Peruvian upwelling plankton respiration: calculations of carbon flux, nutrient retention efficiency, and heterotrophic energy production

Oceanic depth profiles of plankton respiration are described by a power function, R-CO2 = (R-CO2)(0)(z = z(0))(b), similar to the vertical carbon flux profile. Furthermore, because both ocean processes are closely related, conceptually and mathematically, each can be calculated from the other. The exponent b, always negative, defines the maximum curvature of the respiration-depth profile and controls the carbon flux. When vertical bar b vertical bar is large, the carbon flux (F-C) from the epipelagic ocean is low and the nutrient retention efficiency (NRE) is high, allowing these waters to maintain high productivity. The opposite occurs when vertical bar b vertical bar is small. This means that the attenuation of respiration in ocean water columns is critical in understanding and predicting both vertical F-C as well as the capacity of epipelagic ecosystems to retain their nutrients. The ratio of seawater R-CO2 to incoming F-C is the NRE, a new metric that represents nutrient regeneration in a seawater layer in reference to the nutrients introduced into that layer via F-C. A depth profile of F-C is the integral of water column respiration. This relationship facilitates calculating ocean sections of F-C from water column respiration. In an F-C section and in a NRE section across the Peruvian upwelling system we found an F-C maximum and a NRE minimum extending down to 400 m, 50 km off the Peruvian coast over the upper part of the continental slope. Finally, considering the coupling between respiratory electron transport system activity and heterotrophic oxidative phosphorylation promoted the calculation of an ocean section of heterotrophic energy production (HEP). It ranged from 250 to 500 J d(-1) m(3) in the euphotic zone to less than 5 J d(-1) m(3) below 200m on this ocean section.