What are brown tides?

Brown (and red) tides can occur when certain algae species reach high concentrations, or "blooms," that discolor water. Some of these algae species can harm some marine life under certain conditions. Scientists call such events "harmful algal blooms." Not all algal blooms that discolor the water are harmful to the environment. The organism that causes brown tides on the east coast, Aureococcus anophagefferens, was first discovered in 1985.

What are the impacts of brown tides?

There are no known human health impacts from brown tides! They also do not appear to harm fish - fish simply avoid bloom areas. Brown tide blooms have been categorized into three categories based on the ecologic impacts with densities greater than 200,000 cells per milliliter (ml) being documented as having severe impacts on bay life that cannot move away from the blooms, especially eelgrass (Zostera marina) and bivalves such as bay scallops (Argopecten irradians) and blue mussels (Mytilus edulis).

Overall, the impact on eelgrass appears to be mainly associated with reduced light penetration (the blooms are so dense that they prevent adequate light from reaching the eelgrass). The impact on bivalves may be by both preventing the animals from feeding on more nutritious forms of algae, and through toxins that may be produced by the algae (specific toxins have not yet been identified in Aureococcus, but toxins in similar species of algae have been documented). Aureococcus blooms also coincide with the spawning season of several commercially important bivalves in mid-Atlantic estuaries, thus threatening their reproductive success and population growth. The short life span of bay scallops (most adults do not survive to a second annual reproductive season) makes them particularly vulnerable to harmful algal outbreaks.

What conditions favor Aureococcus anophagefferens blooms?
Although there is a widespread distribution of brown tide cells from the Gulf of Maine to New Jersey, development of blooms has been restricted to shallow, relatively well mixed estuaries. Research has implicated low rainfall (which reduces flushing), elevated salinities, and elevated nutrients (in particular dissolved organic nitrogen) in bloom initiation. Despite these findings, the combination of factors that triggers the onset of brown tides is not well understood.
Aureococcus blooms typically peak in June-July, although cells may be present in very low numbers in the water year-round. This is probably due to their optimal temperature for growth being between 20 and 25^oC (68-77^oF) and their ability to sustain slow growth at low temperatures (allowing seed populations to overwinter in mid-Atlantic waters).

Decreased consumption of brown tide algae by filter feeders (such as clams, oysters, and scallops) and the ability to adapt to low light environments, may allow brown tide blooms to persist in shallow estuaries. Research has shown that dissolved organic nitrogen (which results from the breakdown of animal and plant material) is the preferred nitrogen source and may contribute to sustaining a bloom. Blooms of Aureococcus may also be maintained for long periods of time (weeks to months) in low light environments by supplementing photosynthesis with the ingestion and breakdown of organic matter to obtain nourishment.

For more information contact Cathy Wazniak (410-260-8638) 
or Dave Goshorn (410-260-8639) in DNR’s Tidewater Ecosystem Assessment Division.

Return to the Maryland DNR Home Page.
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