New vs. export production on the continental shelf

The uptake of atmospheric CO2 by the oceans via the biological pump and the sustainability of fish catch are driven by new production of organic matter and its export into deep waters or its consumption by organisms of higher tropic level. The f-ratio based on 15N measurements of discrete samples for continental shelf waters is around 0.4. However, the system-level constraints imposed by mass balance indicate that this f-ratio may be too high. Specifically, it is more than twice as the value determined on the basis of net export of organic carbon. Such a discrepancy is caused by the fact that, unlike the open-ocean system where remineralized nutrients below the euphotic zone do not return easily, the remineralized nutrients on the shelf reenter the euphotic zone quickly. Since new production based on direct 15N measurements on the shelf actually utilizes nutrients remineralized on the shelf as well, the value is higher than the net export of organic carbon sustained on external sources of nutrients. It is the net export of carbon, however, that sequesters CO2 and sustains productivity on the shelf.