Interactive effects of nutrients and temperature on herbivorous predation in a coastal plankton community

Marine microbial communities in coastal environments are subject to both seasonal fluctuations and anthropogenic alterations of environmental conditions. The separate influences of temperature and resource-dependency on phytoplankton growth, community, and ecosystem metabolism are relatively well understood. However, winners and losers in the ocean are determined based on the interplay among often rapidly changing biological, chemical and physical drivers. The direct, indirect, and interactive effects of these conditions on planktonic food web structure and function are poorly constrained. Here, we investigated how simultaneous manipulation of temperature and nutrient availability affects trophic transfer from phytoplankton to herbivorous protists, and their resulting implications at the ecosystem level. Temperature directly affected herbivorous protist composition; ciliates dominated (66%) in colder treatment and dinoflagellates (60%) at warmer temperatures. Throughout the experiments, grazing rates were < 0.1 d(-1), with higher rates at subzero temperatures. Overall, the nutrient-temperature interplay affected trophic transfer rates antagonistically when nutrients were amended, and synergistically, when nutrients were not added. This interaction resulted in higher percentages of primary production consumed under nutrient unamended compared to nutrient amended conditions. At the ecosystem level, these changes may determine the fate of primary production, with most of the production likely exported out of the pelagic zone in high-temperature and nutrient conditions, while high-temperature and low-nutrient availability strengthened food web coupling and enhanced trophic transfer. These results imply that in warming oceans, management of coastal nutrient loading will be a critical determinant of the degree of primary production removal by microzooplankton and dependent ecosystem production.

Automated summary

Marine microbial communities in coastal environments are influenced by seasonal fluctuations and anthropogenic alterations. The effects of temperature and nutrient availability on phytoplankton growth and ecosystem metabolism are well understood, but the impacts on food web structure and function are not well constrained. This study investigated the interactive effects of temperature and nutrient availability on trophic transfer and found that the interplay of these factors influenced the consumption of primary production. The results suggest that nutrient management will be critical in determining the fate of primary production and ecosystem production in warming oceans.