Presence of two eddies in close proximity drives large spatial and temporal heterogeneity in the euphotic zone

Abstract

Oceanic mesoscale structures, such as eddies, play a fundamental role in ocean circulation, ocean biogeochemical cycles and plankton ecology. They cause lateral and vertical advection, as well as interact with vertical mixing, which is predicted to promote episodic fluxes of macronutrients to the surface ocean. However, the interactions between mesoscale eddies can generate submesoscale fronts and filaments occurring over short temporal and spatial scales and thus their impact on ocean biogeochemistry has been difficult to characterize. During an expedition to the Porcupine Abyssal Plain (PAP) site in the Northeast Atlantic in June 2013, we studied the interface between a cyclonic and an anticyclonic eddy, measuring nutrient and chlorophyll-a concentrations, zooplankton abundance and community structure, and marine snow aggregate abundance and sinking velocities. We observed that eddy rotation and a storm event induced, respectively, lateral stirring and vertical mixing of the two distinct water masses, driving spatial and temporal biogeochemical heterogeneity at the PAP site. Furthermore, we observe that diel and vertical variations in aggregate type and abundance were closely linked to the vertical distribution and abundance of zooplankton, suggesting that zooplankton were the main gatekeepers of carbon flux. Our findings suggest that the interactions between mesoscale structures could significantly modify organic carbon export, as well as provide sustenance for higher trophic levels, processes that have implications for fisheries and global climate.