Biogeochemical Anatomy of a Cyclonic Warm-Core Eddy in the Arctic Ocean

Abstract

In the oligotrophic Arctic Ocean, previous studies have implied positive impacts of eddies on phytoplankton biomass. However, direct observations for estimating vertical nutrient fluxes in various parts of eddies are still lacking; these could explain the mechanism of high phytoplankton biomass associated with eddies compared to their surroundings. Here we conducted conductivity-temperature-depth surveys with water sampling to examine the characteristics of a cyclonic warm-core eddy found over the Chukchi shelf slope in late summer 2015. Furthermore, we measured ocean microstructures to estimate vertical nutrient fluxes and their contributions to nutrient uptake by phytoplankton. The results imply that nutrients were supplied to the euphotic zone from a lower layer through vertical shear mixing near the center of the eddy and through double-diffusive mixing associated with an interleaving structure at the rim of the eddy. Phytoplankton size structures differed markedly between the center and rim of the eddy.