SIZE-DEPENDENT PHYTOPLANKTON RESPONSES TO ATMOSPHERIC FORCING IN LAKEBIWA

Size-dependent changes in chlorophyll a and uptake of inorganic carbon (C) and nitrogen (N) by phytoplankton were measured in the shallow So uth Basin of Lake Biwa, before and during a period of typhoons (high w inds). The latter period was characterized by complete mixing of the w ater column, a major decline in underwater irradiance, and a transient increase in dissolved reactive N and phosphorus (P). Nutrient concent rations, seston N:P ratios and uptake rates indicate that P and not N limited phytoplankton over the whole study period. The typhoon-induced increase in phosphorus supply resulted in Blackman-type (increased C- and N-specific growth rates) and Liebig-type (increased biomass yield ) responses by the phytoplankton. Picoplankton were dominant in the re latively stable and clear water column prior to the typhoons, but were rapidly outgrown by larger cells during and after wind-induced mixing . The slower response of picoplankton indicates lower maximum intrinsi c growth rates and photosynthetic efficiency. These observations are c ontrary to the view that picoplankton have higher light-harvesting abi lities and growth rates than larger cells. Typhoon-induced mixing stim ulated the shift to fast-growing, dim light-adapted larger cells. The results question the allometric paradigm of increasing growth rates an d light-capturing efficiency with decreasing cell size, but are consis tent with the oceanographic view that large-celled phytoplankton contr ol the major fluctuations in biomass and primary productivity, while p icoplankton account for a comparatively stable background productivity .