Abstract

Rising ocean temperatures affect marine microbial ecosystems directly, since metabolic rates (e.g., photosynthesis, respiration) are temperature-dependent, but temperature also has indirect effects mediated through changes to the physical environment. Empirical observations of the long-term trends in biomass and productivity measure the integrated response of these two kinds of effects, making the independent components difficult to disentangle. We used a combination of modeling approaches to isolate the direct effects of rising temperatures on microbial metabolism and explored the consequences for food web dynamics and global biogeochemistry. We evaluated the effects of temperature sensitivity in two cases: first, assuming that all metabolic processes have the same temperature sensitivity, or, alternatively, that heterotrophic processes have higher temperature sensitivity than autotrophic processes. Microbial ecosystems at higher temperatures are characterized by increased productivity but decreased biomass stocks as a result of transient, high export events that reduce nutrient availability in the surface ocean. Trophic dynamics also mediate community structure shifts resulting in increased heterotroph to autotroph ratios at higher temperatures. These ecosystem thermal responses are magnified when the temperature sensitivity of heterotrophs is higher than that of autotrophs. These results provide important context for understanding the combined food web response to direct and indirect temperature effects and inform the construction and interpretation of Earth systems models used in climate projections.

Key Points

• Warming results in increased productivity, but decreased biomass, in marine microbial food webs due to the thermal dependence of metabolism

• Higher temperatures disproportionately favor higher trophic levels, increasing the ratio of heterotrophs to autotrophs

• Thermal responses are amplified if heterotrophic and autotrophic processes have different temperature sensitivity coefficients

Plain Language Summary

Warming oceans cause a myriad of changes to marine ecosystems, including both biological changes to the organisms themselves and physical changes to the environment. Here, we use mathematical models to isolate the effects of warming that arise directly from temperature's effect on metabolic rates, and the resulting changes to marine food webs and the global carbon cycle. We focus on how different metabolic rates (e.g., photosynthesis, grazing) may have different temperature sensitivities and the consequences of those differences on the overall thermal sensitivity of marine ecosystems. We found that marine food webs had higher productivity, but less overall biomass, when temperatures increase. These effects were amplified when grazing had greater temperature sensitivity than photosynthesis. Increased temperature also had effects on community and food web structure. These results provide important context for the kinds of global models that are used in climate change projections.