CO2 CYCLING IN THE COASTAL OCEAN .1. A NUMERICAL-ANALYSIS OF THE SOUTHEASTERN BERING SEA WITH APPLICATIONS TO THE CHUKCHI SEA AND THE NORTHERN GULF-OF-MEXICO

A quasi-two dimensional model of the carbon and nitrogen cycling above the 70m isobath of the southeastern Bering Sea at 57-degrees-N replicates the observed seasonal cycles of nitrate, ammonium, SIGMACO2, pCO2, light penetration, chlorophyll, phytoplankton growth rate, and primary production, as constrained by changes in wind, incident radiation, temperature, ice cover, vertical and lateral mixing, grazing stress, benthic processing of phytodetritus and zooplankton fecal pellets, and the pelagic microbial loop of DOC, bacteria, and their predators. About half of the seasonal resupply of nitrate stocks to their initial winter conditions is derived from in situ nitrification, with the rest obtained from deep-sea influxes. Under the present conditions of atmospheric forcing, shelf-break exchange, and food web structure, this shelf ecosystem serves as a sink for atmospheric CO2, with storage in the forms of exported DOC, DIC, and unutilized POC (phytoplankton, bacteria, and fecal pellets). As a consequence of just the rising levels of atmospheric pCO2 since the Industrial Revolution, however, the biophysical CO2 status of the Southeastern Bering Sea shelf may have switched over the last 250 years, from a prior source to the present sink, since this relatively pristine ecosystem has undergone little eutrophication. Such fluctuations of CO2 status may thus be reversed by the physical processes of: (1) reduction of atmospheric pCO2, (2) increased onwelling of deep-sea SIGMACO2, and (3) warming of shelf waters. Based on our application of this model to the Chukchi Sea and the Gulf of Mexico, about 1.0-1.2 gigatons C y-1 of atmospheric CO2 may now be sequestered by temperate and polar shelf ecosystems. When tropical systems are included, however, a positive net sink of only 0.6-0.8 x 10(15)g C y-1 may prevail over all shelves.