What is the primary difference between marine and freshwater biomes?

1. What is the primary difference between marine and freshwater biomes?
2. Why are estuaries considered highly productive ecosystems?
3. The diagram below shows the relationship between salinity levels and biodiversity in different aquatic ecosystems. What pattern does this reveal?
A) Higher salinity always leads to higher biodiversity.
B) Freshwater ecosystems always have the lowest biodiversity.
C) Moderate salinity levels (i.e., estuaries) support the highest biodiversity.
D) Salinity has no relationship to biodiversity in aquatic systems.