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, nonheterocystous cyanobacteria, and chlorophytes was studied along a trophic 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 chlorophytes often were dominant at high TP (>1 mg P.L(-1)). In contrast with many earlier findings, heterocystous cyanobacteria were not dominant at 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 photic zone to mixing zone ratio. We suggest that chlorophyte dominance in hypertrophic shallow lakes is attributable to continuous input of nutrients and carbon from the sediment and external sources. This renders the fast-growing chlorophytes a superior competitor compared with the relatively slow-growing cyanobacteria, even when inorganic nutrient concentration is low and pH high. New predictive models relating phytoplankton dominance to TP in shallow lakes were developed, as former models failed to predict our observations satisfactorily.