Abstract
Abstract Filter‐feeding zooplankton in lakes feed on a mixture of phytoplankton, bacteria, and terrestrial particles and the proportions and nutritional value of these components can be highly variable. However, the extent to which food quality interacts with food quantity in affecting overall zooplankton performance is not yet fully resolved. Here we performed laboratory feeding experiments to test how the performance of the unselective filter feeder Daphnia galeata was affected if various quantities of high‐quality food (the phytoplankton Rhodomonas) were diluted with low‐quality food such as heterotrophic bacteria (Pseudomonas) or terrestrial detritus particles (t‐POM) from the riparian zone of a boreal forest stream. We hypothesised: that increased proportions of bacteria and t‐POM in the diet will lead to decreased survival, somatic growth; and reproduction of Daphnia despite the presence of phytoplankton; that these effects are more pronounced for t‐POM than for heterotrophic bacteria; and that this response is stronger when phytoplankton availability is low. Increasing the concentrations of Pseudomonas affected Daphnia survival, growth, and reproduction negatively when Rhodomonas was available at intermediate (0.37 mgC/L) and high (0.55 mgC/L) quantities. When Rhodomonas quantity was low (0.22 mgC/L), the addition of Pseudomonas generally resulted in better Daphnia performance except at very high concentrations of the bacterium relative to Rhodomonas. In contrast, the addition of t‐POM was detrimental for overall Daphnia performance at all Rhodomonas concentrations. Daphnia performance was best described by a model including the interaction between food quality and quantity, with stronger negative effects on Daphnia when high‐quality food was supplemented with t‐POM than with Pseudomonas. The results indicate that the ability of zooplankton to use low‐quality food is affected by the concurrent availability of high‐quality food. Furthermore, food sources that can be used but do not fulfil dietary requirements of grazers (e.g. bacteria), may still provide nutritional benefits as long as other complementary food components are available in sufficient quantities to compensate for biochemical deficiencies. Therefore, we conclude that heterotrophic bacteria, but not peat layer t‐POM, can be an important component of zooplankton diets in boreal lakes, especially if the concentration of phytoplankton is low.
Highlights
Zooplankton production in lakes is supported by both autochthonous and allochthonous carbon sources
Daphnia performance was best described by a model including the interaction between food quality and quantity, with stronger negative effects on Daphnia when high-quality food was supplemented with Terrestrial particulate organic material (t-POM) than with Pseudomonas
Our results suggest that the availability of high-quality phytoplankton biomass is critical for Daphnia survival, growth, and reproductive output
Summary
Zooplankton production in lakes is supported by both autochthonous and allochthonous carbon sources. The chemical character of the t-POM varies depending on the characteristics of the terrestrial ecosystem but most of this OM is expected to have a low nutritional value for metazoans due to its high carbon:phosphorus (C:P) stoichiometry and lack of essential biochemicals. Boreal lakes with high inputs of t-OM often only support a low phytoplankton biomass and algal carbon concentrations rarely exceed 0.1 mgC/L (Arvola et al, 1999; Jansson et al, 2003). Heterotrophic bacteria are directly consumed by a variety of non-selective filter feeding zooplankton, e.g. Daphnia (Wylie & Currie, 1991) Owing to their low C:P stoichiometry, bacteria may be considered a high-quality food for organisms with high somatic P content, e.g. daphnids (Sterner & Elser, 2002).
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