An evaluation of ecological stoichiometry in pelagic systems

Abstract

Ecological stoichiometry is the study of the balance of chemical substances in ecosystems. In freshwaters, research has focused on how the ratios of carbon, nitrogen and phosphorus in organisms and their environment affect ecosystem processes. Because autotrophs have variable stoichiometry, particulate C:N:P ratios are used to assess nutrient availability in lakes. Zooplankton have relatively fixed stoichiometry and so differences between their body stoichiometry and the stoichiometry of their food can constrain their growth. Ecological stoichiometry predicts that zooplankton with low C:P body ratios (e.g., Daphnia) will be limited by the P content of their food in lakes where seston C:P is high. The stoichiometric theory of consumer-driven nutrient recycling (CNR) predicts that the stoichiometry of a consumer will influence the stoichiometry of the nutrients they regenerate through such processes as egestion and excretion. In lakes, zooplankton with a low body N:P are expected to regenerate nutrients in a high N:P ratio, potentially shifting nutrient limitation of the food web from N to P limitation. I used data from 99 Canadian lakes to test the following: a. Are particulate C:P and N:P ratios consistent with other P deficiency indicators? b. Do seston C:P and N:P ratios affect zooplankton community composition? c. Does zooplankton community composition affect plankton P limitation as predicted by CNR? Particulate C:P and N:P ratios generally agreed with other P deficiency indicators, except dissolved phosphate turnover times (TTPO4). C:P and N:P suggested P sufficiency more often than TTPO4, possibly because these two indicators respond to P deficiency over different time scales. Most zooplankton biomass parameters were negatively related to seston C:P ratios consistent with improved food quality at lower seston C:P. There was, however, no evidence that Daphnia were more strongly affected than any other zooplankton. Turnover times of particulate P in the whole plankton assemblage were not related to zooplankton community structure parameters. However, particulate P turnover in the >200 µm size fraction increased with increasing zooplankton biomass. There was no evidence for a particular effect of Daphnia on particulate P turnover. Phosphorus deficiency indicators showed a trend of relaxing P deficiency as zooplankton biomass and the proportion of Daphnia increased. This contradicts the predictions of CNR which suggest that Daphnia should cause greater P deficiency in lakes.