Poor direct exploitation of terrestrial particulate organic carbon by Daphnia galeata

Background/Question/Methods

Terrestrial organic carbon can constitute an important subsidy to lake ecosystems. While support of heterotrophic bacteria by dissolved organic carbon (t-DOC) has been shown, it is not clear, to what extent the direct usage of the particulate form (t-POC) can support somatic growth and reproduction of zooplankton. High terrestrial carbon signals found in metazoans after stable isotope analysis have partly been attributed to direct consumption of t-POC by metazoans. However, the only direct experimental studies to date that tested t-POC bioavailability and quality for zooplankton nutrition used pollen or ground leaves as t-POC representatives and did not find any beneficial effects on zooplankton performance. As neither pollen nor leaves are comparable to what zooplankton will experience in boreal unproductive lakes, we extracted t-POC from boreal peatland soils in Northern Sweden and conducted two growth experiments with Daphnia.

The first experiment tested how different ratios of peatland t-POC and the cryptophyte Rhodomonas affect Daphnia galeata performance at a non-limiting food concentration (1 mgC L^-1). A second experiment investigated effects of 50:50% mixtures of t-POC or the heterotrophic bacterium Pseudomonas and Rhodomonas over a food concentration gradient, ranging from low (0.01 mgC L^-1) to high food quantity (1.28 mgC L^-1).

Results/Conclusions

Experiment 1 showed that a pure t-POC diet could not sustain survival, growth and reproduction of D. galeata. Having 10% Rhodomonas in the diet led to survival of daphnids but at least 20% was necessary for the production of viable offspring. In Experiment 2 mixtures of Pseudomonas and Rhodomonas were a better quality food than mixtures of t-POC and Rhodomonas. However, best Daphnia performance always occurred on pure Rhodomonas diets.

Taken together, Rhodomonas was a higher quality food than Pseudomonas which in turn was a better food source than t-POC. Quantification of phosphorus (P) and essential biochemicals (e.g. fatty acids) revealed that Rhodomonas had the highest contents of all these components. Pseudomonas, while rich in P, contained few essential biochemicals and t-POC had low concentrations of both, P and biochemicals. We therefore suggest that the poor food quality of t-POC was due to its suboptimal mineral and biochemical composition.

Our results imply that high terrestrial carbon signals often described in zooplankton must, to a large extent, derive from incorporation of t-DOC by bacteria and subsequent transfer to higher trophic levels through the microbial loop. The direct contribution of t-POC to zooplankton nutrition, however, is probably comparably small in boreal lakes.