Influence of atmospheric nutrients on primary productivity in a coastal upwelling region

Atmospheric deposition is an important source of nutrients to the coastal and open ocean; however, its role in highly productive upwelling regions like coastal California has not been determined. Approximately 0.1%-0.2% of new production is attributable to atmospheric deposition of nitrogen (N) annually, but if the estimate is expanded to encompass the effects of iron (Fe), aerosols may support 1%-2% of new production on average, and up to 5% on days with high deposition fluxes. Laboratory culture and in situ incubation experiments confirm the bioavailability of N from dry deposition in this region. A significant positive relationship between aerosol optical thickness and chlorophyll a derived from the Moderate Resolution Imaging Spectroradiometer is observed for the summer months and is stronger offshore than near the coast. Moreover, the portion of productivity supported by atmospheric deposition is higher on days without upwelling and during El Nino periods when nutrient input from upwelling is suppressed, a phenomenon that could become more prevalent due to climate warming. Expanding the results from California, we estimate that dry deposition could increase productivity in other major coastal upwelling regions by up to 8% and suggest that aerosols could stimulate productivity by providing N, Fe, and other nutrients that are essential for cell growth but relatively deplete in upwelled water.