December 3, 2010

Blooms Beget Blooms: Harmful Algae Could Block Restoration Efforts in Chesapeake

Cyanobacteria bloom on the Sassafras River. Photo from Jeffrey Cornwell's lab.

Given the right set of conditions, blooms of harmful algae in the Chesapeake’s tidal estuaries may be self-perpetuating. This could pose a significant problem for estuary and Bay restoration efforts, say Diane Stoecker and Jeffrey Cornwell, two researchers who have been studying the circumstances under which these potentially detrimental algae blooms thrive.

The researchers, both from the University of Maryland Center for Environmental Science Horn Point Laboratory, are studying how blooms of cyanobacteria develop and why they persist in the Sassafras River, located on the border of Cecil and Kent Counties.

Cyanobacteria include a number of blue-green algae types that can release ammonia and other toxins into the environment, degrading water quality and harming aquatic vegetation vital to water-based habitats. The Sassafras River experiences blooms of cyanobacteria on an almost annual basis, usually during the summer, according to Stoecker.

In a study funded by Maryland Sea Grant, Stoecker and Cornwell focused on two specific algae types in the Sassafras River:  Anabaena, a species that can fix nitrogen and produce toxins, and Microcystis, a smaller species that can also produce toxins but cannot fix nitrogen independently. Nitrogen fixation is a process by which cyanobacteria convert nitrates and other nutrients into ammonia, which is released into the surrounding environment.

When it comes to blooms, high pH appears to play a key role in both cause and effect. Dense blooms of cyanobacteria generate higher than normal levels of photosynthetic activity, which depletes carbon dioxide levels and increases water pH. In the Sassafras River, said Cornwell, excess nutrients like nitrogen and phosphorus build up due to agricultural and stormwater runoff.

Cyanobacteria use excess nutrients to fix even more nitrogen. The ammonium by-product created as a result of this process in turn becomes more toxic in high pH water systems, according to William A. Wurts, Senior State Specialist for Aquaculture at Kentucky State University.

In their study, Stoecker and Cornwell found that nitrogen fixation increased significantly in a water system with high pH induced by cyanobacterial activity. But in a low pH system, nitrogen fixation actually decreased. This study represents the first time nitrogen release could be traced directly to cyanobacterial influence in a high pH water column.  
 
In shallow water systems, like the Sassafras, fluctuations in water column pH can stir up sediments, providing nutrients like phosphorus and nitrogen that allow cyanobacterial blooms to grow larger. As these blooms grow, they continue to raise water pH levels, increasing ammonia toxicity in the water column. And so the feedback cycle continues.

Sedimentation also contributes to the “road to ruin” in estuaries, says Stoecker. “If you’re adding sediments and materials to the system you’ll be making it even shallower, perpetuating the conditions under which [blooms] will come back next year,” says Stoecker.

If these blooms persist on a yearly basis, restoration efforts could prove near impossible to complete, researchers say. Stoecker stressed that even if nutrient loads resulting from polluted runoff were interrupted entirely, existing loads within the estuary would not allow any significant improvement in water quality.

“Say somehow you could magically cut off the nutrient inputs to the system next year,” she says. “Our study shows us that there’s just so much residual in the system that even if you change the nutrient inputs, it’s still going to take a long time to fix the system and we’re not sure how long that could take.”