Beyond monoculture stoichiometry studies: assessing growth, respiration, and feeding responses of three Daphnia species to P-enriched, low C:P lake seston

Numerous studies in ecological stoichiometry have assessed responses of Daphnia to single- and multi-species food resources of varying nutritional quality. Diets based on P-limited algae with a high biomass carbon to phosphorus (C:P) ratio impose decreased growth on Daphnia because P demands for growth are not met. More recently, laboratory studies have shown that P-rich algal food also results in decreased growth rates for Daphnia; however, no published study has tested the impact of low food C:P on Daphnia under field conditions. To address this, we measured growth rate as well as respiration and ingestion rates for D. magna, D. pulicaria, and D. pulex that were fed lake seston experimentally enriched with phosphate (PO43−). Lake seston during the experimental period successfully took up PO43− enrichment treatments and was dominated by cyanobacteria and haptophytes, primarily those from the genus Prymnesium. Growth rate reductions for D. magna were strong in response to high-P food, most likely as a result of decreased ingestion rate; however, growth rate responses for D. pulex and D. pulicaria were modest and not statistically significant, although significant responses in respiration rates were observed for all species. Our study extends laboratory findings that P-rich food can have deleterious impacts on Daphnia to field conditions. We also found diverse responses among species, however, possibly caused by variations among taxa in body stoichiometry or physiology, differences in ambient and treatment seston C:P for the various experiments, or the effects of diet diversity under high-P conditions.