The Maryland Department of Natural Resources (DNR) Chesapeake Bay Monitoring Program identified bloom densities of the dinoflagellate Karlodinium micrum (16,165 cells/milliliter) on Wednesday, July 17, 2002 near the mouth of Eastern Bay. Additional reports of large stretches and smaller patches of a maroon to coffee colored water were provided by watermen from the region offshore of Herring Bay between Holland and Battees Point, east of Deale and Fairhaven (Anne Arundel County). Charter boat captain Jim Brincefield provided the DNR with a surface water sample also collected on Wednesday, offshore from Herring Bay, that showed similar densities of K. micrum (11,271 cells/milliliter) to that found near Eastern Bay.

Karlodinium micrum is a common dinoflagellate in the Bay during the summer season and there is evidence that the species can produce toxins that will kill fish. The highest concentrations observed for K. micrum over the last 20 years was on Sept. 9, 1982 in Bodkin Creek (1,322,500 cells/milliliter). Laboratory studies (Drs. Allen Place and Jonathan Deeds, Center of Marine Biotechnology, University of Maryland Biotechnology Institute, in cooperation with DNR and other researchers) have shown that densities above 20,000 cells/milliliter can be acutely toxic to fish. Large algal blooms such as this in the Bay also have the potential to produce low dissolved oxygen events that are harmful to Bay life.