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Eyes on the Bay

"Black Tide Bloom" in Lower Patuxent River
Algal blooms can affect local water quality and have tremendous impacts on living resources. Algae are present in the water all year round. However, in the cooler months of the year their population numbers (measured as the number of cells per ml of water) are low. As the waters warm in the spring and summer months of the year, algae begin to undergo natural population increases, known commonly as blooms. The level of these blooms can be exacerbated by the presence of excess nutrients in the water. This problem is known as eutrophication and is a common problem in the Chesapeake and Coastal Bays.

When algae bloom, oxygen levels in the water increase initially as oxygen is produced by the algae through the process of photosynthesis. Oxygen levels may decline on cloudy days and at night as more oxygen is consumed (known as respiration) than is produced by the algae. The processes of photosynthesis and respiration can also cause increases in the pH of the water. The amount of chlorophyll (a photosynthetic pigment) in the water increases as the number of algae in the water increase. As the algae die and begin to decompose, oxygen levels tend to plummet. This decline in oxygen levels can have detrimental impacts on living resources and will occasionally result in fish kills.

The effects of localized algal blooms on water quality are routinely captured by Continuous Monitoring Data. During the summer months of 2003, a bloom of Karlodinium micrum (also known as "black tide") occurred in the lower Patuxent River. The continuous monitoring station on the Patuxent River at the Chesapeake Biological Laboratory detected the initial increases in dissolved oxygen and pH associated with this bloom between the 21st and 27th of June (Figures 1 & 2). Increases in chlorophyll were also detected over this time period (Figure 3). Between June 27th and July 3rd the Karlodinium bloom reached its highest concentrations and then died off, causing oxygen levels in the river to drop below 5 mg/l. The bloom continued over the following weeks and intensified again after July 9th. Once again, oxygen levels increased temporarily, then dropped below 5 mg/l as the bloom died off. Luckily, only a few small fish and crab kills were observed during these periods of low dissolved oxygen.