Abstract
Abstract. Phytoplankton blooms in the northwestern Mediterranean Sea are seasonal events that mainly occur in a specific area comprising the Gulf of Lion and the Provençal basin, where they are promoted by a general cyclonic circulation, strong wind-driven mixing and subsequent re-stratification of the water column. At the southern boundary of this area, a persistent density front known as the north Balearic front can be found. The front is presumed to cause an early phytoplankton bloom in its vicinity because (a) it enhances the transport of nutrients into the euphotic layer and (b) it promotes the speedy re-stratification of the water column (through frontal instabilities). In February and March 2013, a glider, equipped with a CTD (conductivity, temperature, and depth device) and a fluorometer, was deployed on a mission that took it from the Balearic Islands to Sardinia and back. The frontal zone was crossed twice, once during the outbound leg and the once on the return leg. The data provided by the glider clearly showed the onset of a bloom soon after a decrease in wind-driven turbulent convection and mixing. The in situ observations were supported and confirmed by satellite imagery. It is shown that frontal dynamics play a key role in the promotion and acceleration of re-stratification, which is a necessary pre-conditioning factor for the onset of blooms much like other relevant processes such as an enhanced biological pump. Swift re-stratification stimulates new production by inhibiting mixing. Finally, viewing the blooming phenomenon from a regional perspective, it seems that Sverdrup's critical depth model applies in the northern well-mixed area whereas, in the south, front-related re-stratification seems to be the principal cause.
Highlights
A debate is underway in the oceanographic community concerning the physical/ecological mechanisms that trigger the seasonal phytoplankton bloom in seas and oceans
The critical depth (CD) hypothesis states that when the mixed layer (ML) depth is shallower than the CD, assuming favourable environmental conditions, blooms are triggered and maintained thanks to the residence, persistence and growth of phytoplanktonic populations in the euphotic layer
Taylor and Ferrari (2011b), extending the work of Huisman et al (1999) which first contraposed the concept of critical turbulence (CT) to the CD explanation, found that while the primary driver is the atmospheric forcing like in the Sverdrup paradigm, the regulating factor is the reduction of vertical turbulence and not the shoaling of the ML
Summary
A debate is underway in the oceanographic community concerning the physical/ecological mechanisms that trigger the seasonal phytoplankton bloom in seas and oceans. Mahadevan et al (2012) have shown that, at high latitudes in the North Atlantic, the stratification required for triggering a bloom is often not initiated by the seasonal warming but by eddy instabilities in areas of strong density gradients. Such instabilities, in the absence of strong wind forcing, cause lighter water to stratify over denser water even when heating is nonexistent. This work presents the results from a glider survey that was carried out to investigate the impact of mixing and restratification on the triggering of the seasonal bloom in the central part of the Algero–Provençal basin (northwestern Mediterranean Sea) in the area of the north Balearic front (hereafter referred to as NBF). There are reports in the literature Sackmann et al (2008) that the quenching effect is quite small in winter
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