For two straight years, huge amounts of fresh water bearing huge amounts of nutrients to feed algae blooms were flushed into the Bay, resulting in some of the worst water-quality conditions ever observed in the Chesapeake.
Water quality managers hoped for a break in that trend this year. They got their wish. After a mild winter, the Bay did, indeed, get low flows. April set a record for low discharges from the Susquehanna, the largest tributary to the Chesapeake.
But managers also learned there was truth in an old adage: Be careful what you wish for, because you may get it.
Instead of good water everywhere, Bay Program monitoring efforts found that areas in the upper Chesapeake were even more oxygen-starved this spring than during the previous two years.
That was a surprise to us, said Rob Nelson, a data analyst with the Maryland Department of Natural Resources working with the EPAs Chesapeake Bay Program office. The algal blooms were not flushed down the Bay like they would be in a normal year by the strong Susquehanna flows.
While the severity of the upper Bay oxygen problem was a surprise, preliminary data for portions of the Bay farther south indicated improvement over the last two years. When you take the Bay in total, its a little bit better than normal, as we predicted, said Rob Magnien, director of the Tidewater Ecosystems Assessment Division of the DNRs Chesapeake and Watershed Assessment Administration.
The Bay Program has a goal of reducing the levels of two nutrients phosphorus and nitrogen 40 percent by the turn of the century. That goal was based on computer modeling which indicated that such a reduction in an average runoff year would significantly reduce the amount of low-oxygen water in the Bay. The amount of low-oxygen is considered to be a good overall indicator of Chesapeake water quality.
This years results indicate that while nutrient reductions can help improve water quality, the actual health of the Bay in a given year is largely dependent on weather conditions primarily the amount and timing of freshwater flows into the Chesapeake. Every year is special, noted Marcia Olson, of the National Oceanic and Atmospheric Administrations Chesapeake Bay Office.
In 1993 and 1994, high and often record-setting flows flushed huge amounts of nutrients off city streets, suburban yards and agricultural lands and into the Bay. Those nutrients fed the growth of large algae blooms. There was more algae than fish and other species could consume, and the excess sank to the bottom, died and decomposed in a process that depleted the bottom water of oxygen.
By contrast, lower flows generally mean less nutrients are flushed into the Bay and less algae is produced. That, in turn, generally means that oxygen levels remain higher in the Bay.
This year, those flows may have been too low.
In April, the flow down the Susquehanna, which normally provides about half of all the fresh water to the Bay, averaged only 35,700 cubic feet per second a record low for the month. Flows for March and May were also far below average.
In any given year, the spring flow contains enough nutrients to cause an algae bloom. Normally, though, the flows are strong enough to push that bloom to the Annapolis Bay Bridge and farther south where the Bay becomes wider and deeper. If the algae production is sent farther and farther downstream, the effects get more and more diluted, Olson said.
But this year, there was little power to push the bloom anywhere, so it blossomed north of the Bay Bridge. In that area, the Bay is both narrower and shallower, so the low dissolved-oxygen problem was confined to a smaller area, amplifying its impact on local water quality.
It wasnt a very unusual bloom, Magnien said. You had a typical spring bloom level, but the position of the bloom was farther up the Bay and a bit earlier than normal.
Measures of algae concentrations in that region were several times what is normally detected, Nelson said.
Not only that, the top layer of fresh water flowing into the Bay from the Susquehanna and other tributaries was strongly stratified from the bottom layer of salt water which comes up the Chesapeake from the Bays mouth.
When the water is strongly stratified, the pycnocline the line that separates the salty bottom water from the fresh top water becomes a barrier that prevents the layers from mixing. As a result, when oxygen becomes depleted in the lower layer as algae decays, it can not be replenished by mixing with the top layer.
That combination an algae bloom
taking place farther north than normal, and a strong stratification that prevented oxygen-starved bottom water from mixing with oxygen-rich top water created the conditions in which the deep channel of the upper portion of the Bay became severely depleted of oxygen.
In April, oxygen levels fell below 5 parts per million, the level at which the Bays living resources become stressed, in the deepest areas of the Bay north of the Bay Bridge. Conditions worsened in May and June, with oxygen concentrations in some portions of the upper Bay dipping below 3 ppm.
At the same time, stratification appeared to weaken farther south, and water quality as measured by oxygen concentrations in the water appeared better than the previous two years.
Nonetheless, the finding caused concern because the upper Bay is a particularly important spawning area. While the low oxygen levels are thought to have occurred outside important spawning areas, they did take place in an area used by juvenile fish after spawning. Newly spawned fish and juveniles tend to be particularly vulnerable to low oxygen levels, which can kill them outright, affect their growth, or make them more susceptible to disease or impacts from other pollutants.
After this spring, scientists are anxious to see how the summer oxygen levels compare with those of the last two years. Oxygen levels are usually at their lowest in the summer. During the last two summers, the Bay had its worst oxygen levels since the Chesapeake Bay monitoring program a cooperative effort of government agencies and research institutions began more than a decade ago.
NOAAs Olson said that in 1993, about 40 percent of the Bay was below the pycnocline. Of that, oxygen levels fell below 3 ppm in 37 percent, and water went anoxic lost all oxygen in 14 percent.
In 1994, about 33 percent of the Bay was below the pycnocline. Of that, oxygen levels dropped below 3 ppm in about 35 percent, and about 9 percent went anoxic.
Monitoring data is not yet available to show what impact the late spring storms that caused flooding in western Virginia and some other areas have had on the Bay and its tributaries.
Lower Bay tributaries tend to have less of an effect on dissolved oxygen in the Chesapeake than the more northern rivers particularly the Susquehanna but Magnien said it was possible that the huge influx of water and nutrients could cause algae blooms and stronger stratification that would deplete oxygen in the lower portions of some Virginia rivers.
Heavy rains in June dumped more than 6 inches of water on parts of western Virginia, causing flooding that left several people dead and forced thousands to flee their homes.