What is to blame for harmful algal blooms?
Primary Article: Zhang, L. et al. 2020. Unique surface density layers promote formation of harmful algal blooms in the Pengxi River, Three Gorges Reservoir. Freshwater Science (2020) 39:4, doi: https://doi.org/10.1086/710332.
Featured Image: The Three Gorges Dam, Wikipedia
Artificial reservoirs: intended benefits and unintended consequences
Over half of the world’s large rivers are broken up by dams. Dams provide 16% of the earth’s electricity, and 71% of renewable energy. The world’s largest hydroelectric power plant, the Three Gorges Dam in China, has reduced coal dependence, decreasing the country’s overall CO2 emissions. However, the reservoir has unintended consequences as well: it has transformed the hydrology and ecology of the Yangtze River. One example of these changes can be seen in algal blooms, which increased in size and frequency after the reservoir was constructed.
The term “algae” refers to a diverse group of photosynthetic organisms, generally divided into 2 groups: macroalgae, which are large enough to see with the naked eye, and microalgae, which are too small to be seen without a microscope. Under certain environmental conditions, some types of microalgae can grow out of control and produce blooms. Some blooms produce toxins, which may have harmful effects on animals or humans that swim in the water. Even non-toxic blooms may simply suffocate other species by sheer density. Blooms that cause these negative effects are called Harmful Algal Blooms, or HABs. They may be caused by environmental factors such as excess nutrients (e.g., from agricultural fertilizer runoff) and high temperatures.
Investigating HABs in the Three Gorges system
A group of collaborating scientists based in China and Canada aimed to determine where, when, and why HABs occur within the river system that is home to the Three Gorges Reservoir. In order to do so, they investigated correlations between HAB occurrence and other variables, including nutrients, temperature, and river geology. While previous studies have been conducted on HAB occurrence in the Three Gorges system, these studies were limited in scope and often focused on only a single tributary. In this study, scientists examined the variables of interest at multiple spatial scales: within the entire reservoir, within a single tributary, and in the upper reaches of that same tributary. The scientists conducted field surveys over a year, where they collected microalgae, and measured hydrological, chemical, and biological data.
The group found that harmful algal blooms do not coincide with high levels of nutrients, which are known to have caused HABs elsewhere. However, hydrological variables such as water flow and geological morphology do correlate with HABs. HABs occurred more often in mid and upper reaches of tributaries, and less often in the main stem, where water flows faster and more consistently.
Bodies of water tend to be made up of layers, with warm water near the surface, and cold, nutrient-rich water at greater depths. Bloom-forming algae thrives closest to the sun and in warm temperatures, so blooms typically form in temperate lakes during the peak summer months. Wind can promote mixing of lake layers, breaking up algal blooms. However, the Three Gorges Dam is located amongst mountains which block the wind, promoting the formation and retention of layers in the water column. Since wind never causes the cold water to mix with the warm, the algae remain undisturbed and form large blooms. This series of events is more likely to occur where water flow is slow, i.e., the middle and upper reaches of tributaries.
The scientists recommended specific management strategies based on their findings. They suggested regulating water level fluctuations governed by the dam, and encouraged artificial mixing of the water layers in the middle and upper reaches of tributaries. They also suggested the development of management plans for specific tributaries rather than for the entire reservoir, based on varying likelihoods of HAB formation across tributaries. If taken into account, these science-based strategies may help improve drinking water quality and support a healthy, biodiverse ecosystem.