A mahogany tide was detected in Baltimore Harbor (Patapsco River) on April 17, 2002. Concentrations of Prorocentrum minimum measured 109,180 cells/milliliter. The density of P. minimum is among the highest densities observed by Maryland Department of Natural Resources (DNR) in Chesapeake Bay over the past twenty years of monitoring. While conducting routine sampling of fish, biologists from the Maryland Department of the Environment noticed discolored water in much of the Patapsco River and collected a sample of water from the Middle Branch near the Hanover Street bridge.

On April 11, 2002, Maryland DNR was conducting routine water quality monitoring and found P. minimum approaching bloom concentrations at 4,187 cells/milliliter near Key Bridge. It is likely that the sunny and warm weather of the past few days allowed this algal species to multiply rapidly and reach bloom proportions.

Blooms of P. minimum are frequently observed in the Patapsco River, other Bay tributaries and mid-salinity portions of the mainstem during spring. In 2000 we experienced the most extensive P. minimum bloom since records were kept over 20 years ago.