A striking satellite image, captured on October 1, 2020, reveals a surge of tea-like ‘blackwater’ flowing into Winyah Bay, South Carolina. This phenomenon, captured by Landsat 8, is a direct consequence of Hurricane Sally, which made landfall two weeks earlier. The storm, with its torrential rains, caused widespread flash flooding, ultimately leading to the release of this ‘blackwater’ into the bay.
Winyah Bay, located near Georgetown, South Carolina, is a coastal estuary that connects to the Atlantic Ocean. It is fed by four blackwater rivers: the Waccamaw River, the Pee Dee River, the Black River, and the Sampit River, all of which traverse the eastern part of the state. These rivers flow through swamps, wetlands, and forests, ecosystems teeming with decaying vegetation. As this organic matter decomposes, it releases chemicals like tannin, phenol, and humic acid, giving the water its distinctive brown hue, similar to brewed tea. This staining material is scientifically referred to as colored dissolved organic matter (CDOM).
Heavy rains amplify the presence of CDOM, as floodwaters gather more of this material and flow into nearby rivers, eventually reaching Winyah Bay and subsequently the ocean. In the aftermath of Hurricane Sally, monitoring stations within Winyah Bay detected CDOM levels exceeding 50% of their average for that date. This surge of CDOM significantly altered the ocean’s light penetration.
The increased CDOM levels in the ocean obstruct the passage of blue light deep into the water column. Only red light penetrates the depths, lending the water a rusty appearance when viewed from above. This alteration in light availability directly impacts some photosynthetic algae.
According to Tammi Richardson, an oceanographer at the University of South Carolina, phytoplankton lacking pigments capable of absorbing red light are unable to photosynthesize. This can disrupt the delicate balance of coastal food webs. However, some plankton, such as cryptophytes and diatoms, can still thrive in this blackwater environment due to their ability to absorb red light.
The ‘blackwater’ event in Winyah Bay highlights the profound impact of heavy rainfall and natural disasters on coastal ecosystems. As the CDOM flows into the ocean, it triggers a cascade of effects, influencing light penetration, photosynthetic processes, and the intricate relationships within marine food webs.
