Blackwater Rivers | July 14: Blackwater seeps from the vast lowland swamps that cradle the headwaters of Florida rivers. At times turning them so dark they shimmer like a mirror reflecting the passage of herons in oak canopies, hogs stealing a drink on the shoreline, and hyacinth in summer bloom. The canonical blackwater river is the Suwannee. It drains millions of acres from eight Florida counties in its southwest meander from Cypress Creek in the north to Cedar Key on the Gulf—separating the panhandle from the rest of the peninsula1. In the Unites States, the longest blackwater river lives in South Carolina—the Edisto. Flowing some 250 miles from inland sandhill counties to its mouth in the ACE (Ashepoo-Combahee-Edisto) Atlantic basin2. The Edisto is dwarfed by the Rio Negro of Brazil—the largest blackwater river in the world. From its headwaters in the Puinawai rainforests it travels southeast 1,400 miles to its convergence with the Amazon.
Rivers can be darkened by the presence of metal sulfides and tannins. In rivers that end in tidal flat estuaries, sulfur byproducts of bacterial metabolism combine with metals (such as iron) to form dark insoluble complexes in the sediment3,4. These are sometimes referred to as black-mud rivers. But, for true blackwater rivers it’s the water that’s black, not the sediment. True blackwater rivers drain bottomland basins5. Lazily meandering over channels of shifting sand, their waters are stained by tannins that leach from the leaves of oak and willow, cypress knees, and ash samaras that steep in the stale waters of riverside backswamps.
There are hundreds of tannins. Many have been used for centuries in the tanning of animal hides6 and medicinally as analgesics7 and antimicrobials8. The mother-tannin of blackwater is tannic acid. It has a sugar core contained by a ring of gallic acid residues, weighing-in at 1.7 kilos per mole with a whopping 72 carbons9. Gallic acid consists of tandem hydroxylated phenols—a chemistry that determines the high solubility of tannic acid at nearly 3 kilos per liter. This is equivalent to 23 pounds of tannic acid being dissolved in a gallon of river water, or 8-times the solubility of salt.
Tannins contribute significantly to the high carbon load carried by blackwater rivers. It’s been estimated that the Suwannee contains as much as 10 times more dissolved organic carbon than comparable spring-fed, or alluvial rivers10. This surfeit of tannins determines the river’s ecological pulse. Many tannins are weakly acidic, lowering pH and changing river hydrochemistry as they mix into the water column, while blooms of tannin-consuming microorganisms gobble-up oxygen, limiting the availability of essential nutrients and light to plants, insects, crustaceans, and fish. As such, naturalists of the past have inferred blackwater rivers to be depauperate environments5, 11.
Yet, healthy blackwater rivers are richly diverse. A study of the Upper Three Runs River (Aiken Co. South Carolina) identified more species-richness (biodiversity) than all allied rivers in North America at the time12. Blackwater rivers thus have a unique ecological signature that has aligned itself with the demands of tannins.
1. Suwannee River Watershed: Conserving the Georgia/Florida Connection. North Florida Ecosystem Team. Retrieved in June 2021 from: https://www.fws.gov/northflorida/Documents/NFL_Suwanee_factsheet.pdf
2. Beasley et al. 1996. The Edisto River Basin Project Report. South Carolina Department of Natural Resources.
3. Juergen Schieber 2011. Iron sulfide formation. Encylopedia of Geobiology, Springer Verlag et al. (Eds.), p. 486-502.
4. Liang et al 2017. Blackening and odorization of urban rivers: a biogeochemical process. FEMS Microbial Ecology: https://doi.org/10.1093/femsec/fix180
5. Judy Meyer 1990. A blackwater perspective on riverine ecosystems. BioScience, vol. 40 (9): 643.
6. Tannins. USDA US Forest Service, Ethnobotany. Retrieved in June 2021 from: (https://www.fs.fed.us/wildflowers/ethnobotany/tannins.shtml).
7. Viana et al 1998. Analgesic and anti-inflammatory effects of the tannin fraction from Myacrodruon urundeuva. Phytotherapy Research: doi.org/10.1002/(SICI)1099-1573(199703)11:2<118::AID-PTR38>3.0.CO;2-J
8. Jaya Kurhekar 2016. Tannins: antimicrobial chemical components. Intl J of Tech Sci, vol 9(3): 5-9.
9. Molecular of the week archive: Tannic acid. 2018. American Chemical Society. Recovered from the web in June 2021: https://www.acs.org/content/acs/en/molecule-of-the-week/archive/t/tannic-acid.html
10. Laura Rocchio 2015. A blackwater river meets the sea. NASA Landsat Science Outreach, US Geological Society.
11. Frank Egerton 2012. History of ecological sciences, part 41: Victorian naturalists in Amazonia—Wallace, Bates and Spruce. Univ. of Wisconsin, Department of History.
12. Paller et al 2010. Ecological reference models for blackwater streams: a prerequisite for successful ecosystem recovery and management. Proceedings of the 2010 South Carolina Water Resources Conference.
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