For many years, the nutrient reductions predicted by computer models were not seen in actual water quality monitoring. Instead, monitoring stations in the nontidal rivers that feed the Bay showed little improvement in the total amount of nutrients exported.
That has begun to change. In 2002, data showed downward trends in nitrogen and phosphorus at 18 long-term monitoring sites in nontidal rivers, while nine showed no change, and two had an increasing trend. [Not all sites are shown in adjacent maps.]
That was a big difference from 2001, when monitoring showed downward trends at only nine sites for nitrogen, and four for phosphorus. The year before that, hardly any monitoring stations showed a downward trend.
The trends, in part, reflect a drier than normal period from 1999 through 2002. Dry conditions result in fewer nutrients being washed off the land. As a result, recent water quality trends are likely to be reduced or reversed as river data becomes available from 2003, which had near-record freshwater flows into the Bay.
While computer models offer a “virtual” look at the Bay under average conditions, the monitoring offers a look at the conditions actually facing fish, underwater grasses and other resources. Many of those thrived during recent dry years with reduced nutrient inputs—2002 saw the greatest extent of underwater grass beds since aerial surveys began in 1984.
On the other hand, the high flows and nutrient loads last year combined to create one of the worst years for dissolved oxygen on record. In fact, the amount of water that was “severely hypoxic”—areas where few fish and shellfish can survive—was the greatest observed since Baywide monitoring began in 1985.
So is the nutrient load entering the Chesapeake totally dependent on the weather? No. The loads represent the average concentration of nutrients in the water multiplied by the total volume of water entering the Bay.
In many—though not all—rivers, monitoring has begun to show decreases in nutrient concentrations. That means less is entering the waterways during “average” conditions. But during many wet years in the 1990s, that decreasing trend was overwhelmed by increased flows.
If nutrient reductions further reduce concentrations, it will help mitigate some of the impacts of high flow years in the future.
Reducing nutrients enough so water quality is not determined primarily by weather patterns is important to the cleanup effort. And, ultimately, the determination of whether the Bay attains its new water quality standards will be determined by real-world monitoring—not virtual-world models.