Phytoplankton growth rate as a function of cell size: An experimental test in Lake Biwa

It is well known that algal growth rates decrease with increasing cell size. Most of these findings were, however, obtained under laboratory conditions. It is not clear if these allometric relationships are also applicable to in situ conditions. In the present study, the relationship between growth rates and cell size of algal species was examined seasonally in Lake Biwa by in situ dilution bioassays. The bioassays revealed that the highest growth rate of each species throughout the experiments was negatively correlated with cell size consistent with known allometric relationships. At each incubation experiment, however, growth rates were not necessarily correlated with cell size. This was true even when both macro- and micronutrients were added, although a substantial number of species responded to nutrient enrichment. These results showed that nutrient supplies affected algal species differently regardless of cell size and that factors other than nutrient supplies limited the growth rate of some algal species. Due to such species-specific differences in limiting factors, at any given time in situ growth rates of algae are not determined exclusively by cell size.