But in seawater, there is more sodium, potassium and chlorides, while freshwater has more carbonates and bicarbonates. In seawater, much of the carbonates are removed by sea creatures to make their shells. Coral reefs remove a large proportion of these minerals so they are what makes the sea taste salty. While water plants release oxygen with photosynthesis during the day, they remove oxygen during the night and can seriously deplete oxygen levels during this time. Often, though, these two merge into one another.
Rivers change along their length.
Plants and animals living in fast-flowing waters have special adaptations to avoid being washed away. Aquatic insect larvae have claws to hang on with, some fishes have modified fins which form suckers. See our page on fishes adapted to murky water . They are wet all year although the water level may change seasonally. These also form at the edges of slow-moving streams or rivers, edges of lakes and in floodplains of rivers in the lowlands.