Scientists say this summer was a mixed bag for the Chesapeake Bay, with dissolved oxygen levels turning out to be slightly better than expected while large areas continue to be plagued with worsening water clarity.

Overall, summer conditions for many areas assessed in the annual scientific review appeared to be near the average observed over the last two decades, although they still paint a picture of a severely degraded Bay with scattered harmful algae blooms, fish kills and increasingly murky water.

Anoxic water-the area depleted of oxygen-is one of the most closely watched indicators of Bay health, and was present in 2.5 percent of the Bay this summer. The 1.3 cubic kilometer dead zone was exactly the average since Baywide monitoring began in 1985.

That was slightly better than expected. This spring, scientists had predicted the amount of anoxia would be the fifth worst on record; instead it was 11th worst-or exactly average.

The anoxia forecast is based on the historic relationship between late winter and early spring flows from the Susquehanna River, nutrient loads from the upper Bay and the amount of anoxic water in the Bay. High flows during that time, along with the nutrients they carry, are closely associated with the amount of summertime anoxia in deep areas of the Bay.

This year's forecast was driven by higher than normal flows from the Susquehanna during February and March.

The lower level of anoxia stemmed from better than expected conditions in July, when the amount of anoxia usually peaks. This year the amount of oxygen-starved water in July was smaller than normal. Scientists speculate the reason was a wedge of saltwater that moved into the Bay from the ocean in midsummer, pushing oxygen-rich water from the southern Chesapeake into the mid Bay, limiting the anoxic zone.

By early August, the event dissipated and anoxic water conditions returned to normal levels for the month.

The anoxia figures only cover the "mainstem" of the Bay, not localized anoxic events that take place in tidal tributaries during the course of the summer.

Low dissolved oxygen is thought to have contributed to two large fish kills in Maryland, one in the South River and one in Marley Creek, a tributary of the Patapsco. Together, the two claimed more than 100,000 fish.

A number of smaller fish kills, involving fewer than 3,000 fish, were reported in other tidal rivers around the Bay, according to the summary.

Meanwhile, the amount of hypoxic water-areas with low levels of oxygen-was slightly larger than average. While not as lethal as anoxia, hypoxic water makes large areas uninhabitable for most aquatic life.

An average of 5.4 cubic kilometers of the Bay, or about 10 percent, had concentrations of less than 2 milligrams of oxygen per liter of water during the summer months, compared with an average of 4.7 cubic kilometers since 1985. Unlike anoxia, hypoxia followed the typical summer pattern of worsening through late July, with the volume of hypoxic water decreasing in August.

The scientists said a major area of concern is water clarity, which continued a worsening trend throughout most of the Bay. "There does not seem to be a difference between the upper, mid and lower Bay, just a general poor water clarity year," the scientists said. "This follows the same trend as last year."

The trend is all the more surprising because summer conditions were drier than average both this year and last, which should mean fewer bloom-fueling nutrients entering the Bay. Still, a few areas were better than average, including the tidal fresh areas of both the Choptank and James rivers.

Dry conditions may have limited the extent of some harmful algae blooms, such as Microcystis-which has been a major problem in the Potomac and other rivers in recent years-because of increased salinity.

Bay tributaries still had numerous harmful algae blooms this summer. Although not as severe as some years, blooms in the Corsica and Patapsco rivers are thought to have led to fish kills. Other significant blooms turned up in the Elizabeth and lower James rivers.

Of particular concern was the detection of Alexandrium monilatum in the lower York River. "Alexandrium is a cyst-forming dinoflagellate that can be toxic to fish and humans and is known to cause shellfish to stop pumping water during blooms," according to the summary. "Primarily a warmwater species, this dinoflagellate is more common in the Gulf of Mexico and Florida waters."

The annual summary was compiled by Ecocheck, a partnership between the National Oceanic and Atmospheric Administration's Chesapeake Bay Office and the Integration and Application Network at the University of Maryland Center for Environmental Science. It is based on input from the Bay Program's Tidal Monitoring and Analysis Workgroup.

The full summer review can be found online at www.eco-check.org/summerreview/chesapeake/