Optimal conditions for primary production in a polymictic tropical lake (Lake Xolotlan, Nicaragua)

From 1987 to 1993 we assessed the variation of phytoplankton biomass, underwater irradiance and primary production in Lake Xolotlan (L. Managua, Nicaragua). Chlorophyll-a averaged 65 mg m(-3) and maximum and minimum concentrations were 120 and 30 mg m(-3), respectively. The variability over depths and weeks was low (CV < 20%). There were strong correlations between particulate carbon and chlorophyll-a (the ratio approximate to 100: 1) and between particulate carbon and particulate nitrogen and phosphorus (the ratio approximate to 100: 11: 1). Gross primary production averaged 6.8 g C m(-2) d(-1) and was stable over the years (CV approximate to 10%). Algal cell growth was approximately 4-5 g C m(-2) d(-1). Productivity was limited only by the availability of underwater light and the depth of the photic zone was mainly regulated by the chlorophyll-a concentration. Therefore, areal photic zone chlorophyll-a was the only factor directly correlated to the integral photosynthetic activity but, contrary to theoretical models, the production did not increase in proportion to chlorophyll-a. Data from African lakes show a similar pattern.