Intrusion of Rhone River diluted water into the Bay of Marseille: Generation processes and impacts on ecosystem functioning

Abstract

The Rhone River provides the largest inputs of terrestrial freshwater and nutrients into the Mediterranean Sea. The Rhone River diluted water intrusions into the Bay of Marseille were investigated, examining their physical generation processes and associated biogeochemical impact by using in situ observations, remote sensing data, and a three-dimensional physical/biogeochemical coupled model. During our study period from 2007 to 2011, Rhone River intrusions occurred on average 7.6 times per year and affected more frequently the northern part of the bay. A classification of intrusion events in three categories is proposed (short lived, big, and small) as a function of their duration and spatial extent. The intrusions appeared to be driven by: (i) wind forcing, (ii) the presence of a mesoscale eddy, (iii) the Rhone River discharge volume, and (iv) the variation in thermocline depth. Typically, a combination of these favorable factors was necessary to induce an intrusion. An intrusion strongly impacts the biogeochemical functioning of the Bay of Marseille by bringing large quantities of nutrients into the bay. Mass balances were computed allowing us to quantify this impact on the Bay of Marseille. The results show that the ecological impact depends very much on the type of intrusion, with big intrusions having the highest impact.

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

“…This increase is in agreement with the wind setup mechanism proposed by Csanady (1982), which characterizes a balance between the wind-stress and the hor- (Fig. 6, panel c), consistently with Fraysse et al (2014). This freshwater plume is then advected westward past 4 • E by the intruding current (Fig.…”

“…In addition to wind forcing, the Gulf of Lion is subjected to the freshwater inputs of the Rhône River. Its plume can extend over large areas when a strong volume discharge is combined with a shallow thermocline (Fraysse et al, 2014). The associated changes in the density field can impact the shelf circulation (Estournel et al, 2001; Reffray et al, 2004;André et al, 2005).…”

Occurrence of perfluoroalkyl substances in the Bay of Marseille (NW Mediterranean Sea) and the Rhône River

“…4). The maximum value simulated at l'Estaque on November 16th occurred during south-south-easterly winds, associated with the anticyclonic Marseille eddy, which is necessary to advect the diluted Rhône River waters toward the northern part of the Bay (Fraysse et al, 2014). In contrast the south-westerly winds that occurred for example on October 31st moved the panache eastwards without generating the eddy necessary to reach the Bay of Marseille.…”

“…The Bay of Marseille, and more generally the Gulf of Lions, is also subject to high solar radiation (Sempéré et al, 2015) and strong wind regimes, which often make plant and plastic debris accumulate on the coast. Depending on wind forcing (among other factors), the Bay of Marseille can also be influenced by the Rhône River via freshwater intrusions (Fraysse et al, 2014;Gatti et al, 2006;Para et al, 2010).…”

Seasonal freshening of NW Mediterranean surface water impacts microbial heterotrophic activity and dissolved organic matter

“…Intrusions of Rhone river water were detected in 5-20% of a 4-year bi-monthly survey in the Bay of Marseille (Fraysse et al, 2014), indicating that these events are frequent phenomena in the coastal Mediterranean Sea. Diluted structures with low salinity waters (LSW) detached from the Rhone river plume can be transferred to open waters (Barrier et al, 2016, Diaz et al, 2008, Nencioli et al, 2011, Petrenko et al, 2017.…”