Recent dry conditions have helped produce clearer water in many areas of the Bay, but it appears they won’t make it much easier for fish, crabs and other aquatic organisms to breathe in the Chesapeake this summer.
Scientists predict that the low-oxygen “dead zone” will affect about 1.58 cubic miles, or more than 12 percent, of the Bay this summer. That’s a slightly greater volume of poor water than the long-term average over the last three decades.
The portion of the Bay totally starved of life-sustaining oxygen is predicted to be slightly smaller, though. Anoxic conditions will cover just 0.28 cubic miles in early summer, though that is expected to grow to 0.31 cubic miles by late summer, according to the annual forecast. That would encompass a bit more than 2 percent of the Bay.
The annual summer forecasts, released Monday, come from computer models that predict the size of the dead zone, which plagues deep areas of the mid-Bay each summer. The forecast is based on estimates of the amount of nitrogen washing into the Bay from the Susquehanna River, the strength of its flow in May, wind, and other factors.
Excess nutrients from wastewater, stormwater and farm runoff spur blooms of algae which eventually die, sink to the bottom and are decomposed by bacteria in a process that depletes the water of oxygen.
River flows are also important because they affect how well the Bay’s water mixes and absorbs oxygen from the air. Fresh water flowing in from the Susquehanna and other rivers tends to sit on top of heavier, salty water coming in from the Atlantic Ocean. A barrier, known as a pycnocline, forms between the fresh water on the surface, which has more dissolved oxygen in it, and the saltier water in the depths of the Bay. Thus, the oxygen consumed in deep water by decaying algae is not easily replaced, exacerbating low oxygen problems in deep water.
Low river flows during dry weather typically result in more mixing of the waters, as well as fewer nutrients from runoff, while high flows bring more nutrients, less mixing because of a stronger pycnocline, and often larger dead zones.
This year presented a little of both. Susquehanna flows were low much of the spring, and well below average in March and April. U.S. Geological Survey monitoring showed the amount of nitrogen entering the Bay from the river during the first five months of the year was 17 percent less than average. But river flows in May — which greatly influence the dead zone— were higher than average.
Those conditions, when fed though the models, showed the amount of hypoxic water — with less than 2 milligrams of oxygen per liter — would be on par with the average since 1985. But the volume of anoxic water with no oxygen at all should be about 10 percent less than the long-term average.
Low oxygen stresses fish, and it can make them more vulnerable to predators and disease. No oxygen can be lethal, particularly for immobile shellfish on the bottom.
The models used in the forecast were developed by researchers at the University of Maryland Center for Environmental Science and the University of Michigan. Funding for the effort comes from the National Oceanic and Atmospheric Administration.
Weather conditions through the summer, especially if any tropical storms hit or pass nearby, could affect the accuracy of the forecast. Researchers will track oxygen levels from water samples taken by Maryland and Virginia agencies to monitor the actual size of this year’s dead zone.
UMCES President Donald Boesch said oxygen conditions in the Bay have tended to improve during late summer in recent years. He called that a hopeful sign, but added, “it’s no reason to be complacent — we have a long way to go to finish the job.”
Beth McGee, senior scientist with the Chesapeake Bay Foundation, noted that the size of the dead zone is largely linked to pollution from the Susquehanna River, which mostly originates in Pennsylvania where nutrient reduction efforts are lagging.
“CBF believes that an average-sized dead zone is still unacceptable, and that Pennsylvania and the other Bay states must implement the plans they developed to reduce pollution and restore water quality in local rivers, streams and the Chesapeake Bay,” she said.