SEASONAL VARIABILITY OF PHYTOPLANKTON SIZE STRUCTURE IN A HYPERTROPHIC LAKE

In this paper, we describe the time dynamics of phytoplankton size-bio mass spectra over a 60 week period in a hypertrophic lake (El Porcal, near Madrid, Spain). Average total phytoplankton biomass was 59.5 mg l -1, and 58 taxa were present which had between 1.8 and 38.5 mum equiva lent spherical diameter (ESD). Average biomass of pico-, nano- and mic roplankton was 0.04, 23 and 36.2 mg l-1, respectively. The average siz e-biomass spectrum was characterized by a higher amount of biomass in larger cells (biomass increase as the 0.59 power of body volume). The multivariate analysis generated five conspicuous groups of normalized biomass spectra which led to understanding of global dynamics as a tim e sequence of size structures in the studied lake. During a long equil ibrium period, normalized biomass spectra change towards less negative slopes, i.e. an increase of larger cells. During the periods with a h igher frequency of perturbations, the spectra evolve towards more nega tive slopes (increase of smaller cells). A comparison of this study wi th data from other aquatic systems suggests that the size-biomass spec trum is related to trophic state. In El Porcal Lake, size-spectrum des criptions are an adequate tool in the dynamic study of the phytoplankt on community.