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

Abstract. Oceanic depth profiles of plankton respiration are described by a power function, R_CO2 = (R_CO2)₀ (z/z0)^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 |b| 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 |b| 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 200 m on this ocean section.