A trend analysis of the first eight years of nutrient data collected by the Bay Program’s monitoring program has found a significant decline in phosphorus concentrations in Chesapeake waters, but no change in nitrogen concentrations or the amount of oxygen in the water.

Trend analyses are important tools because they provide a gauge by which progress toward restoring the Bay can be measured. The findings seem to verify estimates of nutrient loads by the states and the Bay Program which indicate more progress has been made in controlling phosphorus than nitrogen. The new analysis, which will presented in a soon-to-be-published report, analyzed data collected from 1984 to 1992.

Reducing the amounts of nitrogen and phosphorus has been a central part of the Bay restoration effort.

In the 1987 Bay Agreement, the states of Maryland, Virginia and Pennsylvania, and the District of Columbia committed to reducing the amounts of those nutrients entering the Bay by 40 percent by the year 2000. The jurisdictions are now developing nutrient reduction strategies for the Bay’s major tributaries to fulfill that commitment.

Excess nutrients are considered to be the Bay’s main water quality problem. Too many nutrients stimulate the growth of more algae than fish and other predators can consume. The excess dies and sinks to the bottom where it decomposes in a process that depletes the water of oxygen needed by fish and many other organisms. That reduces the amount of habitat available. In addition, the algae can cloud the water, blocking sunlight needed by Bay grasses, which also provide important habitat for many species.

The report found that phosphorus concentrations in the Bay had declined 16 percent over the eight-year period. That was slightly lower than an earlier estimate of 19 percent, in part because some information used in the earlier estimate was discarded because of quality assurance problems. The greatest declines were seen in the upper and lower portions of the Bay.

Declines in the upper Bay appear to be linked to similar phosphorus declines detected in Susquehanna River monitoring. The reason for the declines in the lower Bay is less clear, the report says, because none of the nearby tributaries — the Rappahannock, York, and James) had declining phosphorus water quality trends from 1984 through 1991. The decline, the report said, might be caused by changing concentrations in ocean water that washes in and out of the lower Bay.

The phosphorus numbers are substantiated by estimates of reduced phosphorus loads from a computer model, said Joseph Macknis, monitoring coordinator with EPA’s Chesapeake Bay Program Office. The model, which estimates load reductions based on implementation of improvements at sewage treatment plants, use of runoff control practices on farmland, and other factors, has estimated that total phosphorus loads to the Bay dropped from 27 million pounds a year in 1985 to 22.8 million pounds in 1992.

For nitrogen, the eight years of data shows no trend. The only section of the Bay with a significant trend was Mobjack Bay in Virginia, where loads increased by about 10 percent, apparently related to increased loads in the York River and other nearby tributaries.

Again, the monitoring results correspond closely with model estimates that indicate nitrogen loads to the Chesapeake barely fell during the period, Macknis said. The model estimated that nitrogen loads were 376 million pounds in 1985 and 364 million pounds in 1992, he said. The lack of significant reductions stems largely from the fact that major efforts to control nitrogen throughout the Bay began later than phosphorus control efforts.

Also, the monitoring data found no significant trend for dissolved oxygen in the Bay — a measure of the amount of oxygen available in the water, particularly in deeper areas more susceptible to oxygen depletion.

The lack of improved dissolved oxygen conditions, the report said, could be based on several possibilities:

  • Nutrient declines have not been great enough to influence dissolved oxygen levels, or have not affected enough of the Bay. Also, dissolved oxygen may be more linked to the spring freshet and stratification than nutrients.
  • There may be a time lag between nutrient reductions and dissolved oxygen improvements.
  • Nitrogen has not been reduced. Some computer modeling efforts indicate that phosphorus reductions without corresponding nitrogen reductions will have little impact on dissolved oxygen levels in the Bay.

In one other water quality parameter — secchi depth trends, which measure water clarity — improvements were seen in the upper portions of the Bay. That could help in efforts to restore submerged aquatic vegetation in those areas, the report noted. The abundance of Bay grasses is closely linked to the amount of light the plants receive.

The report analyzed the eight years of data gathered since the monitoring program began in 1984. The data was collected from 49 stations throughout the Bay, where samples are taken once or twice a month, depending on the season, and analyzed for more than 20 water quality parameters.

That data was used to calculate trends for 10 geographic regions of the Chesapeake, each of which have similar salinity, circulation, and geomorphic conditions.

The results are presented in a report, “Trends in Phosphorus, Nitrogen, and Dissolved Oxygen in the Chesapeake Bay 1993.” The report is expected to be available from the Bay Program in the spring.