Question

An aquatic ecosystem was rich in benthic microalgae and sea grasses. Over the years, industrial pollutants have reduced these populations considerably. How would this affect the ecosystem?

Answer 1

The answer you are looking for is:
There would be less energy transferred to the higher trophic levels.
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Answer 2

Answer: This would affect the whole marine ecosystem. It would also effect the oxygen concentration inside the sea and in outer environment.

It contributes to a huge proportion of oxygen in the environment and if there is some sought of pollution in the sea then the aquatic animals will die and it will effect the other animals present in sea.

The marine ecosystem will be disturbed and the food web inside the sea will also be hampered.