Empirical analysis of the effect of phosphorus limitation on algal food quality for freshwater zooplankton

Herbivorous zooplankton production is often constrained by algal food quality, and Variation in algal food quality has been attributed to its phosphorus (P) content. To test the hypothesis that mineral P limitation is an important constraint on freshwater herbivorous zooplankton production, we conducted a quantitative analysis of published data on lake seston carbon to phosphorus (C:P) ratios and daphnid growth responses to P-limited algal diets. Our summary of 276 observations of mean lake seston C:P ratios showed 38% exceeded a critical threshold of 300, above which daphnid production is often predicted to be limited by algal P content. Our analysis of 91 observations of daphnid growth responses to nutrient-limited algal diets indicates the true daphnid critical threshold lies within the range of 225-375. Algal food quality is consistently reduced at algal C:P ratios above 300; however, the predicted reduction in algal food quality with P limitation is greater than the observed reduction at critical thresholds <390. Mineral P limitation of Daphnia production may be quite strong in some lakes; however, the production of most zooplankton taxa would not be expected to be limited by mineral P in a large proportion of lakes. After correcting for the algal C:P ratio, large differences in food quality were found between the major algal taxa, with diatoms and cryptophytes high food quality, chlorophytes intermediate food quality, and small ingestible cyanophytes very low food quality. A Monte Carlo simulation indicated that shifts in aIgal community composition similar to those seen during seasonal succession or across lake trophic state gradients explained four times more variability in daphnid growth rates across a range of seston C:P ratios representative of natural lakes. Overall these results tend to support the mineral P Limitation hypothesis for daphnids, but they also suggest other factors will be the primary determinants of algal food quality for most zooplankters in most lakes.