Fronts Potentially Drive Variability in Phytoplankton Productivity in the HNLC Subarctic Northeast Pacific.

Quantifying the biological pump and determining what causes variability is important to understanding the carbon cycle. Our study explores the cause of small differences in productivity rates on mesoscales in the offshore high nutrient low chlorophyll subarctic NE Pacific, where iron limits productivity. We utilized satellite, model, and shipboard measurements in 2014-2017 to explore the coupling between the physical environment and phytoplankton. We hypothesize that fronts, which we define as a 1°C temperature change over 1/3 degree distance, drives variability in both chlorophyll and production. Surface chlorophyll is more variable in this region than implied by historical studies and, in fact, has cohesive structure. The higher chlorophyll correlates with lower temperature and higher salinity, suggesting water is coming from below the surface. The higher chlorophyll is also associated with increased primary production, gross primary production, and carbon export. Higher chlorophyll regions tend to have more diatoms present and increased seasonal silicic acid drawdown indicating iron inputs. Further, when there were no fronts present in June 2017, the entire subarctic NE Pacific had low, invariant chlorophyll. We examine June 2015 in particular, where two fronts caused water to upwell that had higher iron concentrations, fueling production, and then geostrophic currents transported the water away from the front. Fronts are not the only mechanism that can explain chlorophyll patterns; however, fronts can explain much of the variability that was observed in the area. Our study adds to the mechanistic understanding of the coupling between physics and biology, enhancing future ability to predict spatial and temporal trends.