IMPACT OF NUTRIENTS AND PHYSICAL FACTORS ON THE SHIFT FROM CYANOBACTERIAL TO CHLOROPHYTE DOMINANCE IN SHALLOW DANISH LAKES

Phytoplankton dominance (as biomass) by heterocystous cyanobacteria, n onheterocystous cyanobacteria, and chlorophytes was studied along a tr ophic gradient (0.011-2.2 mg P.L(-1)) by analyzing regularly collected semiquantitative data from 178 shallow Danish lakes (mean depth <3 m) and quantitative data from 32 lakes. Heterocystous cyanobacteria were dominant at low total P (TP) (<0.25 mg P.L(-1)) and nonheterocystous cyanobacteria at intermediate TP (0.25-0.8 mg P.L(-1)), while chloroph ytes often were dominant at high TP (>1 mg P.L(-1)). In contrast with many earlier findings, heterocystous cyanobacteria were not dominant a t low total N (TN):TP or low inorganic N concentrations; chlorophytes were dominant at extremely high pH, and the shift from cyanobacterial to chlorophyte dominance could not be explained by a change in the pho tic zone to mixing zone ratio. We suggest that chlorophyte dominance i n hypertrophic shallow lakes is attributable to continuous input of nu trients and carbon from the sediment and external sources. This render s the fast-growing chlorophytes a superior competitor compared with th e relatively slow-growing cyanobacteria, even when inorganic nutrient concentration is low and pH high. New predictive models relating phyto plankton dominance to TP in shallow lakes were developed, as former mo dels failed to predict our observations satisfactorily.