The possible reason for the decrease in average water clarity from 1990 to 2015 could be eutrophication.
Eutrophication is a process where a body of water becomes overly enriched with nutrients, leading to excessive growth of algae and other aquatic plants. This excessive growth can reduce water clarity as the algae and plants block light from penetrating the water, making it cloudy or murky. As a result, the decrease in water clarity could be attributed to eutrophication during the period mentioned.
Another possible reason for the decrease in water clarity could be a change in the amount of oxygen in the water. When oxygen levels in water decrease, it can lead to the decomposition of organic matter, which in turn can cause turbidity and reduce water clarity.
Therefore, both eutrophication and changes in oxygen levels can be contributing factors to the decrease in average water clarity observed from 1990 to 2015.
