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

Frost, Paul C., Marguerite A. Xenopoulos, James H. Larson

Limnol. Oceanogr., 49(5), 2004, 1802-1808 | DOI: 10.4319/lo.2004.49.5.1802

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:NH₄ 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.