Untangling the uncertainties about combined effects of temperature and concentration on nutrient uptake rates in the ocean

摘要

I show that assumptions about how uptake rates depend on concentration strongly impact the interpretation of field observations, specifically with respect to the combined effects of temperature, T, and nitrate concentration, [NO_3], on nitrate uptake. The standard assumption that maximum uptake rate, V_(max), is independent of ambient nutrient concentration, Sa, contrasts with the prediction of the recently developed Optimal Uptake kinetics that V_(max) should increase hyperbolically with Sa. Assuming Arrhenius T dependence, I fit the respective equations to field observations of chlorophyll-specific V_(max), T and [NO_3]. The inferred sensitivity to T differs by a factor of two. Considerable uncertainty therefore remains about the T dependence of uptake rates, and therefore about biological production and biogeochemical cycles. Given that both climate change and anthropogenic nutrient inputs are altering the relationship between T and nutrients in the ocean, these uncertainties limit our understanding of the direct effects and associated feedbacks.