The stoichiometry of dissolved organic carbon, nitrogen, and phosphorus release by a planktonic grazer, Daphnia

Abstract

We examined the relationship between Daphnia release rates and ratios of dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P) and the elemental composition of algal food. We constructed a three-element (C, N, and P) mass-balance model to explore the relationships between food elemental composition and Daphnia release of DOC, N, and P. The model predicts that Daphnia DOC:N and DOC:P release ratios will increase with increasing food C:N and C:P ratios, respectively. These effects on DOC:N and DOC:P ratios are dependent, however, on the availability of the third element (P and N respectively) in our model. In addition, we experimentally measured the mass-specific release of DOC, ammonia (NH4), and soluble reactive phosphorus (SRP) from Daphnia magna fed food of contrasting C:N:P ratios. Daphnia release rates of DOC were not significantly affected by algal C:N or C:P ratios. However, high C:N and C:P ratios in algal food raised the DOC:NH 4 and DOC:SRP ratios (respectively) released from Daphnia. Our results, experimental and modeling, show a strong dependence of DOC: N and DOC:P release ratios by Daphnia on food C:N and C:P ratio. Thus, elevated C:N or C:P ratios in food can cause imbalanced release of DOC and NH4 or SRP, respectively, by Daphnia with potentially strong feedback on microbial heterotrophs in planktonic food webs.