DIRECT AND INDIRECT EFFECTS OF ZOOPLANKTON ON SESTON STOICHIOMETRY

Sestonic particles composed of phytoplankton, bacteria, and detritus are a potential food source for most zoo-plankton. In the present study, the effect of zooplankton on the stoichiometry of seston was examined in a small eutrophic pond from April to August. because the relative elemental contents of seston relates with phytoplankton growth rate and food quality. In the pond, most of the sestonic particles were edible size (< 30 mu m), and sestonic C:P and N:P ratios were high at low zooplankton biomass but low at high zooplankton biomass, suggesting that the zooplankton activity is one of the important factors inducing changes in seston stoichiometry. This possibility was substantiated by grazer gradient experiments that showed a significant increase in the relative P content of the seston with increasing zooplankton biomass. However, slopes of the regression line of the elemental ratios against the zooplankton biomass varied between seasons, and were greater at lower nutrient concentration and higher seston abundance. The relationship implies that the magnitude of the mass-specific effect of zooplankton depends on the relative nutrient concentration available for phytoplankton. In parallel with the grazer gradient experiments, zooplankton grazing was simulated by artificially eliminating some fraction of seston in order to separate the direct (eliminating particles) and indirect (nutrient release) effects on the seston stoichiometry. The response of sestonic elemental ratios to simulated grazing revealed that the zooplankton can affect seston stoichiometry by increasing the per capita availability of a deficient nutrient for phytoplankton through grazing as well as nutrient release. These results suggest that the effect of zooplankton on seston panicles is feedback to their growth rate by changing not only food abundance but also food quality.