Temperature effects on export production in the open ocean

A pelagic food web model was formulated with the goal of developing a quant itative understanding of the relationship between total production, export production, and environmental variables in marine ecosystems. The model ass umes that primary production is partitioned through both large and small ph ytoplankton and that the food web adjusts to changes in the rate of allocht honous nutrient inputs in a way that maximizes stability, i.e., the ability of the system to return to steady state following a perturbation. The resu lts of the modeling exercise indicate that ef ratios, defined as new produc tion/total production = export production/total production, are relatively insensitive to total production rates at temperatures greater than similar to 25 degreesC and lie in the range 0.1-0.2, At moderate to high total prod uction rates, ef ratios are insensitive to total production and negatively correlated with temperature. The maximum ef ratios are similar to0.67 at hi gh rates of production and temperatures of 0 degrees -10 degreesC. At tempe ratures less than similar to 20 degreesC, there is a transition from low ef ratios to relatively high ef ratios as total production increases from low to moderate values. This transition accounts for the hyperbolic relationsh ip often presumed to exist between ef ratios and total production. At low r ates of production the model predicts a negative correlation between produc tion and ef ratios, a result consistent with data collected at station ALOH A (22 degrees 45'N, 158 degreesW) in the North Pacific subtropical gyre. Th e predictions of the model are in excellent agreement with results reported from the Joint Global Ocean Flux Study (JGOFS) and from other field work. Tn these studies, there is virtually no correlation between total productio n and ef ratios, but temperature alone accounts for 86% of the variance in the ef ratios. Model predictions of the absolute and relative abundance of autotrophic and heterotrophic microorganisms are in excellent agreement wit h data reported from field studies. Combining the ef ratio model with estim ates of ocean temperature and photosynthetic rates derived from satellite d ata indicates that export production on a global scale is similar to 20% of net photosynthesis. The results of the model have important implications f or the impact of climate change on export production, particularly with res pect to temperature effects.