The Bay Program uses powerful computer models to estimate the impact of nutrient and sediment reductions on tributaries. The models divide tributaries, such as the Rappahannock, shown here, into grids, or segments.

The model can calculate how each grid responds to various levels of nutrient and sediment reduction — from no action, to extreme “limit of technology” reductions, to more plausible scenarios in between — for a variety of factors.

For each grid, it can calculate whether grass beds would return, and how dense those grass beds would be. Generally, thicker grass beds provide better habitat and are more stable; they can better withstand periodic pulses of poor water quality.

It also calculates algae concentrations for each grid; too little algae, and there is not enough to serve as “fish food;” too much and it can block sunlight to grass beds.

Also calculated for each grid is bottom dissolved oxygen concentrations, measured in milligrams per liter (mg/l). Less than 1 mg/l is lethal to many bottom-dwelling organisms; 2-3 mg/l is suitable for bottom dwelling clams and worms (fish food); 3-5 mg/l is suitable for bottom-feeding fish and crabs; and more than 5 mg/l is considered fully suitable for fish.

Summaries of Model Results

James River

Upper Tidal James River:

  • Some reduced algal concentrations and increased Bay grass density as a result of feasible nutrient and sediment reductions.
  • Algal reduction is primarily due to nutrient reductions.
  • Sediment plays a minor role in algal reductions, but a major role in Bay grass restoration.
  • Algal and Bay grass response in the upper tidal James is due to nutrient and sediment reductions made from the upriver areas, and the surrounding upper tidal watershed, not from the lower James.

Lower Tidal James River:

  • Current dissolved oxygen conditions are not a living resource habitat concern, with the possible exception of parts of the Elizabeth River.
  • Little computed Bay grass recovery even at “limit of technology” and “extreme sediment reduction” scenarios.
  • Minimal influence on lower Chesapeake Bay water quality.

Rappahannock River

  • Significant estimated reduction in anoxic (no oxygen) and hypoxic (low oxygen) conditions within the feasible region of nutrient reduction.
  • Nutrients from the Potomac and more northern river basins influence low oxygen conditions in the lower tidal Rappahannock.
  • Bay grasses estimated to reach stable, sustainable densities with feasible nutrient and sediment reductions.
  • Rappahannock nutrient and sediment sources have a greater effect on Bay grass response than loads outside the Rappahannock.

Virginia’s Eastern Shore

  • No dissolved oxygen concerns
  • Stepwise increases in Bay grass density with more beds in the category of thick, deep, healthy grass beds with increasing nutrient/sediment reductions.
  • Local reductions seem to be the reason for increases in density; reductions from the Potomac and more northern river basins had minor influence only on grass beds off Tangier Sound.

York River

  • Significant reduction in low-oxygen conditions with feasible nutrient control reductions. Nutrients from the Potomac and more northern river basins have some influence on low oxygen conditions in the lower tidal York.
  • Bay grass density and area increase with increasing nutrient and sediment reductions.