Size-dependent phytoplankton responses to atmospheric forcing in Lake Biwa

Size-dependent changes in chlorophyll a and uptake of inorganiccarbon (C) and nitrogen (N) by phytoplankton were measured inthe shallow South Basin of Lake Biwa, before and during a periodof typhoons (high winds). The latter period was characterizedby complete mixing of the water column, a major decline in underwaterirradiance, and a transient increase in dissolved reactive Nand phosphorus (P). Nutrient concentrations, seston N:P ratiosand uptake rates indicate that P and not N limited phytoplanktonover the whole study period. The typhoon-induced increase inphosphorus supply resulted in Blackman-type (increased C- andN-specific growth rates) and Liebig-type (increased biomassyield) responses by the phytoplankton. Picoplankton were dominantin the relatively stable and clear water column prior to thetyphoons, but were rapidly outgrown by larger cells during andafter wind-induced mixing. The slower response of picoplanktonindicates lower maximum intrinsic growth rates and photosyntheticefficiency. These observations are contrary to the view thatpicoplankton have higher light-harvesting abilities and growthrates than larger cells. Typhoon-induced mixing stimulated theshift to fast-growing, dim light-adapted larger cells. The resultsquestion the allometric paradigm of increasing growth ratesand light-capturing efficiency with decreasing cell size, butare consistent with the oceanographic view that large-celledphytoplankton control the major fluctuations in biomass andprimary productivity, while picoplankton account for a comparativelystable background productivity.