Abstract
Lakes are undergoing striking physicochemical changes globally, including co‐occurring increases in dissolved organic carbon and nutrient concentrations, water color, and surface temperature. Although several experimental studies of lake browning and warming have been conducted over the last decade, knowledge remains limited as to the structural and functional responses of multitrophic plankton communities, especially under environmentally relevant physicochemical conditions. Using reverse osmosis to manipulate naturally occurring dissolved organic matter (DOM), we performed an enclosure experiment to evaluate the response of a planktonic food web (zooplankton–phytoplankton–bacterioplankton) to individual and combined increases in DOM and temperature, while accounting for changes in inorganic nutrients associated with DOM enrichment. We found that concomitant increases in DOM and temperature or inorganic nutrients elicited substantially greater biotic effects, but infrequently led to interactive effects. Overall, major plankton groups responded differently to manipulated factors, with most effects observed in standing stocks, community composition, and trophic structure, while metabolic (primary production and respiration) rates appeared to be generally less responsive. DOM enrichment had a clear stimulatory effect on phytoplankton, but weakly affected zooplankton. More specifically, DOM enrichment, alone or combined with inorganic nutrient amendments, decreased zooplankton : chlorophyll a ratios, implying a reduced trophic transfer efficiency and altered trophic structure. Warming generally increased bacterial abundance and cyanobacterial dominance, especially under DOM‐enriched conditions. Collectively, these results demonstrate that increasing local DOM, even by only ~ 2 mgC L −1 , can enhance plankton responses to rising temperature or inorganic nutrients in the near‐surface layer of a clearwater lake, with potential implications for ecosystems facing co‐occurring environmental changes.