An organic carbon budget for coastal Southern California determined by estimates of vertical nutrient flux, net community production and export

Abstract

Organic carbon export and burial in coastal upwelling regions is an important mechanism for oceanic uptake of atmospheric CO2. In order to understand how these complex systems will respond to future climate forcing, further studies of nutrient input, biological production and export are needed. Using a 7Be-based approach, we produced an 18-month record of upwelling velocity estimates at the San Pedro Ocean Time-series (SPOT), Southern California Bight. These upwelling rates and vertical nutrient distributions have been combined to make estimates of potential new production (PNP), which are compared to estimates of net community oxygen production (NOP) made using a one-dimensional, two-box non-steady state model of euphotic zone biological oxygen supersaturation. NOP agrees within uncertainty with PNP, suggesting that upwelling is the dominant mechanism for supplying the ecosystem with new nutrients in the spring season, but negligible in the fall and winter. Combining this data set with estimates of sinking particulate organic carbon (POC) flux from water column 234Th:238U disequilibrium and sediment trap deployments, and an estimate of the ratio of dissolved organic carbon (DOC):POC consumption rates, we construct a simple box model of organic carbon in the upper 200m of our study site. This box model (with uncertainties of ±50%) suggests that in spring, ~28% of net production leaves the euphotic zone as DOC, of this, ~12% as horizontal export and ~16% via downward mixing. The remaining ~72% of net organic carbon export exits as sinking POC, with only ~10% of euphotic zone export reaching 200m. We find the metabolic requirement for the local heterotrophic community below the euphotic zone, but above 200m, is ~105±50mmolCm−2d−1, or ~80% of net euphotic zone production in spring.