The appearance of deep red or reddish-brown water in Lake Eufaula, Oklahoma, is a visually arresting phenomenon that raises immediate questions about its cause and safety. This striking discoloration is not caused by sediment or pollution but is a biological event resulting from a massive population explosion of microscopic organisms. When environmental conditions align, these life forms multiply rapidly, creating what scientists call a bloom that transforms the water’s appearance. The coloration is a natural process tied directly to the specific biology and chemistry of the reservoir.
The Biological Source of the Red Color
The red color in Lake Eufaula is caused by a dense concentration of cyanobacteria, which are often referred to as blue-green algae, even though they are technically bacteria. These organisms contain specialized photosynthetic pigments that allow them to harvest light energy from the sun. The key pigment responsible for the red or reddish-brown hue is called phycoerythrin.
While many cyanobacteria contain the blue-green pigment phycocyanin, species known for red blooms, such as those in the genus Planktothrix, contain high amounts of this accessory red pigment. The red tint becomes visible when the cyanobacteria rise to the surface, creating a thick layer, or when the bloom begins to decay. As the cells break down, the concentrated phycoerythrin is released, coloring the surface water a shade of red or pinkish-red. A bloom can also appear reddish-brown or purple-red, depending on the specific species present.
Environmental Conditions Driving the Bloom
The proliferation of cyanobacteria into a visible red bloom is directly linked to specific physical and chemical conditions within the lake. High concentrations of nutrients, specifically phosphorus and nitrogen, act as fertilizer for the cyanobacteria, fueling their explosive growth. Lake Eufaula receives significant nutrient loading from its extensive watershed, including runoff from agriculture and municipal wastewater discharges.
Nonpoint source pollution, such as fertilizer and animal waste, contributes substantial amounts of these nutrients. Point sources, like discharges from sanitary waste treatment facilities located upstream, also provide a steady influx of phosphorus and nitrogen. These excessive nutrients accumulate in the reservoir, leading to eutrophication, which favors the growth of bloom-forming organisms.
The blooms are further encouraged by periods of warm weather, typically in late summer and early fall, when water temperatures rise significantly. This warmth, combined with calm winds, allows the water column to stratify, meaning warmer water sits atop cooler water. Cyanobacteria regulate their buoyancy to remain concentrated in the warmer, well-lit surface layer, maximizing photosynthesis and resulting in the visible surface bloom.
Assessing Water Safety and Public Health
The primary concern associated with any large cyanobacteria bloom is the potential for cyanotoxins. Although not all blooms are toxic, many bloom-forming species found in Oklahoma waters, such as Microcystis and Anabaena, can produce potent toxins that pose a hazard to humans, pets, and wildlife. These toxins include microcystins, which damage the liver, and saxitoxins, which can affect the nervous system.
Contact with bloom water can cause immediate health issues, such as skin irritation, rashes, and gastrointestinal distress. Public health officials, including the Oklahoma Department of Tourism and the U.S. Army Corps of Engineers, monitor the lake and issue advisories when cell counts or toxin levels exceed established thresholds. A water body may be placed under a caution advisory when cell densities reach 20,000 cells per milliliter, with higher counts triggering a no-contact warning.
The advisories recommend that people and pets avoid swimming or recreating in areas where the water is discolored or has visible scum. Toxin levels are compared against guidelines, such as the EPA’s 10-day Health Advisory levels for drinking water, to ensure the safety of public water supplies drawn from the lake. It is always best to avoid any water that appears unusually red, green, or otherwise discolored.