Testing zooplankton food limitation across gradients of depth and productivity in small stratified lakes

We examined the significance of vertical structure in lake plankton to grazer-resource coupling by quantifying how the intensity and mechanism of grazer food limitation varies with depth. We used Daphnia growth assays and also sampled resources (chlorophyll, particulate carbon, and phosphorus) and the day/night depth distributions of Daphnia spp. during summer in four depth strata of lakes that differ in trophic state. The growth assays included treatments with natural seston and seston supplemented with phosphate or Synechococcus, an energy- and P-rich cyanobacterium highly deficient in essential fatty acids and sterols. Results from growth assays and measurements of Daphnia P balance indicate strong energy limitation and weak and sporadic P limitation, despite seston C:P ratios in the upper depth strata above the predicted threshold for Daphnia P limitation. Growth improved with depth and was very high with resources collected from a deep layer of sulfur bacteria that was partially protected from grazing by low dissolved oxygen. Strong positive responses to Synechococcus supplements and high growth with resources from the sulfur bacteria layer are both contrary to limitation by essential lipids. The intensity of food limitation was independent of lake trophic state but was influenced by the quality of the deep-water refuge. Across depths and lakes, food limitation was dependent on Daphnia density; strong food limitation was associated with high Daphnia biomass (>100 mug L-1) and Daphnia grazing >20% per day.