Estimation of f-ratios in oceans based on phytoplankton size structure

Abstract

Several equations to estimate the vertical export of particulate organic carbon from the ocean's euphotic zone (POC(E)) use variables that are determined at sea or are derived from remote sensing. One of the approaches requires reliable estimates off-ratios (NO3- uptake/total N uptake) that can be either determined directly from N uptake by phytoplankton or derived from total phytoplankton Chl a biomass or production (P(T)) or from NO3- concentrations. By using a combination of theoretical considerations and field measurements, we show that f-ratios are linear functions of size-fractionated (>5 μm/total) phytoplankton production (P(L)/P(T)) and biomass. Comparison of our model with the more usual f-ratio = f(P(T)) shows that the large residuals are spread over the range of P(L)/P(T) in the former, whereas they are concentrated at low P(T) in the latter. Because P(T) is low in most of the world oceans most of the time, use of our model may significantly improve the estimates off-ratio and thus of POC(E).