Steady-state DCM dynamics in Canaries waters

This paper concerns the deep chlorophyll maximum (DCM) dynamics in a steady state, using primarily data from the Canary Islands Area of Filament and Eddy eXchange obtained in August 1999 during a cruise between oligotrophic waters west of La Palma and the north-west African coastal upwelling. CTD-fluorometer observations of the deep fluorescence maximum (DFM) were confirmed by water samples from which chlorophyll was extracted. The DFM–DCM was perturbed at many stations by island-generated eddies and similar features, and the paper focuses on unperturbed stations, which were identified by the occurrence of the DFM close to the 26.4 kg m−3 isopycnal. The DFM at these stations occurred at the top of the nitracline, in the presence of 0.3–2 μM nitrate+nitrite, and at 24 h mean isolumes of 10–20 μE m−2 s−1. The classical compensation depth model predicts the occurrence of the DFM–DCM at less illumination and hence at too great a depth, making it necessary to take into account additional losses, especially those due to the respiration of microheterotrophs in biomass equilibrium with phytoplankton. The depth of the steady-state DCM is compatible with the predictions of a microplankton model, given (i) a ratio of 0.3–0.6 of microheterotroph to total microplankton biomass, and (ii) other losses (due to meszooplankton grazing and vertical mixing) of about 0.1 d−1.