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A water sample collected 4/20/09 by DNR from routine monitoring stations in the St. Martin River, located near Ocean City, contained 13,716 and 19,685 cells/ml of Prorocentrum minimum at the water surface. Prorocentrum blooms at levels approaching 3,000 cells/ml and greater typically give the water a reddish tinge, and are referred to locally as a Mahogany tide.' Follow-up sampling in Bishopville Prong on April 29 showed the algae had a concentration of 64,897 cells/ml. This is one of the largest blooms recorded from this river (the other occurred in January 2007). Additional accounts of dark brown water have been reported in Herring and Turville creeks. Mahogany Tide is known for discoloring the water a coffee to orange-brown color. Occasional fish kills have been associated with Prorocentrum blooms in the Chesapeake Bay. Large blooms in spring can result in a decrease in the amount of light reaching submerged aquatic vegetation (SAV) for days to weeks and negatively affect plant growth and survival. Decomposition of localized blooms can cause hypoxic (low dissolved oxygen) or anoxic (no dissolved oxygen) conditions negatively impacting living resources. Literature suggests Prorocentrum minimum may sometimes be benign for shellfish but at other times can be detrimental to their health; at least one seed oyster kill in the lower Potomac River during spring 2000 was associated with bloom waters of Prorocentrum minimum and other anecdotal reports of impacts to commercial shellfish operations have been reported. Such aquaculture shellfish kills produce economic impacts to the growers. No human health effects related to Prorocentrum blooms have been reported for Chesapeake Bay region.
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