Abstract
Atmospheric deposition is an important source of phosphorus (P) to the tropical North Atlantic, where phytoplankton productivity is often limited by its availability. In this study, the seasonality of soluble phosphorus deposition was investigated using aerosol samples collected at Ragged Point, Barbados during two Sub-Saharan aerosol transport regimes: winter characterized by elevated smoke influence and summer characterized by substantial dust loading. Soluble reactive phosphorus (SRP), a proxy for dissolved inorganic phosphorus, was measured from seawater leaches of aerosol samples, and these results were compared to previously characterized aerosol properties, including dust mass, smoke number fraction, and ion concentrations. Despite much higher dust mass concentrations in summer, SRP concentrations were of similar magnitude between seasons. When normalized to dust mass, however, winter aerosols exhibited SRP:dust ratios approximately two times higher than summer aerosols, indicating more soluble phosphorus per unit dust. During MAGPIE, SRP measurements were consistent with dust as the dominant source of soluble phosphorus by abundance. In contrast, winter EUREC4A aerosols showed a more mixed signal: SRP was strongly related to smoke number fraction, NO3- , and nss-K+, but not to nss-Ca2+ or nss-SO42-. These results suggest that dust and biomass burning influence soluble phosphorus deposition through different pathways, with dust acting as a quantity-driven source and biomass burning exerting a stronger influence on the soluble phosphorus fraction. Overall, this work shows that seasonality and aerosol type influence not only the P delivered to the tropical North Atlantic, but also the fraction available to marine phytoplankton, with implications for marine productivity and carbon sequestration.