Rates and drivers of Red Sea plankton community metabolism

Abstract. Resolving the environmental drivers shaping planktonic communities is fundamental for understanding their variability, in the present and the future, across the ocean. More specifically, addressing the temperature-dependence response of planktonic communities is essential as temperature plays a key role in regulating metabolic rates and thus potentially defining the ecosystem functioning. Here we quantified plankton metabolic rates along the Red Sea, a uniquely oligotrophic and warm environment, and analysed the drivers that regulate gross primary production (GPP), community respiration (CR), and net community production (NCP). The study was conducted on six oceanographic surveys following a north–south transect along the Saudi Arabian coast. Our findings revealed that GPP and CR rates increased with increasing temperature (R2=0.41 and 0.19, respectively; p<0.001 in both cases), with a higher activation energy (Ea) for GPP (1.20±0.17 eV) than for CR (0.73±0.17 eV). The higher Ea for GPP than for CR resulted in a positive relationship between NCP and temperature. This unusual relationship is likely driven by the relatively higher nutrient availability found towards the warmer region (i.e. southern Red Sea), which favours GPP rates above the threshold that separates autotrophic from heterotrophic communities (1.7 mmol O2 m−3 d−1) in this region. Due to the arid nature, the basin lacks riverine and terrestrial inputs of organic carbon to subsidise a higher metabolic response of heterotrophic communities, thus constraining CR rates. Our study suggests that GPP increases steeply with increasing temperature in the warm ocean when relatively high nutrient inputs are present.