Ageostrophic frontal processes controlling phytoplankton production in the Catalano-Balearic Sea (Western Mediterranean)

Plankton production in marginal seas is characterized by significant mesoscale variability, as often inferred by satellite observations. The Catalano-Balearic Sea in the Western Mediterranean is one of such areas and is the focus of this coupled physicalbiological process study. We use a three dimensional biophysical model to understand how the mesoscale variability of the regional characteristics was controlled by quasigeostrophic (weakly nonlinear) and ageostrophic (moderately nonlinear) dynamics of the coastal boundary current and associated instabilities. Ageostrophic motion was most effective within the slope area where more pronounced meanders promoted enhanced upward pumping of subsurface nutrients into the euphotic zone and thus led to higher and broader coverage of phytoplankton biomass on anticyclonically-dominated onshore side of the front. On the contrary, the circulation further offshore, in the Balearic basin located further south, was controlled mainly by the quasi-geostrophic cyclonic eddies that were associated with a weak upward motion and thus unable to provide sufficiently high nutrient injections into the euphotic layer. However, these eddies played an effective role redistributing phytoplankton biomass that originated from the highly productive Iberian shelf and led to a patchy distribution over the basin, as seen in satellite imagery. In addition, the anticyclonic eddies developed in close proximities of the Balearic islands provided occasionally localized high phytoplankton patches in response to the agestrophic dynamics. It appears that the relatively low surface chlorophyll concentration shown by the satellite data does not necessarily imply poor phytoplankton production but may reflect an efficient energy flow towards higher trophic levels as supported by the zooplankton and fishery observations in the Catalano- Balearic Sea.

JRC.H.1 - Water Resources