Nutrient limitation reduces food quality for zooplankton: Daphnia responseto seston phosphorus enrichment

Laboratory studies are increasingly indicating that the quality of nutrient -limited algae is suboptimal for zooplankton production. However, little is known about how quality is affected by nutrient limitation of phytoplankto n in more natural situations. To test for phosphorus (P) limitation of zoop lankton growth under realistic food conditions, we performed a set of 5-d e xperiments using Daphnia dentifera and suspended particulate matter (seston ) from three lakes at the Experimental Lakes Area (Ontario, Canada). Neonat e Daphnia fed for 6 h per day on freshly collected seston enriched or unenr iched with PO, and spent the rest of the day feeding on unaltered natural s eston. PO, enrichment did not affect food abundance or concentrations and c omposition of essential fatty acids but dramatically lowered seston C:P rat io and significantly stimulated Daphnia growth. These results demonstrate t hat, even with field-collected seston, the effects of algal phosphorus limi tation can extend to herbivores through reduced food quality.