Effect of anthropogenic warming on microbial respiration and particulate organic carbon export rates in the sub-Antarctic Southern Ocean

Abstract

Microbial respiration of particulate organic carbon (POC) is one of the key processes controlling the magnitude of POC export from the surface ocean and its storage on long timescales in the deep. Metabolic processes are a function of temperature, such that warming sea temperatures should increase microbial respiration, potentially reducing POC export. To investigate this in the Southern Ocean, we measured microbial oxygen consumption of large particles over a 10°C temperature range (summer maximum + 8°C) to then estimate the decrease in export by 2100. Our results showed that POC-normalised respiration increased with warming. We estimate that POC export (scaled to primary production) could decrease by 17 ± 7% (SE) by 2100, using projected regional warming (+1.9°C) from the IPCC RCP 8.5 ('business-as-usual' scenario) for our sub-Antarctic site. Increased microbial respiration is one of many processes that will be altered by future climate change, which could all modify carbon storage in the future. Our estimate of the potential decline in carbon sequestration is within previous estimates from lab and field experiments, but higher than simple mechanistic models. To explore our results further, we used the metabolic theory of ecology (MTE) to determine the activation energy of microbial respiration, which was 0.9 eV. This is higher than classical MTE (0.6−0.7 eV), suggesting that sub-Antarctic microbes are particularly sensitive to temperature change. Such regional characteristics in the response of organisms to increased temperatures should be accounted for in large-scale or global model analyses to ensure that the results do not underestimate microbial responses to warming.

The Influence of Plankton Community Structure on Sinking Velocity and Remineralization Rate of Marine Aggregates

“…The relatively low values are somewhat surprising as our incubation temperature ( T ) of 22 °C was high compared to other studies (Table ). For instance, C remin was at least an order of magnitude higher for particles collected at the Southern Ocean Time Series, even though T was 10 °C lower (Table ; Cavan & Boyd, ). Likewise, our values were at least 4 times lower than C remin measured in situ at the Bermuda Atlantic Time‐series Study (Table ; Mcdonnell et al, ) even though the oceanographic setup is relatively similar.…”

“…Minimizing concentration changes in the course of the incubation is important because the O 2 concentrations itself has an influence on the measured rate (Holtappels et al, ; Ploug & Bergkvist, ). O 2 consumption rates (μmol · O 2 · L · day) were converted to CO 2 production rates (μmol · C · L · day) assuming a widely used respiratory quotient (RQ) of 1‐mol O 2 consumed: 1‐mol CO 2 produced (Belcher et al, ; Cavan & Boyd, ; Iversen & Ploug, ; Ploug & Grossart, ). The C‐specific remineralization rate ( C remin in d −1 ) was calculated as where POC inc is the POC content (μmol · C · L) measured in the incubation bottles and r sediment and r control are the O 2 consumption rates in the bottles with and without sediment trap material, respectively (Figure S2).…”

“…Minimizing concentration changes in the course of the incubation is important because the O 2 concentrations itself has an influence on the measured rate (Holtappels et al, 2014;Ploug & Bergkvist, 2015). O 2 consumption rates (μmol · O 2 · L · day) were converted to CO 2 production rates (μmol · C · L · day) assuming a widely used respiratory quotient (RQ) of 1-mol O 2 consumed: 1-mol CO 2 produced (Belcher et al, 2016;Cavan & Boyd, 2018;Iversen & Ploug, 2013;Ploug & Grossart, 2000). The C-specific remineralization rate (C remin in d −1 ) was calculated as…”

The Sensitivity of Subsurface Microbes to Ocean Warming Accentuates Future Declines in Particulate Carbon Export

“…Warming is projected to increase metabolic rates (Brown et al, 2004), which will increase both primary production (more carbon available to sink to deep ocean) and the metabolic rates of heterotrophs that consume phytoplankton and sinking POC (decreasing the carbon sink) (López-Urrutia et al, 2006;Taucher and Oschlies, 2011;Cavan and Boyd, 2018). In parallel warming-induced stratification and shoaling of the mixed layer will reduce nutrient inputs to the sunlit upper ocean (Bopp et al, 2001), expanding oligotrophic waters thus likely favoring smaller phytoplankton species and ultimately reducing primary production (Bopp et al, 2005).…”

“…MTE states all organisms have an activation energy of 0.6-0.7 eV (Gillooly et al, 2001;Brown et al, 2004), thus there is little natural variation in temperature sensitivity. Activation energies higher than this range suggest organisms are more sensitive to changes in temperature, as shown in Arctic, and Antarctic zooplankton [E a = 1.29 eV, (Alcaraz, 2016)], mesopelagic heterotrophs [E a = 0.9 eV, (Brewer and Peltzer, 2016)] and a mixed heterotrophic microbial community from the sub-Antarctic [E a = 0.9 eV, (Cavan and Boyd, 2018)]. As primary production and respiration (remineralisation) are both metabolic rates, MTE can be used to describe how they might change with future warming in the global oceans, and thus their influence on POC export.…”