Size-dependent phytoplankton responses to atmospheric forcing in Lake Biwa

Abstract

Size-dependent changes in chlorophyll a and uptake of inorganic carbon (C) and nitrogen (N) by phytoplankton were measured in the shallow South Basin of Lake Biwa, before and during a period of typhoons (high winds). The latter period was characterized by complete mixing of the water column, a major decline in underwater irradiance, and a transient increase in dissolved reactive N and phosphorus (P). Nutrient concentrations, 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 sup- ply 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 relatively 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 intrinsic growth rates and photosynthetic efficiency. These observations are contrary to the view that picoplankton have higher light-harvesting abilities and growth rates than larger cells. Typhoon-induced mixing stimulated the shift to fast-growing, dim light-adapted larger cells. The results question the allometric paradigm of increasing growth rates and light-capturing efficiency with decreasing cell size, but are consistent with the oceanographic view that large-celled phytoplankton control the major fluctuations in biomass and primary productivity, while picoplankton account for a comparatively stable background productivity.