Where freshwater meets the sea: satellite–model insights into salinity dynamics in the Venice Lagoon

Abstract

Salinity is a key descriptor of transitional water bodies at the land–sea interface, and resolving its spatiotemporal dynamics is essential for understanding water-mass exchange, habitat distribution, and ecosystem functioning. However, available observations remain limited in their ability to resolve dynamic freshwater–marine interactions at relevant spatial scales. The Venice Lagoon, a complex transitional system exposed to interacting natural and anthropogenic pressures, offers a valuable setting for investigating these dynamics. This study examines surface salinity variability using coloured dissolved organic matter (CDOM) as an optical proxy. Multi-year in situ measurements of remote sensing reflectance (R rs ), CDOM absorption, and salinity collected between 2018 and 2024 showed a persistent inverse relationship between CDOM and salinity, consistent with freshwater-marine mixing processes. A regionally calibrated model was developed from the in situ dataset, using the reflectance band ratio between 665 and 490 nm to estimate the CDOM absorption coefficient at 440 nm, a C D O M 440 , and showed good agreement across the lagoon–coastal gradient. The model was applied to atmospherically corrected Sentinel-3 Ocean and Land Colour Instrument (OLCI) imagery to generate satellite-derived a C D O M 440 . These estimates were then combined with concurrent salinity measurements from the continuous monitoring network (SAMANET) to develop a CDOM-based salinity retrieval model. The retrieval was further refined by accounting for wind and river discharge regimes, improving the representation of salinity variability under different environmental conditions. Resulting salinity maps captured short-term tidal redistribution, freshwater inputs during spring, and the persistent contrast between freshwater-influenced landward sectors and more saline waters near the tidal inlets. The System of Hydrodynamic Finite Element Module (SHYFEM), an established hydrodynamic model, captured the broader salinity patterns but underestimated values near the Dese river mouth. The proposed framework demonstrates the value of regionally calibrated ocean-colour observations for monitoring optically complex transitional waters and could support adaptive management strategies for environmentally sensitive lagoon systems.