Candy Land’s molasses swamp and an ice cream sea may be a bit of a stretch in the real world, but some of the shady streams in Georgia’s coastal plain steep like tea and make blackwater rivers.
“It’s all these tree leaves and all this different organic matter in the soil and things like that,” said Andrew Mehring, a postdoctoral scholar at the Scripps Institution of Oceanography in La Jolla, Calif., and formerly a postdoctoral researcher at the University of Georgia. “Once it gets in contact with waters it’s just releasing all these dissolved carbon compounds and that’s coloring the water — that tea color or coffee color that you see.”
Mehring and other researchers published an article in the Journal of Geophysical Research about the effects of drought on dissolved organic carbon, or DOC, which comes from these leaves and other debris when they decompose in the water. DOC is an important part of the food pyramid in these environments, feeding microorganisms which in turn feed insects which in turn feed birds and other animals.
Mehring said in years of drought, the amount of DOC flowing through a blackwater river goes down.
“The concentrations could be pretty high, depending on what the river flow is at that time,” he said. “It’s not the concentration that’s gone down so much in a drought, it’s actually the amount that’s getting moved just because there’s less water flowing.”
DOC can then increase beyond normal levels during the year the drought ends. Mehring said his guess that was because the amount of carbon-rich materials had built up over time before they could be exposed to water, decompose and was downstream.
He said DOC trends as they pertain to drought should continue to be monitored because the number and length of droughts noted in the trial river had gone up over the last 37 years.
“If we are having these more extreme, longer dry periods which we’ve seen so far, if those continue into the future, then that’s a pretty big deal,” he said.
Another reason for better understanding how drought affects DOC and dissolved oxygen (DO) levels in blackwater rivers is the amount of money it could save the tax payers.
George Vellidis, a professor of crop and soil sciences at UGA’s Tifton campus, and his research team have received funding from the U.S. Department of Agriculture’s National Institute of Food and Agriculture to study blackwater rivers extensively.
He said in 1972, with the passage of the Clean Water Act, states were required to manage water quality in their surface waters – in this case the task fell to the Georgia Department of Natural Resources’ Environmental Protection Division — which included DO levels.
“Because our state, and many other states, did not have a lot of research done to figure out what kind of dissolved oxygen standards we need for these blackwater rivers, they just applied the same water quality standards to all the rivers and streams in the state,” Vellidis said. “Whether you’re in the mountains or in the coastal plain it was the same water quality standard. And that was a really high water quality standard.”
He said because blackwater rivers weren’t meeting the set requirements, the state would have to do a comprehensive examination of the entire watershed, all possible points of contamination and then develop a management plan to get the DO content back to what was considered normal levels. These efforts would take considerable time and manpower, Vellidis said.
But, he said, DO levels are naturally low in blackwater streams because the amount of DOC from the decomposing leaves and other organic matter is so high. He said once researchers were able to show what was happening, the Environmental Protection Division was able to change their standards.
“They haven’t set an established value, but they’re saying that it’s naturally low so if the values are not just abysmally low the streams are not considered to be in violation of the old standards anymore,” Vellidis said.
He said he would like to see funding for a long-term monitoring project for a range of acceptable DO levels for all seasons.
“What would really be nice would be to see for different parts of the coastal plain what would be an approximate number for DO at different times of the year,” he said.