Observations of a phytoplankton spring bloom onset triggered by a density front in NW Mediterranean

摘要

Phytoplankton blooms in the northwestern Mediterranean Sea are seasonalevents that mainly occur in a specific area comprising the Gulf of Lion andthe Proven?al basin, where they are promoted by a general cycloniccirculation, strong wind-driven mixing and subsequent re-stratification ofthe water column. At the southern boundary of this area, a persistent densityfront known as the north Balearic front can be found. The front is presumedto cause an early phytoplankton bloom in its vicinity because (a) it enhancesthe transport of nutrients into the euphotic layer and (b) it promotes thespeedy 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, wasdeployed on a mission that took it from the Balearic Islands to Sardinia andback. The frontal zone was crossed twice, once during the outbound leg andthe once on the return leg. The data provided by the glider clearly showedthe onset of a bloom soon after a decrease in wind-driven turbulentconvection and mixing. The in situ observations were supported and confirmedby satellite imagery. It is shown that frontal dynamics play a key role inthe promotion and acceleration of re-stratification, which is a necessarypre-conditioning factor for the onset of blooms much like other relevantprocesses such as an enhanced biological pump. Swift re-stratificationstimulates new production by inhibiting mixing. Finally, viewing the bloomingphenomenon from a regional perspective, it seems that Sverdrup's criticaldepth model applies in the northern well-mixed area whereas, in the south,front-related re-stratification seems to be the principal cause.