Particle aggregation in anticyclonic eddies and implications for distribution of biomass

Abstract

Abstract. Acoustic measurements show that the biomass of zooplankton and mesopelagic fish is redistributed by mesoscale variability and that the signal extends over several hundred meters depth. The mechanisms governing this distribution are not well understood, but influences from both physical (i.e. physical redistribution) and biological processes (i.e. nutrient transport, primary production, active swimming, etc.) are likely. This study examines how hydrodynamic conditions and basic vertical swimming behavior act to distribute biomass in an anticyclonic eddy. Using an eddy-resolving 2.3 km-resolution physical ocean model as forcing for a particle-tracking module, particles representing passively floating organisms and organisms with vertical swimming behavior are released within an eddy and monitored for 20 to 30 days. The role of hydrodynamic conditions on the distribution of biomass is discussed in relation to the acoustic measurements. Particles released close to the surfaces tend, in agreement with the observations, to accumulate around the edge of the eddy, whereas particles released at depth tend to distribute along the isopycnals. After a month they are displaced several hundreds meters in the vertical with the deepest particles found close to the eddy center, but there is no evidence of aggregation of particles along the eddy rim. All in all, the particle redistribution appears to result from a complex mixture of strain and vertical velocity. The simplified view where the vertical velocity in eddies is regarded as uniform and symmetric around the eddy center is therefore not a reliable representation of the eddy dynamics.