Particle aggregation at the edges of anticyclonic eddies and implications for distribution of biomass

Acoustic measurements show that the biomass of zooplankton and mesopelagic fish is redistributed by mesoscale variability and that the signal extends over sev- eral hundred meters depth. The mechanisms governing this distribution are not well understood, but influences from both physical (i.e. redistribution) and biological processes (i.e. nutrient transport, primary production, active swimming, etc.) are likely. This study examines how hydrodynamic con- ditions and basic vertical swimming behavior act to dis- tribute biomass in an anticyclonic eddy. Using an eddy- resolving 2.3 km-resolution physical ocean model as forc- ing for a particle-tracking module, particles representing pas- sively floating organisms and organisms with vertical swim- ming 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 surface tend, in agreement with the observations, to accumulate around the edge of the eddy, whereas particles released at depth gradu- ally become distributed 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 and the shallowest close to the edge. There is no evidence of ag- gregation of particles along the eddy rim in the last simula- tion. The model results points towards a physical mechanism for aggregation at the surface, however biological processes cannot be ruled out using the current modeling tool.