Non-redfield carbon and nitrogen cycling in the Sargasso Sea: pelagic imbalances and export flux

An ecosystem model embedded in a one-dimensional physical model is used to study the stoichiometry of carbon and nitrogen cycling at the Bermuda Atlantic Time Series site. The model successfully provides a budget for the processes contributing to the drawdown of dissolved inorganic carbon (DIC) that is observed in surface waters in the absence of detectable nitrate throughout much of the summer. The modeled drawdown is initially driven by export fueled by in situ N and accumulation of dissolved organic carbon, with continued DIC consumption after nutrient exhaustion resulting largely from nitrogen fixation and outgassing of CO2 to the atmosphere. The modeled export flux of organic C at 300 m is dominated by particles (81%), with a nevertheless significant fraction (19%) due to dissolved organic matter. The predicted combined C/N of particulate and dissolved export increases from 10.8 at 70 m to 14.3 at 300 m, because of preferential remineralization of N. In theory, at least as a first approximation, the ratio of net consumption of DIC and nutrients in the euphotic zone is equivalent to this C/N of export. However, the C/N of consumption during DIC drawdown averaged 23.5 (>10.8), indicating that this assumption is not always valid and C/N ratios of nutrient consumption cannot reliably be used to estimate the export ratio, which is difficult to measure directly. The work highlights the complex interplay between the cycling of C and N the upper ocean and the resultant export flux.