Carbon flux, nutrient retention efficiency and the curvature of respiration depth-profiles in the ocean water column

Abstract

Respiration–depth profiles of microplankton and zooplankton below their respiration maximum in the ocean often can be described mathematically by a power function, RCO 2 = (RCO 2 )0 (z/z0)-b. The magnitude of “b” defines the curvature of the depth-profile. When b is greater than zero, (RCO 2 ) 0 (z/z 0 )- b decreases with depth at a rate, d[(RCO 2 ) 0 (z/z 0 )-b]/dz, proportional to the magnitude of b. This means that even if water columns have the same RCO 2 at their respiration maximum, the profile with the highest b will have the lowest RCO 2 at 100 m. The significance of this variability is that when b is small the carbon flux from the epipelagic ocean is high, there is little loss to respiration between depths, and the nutrient retention efficiency (NRE) in the epipelagic ocean is low. When the difference in the carbon flux between two layers normalized by the carbon flux of the upper layer is high the NRE is high, when this relative flux difference is low the NRE is low. The b value for the respiration profiles is inversely related to carbon flux transfer efficiency. When the value for b is high, the relative carbon flux difference between two depths is great, but the transfer efficiency is low. The crux of this is that the attenuation of respiration is critical to understanding both vertical carbon flux in ocean water columns as well as understanding the differences in the capacity of epipelagic ecosystems in retaining their organic carbon and their vital nutrients.