Relationships between resource ratios and phytoplankton species composition during spring in five north German lakes

Phytoplankton species composition and the availability of potentially limiting resources were inves-tigatcd in five northern German lakes (Suhrer See, Kellersee, Behler See, PluBsee, and Krummsee) during spring 1988. Species composition was related to resource ratios to test Tilman’s resource-ratio hypothesis. The clearest results were obtained for Aulacoseira spp. Both the time-courses within the lakes and the comparison among the lakes suggest a strong tendency of this genus to become dominant at high Si: light ratios. Fragilariaceae occupied the next position on the Si: light gradient. Such conditions permitted them to become important even when Si: P ratios were quite low. With the onset of stratification diatoms were replaced by flagellates. The resource-ratio hypothesis is central to Tilman’s competition theory (Tilman 1977, 1982); it has been well-tested experimentally in the laboratory with algae from unialgal strains and with natural phytoplankton com-munities (Kilham et al. 1977; Kilham 1986