The effects of estuary salinization on benthic assemblages: a richer and treacherous future

Estuarine salinization is emerging as a major consequence of climate change, yet its long-term ecological effects on tropical benthic assemblages remain poorly understood. Using nearly two decades of monitoring across three tropical estuaries, we evaluated temporal changes in benthic macroinfauna structure along their salinity gradients. We tested whether salinization would increase richness, change assemblage composition and cause shifts in benthic invertebrate distribution along the estuarine gradient. In two estuaries (Jaguaripe and Subaé) richness increased over time. Multivariate analyses showed that salinity remained the primary driver of benthic structure. Although some homogenization has occurred in middle and upper estuary zones in recent years, we found no consistent evidence that lower and middle estuarine taxa expanded landward. One estuary (Subaé) exhibited weaker temporal trends, possibly owing to historical contamination masking salinity effects. Our findings highlight that estuarine benthic assemblages are indicators of salinization, and increasing richness does not necessarily imply ecological improvement. Shifts in salinity regimes may compress estuarine zones, threaten oligohaline assemblages and alter ecosystem functions. Long-term monitoring, especially in tropical estuaries where climate extremes and human pressures are intensifying, is essential to detect tipping points and guide adaptive management and conservation strategies.