NUTRIENT LIMITATION REDUCES FOOD QUALITY FOR ZOOPLANKTON:DAPHNIARESPONSE TO 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 phytoplankton in more natural situations. To test for phosphorus (P) limitation of zooplankton growth under realistic food conditions, we performed a set of 5-d experiments using Daphnia dentifera and suspended particulate matter (seston) from three lakes at the Experimental Lakes Area (Ontario, Canada). Neonate Daphnia fed for 6 h per day on freshly collected seston enriched or unenriched with PO4 and spent the rest of the day feeding on unaltered natural seston. PO4 enrichment did not affect food abundance or concentrations and composition of essential fatty acids but dramatically lowered seston C:P ratio and significantly stimulated Daphnia growth. These results demonstrate that, even with field-collected seston, the effects of algal phosphorus limitation can extend to herbivores through reduced food quality.