Small-Scale Variability of New Production and Particulate Fluxes in the Ocean

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Abstract
Particulate carbon losses from the surface waters of the ocean can be estimated from short-term (.ltoreq. 1 d) determinations of the contribution of nitrate to total nitrogen uptake (f(t)), under the assumption that the nitrate flux represents the supply of N from external sources. We used numerical modeling of the euphotic food web to explore the relationship between short-term variability in f(t) and the long-term export normalized to production, (f). We found that modeling sedimentation as a nonlinear process was necessary to simulate variability in f(t). The resulting model generated nonlinear relationships of f(t) to PT and f(t) to [NO3]that, to date, have only been empirically described. The shape of the f ratio curves depends on an exponent n that expresses a power law dependence of the sedimentation rate on the concentration of suspended material in the euphotic zone, for which there is increasing empirical support. Simulations of the model with a temporally variable NO3 supply show that particulate losses can amount to 30-40% of production in seasonal oligotrophic systems. The model suggests that (f) in N-limited ocean provinces depends on the variance of the nutrient field and on the sensitivity of the loss rate to variations in the plankton biomass in the euphotic zone.