The calm, shallow waters of coastal estuaries are easily polluted by nutrient-rich sewage  When estuary waters become overnutrified as a result, algae proliferate
The abundant algae, in turn, sometimes provide a rich food source for microorganisms that are toxic to fish, thereby killing most of the fish in the estuary
Overnutrifying estuary waters by sewage can result in the death of most of the fish in the estuary.