The nutrients nitrogen and phosphorus are considered the main threats to the health of the Bay because they spur algae blooms that cloud the water, resulting in the die-off of underwater grass beds, which provide critical food and habitat for a host of fish, shellfish and waterfowl. When the algae die, they decompose in a process that depletes oxygen from the water, rendering huge areas off-limits to aquatic life.
Reducing nutrients has been a goal of the Bay Program since the mid-1980s.
In the Chesapeake 2000 agreement, the Executive Council committed itself to restoring Bay water quality by 2010. To achieve water quality standards for the Bay, computer models estimate that:
- The amount of nitrogen entering the Bay must be reduced to 175 million pounds a year. In 1985, the baseline for measurements, about 337 million pounds entered the Bay.
- The amount of phosphorus entering the Bay must be reduced to 12.8 million pounds a year. In 1985, about 27.13 million pounds entered the Bay.
The Bay Program, based on activities reported by the states, estimates that actions taken since 1985 would achieve 44 percent of the nitrogen and 60 percent of the phosphorus goals.
Water quality monitoring, though, indicates little significant change in the actual amount of nitrogen actually entering the Bay, although phosphorus levels have decreased.
Several factors may contribute to the discrepancy. Many runoff control practices have significant "lag times" before they become fully effective. Some runoff control practices may not be as effective as assumed. Also, the reduction goals are based on "average" rainfall years. Since the mid-1980s, there has been a trend toward wetter years, which flush more nutrients off the land and into the Bay.