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

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. Plain Language Summary Eddies are ubiquitous in the world's oceans, and, especially in the Arctic Ocean, eddy activity (number, volume, and life span) appears to be increasing due to the recent loss of sea ice. Previous studies have implied that eddies may increase phytoplankton biomass in the barren Arctic marine ecosystem. However, the detailed structures of eddies, including ocean microstructures (<1 m) and associated phytoplankton distribution, have not been well studied. In this study, we conducted hydrographic and biogeochemical observations and collected microstructure measurements across a cyclonic warm-core eddy off the northern Alaskan coast in the Arctic Ocean. The results indicate that microscale mixing processes control nutrient supply to the euphotic zone and induce quite different phytoplankton size structures between the center and rim of the eddy. Close to the center of the eddy, we detected abundant large phytoplankton (> 20 mu m), which could transport carbon to deeper layers more effectively than smaller phytoplankton. Our findings may contribute to improved numerical modeling methods, which currently do not resolve these features, and help to elucidate marine ecosystem changes in response to decreasing sea ice and warming in the Arctic Ocean.