Seasonal seston stoichiometry: Effects on zooplankton in cyanobacteria-dominated lakes

Abstract

Seasonal dynamics in elemental composition [carbon (C), nitrogen and phosphorus (P)] of seston and zooplankton were studied over several years in three hypereutrophic Dutch lakes with persistent dominance and high biomass of cyanobacteria. In all three lakes, there was a strong pattern with decreased P-content and increased C:P ratio in seston (<150 μm) coinciding with the increase in water temperature. The seston C:P ratios (at:at) were more than doubled with the rising temperature, i.e. from ∼200 (at:at) in winter to 500 in summer. Sestonic C:P ratios increased over the growing season, suggesting that seasonal dynamics among autotrophs with high P-uptake in winter and support of subsequent phytoplankton growth by consumption of internal cellular P (P-quota) was the main cause of low sestonic P contents in late summer. This could, however, occur in concert with a physiologically driven decrease in cell-specific P at higher temperatures in summer. In contrast, the annual variation of C:P ratios of the zooplankton fraction was only 10% of that of seston. The variations of C:P ratios of the zooplankton were, nevertheless, strongly correlated with those ofseston. For most of the summer, seston C:P ratios were far above the threshold ratio for P-limitation in Daphnia and other P-demanding species. This will pose further constraints on growth performance of Daphnia in these lakes, thus adding to the fish predation pressure and the poor food quality of cyanobacteria per se. The low grazing pressure causes a high biomass of low-quality autotrophs, promoting a stable state with low trophic transfer efficiency. © The Author 2005. Published by Oxford University Press. All rights reserved.