Final answer:
Freshwater runoff into the sea reduces the sea's salinity and can impact marine ecosystems.
Step-by-step explanation:
Freshwater runoff into the sea reduces the sea's salinity. When freshwater from rivers, lakes, or streams flows into the ocean, it dilutes the saltwater, resulting in a decrease in salinity. This can have various impacts on marine ecosystems, such as affecting the distribution and abundance of marine organisms and disrupting the balance of the ecosystem.