Answer:
The most significant challenge is change in salinity.
Step-by-step explanation:
Estuaries are apparently stressful for some organis, the estuaries are very dynamics and they are ecosystems with an abrupt salinity change because seawater and fresh water meet.
The changes in salinity cause internal osmotic stresses. Benthic organisms for example use a number of strategies like modification of physical systems (oysters close their shells in low salinity water).