Saltier Than Expected – Small Brine Discharge Increases Salt Intrusion in the Tidal Ems Estuary, North Sea

Abstract This study investigates the long‐term salinity dynamics of the partially mixed Ems Estuary, focusing on the influence of brine inflow on salt intrusion. Salinity time series from eight monitoring sites (2000–2023) were analyzed to assess variations due to river discharge and brine inflow from two injection sites. Tidally‐averaged salinity was evaluated by tracking the positions of the 2 and 13 gkg −1 isohalines. Brine effects were identified by separating the data set into four subsets: two no‐brine, one low‐brine, and one high‐brine period. For each, we computed temporal averages, absolute and relative salinity changes, and horizontal salinity gradients. Salinity showed strong seasonal variation driven by river discharge, both with and without additional salt input. The effect of brine inflow was superimposed on river discharge‐related variability and depended on prevailing and previous discharge conditions. Although brine volumes were, on average, five orders of magnitude smaller than flood tidal volumes, they increased salt intrusion during low discharge conditions. Compared to no‐brine scenarios, salt intrusion shifted up to 10 km further upstream during the high‐brine period. Estuarine transport processes caused the salinity to increase 15–35 km upstream (exceeding the tidal excursion) of the brine inflow sites. The longitudinal location of the brine release affected salt dispersion, as demonstrated by a movement of the maximum salinity gradient. These findings emphasize the sensitivity of estuarine systems to small but persistent salt sources, with implications for estuarine ecosystems and management globally under increasing salinity stress from freshwater scarcity, land‐use pressure, and climate change.