Which aquatic ecosystem typically forms along coastlines where freshwater and saltwater meet, leading to varying degrees of salinity

Estuary

An estuary can be defined as an ecosystem where a fresh water source like stream of river get mixed with an ocean. The water in the estuary is brackish water. In estuaries, the water level and salinity rise and fall under the influence of oceanic tides, river flow and seasons. The oceanic tides creates the largest flow of saline water whereas, rivers creates largest flow of fresh water. During rainy season the rivers will flood the estuary with fresh water and in the dry season the estuary shrinks and becomes more saline. Therefore, estuary water varies in degrees of salinity.

i think its the first one