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.

The present paper is specifically focused on enclosed or semi-enclosed basins where the wind is the dominant driver of water surface tilting, leading to the so-called wind tide contributing to water levels rise. Wind-induced free surface tilting is studied using the 1-D steady form of the depth-averaged shallow water (Saint-Venant) momentum equation which reflects the depth-averaged local balance between surface slope and wind stress. Two contrasted field sites, the Berre and Vaccarès lagoons, have been monitored providing water level data along a reference axis. This study highlighted the occurence of wind tides at the two field sites. The bimodal wind exposure ensured the robustness of the observations, with non-linear but symmetric behaviors patterns observed in winds from opposite directions. It is observed that the higher the wind speed, the steeper the slope of the free surface in accordance with the well known basic trend. In addition, a significant effect of depth is observed, with greater surface tilting in the shallower lagoon. The data analysis confirmed the robustness of such a simple approach in the present context. Using the additional assumption of constant, i.e. wind-independent, drag coefficients (C D ) allowed a good match with the observations for moderate wind speeds for both sites. However, the depth effect required the C D to be increased in the shallower basin. Classical empirical wind-dependent C D parameterizations provide better wind-tide predictions than the constant-C D approach in very strong wind conditions but totally failed in predicting surface tilting in the shallower site, suggesting that physical parameters other than wind speed should be taken into account for the C D parameterization in very shallow lagoons.

<p>Over the past centuries, coastal marsh areas have been declining, mostly as a consequence of human impacts, including direct wetland conversion and land reclamation. More recently, accelerated global sea-level rise poses an additional challenge for the longevity of existing coastal marshes. This risk is further compounded by densely populated coastal zones, where coastal infrastructure inhibits the capacity of coastal marshes to migrate inland in response to rising sea levels (coastal squeeze). In the Mediterranean, coastal wetlands, incl. saltmarshes, are important contributors to the region’s high biodiversity, and provide a set of invaluable ecosystem services. Here, we present a study on the modelling of the future development of Mediterranean coastal marshes, taking into account both their ability to vertically adjust to increasing sea levels through sediment accumulation and their capacity to migrate inland in response to rising sea levels where sufficient inland migration space is available. In contrast to previous global studies, our preliminary results indicate an overall loss of coastal marshes by 2100 for all climate and management scenarios, even under low sea-level rise scenarios and when abundant inland migration space is available. Total losses are projected between 17% and 94% for RCP 2.6 with maximum available space for inland space; and RCP 8.5 with minimum space, respectively. A total loss of coastal marshes is projected for some Mediterranean countries by 2100. Nevertheless, the implementation of coastal management strategies facilitating the inland migration of coastal marshes as well restoration of catchment-to-coast sediment connectivity and enhancement of sediment trapping capacities can, to some degree, mitigate future coastal marsh losses.</p>