Microcystis UPDATE

The MD state government has received reports in recent weeks regarding a bright green substance on or just beneath the surface of the upper Chesapeake Bay and its tidal tributaries. Samples collected from several sites have confirmed that this is a bloom of the algae, Microcystis aeruginosa. Below are answers to some frequently asked questions regarding this organism.

What is Microcystis?

Microcystis aeruginosa is a type of blue-green algae (also referred to as cyanobacteria). This species is colonial, which means that single cells can join together in groups as colonies which tend to float near the water surface. Colony sizes will vary from a few to hundreds of cells. It is a common bloom-forming algae found primarily in nutrient enriched fresh waters and lower salinity estuaries. Microcystis aeruginosa is one of more than 700 species of algae that may be found in water samples collected from the Chesapeake Bay and usually blooms in mid to late summer in this area.

What does it look like?

Microcystis becomes very noticeable during bloom events. Thick mats of the algae can coat the water so heavily that you cannot see your hand an inch below the surface. In calm freshwater areas, it may look like someone has spilled green paint on the surface. On closer inspection, the mats can be seen to be comprised of small flakes or balls. If the algae is blown by wind or pushed by currents into higher salinity waters where it cannot survive, it generally takes on a greenish-yellow color and a chunky appearance as it dies. If you would like to see what the cells look like, visit our bay life guide.

What are we seeing on the Bay now?

There have been reports of Microcystis blooming in the upper Bay and tributaries (Bohemia, Sassafras, Northeast, and Elk rivers among others), since early August. It is likely that the heavier than normal rains this summer have pushed freshwater farther down tributaries and the Upper Bay than normal, causing blooms over a broader area than previous years.

How does this year’s bloom compare to previous years?

Annual late summer blooms of Microcystis are frequently observed in the tidal freshwater portions of the Potomac River. During the 1970's and early 1980's, this area of the Potomac experienced large blooms. Since the early 1980's blooms have occurred to varying degrees in both the Potomac and Upper Bay. The distribution of this year’s bloom in the Upper Bay appears to be larger than in most recent years, but the Potomac is experiencing smaller blooms than average.

What causes it to bloom?

Microcystis blooms are most frequently associated with warm, fresh, nutrient enriched water. Blooms generally occur in mid to late summer and can carry on into the fall. Excess nutrients (particularly phosphate) that are added to the Bay by human activities may contribute to the frequency and intensity of blooms. In particular, years with wet springs can wash greater than average amounts of nutrients into the Bay and its tributaries that can further fuel a bloom.

Does it pose a threat to animals or people?

Any large algal bloom has the potential to result in fish and shellfish kills by depleting the water of oxygen. Algae can remove oxygen through normal respiration at night or through the decomposition process as the bloom dies. In such situations, there may not be enough oxygen remaining in the water to support fish or shellfish in the vicinity. Furthermore, as these large blooms die and sink to the bottom, they commonly release chemicals that can produce a foul odor and musty taste.

Some strains of Microcystis may produce toxins that have been reported to result in health problems to animals that drink the water, and minor skin irritation and gastrointestinal discomfort in humans that come in contact with toxic blooms.

What is the State doing to monitor the situation?

Maryland Department of Natural Resources has maintained a Chesapeake Bay Monitoring Program that examines algae density and distribution throughout the year in the mainstem of the Bay, its tributaries and embayments since 1985. Data collected as part of this program as well as additional data being collected in direct response to this bloom will be used to track the extent and duration of the bloom. This information will be analyzed with water quality data (nutrient concentrations, salinity, temperature, etc) collected baywide to better understand the environmental factors that have contributed to this bloom and insure that appropriate management actions are being taken to reduce the severity of future blooms.

What should I do if I see a bloom or have more questions?


For more questions on Microcystis: Maryland Department of Natural Resources Tidewater Ecosystem Assessment Division: (410) 260-8630 (normal business hours).

To report a fish kill or fish health event: Maryland Department of Natural Resources Fish Health Hotline: (888) 584-3110 (24 hours a day).

Questions regarding human health impacts of Microcystis: please contact your physician or your local health department.

Questions regarding animal (livestock) health impacts of Microcystis: Maryland Department of Agriculture: (410) 841-5882 (normal business hours)

Posted: 9-13-2000