Brown tide strikes the coastal bays

by Dave Wilson. (Wilson is the public outreach coordinator for the Maryland Coastal Bays Program.)

The seagrass and shellfish-killing brown tide struck some of the highest levels ever recorded in the coastal bays this June. Brown tide, which was blamed for the killing of 600 acres of seagrass in Chincoteague Bay in 2000, again bludgeoned the southern bays early this summer with cell counts at six of 11 water monitoring stations reaching two times the lethal dose of 200,000 cells/ml and 10 of 11 stations exceeding the standard. The coffee brown algae kills shellfish and seagrass when its blooms rob aquatic life of sunlight. It is not harmful to humans.

Scientists consider counts of Aureococcus anophagefferens under 35,000 cells/ml to have no impacts on aquatic life. From 35,000-200,000 cells/ml researchers find a reduction in juvenile hard clam growth and reduced feeding rates in adults. Above this mark, the water becomes yellowish brown, eel grass dies, and mussels, scallops, and clams cease breeding or perish. Fish and crabs also avoid these areas.

Last year both the northern and southern bays were hit when blooms reached all-time highs in Isle of Wight Bay and Manklin Creek. This year all bays north of the Ocean City inlet stayed at or below the 100,000 cell mark while Trappe Creek and Newport and Chincoteague bays were consistently above 300,000 cells/ml in mid-June.

On June 10, Green Point in southern Sinepuxent Bay had a cell count of almost 750,000 cells/ml and Taylors Landing in Chincoteague Bay hit 636,000 cells/ml. Both Trappe Creek and Newport Bay remained between 400,000-500,000 in mid-June. Because brown tide is a cool water alga, all cell counts began receding in early July.

The algae was not monitored in the coastal bays until 1998 when parts of the southern bays were impacted. Preserved water samples show brown tide dating back to 1993. Since then, Newport and Chincoteague bays had been hardest hit until last year’s high numbers in the northern bays.

This year, blooms began in mid-May and persisted longer than had historically been recorded. The longer the microorganism persists, the more impact it has on seagrass, oysters, and clams.

Anyone noticing persistent coffee brown or yellowish water in areas besides the ones already affected should call the Coastal Bays Program at 410-213-2297 or e-mail science@mdcoastalbays.org.