POPULATION-DYNAMICS OF AUTOTROPHIC PICOPLANKTON IN RELATION TO ENVIRONMENTAL-FACTORS IN A PRODUCTIVE LAKE

Spatial and seasonal fluctuations in autotrophic picoplankton (APP) ab undance in a eutrophic, dimictic lake (Lake Aydat, France) were measur ed concurrently with a variety of environmental variables. Cell number ranged from 0.03 to 2.36x10(6) cells . ml(-1) (highest concentrations were > 5-fold higher than in oligotrophic lakes) and averaged 24 +/- 7% of total picoplankton abundance (APP + heterotrophic bacteria). APP abundance (1) peaked in spring simultaneously with heterotrophic flag ellate and ciliate densities, (2) decreased during the nitrogen-limite d and summer stratification period, and (3) increased with fall turnov er. In summer-autumn, the contribution of single-cell eukaryotic (up t o 66%) and colonial prokaryotic (18%) forms to total abundance peaked in the bottom waters. Multivariate regression analyses suggest that > 40% variance in APP number changes may be explained by ciliate abundan ce (at 0-4 m depth-range), heterotrophic flagellate number and oxygen concentration (5-9 m), and ciliate carbon biomass (10-14 m). The model accounting for changes in heterotrophic bacterial abundance (5-9 m) i ndicates chlorophyll a concentration (r(2)=58%) and ciliate abundance (r(2)=34%) as dominant covariates. The data presented here suggest tha t micrograzers control APP abundance in Lake Aydat.