Seasonal Variation of Phytoplankton Biomass and Photosynthesis in the High-Mountain Lake La Caldera (Sierra Nevada, Spain)

Abstract

Studies on lake La Caldera (Sierra Nevada, Spain) were carried out to follow changes in nutrients (N and P), phytoplankton biomass, photosynthesis and chloropyll a content. Differences were observed between two periods, 1974 and 1976. An increase in the PO₄ content of the lake occurred which could not be satisfactorily explained. Phytoplankton biomass was as much as twice (maximal values of 500 and 1000 mg m⁻³, respectively) during the second year as the first. Photosynthesis also increased, although less than biomass; values near the maximum (2.5 mg C m⁻³ h⁻¹) at midday) were more frequent and production was more uniformly distributed in the second year. Biomass and production increases are attributable to PO₄ increase. Chlorophyll content per unit biomass varied between 0.3 and 1.6%, the lowest values belonging to the beginning and the end of the period. The relationship C ass h⁻¹/mg Chl a for each depth varied between zero (no production, under ice cover) and 1.1, 50% of the quotient values falling between 0.02 and 0.2, these low values being characteristic for an oligotrophic lake.