The Vaccarès Lagoon System, located in the central part of the Rhône Delta (France), is a complex shallow coastal lagoon, exposed to a typical Mediterranean climate and a specific hydrological regime affected by man-controlled exchanges with the sea and agricultural drainage channels. In this article, we report the results obtained by a series of monitoring programs, with different spatial and temporal resolutions. Long-term datasets from 1999 to 2019 with data collected on a monthly basis and a high spatial resolution highlighted the significant spatial heterogeneity in salinity regimes, and helped to determine the long-term evolution of the total mass of dissolved salt. High-frequency surveys allowed to characterize the water levels and salinity dynamics seasonal response to (i) the exchanges with the Mediterranean Sea, (ii) the exchanges with agricultural drainage channels, and (iii) the rain and evaporation. In addition, wind effects on salinity variations are also explored. This work shows how different spatial and temporal monitoring strategies provide complementary information on the dynamic of such a complex system. Results will be useful and provide insight for the management of similar lagoon systems, accommodating for both human activities and ecological stakes in the context of global change.
A dense network of instruments has been deployed within harbours along the Mediterranean coast, in the Toulon Metropole area, between the Hyères islands and the Sanary Bay in the framework of the observation network HTM-NET. Each station is equipped with two piezometric sensors, the first immersed and the second emerged, which allows the calculation of the water level. Both piezometric sensors are also equipped with a temperature sensor. Water level and temperature data are analyzed and discussed, also considering meteorological data provided by Météo-France stations. The tide gauges provide information about tide harmonic components, extreme water level and seiching. Moreover, significant differences are observed between sheltered zones in enclosed bays and offshore zones, such as between the back of the Bay of Toulon and at the Port-Cros Island. Differences in water level up to 0.10m are indeed observed under windy conditions, of same order as the tidal range (order of 0.20m) or the annual level variability due to the volumetric expansion (order of 0.10m). Water level variations, up to about 1m, are found to be mainly due to atmospheric effects, with a more or less isostatic behavior according to the weather events. In addition, seiching with an amplitude of few centimeters is observed within the Little Bay of Toulon, for east wind conditions. The near-surface water temperature is measured at the submerged piezometer location (depth of immersion range 0.10m -1.80m according to the station and to the water level). The analysis of the temperature associated with the weather conditions allows to detail a strong variability of the upwelling intensity under Mistral wind conditions in summer, leading to more or less pronounced temperature drops according to the shore configuration. The Bay of Toulon is more prone to the generation of upwellings than the neighboring bays. During winter, water exchanges between the Little Bay of Toulon and offshore are also clearly observed during windy conditions. The HTM-NET long-term observation network thus provides useful insights to increase our knowledge of the hydrodynamics and mass fluxes, and therefore enhances our modeling capacity and risk assessment at the scale of a bay.
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