Effects of phosphorus enrichment on <i>Daphnia</i>–algae interactions in laboratory microcosms

We examined the effects of phosphorus (P) on algal biomass and stoichiometry and, subsequently, alterations to zooplankton populations. We modified P supply in microcosms and tracked changes in algal and <i>Daphnia</i> populations, and phosphorus concentrations. Even though we found algal biomass increased over the experiment in low P containers, greater food abundance did not increase <i>Daphnia</i> abundance. In low P containers, a high algal biomass was accompanied with elevated C:P ratios, very low soluble reactive P concentrations and low <i>Daphnia</i> fecundity. High algal C:P ratios and low soluble reactive P concentrations in these microcosms indicated a strong P-limitation of algae and food quality constraints on consumer populations. In high P containers, algal biomass initially increased, which led to an increased <i>Daphnia</i> abundance. In most of the high P microcosms, rapid increases in <i>Daphnia</i> populations led to reduced algal biomass and increased concentrations of soluble reactive P. However, in an outlier high P container, we found a different pattern with elevated algal biomass, low soluble reactive P concentrations and a very large <i>Daphnia</i> population. Our results show that P supplies can strongly affect <i>Daphnia</i>-algae interactions, but the nature of these effects are likely complicated by internal feedbacks that affect the gain and loss of both populations.