Investigating the Nutrient Landscape in a Coastal Upwelling Region and Its Relationship to the Biological Carbon Pump

We investigated nutrient patterns and their relationship to vertical carbon export using results from 38 Lagrangian experiments in the California Current Ecosystem. The dominant mode of variability reflected onshore-offshore nutrient gradients. A secondary mode of variability was correlated with silica excess and dissolved iron and likely reflects regional patterns of iron limitation. The biological carbon pump was enhanced in high-nutrient and Fe-stressed regions. Patterns in the nutrient landscape proved to be better predictors of the vertical flux of sinking particles than contemporaneous measurements of net primary production. Our results suggest an important role for Fe-stressed diatoms in vertical carbon flux. They also suggest that either preferential recycling of N or non-Redfieldian nutrient uptake by diatoms may lead to high PO43-:NO3- and Si(OH)(4):NO3- ratios, following export of P- and Si-enriched organic matter. Increased export following Fe stress may partially explain inverse relationships between net primary productivity and export efficiency. Plain Language Summary The productivity of marine ecosystems is limited by the availability of macronutrients (nitrogen and phosphorus) and trace elements (iron) in the sunlit surface ocean. The ocean's ability to absorb atmospheric carbon dioxide through the biological carbon pump is further constrained by the rates at which oceanic upwelling naturally fertilizes the surface ocean with new nitrogen contained in nutrient-rich deep water. We investigated patterns in nutrient distributions during 38 experiments in the California Current Ecosystem. In these experiments, we followed biological communities as they were transported with the currents and measured nutrients, primary productivity, and the export of organic carbon contained in sinking particles. Our results indicate that nutrient concentrations may be a useful predictor of rates of carbon export and that iron stress increased the efficiency with which organic carbon created by algae is transported to the deep ocean. This increased carbon export efficiency likely results from physiological changes within diatoms that lead to thicker silica shells relative to organic carbon content.