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

Abstract

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, [NO3], on nitrate uptake. The standard assumption that maximum uptake rate, Vmax, is independent of ambient nutrient concentration, Sa, contrasts with the prediction of the recently developed Optimal Uptake kinetics that Vmax should increase hyperbolically with Sa. Assuming Arrhenius T dependence, I fit the respective equations to field observations of chlorophyll-specific Vmax, T and [NO3]. 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. Copyright © 2010 by the American Geophysical Union.