Biological response to frontal dynamics and mesoscale variability in oligotrophic environments: Biological production and community structure

Abstract

Two food web models, a single‐species and a multispecies formulation, are incorporated into a three‐dimensional eddy‐resolving model of an unstable frontal jet to investigate the effects of mesoscale instabilities on biological production and community structure in oligotrophic ocean environments. The growing instability wave triggers the formation of divergence and convergence and, consequently, upwelling and downwelling zones along the jet. Most of the biological production takes place near the surface on the cyclonic (northern) side of the front and on the upstream region of meander crests in response to nutrient upwelling. However, strong advection downstream and downward in the convergence regions (downstream side of meander crests) results in accumulation of biomass at depth on the anticyclonic (southern) side of the front, leading to spatial separation between regions of biological growth and regions of biomass accumulation. The large domain and long duration of the runs allow for investigation of diverse phenomena including eddy–eddy and eddy–front interactions for both cyclonic and anticyclonic features. In addition to the traditional cyclonic eddy pumping mechanism we observe enhanced primary production and biomass accumulation on the edges of anticyclonic eddies due to eddy–eddy interactions and biomass filaments trailing behind detached cyclonic eddies propagating away from the front. In the multispecies formulation the mesoscale dynamics drives a sustained shift in the phytoplankton community toward the large size classes that is consistent with observations and that has not been observed in previous similar process‐modeling studies. Because of the higher species diversity, nutrients are used more efficiently in the multispecies system, resulting in higher total plankton biomass for the same amount of total nitrogen in the domain. The fractional increase in total biomass is less pronounced inside the unstable front and eddies where species diversity is lower (large size classes dominate).