Effects of growth condition on succession patterns in tropical phytoplankton assemblages subjected to experimental eutrophication

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We isolated phytoplankton assemblages from five tropical lakes differing in environmental conditions and incubated them under different extreme growth conditions. Conditions were: under light with nutrient addition (L + N), under light with nutrient and organic carbon addition (L + N + OC), and in the dark with nutrient and organic carbon addition (N + OC). These were intended to simulate phytoplankton succession during early, intermediate and late eutrophication stages, respectively. Decreases in species diversity and richness in N + OC assays from all lakes, except the most eutrophic one, suggested consistent diversity and richness declines during late eutrophication stages. Declines in species diversity were generally caused by a stronger reduction in species richness than evenness, indicating that species exclusion is a more probable response to eutrophication than co-existence and dominance of strong competitors. Under N + OC conditions, the osmotrophic chlorophyte Chlamydomonas was generally dominant, indicating that this flagellate may prevail in late eutrophic communities in the tropical lakes studied. The L + N and L + N + OC assays did not differ from each other in species diversity, but exhibited differences in species composition and richness. This suggests that the changing role of the phytoplankton community in whole-lake matter fluxes during early eutrophication stages could be a more important research question, than predicting the community's diversity response in tropical lakes.

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