Effects of morphology and sediment permeability on coastal lagoons’ hydrological patterns

Abstract

Mediterranean coastal lagoons are influenced from a wide variety of external factors such as surface and groundwater flows and climate dynamics. They are also vulnerable to human activities, which have caused a significant loss of these types of habitats. As a result, the EU habitat management have now prioritized restoration of natural wetlands. However, there is a lack of systemic studies on the mechanisms of coastal wetland degradation and ecohydrological processes that determine restored ecological functioning as an end goal, as well as a lack of reference sites to make comparisons. Furthermore, while lagoon morphometry and underlying sediment permeability have been studied extensively, combining these disciplines to evaluate lagoon hydrology and salinity dynamics is yet to be fully explored. The aim of this study was to analyze the hydrological dynamics of 4 newly constructed lagoons and compare them with 2 natural lagoons in the La Pletera salt marshes and evaluate the restoration and conservation efforts. We use the General Lake Model (GLM) to assess water volume fluctuations, salinity variability and lagoon water circulation (groundwater and surface water inflows, outflows and evaporation). We also combine data of the underlying lithological characteristics and lagoon morphometry, to compare and better understand the interplay of these parameters on the hydrological behavior of each lagoon. Results indicated that the older and natural lagoons exhibited more consistent patterns of confinement; with deeper morphologies, lower evaporation effect, lower water circulation, and more annual patterns of salinity fluctuation. The presence of low-permeability layers also resulted in less fluctuation of higher salinity levels. Conversely, three of the four new lagoons had similar, shallower morphologies and higher evaporation fluxes, but exhibited different water circulation patterns due to the presence or absence of low permeability layers. Also, their salinity fluctuations were more influenced by seasonal mixing than by evaporation, indicating more susceptibility to climatic influence in their annual hydrological pattern than in the natural lagoons. This could prove important when constructing and restoring lagoons according to predetermined morphology and underlying sediment patterns, as it could ultimately limit or enhance the success of set objectives and overall ecological functioning in a flooding – confinement driven lagoon ecosystem conditioned by irregular and unpredictable climatic events