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. Here, 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.
Using bathymetric surveys and high-resolution temperature and salinity measurements conducted in seven East Mediterranean micro-estuaries between 2014-2024, we characterized the physical properties of micro-estuaries and established criteria for their identification. We then 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.
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 therefore likely not managed as such. Although these systems are small (1-7 km long, less than 3 m deep), they were characterized by highly stratified water columns; stratification and water quality were the decisive factors shaping biogeochemical conditions such as oxygenation and denitrification levels. The water column structure, in turn, was governed by the morphology of the sandbar and the freshwater inflow regime: most of these estuaries are intermittently connected to the sea, and their function appears to be dictated by a seasonal sandbar which, together with the terrestrial freshwater flow regime, defines the function of the micro-estuary. Residence times of freshwater, seawater, surface water, and bottom water varied considerably, from several days to several months, reflecting differences in hydrography and seasonality.
Because micro-estuaries are especially sensitive to anthropogenic change, regulating water inflow and manipulating outfall morphology are cost-effective management tools unique to these systems. The proximity of micro-estuaries to dense urban populations, alongside significant knowledge gaps, highlights the need for future research on micro-estuaries at a global level.
Citation: Suari, Y., Yahel, G., Diaz, G., Zedaka, H., Sade, T., Amit, T., Blumenfeld, T., Gilboa, M., Ozer, T., & Topaz, T. (2026). Abundance and physical controls of Mediterranean micro-estuaries. Frontiers in Marine Science, 13, 1913361. https://doi.org/10.3389/fmars.2026.1913361