The abundance and composition of freshwater picocyanobacteria in relation to light penetration

The abundance and composition of picocyanobacterial (PC) assemblages were measured in 38 lakes of varying trophic state and dissolved inorganic C concentrations. Light attenuation provided the best prediction of PC abundance, explaining half of the variation in summer abundance. Phycoerythrin‐containing PC were particularly abundant (> l05 cells ml‒1) in oligotrophic to mesotrophic hard‐water lakes, while nonphycoerythrin types were generally found in eutrophic and colored lakes. The percentage of phycoerythrin‐containing PC declined significantly with increasing light attenuation among lakes and within experimental enclosures of one lake where light attenuation was altered by nutrient additions and food‐web manipulations. The relationship may be the consequence of changes in spectral quality as lakes become nutrient enriched or colored by humic material.