This modeling indicated that the Potomac River and the tributaries to the north are the dominant factors in the status of the main Bay's dissolved oxygen. The tributaries to the south of the Potomac, conversely, have a minimal impact on dissolved oxygen in the main Bay. Specific nutrient reduction goals for each of the lower tributaries are being developed to meet the requirements of the plants and animals in each tributary. Those lower tributary goals are being developed through the expansion and refinement of the basic Chesapeake Bay water quality model with results expected in late 1997 or 1998. When accounting for all of the nutrients that enter the Bay, the two largest contributors of both nitrogen and phosphourus are agricultre and point sources, based on 1985 estimates.
Forests are a natrual source of nutrients.
Atmospheric nitrogen pollution that falls directly on the water is displayed as a separate category and accounts for 11 percent of the total nitrogen load. Atmospheric nitrogen also falls on the land and accounts for an additional 16 percent of the total nitrogen load, but is included as part of the forest, agriculture and developed land sources. As a result of the 1992 Amendments to the Chesapeake Bay Agreement, nutrient reduction targets were allocated among Maryland, Virginia, Pennsylvania and the District of Columbia to achieve the greatest improvement in the Bay and its tributaries. The reduction targets use nutrient loads from 1985 - the baseline year for the 1987 Agreement. The loads remaining after the reductions are meant to be a cap to prevent backsliding in the future.