Quantifying Primary Productivity Rates from Coastal to Open Waters in the NE Pacific Ocean: A Contribution to the 2018 EXPORTS Program

Marine phytoplankton contribute ~50% of the carbon fixed annually by primary producers on Earth, and play a critical role in the export of particulate nutrients and carbon to the ocean interior. Phytoplankton are therefore key players in controlling the partitioning of carbon between the ocean and atmosphere, and have a profound effect on Earth’s climate. In particular, diatoms are responsible for ~40% of annual marine carbon fixation, control the cycling of silicon in the ocean, and help in ballasting other nutrients to deep waters via their heavy silica frustule. During a research cruise to the iron-limited NE Pacific in September of 2018, we conducted productivity measurements in the euphotic zone to assess the role of different phytoplankton to total production. We used isotopic tracers and onboard incubations to determine net (¹³C), new (¹⁵NO₃), regenerated (¹⁵NH₄ and ¹⁵N-urea), gross (H₂¹⁸O), and siliceous (³²Si) primary productivity in two size fractions (> and < 5 µm) from the coast of Vancouver Island into the open subarctic NE Pacific (along Line P) and at the NASA EXPORTS program station. In addition, we quantified net community production using in situ O₂/Ar measurements along Line P and in a small grid around the EXPORTS station. We also measured biogenic silica, nitrogen, carbon and chlorophyll a for both size fractions of suspended particles. In contrast to historical data, we found that biogenic silica and the larger fraction (> 5 µm) of chlorophyll a were higher at the EXPORTS station, in HNLC open ocean waters, than in the intermediate stations along Line P. In this presentation, we will compare productivity estimates and assess the contribution of the different phytoplankton size classes and diatoms to vertical nutrient export. These simultaneous measurements of productivity provide a powerful tool to understand and estimate the biological carbon pump in the ocean.