Phytoplankton species composition is more sensitive than OECD criteria to the trophic status of three Brazilian tropical lakes

We compared the structure and function of the phytoplankton community in three tropical systems in Brazil: an oligo-mesotrophic floodplain lake; an eutrophic reservoir; and a hypereutrophic coastal lagoon. Phytoplankton biomass increased along this trophic gradient averaging 2.6, 6.2 and 31.6 mm(3) l(-1) in the lake, reservoir and lagoon, respectively. Similarly, production increased from 427 to 918 gC m(-2) y(-1). Along the trophic gradient, diversity and species richness of phytoplankton decreased. There was, however, no relationship between trophy and rates of change of phytoplankton communities or the ratio of maximum/mean biomass. Phytoplankton size varied between systems with small algae (2-20 mu m) dominating in the hypereutrophic lagoon and larger algae (20-200 mu m) dominating in the other systems. Many of the trophic indices developed for temperate areas could not be applied to these tropical systems since there was no clear relationship between Secchi depth, chlorophyll, or TP and trophic status. Phytoplankton species may be more useful than these trophic indices to the classification of the condition of tropical lakes.