Fluxes of carbon in the upper ocean: regulation by food-web control nodes

We present a new approach to assess the role of upper ocean pelagic food webs on the partitioning of phytoplankton production (P-T) into its 3 principal component fluxes: remineralization to CO2 (i.e. respiration, R), transfer to the pelagic food web (F), and downward export (ET); ET is the sum of its particulate (POC) and dissolved (DOC) organic carbon components (E-T = E-DOC + E-POC). Although it is well known that there are relationships between the size and trophic structure of the planktonic community on the one hand, and the export of organic carbon (OC) from the euphotic zone and its potential sequestration below the permanent pycnocline on the other hand, the causative mechanisms for these relationships are not well understood, Here, we propose that the fluxes of OC in the upper ocean depend on the coexistence of a relatively small POC pool, which is responsible for the fluxes P-T, R, F and E-POC, and a much larger DOC pool, which sustains both bacterial production and E-DOC. In our model, phytoplankton, microbial heterotrophic plankton, and large zooplankton are the 3 food-web control nodes of the 5 carbon fluxes (P-T, R, F, E-DOC and E-DOC). The phytoplankton node controls the downward flux of phytodetritus (mostly from large phytoplankton), which is often the major component of Epoc. The microbial heterotrophic plankton node is responsible for most of the remineralization of OC to CO2 and the uptake and release of DOC. This node therefore controls the size of the DOC pool that can be exported downwards. The large zooplankton node controls both the transfer of POC to large metazoans and part of the downward POC flux (Epoc; faecal pellets and vertically migrating organisms). We implemented our model by estimating export as E-T = P-T - R at 8 sites in different regions of the World Ocean. The functional relationship between E-T and POC was highly significant (r(2) = 0.85): In contrast to other approaches, where export is calculated as a fraction of P-T, we estimate E-T as the difference between 2 independent variables (i.e. PT and R); hence our approach produces some regional values E-T < 0 - these regions are net heterotrophic. Overall, our approach improves our understanding of carbon cycling and export in the upper ocean.