Riverine low salinity buoyant plumes off NW Iberia

Riverine freshwater input is one of the most distinctive processes impacting the coastal ocean, from the physical to the ecosystem level, presenting multiple spatial and temporal variability scales. In situ measurements, satellite imagery, and numerical model solutions are used to study the spatio-temporal distribution of low salinity buoyant plumes off NW Iberia. The in situ observations were carried in the vicinity of the Mondego River estuary (central Portugal) from September 2024 to May 2025 using moored loggers deployed during the fishing operations of a coastal fishing vessel. The measurements from 26 deployments, corresponding to a total of 45 days of valid records of depth, temperature, salinity and turbidity show a strong semi-diurnal signal driven by tidal forcing, superimposed on a seasonal trend. Low salinity values ​​(S < 34.5) were consistently recorded in the coastal zone following the largest precipitation and river discharge event, and the minimum salinity values ​​recorded at low tide after this period were not directly related to the river flow. The observed patterns in high-resolution satellite images support the numerical model solutions and show that the signature of the less saline lens is observable due to suspended particles in the river water and, above all, sediment particles resulting from surface wave breaking, whose distribution serves as a tracer of the currents associated with the plume. The combined analysis of the observations, satellite data and the numerical model solutions showed that the plume's extension offshore and along the coast is primarily linked to the current system in the coastal zone, the local cumulative river discharge and transport of buoyant plumes from neighboring rivers. In particular, the results revealed a significant role of a recirculation cell downstream of Cape Mondego in the vicinity of the river mouth. This cell is associated with the separation of an intermittent equatorward coastal current north of the cape which, at times, also transports low-saline waters from upstream rivers, emphasizing the importance of a regional approach to be able to realistically model the riverine low salinity buoyant plumes off the NW Iberian shelf.