The greatest source of both nitrogen and phosphorus entering most estuaries, such as chesapeake bay, is

The Chesapeake bay is an estuary in United States. The presence of nitrogen and phosphorus in the estuary can cause harm to the aquatic species present there.

The main source of nitrogen in the Chesapeake bay is the agricultural runoff. The pesticides and the herbicides used in the agriculture may come along the runoff and get mixed with large rivers and water bodies. The nitrogen from agricultural runoff accounts approximately 40% of the total sources of nitrogen.

Hence, the correct answer is agricultural runoff.