Decoupling Carbon Effects and UV Protection from Terrestrial Subsidies on Pond Zooplankton

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Studies on spatial subsidies have overwhelmingly addressed their trophic nature. However, certain subsidies might affect recipient communities beyond the feeding pathway. Terrestrial colored dissolved organic matter (DOM) is such a subsidy providing a carbon source for bacteria and heterotrophic nanoflagellates which zooplankton can consume (trophic pathway), but also protecting zooplankton against damaging UV radiation through its color (non-trophic pathway). These mechanisms have been quantified separately, but few studies have attempted to decouple them and evaluate their effects in the context of subsidies. In this study, I experimentally isolated the trophic and non-trophic pathways by which DOM (as maple leaf leachate) affects zooplankton, and also addressed how local food resources (i.e., phytoplankton) could mediate the response of zooplankton to DOM subsidies. Terrestrial subsidies (DOM) effectively shielded zooplankton against damaging UV radiation. Local resources (i.e., phytoplankton), however, did not seem to mediate the fitness response of zooplankton to UV radiation. This study also suggested that the carbon in DOM might be slightly detrimental to zooplankton independent of the UV protection effect. High levels of local resources combined with carbon subsidies from DOM did not translate into higher zooplankton survival, presumably because of the detrimental effects of DOM on zooplankton. This study provides further support for the importance of terrestrial subsidies in affecting communities through non-trophic pathways (UV attenuation) and documents that the trophic pathway might be strongly dependent on subsidy quality. Further studies on the role of subsidies that include the less documented non-trophic pathways are needed to improve our mechanistic understanding of how communities and ecosystems respond to spatial subsidies.


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