In lakes and reservoirs, productivity and biomass responses to nutrient loading are fairly predictable; resultant changes in phytoplankton taxonomic composition are not. Whether enrichment leads to dominance by procaryotic or by eucaryotic phytoplankton is important to lake trophic structures, to ecosystem modelers, and to lake managers. Much of the uncertainty about the effects of enrichment on phytoplankton composition stems from an inadequate understanding of how nuisance bloom-formers succeed in dominating plankton assemblages. The authors contend that investigations of nutrient effects on cyanobacterial buoyancy regulation provide a basis for understanding patterns of cyanobacterial dominance during the course of eutrophication as well as during seasonal successions in stratified, eutrophic lakes. Cyanobacterial dominance in the surface layers of enriched lakes should be more pronounced in HCO3-rich waters where synthesis of the cellular components involved in buoyancy regulation is not limited by nitrogen, phosphorus, or carbon. In mildly acidic and other low-carbon waters, carbon limitation of buoyancy regulation deters nuisance bloom formers, but not non-gas-vacuolate cyanobacteria. The authors also suggest that restoration by artificial mixing or by biomanipulation is less likely to succeed in HCO3-rich than in softwater basins.