Response of zooplankton to inputs of terrestrial dissolved organic matter: Food quality constraints induced by microbes

Terrestrial inputs of dissolved organic matter (DOM) stimulate microbial growth by supplying organic carbon and DOM-associated nutrients, and thus increase food availability for zooplankton. Terrestrial inputs of DOM might decrease the content of eicosapentaenoic acid (EPA) in particular organic matter (POM) due to an increase in biomass of EPA-poor microbes, for example, heterotrophic bacteria and autotrophic cyanobacteria, thereby reducing food quality and filter-feeding zooplankton's biomass. We performed a 26-d mesocosm experiment where terrestrial DOM was manipulated by adding (treatments) and kept without adding (controls) C-13-labeled maize leachate to evaluate the effect of food quality changes induced by terrestrial DOM inputs on zooplankton. DOM addition treatments had significantly higher concentrations of POM and Chlorophyll a relative to controls, but significant lower EPA content of POM, responding to increased biomass of bacteria and chlorophytes. Biomass and EPA content of filter-feeding cladoceran Chydorus sphaericus was significantly reduced by terrestrial DOM addition. Stable isotope analyses showed C-13 enrichments in bacteria and cryptophytes in the treatment of C-13-labeled maize leachate, indicating a growth stimulation of heterotrophic microbes by organic carbon addition. Although the C-13 enrichment in Chydorus evidenced the extensive use of terrestrial DOM by this cladoceran, positive relationships between Chydorus biomass and EPA content of POM suggest that terrestrial DOM inputs increase food quality constrains on filter-feeding zooplankton. Our study suggests that the response of filter-feeding zooplankton to inputs of terrestrial DOM depends on how the terrestrial inputs change the microbial communities and thus the food quality in the receiving aquatic ecosystems.