Blackwater rivers turn Winyah Bay brown •

Today’s Image of the Day from NASA Earth Observatory features a region of eastern South Carolina where four blackwater rivers feed into Winyah Bay.

The blackwater rivers flow through swamps and forests that stain the water brown with organic matter such as decaying leaves and other vegetation. 

According to NASA, the transparent brown color emerges from several plant substances, including tannin, phenol, and humic acid – which is the same process that colors tea.

Heavily stained waters are described by scientists as being high in colored dissolved organic matter (CDOM). 

Eric Smith, the head of the North Inlet-Winyah Bay National Estuarine Research Reserve, said that CDOM levels were roughly 50 percent higher than the long-term daily average on October 1, 2020.

Blackwater rivers can negatively affect aquatic ecosystems by changing the color of the light available to phytoplankton for photosynthesis. 

Tammi Richardson, an oceanographer at the University of South Carolina, explained that the compounds that make up CDOM absorb blue light wavelengths extremely well, so blue light does not penetrate very deep into the water. 

“The red wavelengths are the ones left behind, hence the brownish-red appearance of the water. If phytoplankton do not have pigments that can absorb red light, then they will not be able to photosynthesize.”

Phytoplankton play a central role in the marine food web, which means that CDOM concentrations can weaken productivity throughout Winyah Bay. 

The image was captured on October 1, 2020 by the Operational Land Imager (OLI) on Landsat 8 after Hurricane Sally brought heavy rainfall and flooded many waterways.

Image Credit: NASA Earth Observatory 

By Chrissy Sexton, Staff Writer

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