A one-dimensional simulation of biological production in two contrasting mesoscale eddies in the south eastern Indian Ocean

The question of why anticyclonic eddies appearing along the coast of Western Australia tend to be more productive, and have higher phytoplankton biomass than the cyclonic eddies, remains open. Here we present results from a one-dimensional physical–biological model that explores several possible explanations. In particular, we examine the influence of nutrient conditions during eddy formation and subsequent vertical mixing on the biological production of two counter-rotating mesoscale eddies sampled off Western Australia during October 2003. Observed differences in primary production between the two eddies cannot be adequately explained in the model by differences in mixed-layer depth and vertical nutrient flux. Instead, entrainment of productive shelf waters during the formation of the anticyclonic eddy is suggested to account for up to three quarters of the primary production within the eddy as it moves off-shore. At the same time, the longer residence time of sinking detritus within the deep mixed layer of the anticyclonic eddy is shown to increase the importance of regenerated nitrogen and contribute to its enhanced production. A general under-estimation of production is discussed in the context of biological and physical processes not resolved by the model.