Monitoring dense shelf water cascades: an assessment tool for understanding deep-sea ecosystems functioning

Abstract

Cascading of dense shelf water from continental shelves is a global phenomenon whose effects have been largely underestimated. The north-western Mediterranean is one of the regions of the world where massive dense water formation occurs because of cooling and evaporation of surface waters during winter-time. Concurrent with the well known open-sea convection process over the MEDOC region, coastal surface waters over the wide shelf of the Gulf of Lions also become denser than the underlying waters and cascade downslope until reaching their equilibrium depth. Through this climate-driven phenomenon, dense shelf waters carrying large quantities of particles in suspension are rapidly advected hundreds of meters deep, mainly through submarine canyons. Recent observations within the frame of several research initiatives conducted in the north-western Mediterranean found that major dense shelf water cascades from the Gulf of Lions have a direct effect on the deep water (i.e. WMDW) thermohaline properties and on the long-term fluctuations of deep-sea fisheries, notably the shrimp Aristeus antennatus. Because of the flushing and recurrent behaviour of such cascading events, a continuous monitoring of this phenomenon under a system of Integrated Mediterranean Marine Observatories initiative could be conducted by means of establishing permanent real-time deep-sea observatories at specific key sites in the Mediterranean, in places where this process has been clearly identified (i.e. Gulf of Lions, southern Adriatic and Aegean shelves). Such infrastructures will allow studying this phenomenon using a transdisciplinary approach to assess in detail its effects and implications in the Mediterranean deep-sea ecosystem and living resources.