Parallel forces moving in opposite directions exert

When two forces act in the same direction, they add together. When forces act in opposite directions, they are combined by subtracting the smaller force from the larger force. Balanced forces acting on an object will not change the object's motion. When you add equal forces in opposite direction, the net force is zero. Does this help?

Explanation:

poor water quality affects most estuarine organisms, including commercially important fish and shellfish. the pollutants that have the greatest impact on the health of estuaries include toxic substances like chemicals and heavy metals, nutrient pollution (or eutrophication), and pathogens such as bacteria or viruses.