Retention of plankton within ocean eddies

AimMesoscale ocean eddies (closed circular currents typically 100-300 km in diameter) are ubiquitous features of the world's oceans. They form partially isolated environments with distinct physical and chemical conditions capable of supporting and transporting whole plankton communities. The productivity and biodiversity of these communities is ultimately dependent on an eddy's ability to retain planktonic organisms. Our aim was to estimate eddy retention time-scales across a range of oceanic environments and larval behaviours, with implications for both distributions and future changes in plankton communities. LocationThe Pacific Ocean, Indian Ocean, Southern Ocean and Mediterranean Sea. MethodsA particle-tracking model was forced using ocean currents from a number of validated hydrodynamic models covering environments ranging from shelf seas to the open ocean and equatorial to high-latitude waters. Eddies were seeded with large numbers of particles and their rate of loss from the eddy was used to estimate retention times. The influences of common plankton swimming behaviours were explicitly captured in the model. ResultsEddy retention times of modelled plankton ranged from 5 to 67 days, with a median of 19 days. Retention times were not correlated with latitude or eddy size. However, plankton residing near the surface of eddies rotating cyclonically (anticlockwise in the Northern Hemisphere) had significantly shorter retention times than those residing in the same eddy at depth, and vice versa for eddies rotating anticyclonically. Main conclusionsWe show that ocean eddies have the potential to retain and support planktonic (and even nektonic) communities over many generations and are likely to enhance larval survival for many invertebrate and fish species. Differences in retention with depth suggest that cyclonic and anticyclonic eddies will support differing plankton communities. If their relative geographical distributions change with global climate, then the relative proportions of diatom-based and dinoflagellate-based communities may also change, with potential implications for higher trophic animals.