Last year's heavy spring rains and late summer tropical storms created enough oxygen-starved Bay water to send Baywide dissolved oxygen conditions to their second poorest showing since 1992.

Last year, just 34 percent of the Bay fully met its dissolved oxygen goals. That figure reflected an average of 2009, 2010 and 2011 conditions — the state-federal Bay Program partnership determines whether oxygen goals are met by averaging the most recent three years together, partly to offset the impact of extreme events.

Given last year's extreme weather, the lower score wasn't a surprise, but it was a stark reminder of just how far cleanup efforts have to go, as the three-year average showed that two-thirds of Bay water had too little oxygen to fully support aquatic life.

A decade ago when a multi-year drought resulted in dramatically improved water quality, nearly twice as much of the Bay's area met dissolved oxygen standards. The difference between then and now reflects the dramatic impact weather has on water quality in the Bay.

High rainfall years such as 2011 wash huge amounts of nutrients off the land and into the Chesapeake Bay.

That nitrogen and phosphorus fuels algae growth and, when there is more algae than can be consumed by fish and other predators, the excess sink to the bottom and decompose — a process that removes oxygen from the water.

Compounding the situation is that strong river flows, especially from the Susquehanna, which supplies half of the freshwater to the Bay, help to form a barrier between low-salinity water from tributaries, which floats on the surface of the Bay, and higher salinity — and therefore higher density — water from the ocean, which flows in on the bottom.

That barrier, called the pycnocline, prevents oxygen-starved water on the bottom from mixing with surface water, leading to so-called "dead zones" in some deepwater areas during the summer.

"The Bay ecosystem functions most effectively when fresh and saltwater can mix, just like oil and vinegar need to mix to form a salad dressing," said Peter Tango, a scientist with the U.S. Geological Survey and the monitoring coordinator for the Chesapeake Bay Program.

Last summer had the highest March through May river flows from the Susquehanna on record, washing in large amounts of nutrients and setting up flow conditions that created a strong pycnocline.

Normally, those conditions are relieved in late summer, but the heavy rains associated with Tropical Storm Lee in September resulted in continued strong stratification between surface and bottom layers, causing low levels of dissolved oxygen to persist into the fall.

As a result, last year's dissolved oxygen attainment, if measured by itself, would have been lower than the 34 percent three-year average.

Bay dissolved oxygen standards drive much of the nutrient reduction efforts throughout the watershed and are designed to ensure enough oxygen is present in various habitats to support species that use those areas.

Almost all aquatic life in the Bay requires some level of oxygen to exist, although species in different habitats typically need different amounts — bottom-dwelling worms need less than striped bass swimming in open waters.