Chesapeake Bay Coastal Bays Rivers and Streams Watersheds
Maryland Department of Natural Resources

Mahogany Tide

Like all types of phytoplankton, dinoflagellate species can generate population explosions when habitat conditions (i.e., light, temperature, nutrients, turbulence, predator levels, etc.) are ideal.  High density blooms of dinoflagellates such as the potentially toxic Prorocentrum minimum and Karlodinium micrum may cause water to become discolored a reddish-brown and form Mahogany tides.  Mahogany tides may also severely reduce the amount of oxygen available to living resources at localized bloom sites. 

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Prorocentrum minimum

Prorocentrum blooms occur each spring in the mid-salinity waters of the Chesapeake Bay at varying intensities. It is believed the cells are typically delivered to the upper Chesapeake Bay  in spring by traveling in the up-bay moving, higher salinity bottom waters from the lower Bay. As waters mix and upwell in the mid and upper Bay, cells are delivered to nutrient and light rich surface waters and commonly result in areas of Mahogany tides. Although the blooms are usually most intense during the spring, they have occurred in almost every month of the year.  Prorocentrum has also been identified in Marylandís Coastal Bays.

A bar chart showing the percent of bloom samples by month when Prorocentrum is present.  1985-2001 (n=917), Routine monitoring data, MD DNR.

World-wide, the species is considered potentially toxic to humans with rare cases of Venerupin Shellfish Poisoning (VSP) recorded in the last century. No such cases of shellfish poisoning from Prorocentrum have been reported from Maryland waters.  However, testing over the past two years on blooms found in Maryland suggest that this species has the potential to be toxic to shellfish.

The high biomass of Prorocentrum in blooms may severely reduce the amount of oxygen available to living resources at localized bloom sites. The result can be local fish kills, invertebrate kills and loss of habitat causing fish to move from the area due to low oxygen or irritation from the high cell densities. All these effects may produce local changes in the dynamics of the food web.

Find out more information on blooms in Maryland.

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