A NUMERICAL STUDY OF THE EFFECT OF PERIODIC NUTRIENT SUPPLY ON PATHWAYS OF CARBON IN A COASTAL UPWELLING REGIME

A size-based ecosystem model was modified to include periodic upwellin g events and used to evaluate the effect of episodic nutrient supply o n the standing stock, carbon uptake, and carbon how into mesozooplankt on grazing and sinking flux in a coastal upwelling regime. Two ecosyst em configurations were compared: a single food chain made up of net ph ytoplankton and mesozooplankton (one autotroph and one heterotroph, A1 H1), and three interconnected food chains plus bacteria (three autotro phs and four heterotrophs, A3H4). The carbon pathways in the All-Il si mulations were under stronger physical control than those of the A3H4 runs, where the small size classes are not affected by frequent upwell ing events. In the more complex food web simulations, the microbial pa thway determines the total carbon uptake and grazing rates, and regene rated nitrogen accounts for more than half of the total primary produc tion for periods of 20 days or longer between events. By contrast, new production, export of carbon through sinking and mesozooplankton graz ing are more important in the A1H1 simulations. In the A3H4 simulation s, the turnover time scale of the autotroph biomass increases as the p eriod between upwelling events increases, because of the larger contri bution of slow-growing net phytoplankton. The upwelling period was cha racterized for three upwelling sites from the alongshore wind speed me asured by the NASA Scatterometer (NSCAT) and the corresponding model o utput compared with literature data. This validation exercise for thre e upwelling sites and a downstream embayment suggests that standing st ock, carbon uptake and size fractionation were best supported by the A 3H4 simulations, while the simulated sinking fluxes are not distinguis hable in the two configurations.