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A nitrogen based model for the cycling of carbon dioxide in the subarctic Pacific Ocean Zeng, Jiye

Abstract

This model explores the integration of a physical mixing model with an ecological model for the studies of the CO2 problem in the ocean. The model is specifically for the regional CO2 cycling at Ocean Station P (OSP, 50° N, 145°W) and is composed of a physical submodel for physical processes and a biological submodel for biological processes. By dividing the water column into a number of layers, I have defined physical processes as those affecting material flux between boxes and biological processes as affecting material flow within a box. The direct target of the model is the cycling of nitrogen from which the cycling of CO2 can be derived. The physical submodel basically deals with the turbulent mixing, particulate sinking, and zooplankton migration at OSP. Zooplankton migration is included in the physical submodel because it acts between boxes rather than within boxes. Considering the theoretical simplicity, I used the Mellor and Yamada's level II model for the turbulent mixing. Although available boundary conditions are not sufficient to give a satisfactory continuous time modelling of the mixing, the model did provide a practical formula for estimating the seasonal turbulent mixing coefficient from the temperature profile, with assumptions of a steady-state temperature and constant wind induced current shear. The advantage of this method is that the mixing coefficient in the mixed-layer, which is commonly treated as infinite, can be actually calculated. The mixing coefficient in and below the mixed-layer of OSP obtained by my method is well within the range of those obtained by other methods (Anderson et al., 1977; Denman and Gargett, 1983). My estimation gives a approximate number of 2.5x10-3m2 s-1 for the mixed-layer and 1.0-4.0x10-4m2 s1 for layers below. Using mixing coefficients in an ecological model is one of the advances that this research contributes to the modelling of the biological pump in CO2 cycling. The biological submodel is basically derived from the model of Fasham et al.(1990) and contains the basic components of ammonium, nitrate, phytoplankton, zooplankton, bacterial, DON, PON, and a higher trophic zooplankton. The main difference of this biological model from others is that biological components are allowed to move in the whole water column rather than being confined in one box. The integration of the biological submodel with the physical submodel eliminate the boundary limitation from a one-box model which sets the boundary at the bottom of the mixed-layer. In general, the model reproduces almost all basic ecological characteristics observed at OSP, including (1) Seasonal variation of phytoplankton and zooplankton biomass, (2) seasonal primary production and PON flux, and (3) seasonal ammonium and nitrate concentration and the f-ratio. The model confirmed that the phytoplankton growth at OSP is controlled by zooplankton grazing. Reducing zooplankton grazing would result in high phytoplankton stocks in the spring, as is observed in other regions (Raymont, 1980). However, other unknown factors (e.g. Fe, Martin et al. (1989)) may affect the growth of phytoplankton, especially their preference for ammonium (Price et al., 1991). The model indicates that the food web at OSP has a high efficiency in converting nitrogen from phytoplankton to zooplankton. This factor, combined with the high utilization ratio of ammonium by phytoplankton and upwelling rate, may bean explanation why a high nitrate concentration can remain in such a highly productive open sea. By comparing the simulated results with observations, I conclude that an annualCO2 flux of 100 g m-2 y' would be a reasonable estimate for OSP. But the net uptake of CO2 from the atmosphere can not be estimated accurately by a one-dimensional model. According to Miller et al. (1991), new nutrient input to the euphotic zone of OSP is mainly determined by the upwelling convection. However, the model indicates that the turbulent mixing may play an equal role in this input. The results of the model support the conclusion of Kirchman and Keil (1990) that bacterial growth at OSP is limited by organic carbon. The results also point out that the bacterial production maybe as high as the primary production and therefore sustain about half of the food requirements of zooplankton.

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