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Eyes on the Bay

"Mahogany Tide" responsible for coffee-colored waters in the Magothy and Patapsco Rivers (December 2004)
In early December 2004, reddish-brown waters were noted in the Patapsco River system. Maryland Department of Natural Resources (DNR), Department of the Environment (MDE) and Morgan State University (MSU) sampled the waters on December 6 and 7, 2004. It was determined the colored water was caused by an algal bloom, specifically “Mahogany Tide”, which was being produced by the dinoflagellate organism Prorocentrum minimum. Water samples collected from Bullneck Creek and Bear Creek in the Patapsco River contained high concentrations of Prorocentrum minimum ranging from 140,000 to 215,00 cells per milliliter, which in some areas created bright orange wakes.

On December 9, Maryland Department of the Environment responded to a citizen call regarding water quality on the Magothy River, between Old Man and Cattail Creeks, along North Drive. This investigation revealed another Mahogany Tide bloom with 262,300 cells/ml of P. minimum.

Prorocentrum mininum was also the dominant phytoplankton species identified during monthly phytoplankton monitoring at a mainstem Chesapeake Bay station near the Bay Bridge.

While Prorocentrum frequently blooms in the spring, blooms have been observed in Maryland in all seasons. Water samples collected on November 15 at the Maryland DNR long-term water quality monitoring station in the Severn River yielded a low level bloom (13,303 cells/ml) of Prorocentrum. On December 5, citizen Pierre Henkart noted reddish-brown waters on Saltworks Creek and provided a sample estimate of 220,000 cells/ml, most likely dominated by Prorocentrum minimum.

The effects of the localized Prorocentrum minimum blooms on water quality data were captured by the continuous monitoring station near Fort McHenry on the Patapsco River. Chlorophyll (Fig. 1) and turbidity (Fig 2) increased as the number of Prorocentrum cells increased during the Mahogany Tide event. The highest concentrations of chlorophyll were detected on January 3, 6 and 9. In the early stages of the bloom, oxygen values in the water initially increased as the algae produced oxygen through photosynthesis. As the oxygen was consumed (through respiration), and the algae died and began to decompose, oxygen levels dropped on January 9 and 11 (Fig. 3). This reduction in oxygen can be detrimental to living resources and occasionally can result in fish kills, especially when oxygen values drop below 5 mg/l. The processes of photosynthesis can also cause increases in the pH levels of the water. Peaks in pH coincided (Fig. 4) with peaks in chlorophyll concentrations on January 3, 6 and 9, as well. See the "Our Monitoring Explained" section for more explanations of water and habitat quality measurements.

For more information on Mahogany Tide bloom conditions, please visit our Harmful Algae Bloom page.

Figure 1. Chlorophyll concentrations from the Fort McHenry continuous monitoring station (12/7/04-12/14/04).		Figure 2. Turbidity levels from the Fort McHenry continuous monitoring station (12/7/04-12/14/04).
Figure 3. Dissolved oxygen levels from the Fort McHenry continuous monitoring station (12/7/04-12/14/04).		Figure 4. pH levels from the Fort McHenry continuous monitoring station (12/7/04-12/14/04).