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.

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The validity of Connell's 'intermediate disturbance hypothesis' in phytoplankton communities was tested on data from a hypertrophic, shallow lake, Hjarbsek Fjord, Denmark.The present data from Hjarbaek Fjord demonstrate the difficulties in distinguishing stress from disturbance in a phytoplankton community, and show that great changes in the phytoplankton community can take place within few days.A collapse of blue-green algae in late June 1986 caused remineralization of nutrients and resulted in a rapid increase of fast-growing small chlorococcal green algae and phytoplankton species diversity, without any external disturbances acting on the lake. External disturbances in the form of wind action and brackish water intrusion occurred several days after the onset of these events. Carbon depletion and pH 11.0 were severe stress factors on the phytoplankton community. They were induced by calm, warm weather, but eventually acted as a kind of disturbance to the normally well circulated lake.