Algal Toxins Detected in One-Third of Streams Assessed in Southeastern United States plus 1 more

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Algal Toxins Detected in One-Third of Streams Assessed in Southeastern United States plus 1 more

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Algal Toxins Detected in One-Third of Streams Assessed in Southeastern United States

Posted: 17 Feb 2016 07:00 AM PST

Summary: USGS scientists have detected toxins known as microcystins produced by various forms of algae in 39 percent of the small streams assessed throughout the southeastern United States. Their recent study looked at 75 streams in portions of Alabama, Georgia, North Carolina, South Carolina and Virginia.

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Contact Information:

Keith Loftin ( Phone: 785-832-3543 ); Alex Demas ( Phone: 703-648-4421 );

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USGS scientists have detected toxins known as microcystins produced by various forms of algae in 39 percent of the small streams assessed throughout the southeastern United States. Their recent study looked at 75 streams in portions of Alabama, Georgia, North Carolina, South Carolina and Virginia.

“This is the first systematic stream survey of algal toxins in the southeastern United States,” said Keith Loftin, the USGS research chemist who led the study. “It’s important, because it provides a better understanding of the occurrence of these microcystins in aquatic ecosystems with flowing waters.”

Microcystins are a well-known public health concern. Public health practitioners and medical researchers have observed a range of symptoms in humans after exposure to microcystins.  Symptoms can include nausea, dermatitis and, in severe cases, liver failure.  Toxicity issues have been reported for humans, companion animals, livestock and wildlife. 

Although the maximum microcystin concentration measured in this study (3.2 µg/L) did not exceed World Health Organization moderate risk thresholds (10 µg/L) in the streams sampled, further research is needed to understand the potential effects on water quality and related environmental health concerns in downstream aquatic ecosystems, lakes and drinking water reservoirs.   

Previous research indicated that cyanobacteria, a form of algae capable of producing microcystins, were found in 74 percent of the streams assessed throughout the southeastern United States. However, that research did not include the study of microcystins.

This is the first of several regional assessments of algal toxins, which will provide context for the design of future environmental health studies. These studies will investigate land-use and other factors that may influence or create new environmental pathways of exposures to cyanobacteria and associated toxins.  Ongoing work by the USGS in the Pacific Northwest and planned work in the northeastern United States and California will expand our understanding of cyanobacteria and toxins in a wider variety of aquatic ecosystems. 

More information about this study can be found here. Support for this work was provided by the USGS’ Toxic Substances Hydrology Program and the National Water Quality Assessment Program (NAWQA).

The Fate of Sediment When Freshwater Meets Saltwater

Posted: 17 Feb 2016 07:00 AM PST

Summary: Two recent USGS investigations have measured sedimentation rates along the barely perceptible slope of rivers as they empty into estuaries. The findings of these studies have important implications for the restoration of estuaries — for example, the Chesapeake Bay — and their resilience in the face of sea level rise. 

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Contact Information:

Greg Noe ( Phone: 703-648-5826 ); Jon Campbell ( Phone: 703-648-4180 );

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Two recent USGS investigations have measured sedimentation rates along the barely perceptible slope of rivers as they empty into estuaries. The findings of these studies have important implications for the restoration of estuaries — for example, the Chesapeake Bay — and their resilience in the face of sea level rise. 

The studies compared the sedimentation rates found in upriver tidal freshwater swamps (located at the furthest inland reach of tides) to the rate found in brackish water marshes downstream at the lowest reaches of the rivers. 

Areas like this tidal freshwater swamp, along the Pocomoke River in Maryland, provide important ecosystem services including improving water quality by trapping watershed sediment before it reaches the Chesapeake Bay. However, a sediment shadow along tidal rivers may limit their resilience to the impacts of sea level rise. Photo: Scott Ensign, USGS.

“Sediment trapping in tidal freshwater wetlands is critical for protecting the water quality of estuaries and enhancing the resilience of those wetlands to sea level rise,” said Scott Phillips, USGS science coordinator for the Chesapeake Bay. “These wetlands help reduce nutrients and contaminants from reaching the Bay and also provide critical habitat for waterfowl.” 

A study by Ensign et al demonstrated sediment transport bottlenecks in tidal rivers of Maryland. The bottleneck occurs where watershed sediment is trapped by tidal freshwater swamps at the head-of-tide and where estuarine sediment transported upriver by tidal action is trapped by brackish wetlands in the Chesapeake Bay. 

This process leaves minimal sediment availability to tidal freshwater wetlands just below the head-of-tide, producing a “sediment shadow” that reduces the resilience of wetlands to the impacts of sea level rise. The shadow of reduced sediment accumulation also means that Atlantic Coastal Plain watersheds have very little of their watershed sediment delivered to estuaries and the coastal zone. 

Research by Noe et al found a difference in the basic chemistry of sediment deposited in tidal freshwater swamps compared to brackish wetlands in South Carolina and Georgia, a determination that further supports the conclusion that watershed sediment is trapped out by tidal freshwater wetlands while estuarine sediment is delivered upstream to brackish wetlands. 

Moreover, the Noe study found, sediment accumulation rates have changed over time. Historically, even more sediment was trapped by the upriver tidal freshwater wetlands. The change is likely due to greater availability in the past of “legacy” sediment from post-colonial land use and soil erosion. Modern sediment trapping is greatest overall in downriver brackish wetlands, likely due to sea level rise that has moved the estuarine turbidity maximum upstream. 

Together these studies, along with others, show that tidal freshwater wetlands downstream of the head-of-tide have the lowest sediment accumulation rates along river-to-estuarine gradients. Consequently, these areas may have the least resilience to increased rates of sea level rise. In general, sediment trapping helps tidal wetlands increase in elevation to keep pace with rising sea levels. The effect of excessive saltwater exposure on tidal freshwater swamps is easily seen in places where tree death has produced spindly “ghost forests” that eventually convert into brackish marshes. 

The sediment shadow also means that little of the watershed sediment and associated nutrient loads in lowland coastal rivers actually reaches estuaries. For example, in the smaller rivers that empty into the Chesapeake Bay (characterized by extensive tidal freshwater wetlands in contrast to minimal tidal freshwater wetlands found in large embayed tributaries), a large portion of the watershed sediment load (and associated phosphorus and nitrogen) is removed by tidal wetlands prior to reaching the bay. 

These new insights about the complexity of sediment, carbon, and nutrient transport from watersheds to estuaries can help water quality managers to more accurately forecast the effects of watershed changes on estuarine water quality and improve adaptive management. 

Learn more 

• USGS Chesapeake Bay Activities 
• USGS National Research Program