Thermal responses in global marine planktonic food webs mediated through
temperature effects on metabolism
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, that all metabolic
processes have the same temperature sensitivity, and alternatively, that
heterotrophic processes have higher temperature sensitivity than
autotrophic processes. No other study has explored the direct effects of
temperature on ecosystem provisioning (primary productivity, biomass,
export) independently of the associated changes to the physical
environment that result from warming. Microbial ecosystems at higher
temperatures are characterized by increased productivity, but decreased
biomass stocks as a result of transient, high export events that remove
biomass from the surface ocean. Trophic dynamics also mediate changes to
community size structure, resulting in longer food chains and increased
mean body size 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.