Abundance and physical controls of Mediterranean micro-estuaries

Introduction Estuaries, transitional zones between freshwater and the sea, are vital to global coastal ecology, support biodiversity, and provide ecosystem services. Estuarine research has often focused on larger systems while smaller estuaries (micro-estuaries) remain understudied. Herein, we define micro-estuaries by their hydrological and physical characteristics and investigate their abundance in the Mediterranean Sea, a region experiencing increasing anthropogenic pressure and the impacts of climate change. Methods Using bathymetric surveys and high-resolution temperature and salinity measurements, conducted in seven East Mediterranean micro-estuaries during the years 2014 -2024, we characterized the physical properties of micro-estuaries and established criteria for their identification. Next, we used GIS techniques to identify potential micro-estuaries across the Mediterranean, verified them on satellite imagery, and compared these findings with a published database of estuary locations. Results We identified a high abundance of micro-estuary-like outfalls throughout the Mediterranean (243 in total), about half of which are undocumented as estuaries and are therefore likely not managed as such. Although these systems are small (1 -7 km long, <3 m depth), they were highly stratified; the level of stratification and water quality were the key determinants of biogeochemical conditions such as oxygenation and denitrification. Water column structure was, in turn, controlled by sandbar morphology and freshwater inflow: most of these estuaries are intermittently connected to the sea and appear to be governed by a seasonally built sandbar that, together with the terrestrial freshwater inflow regime, defines the micro-estuary's function. Residence times of freshwater, seawater, surface water, and bottom water varied widely, from days to several months, reflecting these differences in hydrography and seasonality. Discussion Because micro-estuaries are particularly susceptible to anthropogenic modification, altering the inflow and manipulating the outfall shape are cost-effective managerial tools unique to these systems. The proximity of micro-estuaries to dense urban populations and the significant knowledge gaps highlight the need for further research on micro-estuaries globally.