THE ABUNDANCE AND COMPOSITION OF FRESH-WATER 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 (> 10(5) 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.