Interactive effects of higher temperature and dissolved organic carbon on planktonic communities in fishless mountain lakes

1. We tested the hypothesis that higher temperature and dissolved organic carbon (DOC) concentration increase dissolved and particulate carbon (C) relative to phosphorus (P), thereby reducing algal food quality for P‐limited cladocerans while not affecting N‐limited copepods. Also, we expected alpine zooplankton to respond more strongly than those from warmer montane lakes to increased water temperature.
2. Plankton from two alpine lakes and two montane lakes were incubated in vitro for 30 days at 10 or 17 °C and with ambient or +80% DOC, which was achieved by concentrating humic substances from each lake via reverse osmosis.
3. Dissolved organic carbon amendments and warming significantly increase particulate C : P under montane, but not alpine conditions. While higher water temperature and DOC separately reduced phytoplankton abundance, together they increased phytoplankton by stimulating uptake of P. Warming stimulated only Daphnia while suppressing the abundance of the calanoid copepod Hesperodiaptomus when they originated from the three coldest lakes. Particulate C : P was positively correlated with Daphnia abundance and negatively correlated with Hesperodiaptomus, probably due to greater P‐retention by Daphnia.
4. Our findings highlight the importance of interactions between the ecological effects of higher temperature and increased inputs of terrestrial organic matter to forecasts of the net impact of global warming on mountain lakes. Such predictions may be confounded if they are derived solely from the expected sum of single effects by each climatic factor.