Satellite Colour Maps Reveal Venice Lagoon Salinity Through CDOM Optical Proxy

Scientists have developed a new way to map surface salinity across the Venice Lagoon using satellite imagery, turning the optical signature of coloured dissolved organic matter (CDOM) into a proxy for how fresh or marine the water is. The approach, built on multi-year field measurements and Sentinel-3 Ocean and Land Colour Instrument (OLCI) data, offers a regional solution for monitoring transitional waters where conventional satellite salinity retrieval struggles.

Salinity is a fundamental descriptor of transitional water bodies such as lagoons and estuaries, shaping ecosystem function, water quality, and the balance between freshwater and marine influence. Yet direct observations remain sparse in space and time, and existing satellite methods often fail in optically complex coastal waters. The new study addresses this gap by exploiting a persistent inverse relationship between CDOM and salinity, consistent with freshwater-marine mixing processes.

Between 2018 and 2024, researchers collected in situ measurements of remote sensing reflectance, CDOM absorption, and salinity across the lagoon-coastal gradient. These data revealed a strong inverse CDOM-salinity relationship, which the team used to calibrate a regional model. The model estimates the CDOM absorption coefficient at 440 nm (aCDOM440) from the reflectance band ratio between 665 and 490 nm, then converts satellite-derived aCDOM440 into salinity using concurrent measurements from the SAMANET continuous monitoring network.

The retrieval was refined by accounting for wind and river discharge regimes, improving representation of salinity variability under different environmental conditions. Resulting salinity maps captured short-term tidal redistribution, freshwater inputs, and the broader lagoon-coastal gradient. Comparison with the SAMANET hydrodynamic model confirmed the observed freshwater-marine gradient, although larger discrepancies appeared near the Dese River and inner lagoon sectors, possibly reflecting uncertainties in freshwater forcing, local channel geometry, and vertical mixing representation.

The study demonstrates that CDOM-based salinity mapping can provide a practical regional tool for transitional water monitoring. By combining satellite observations with targeted field campaigns and hydrodynamic modelling, the approach offers a scalable solution for tracking salinity dynamics in optically complex coastal systems, supporting ecosystem management and water quality assessment in the Venice Lagoon and beyond.

The choice of CDOM as a salinity proxy rests on a well-established optical principle. Coloured dissolved organic matter is the fraction of dissolved organic material that absorbs light, and in river-influenced systems it is typically delivered to coastal waters by terrestrial runoff. Because freshwater carries high concentrations of CDOM while open marine waters contain comparatively little, the progressive mixing of river water with seawater produces a predictable dilution pattern in which CDOM absorption falls as salinity rises. This conservative behaviour during mixing is what makes the inverse relationship between the two quantities so useful: once it is calibrated locally, an optical measurement of CDOM can stand in for a physical measurement of salinity. The Venice Lagoon, with its network of tributaries, tidal inlets, and sheltered inner basins, provides an ideal setting for testing whether this relationship holds across a full gradient of freshwater and marine conditions.

The technical route from satellite signal to salinity estimate involves several carefully chosen steps. The Ocean and Land Colour Instrument aboard Sentinel-3 measures water-leaving radiance across multiple spectral bands, but the raw signal arriving at the sensor is dominated by light scattered from the atmosphere rather than from the water itself. Atmospheric correction is therefore a critical preprocessing stage, and its accuracy strongly influences the reliability of downstream retrievals in turbid coastal waters. Once corrected, the ratio of reflectance at 665 nanometres to that at 490 nanometres provides a robust empirical basis for estimating the CDOM absorption coefficient at 440 nanometres, a wavelength commonly used to characterise CDOM abundance. The band ratio approach is favoured in optically complex waters because it partially cancels out variations in illumination and suspended particulate matter that would otherwise confound single-band retrievals.

Converting satellite-derived CDOM into salinity requires an additional layer of calibration. The researchers paired satellite observations with concurrent salinity measurements from the SAMANET continuous monitoring network, which maintains fixed stations throughout the lagoon. This pairing allowed them to build a statistical relationship between optical CDOM estimates and in situ salinity, and then to refine that relationship by accounting for wind conditions and river discharge regimes. The refinement matters because the CDOM-salinity relationship is not perfectly constant: wind-driven resuspension of sediments can alter the optical properties of the water column, while seasonal changes in river flow alter the concentration and composition of dissolved organic material delivered from land. By conditioning the retrieval on these environmental regimes, the model becomes more reliable across the range of conditions that characterise the lagoon throughout the year.

The resulting salinity maps reveal patterns that would be difficult to capture through field sampling alone. Short-term tidal redistribution of water masses appears as shifting salinity boundaries that move with the tide, while springtime freshwater inputs produce visible plumes of lower-salinity water extending from river mouths into the lagoon. The persistent contrast between landward sectors, which remain freshwater-influenced, and the more saline waters near the tidal inlets, where marine exchange dominates, emerges clearly in the satellite-derived products. These spatial patterns are ecologically meaningful because salinity gradients structure habitat distribution, influence nutrient cycling, and determine which species can establish and persist in different parts of the lagoon.

Comparison with the SHYFEM hydrodynamic model, an established finite element model of the lagoon’s circulation, provides an independent check on the satellite-derived patterns. The model captures the broader salinity structure but underestimates values near the Dese river mouth, a discrepancy that likely reflects uncertainties in freshwater forcing data, the representation of local channel geometry, and the treatment of vertical mixing in shallow, wind-exposed areas. This kind of cross-validation is valuable because it identifies where each approach performs well and where complementary information is needed. Satellite observations excel at capturing spatial patterns and short-term variability, while hydrodynamic models provide continuous temporal coverage and can simulate conditions when cloud cover prevents optical observations.

The broader significance of this work lies in its demonstration that regionally calibrated ocean-colour observations can extend salinity monitoring into environments where conventional satellite methods fall short. Passive microwave missions such as SMOS and SMAP have transformed open-ocean salinity observation, but their coarse spatial resolution and sensitivity to nearby land surfaces limit their usefulness in narrow coastal zones and lagoon systems. Optical sensors, by contrast, offer spatial detail sufficient to resolve the heterogeneous patterns characteristic of transitional waters, provided that a suitable optical proxy and a locally calibrated retrieval model are available. The Venice Lagoon study shows that this combination can be achieved with existing satellite infrastructure and a modest field campaign, making the approach transferable to other optically complex coastal systems facing similar monitoring challenges.

Salinity is recognised as an Essential Climate Variable because it reflects changes in the hydrological cycle, including precipitation, evaporation, river runoff, and freshwater storage. As climate-driven shifts in these processes alter the balance between freshwater and marine influence in shallow coastal environments, the ability to track salinity dynamics consistently and at appropriate spatial scales becomes increasingly important. The framework developed here, which links satellite optical observations to in situ monitoring and hydrodynamic modelling, offers a practical pathway toward that goal. For environmentally sensitive lagoon systems such as Venice, where ecological condition and human management are tightly coupled, the capacity to observe salinity variability in near-real time could support adaptive management strategies, inform restoration planning, and strengthen the evidence base for protecting transitional ecosystems under changing environmental conditions.

Subject of Research: CDOM-based satellite salinity mapping in the Venice Lagoon

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

Article References: Where freshwater meets the sea: satellite–model insights into salinity dynamics in the Venice Lagoon. (n.d.). https://doi.org/10.3389/frsen.2026.1919861

Image Credits: AI Generated

DOI: 10.3389/frsen.2026.1919861

Keywords: Venice Lagoon, CDOM, salinity, Sentinel-3, OLCI, transitional waters, remote sensing, SAMANET, freshwater, meets, satellite, model