Controls on Iron Bioavailability in the Iron Limitation Mosaic of the California Current System

The California Current System (CCS) is a highly productive marine ecosystem, influenced by seasonally-dynamic coastal upwelling, which leads to pronounced spatial and temporal variations in the physical and chemical environment. Most notably, variations in upwelling strength and continental shelf width affect Fe distribution and speciation seasonally, yielding a mosaic of adjacent high and low primary productivity areas. This study investigated phytoplankton Fe transport in the CCS during July 2014. We aimed to improve our understanding of the controls on Fe bioavailability in this dynamic ecosystem using measurements of short-term, size-fractionated ⁵⁵Fe uptake rates in the presence of in situ organic ligands. We also determined phytoplankton community composition, Fe nutritional status, macronutrient concentrations, dissolved Fe speciation, and colloidal vs. truly dissolved Fe concentrations. In general, Fe uptake rates were a direct function of dissolved Fe concentrations, but other parameters, such as Fe speciation, also influenced Fe uptake rates in some areas. Our in situ Fe uptake rates will be compared with predictions based on laboratory empirical relationships between Fe uptake rates and phytoplankton surface area. The results of this study further our understanding of what regulates Fe bioavailability to phytoplankton in dynamic coastal waters.

•