Relationship between surface dissolved iron inventories and net community production during a marine heatwave in the subarctic northeast Pacific.

From winter 2013-14 to the end of 2015-16, a high pressure atmospheric system induced elevated sea surface temperatures in the offshore subarctic northeast Pacific, resulting in a marine heatwave. Increased stratification due to the heatwave resulted in shoaling of the winter mixed layer and a decrease in nutrient re-supply to the euphotic zone. Here, we investigate relationships between dissolved iron (dFe) and macronutrients, net community production (NCP), (micro)nutrient uptake ratios, and phytoplankton community composition in the winter and summer from 2012 to 2015 to gain insight into coupled biogeochemical responses to the heatwave. Our investigation highlights the importance of external dFe supply during marine heatwave events, as a more shallow mixed layer reduces the transport of essential (micro)macronutrients to the surface layer. We conclude that recycled dFe did not contribute to NCP in 2014, but rather the vertical displacement of dFe rich water unrelated to mixed layer deepening played a major role. In 2015, such transport was not detected, resulting in abnormally low dFe and shift toward higher biomass of pico- and nano-phytoplankton size-classes.