and nutrient environments accounted for a further 8% of the observed variation. Estimates of ammonium supply due to vertical mixing and Artemia monica excretion indicate nitrogen is most likely to limit photosynthetic production during spring and late autumn when A. monica is absent and algal biomass high. Light is also likely to limit production during these periods based on comparisons of the average mixed-layer irradiance to the light intensity at which maximum photosynthetic rates were reached. Light limitation was more pronounced under monomictic conditions when algal biomass was higher.