Nutrient competition experiments were performed with periphytic microa
lgae from the Kiel Fjord, western Baltic Sea. Diatoms were dominant co
mpetitors al high Si:N ratios (under N-limited conditions) and high Si
:P ratios (under P-Limited conditions). Under low silicate supply, cya
nobacteria became dominant at low N:P ratios and chlorophytes at high
ones. Changes in Light intensity influenced the outcome of competition
at the species level but not at the level of higher taxa. The nitroge
n source (ammonium or nitrate) had no impact on the outcome of competi
tion.