How does salinity vary in a lagoon with large amounts of freshwater runoff?

In a lagoon receiving significant freshwater runoff, salinity levels are primarily determined by the balance between that incoming freshwater and the tidal influx from the sea. These environments are typically classified as brackish, and their salinity distribution is characterized by a gradient: values are lowest near the source of the runoff and increase toward the tidal inlets where seawater enters.1

The specific salinity values in such lagoons can vary significantly by season and location. For example, in the Gippsland lagoons, inner-end salinity may range from 0.5 to 5 parts per thousand (ppt), while central areas typically fluctuate between 5 and 20 ppt. Because these bodies of water are often shallow and influenced by local drainage, their salinity is far more variable than that of the open ocean. In contrast to hypersaline lagoons—where high evaporation rates cause salinity to exceed that of seawater—brackish lagoons are defined by this dilution effect, which is particularly pronounced in nearshore regions isolated from the open sea.12