Chesapeake Bay conditions were better than average this summer, with fewer fish kills and sea nettles, and a smaller than average area of low-oxygen water, scientists report.
They said water quality conditions were influenced by above-average river flows into the Bay during the winter, but lower than average flows in the late spring and summer.
High flows during the winter and spring wash nutrients off the land and into the Bay, which can fuel large plankton blooms. When the plankton die, they sink to the bottom and are decomposed by bacteria in a process that depletes oxygen from the water.
Strong flows also cause a layering of the water, making it difficult for bottom water to mix with surface waters, further worsening oxygen conditions in deep areas.
The high flows early this year resulted in a large phytoplankton bloom in portions of the Mid to Upper Bay during much of the spring, which contributed to a moderate area of poor oxygen conditions in deep portions of the Upper Bay in late spring and early summer.
But the low flows that prevailed later in the spring reduced the amount of bloom-fueling nutrients coming into the Bay, and reduced the stratification between surface and bottom waters. In addition, winds blew predominantly from the southeast beginning in mid-summer, a direction that favors water mixing.
Combined, those factors helped to nearly eliminate low-oxygen conditions in the Upper Bay by mid-July. Scientists making the analysis said that this pointed to the importance of the timing of river flows on the Bay's water quality.
They also reported that the Bay experienced relatively minor fish kills in 2010. Seven fish kills were associated with low-dissolved oxygen alone, three kills were associated with low-dissolved oxygen caused by algal blooms, and one kill was associated with a harmful algal bloom that produced fish-killing toxins.
The annual summer Bay conditions review is produced by EcoCheck, a partnership between the Integration and Application Network of the University of Maryland's Center for Environmental Science and the National Oceanic and Atmospheric Administration's Chesapeake Bay Office. For information, visit the EcoCheck website: www.eco-check.org