Anthropogenically Forced Changes in the Balance Between the Supply of and Demand for Organic Carbon in the Upper Ocean: Is the Estimation of Heterotrophic Organic Carbon Demand Biased?

The balance between supply of and demand for organic carbon (OC) in upper ocean can change in response to anthropogenic forcings, and thus influence ecosystem services including carbon sequestration and fisheries. The heterotrophic demand for OC (D) is fulfilled by the net supply of OC, i.e. S – X, where S is the gross supply of OC to upper ocean, and X is OC export. Hence (S – X) ≥ D (Eq. 1). At very short time scales (<1 day), D = (P + R + E) (Eq. 2), where P, R and E are heterotrophic community production, respiration and excretion, respectively; hence (S – X) ≥ (P + R + E) (Eq. 3). In Eq. 3, P and R are generally larger than E and X, hence D ≈ (P + R) (Eq. 4) and S ≥ D (Eq. 5). In oceanic waters, OC is mainly supplied by phytoplankton production (PP), i.e. S = PP (Eq. 6). At long (annual or longer, i.e. steady-state) time scales all OC in the upper ocean is respired or exported: (PP – X) = R (Eq. 7) or PP = (R + X) (Eq. 8), and D = R (Eq. 9). Equations 7 to 9 do not include P or E because at steady state all OC in P and E is either recycled or channelled toward R or X. There is a frequent inconsistency in the estimation of heterotrophic demand for OC in the literature, i.e. for long time scales Eq. 4 is often used instead of Eq. 9, thus leading to incorrect conclusions concerning the balance between OC supply and demand and potentially the trophic status of the ecosystem. Also, R and X are not easy to estimate at large spatial and long temporal scales. Deriving consistent conclusions about the balance between OC supply and demand in the present and future ocean requires that Eqs. 7 to 9 are used when considering long time scales, and Eq. 4 for short time scales only. Using formulations that correctly represent the important processes at the time scale under consideration is required for constraining the ocean’s present and future ecosystem services.