Article: Increasingly Severe Cyanobacterial Blooms and Deep Water Hypoxia Coincide with Warming Water Temperatures in Reservoirs by Smucker and colleagues (2021)
Background: Cyanobacteria are a group of organisms that occur naturally in aquatic environments. They are sometimes referred to as blue-green algae because, like algae, they obtain their energy from the sun through photosynthesis and often turn the water a greenish color. However, cyanobacteria are not true algae but are a dangerous type of bacteria with consequences for both human and environmental health. As such, the increase of cyanobacteria in reservoirs used as public water supply is of extreme concern since cyanobacteria produce bad tastes, odors, and produce toxins that lead to acute and chronic illnesses in humans. In the past 20 years, cyanobacteria blooms, or outbreaks, have significantly increased around the world due to the following two main causes: 1) warmer temperatures due to climate change and 2) increased nutrient availability due to pollution. Smucker and colleagues analyzed 30 years of data from 20 U.S. reservoirs to better understand the consequences cyanobacterial blooms have for human health and the environment.
Methods: Researchers categorized the reservoirs as forested, agricultural, or urban based on land use coverage. Water samples were collected intermittently by the United States Army Corps of Engineers from 1978 to 2018. The samples were analyzed for nitrogen and phosphorus concentrations as well as the number of phytoplankton and cyanobacteria, temperature, and dissolved oxygen. The level of human health risk (high, medium, or low), which is based on cyanobacteria toxin concentration in the reservoirs, was based on the World Health Organization’s (WHO) recommendations.
Findings: In most reservoirs, cyanobacteria posed a low risk to human health prior to 2003. After 2003, levels of cyanobacteria started to increase dramatically. From 2009-2018, 75% of reservoirs posed a high risk to human health according to the U.S. EPA and WHO. This level of risk is associated with a concentration greater than 100,000 cyanobacterial cells in 1 milliliter of water. Increases were most dramatic in agricultural and urban reservoirs, but forested reservoirs experienced heightened cyanobacteria levels as well. High levels of phosphorus and nitrogen, high summer temperatures, and high amounts of precipitation were all linked to cyanobacterial blooms.
Conclusions: Although high nutrient levels and high amounts of precipitation cause cyanobacterial blooms, these two factors remained fairly constant during the course of the study. This suggests that cyanobacterial blooms increased dramatically due to other causes. Smucker and colleagues found that the spike in cyanobacteria levels is associated with longer, hotter summers, which increase water temperatures allowing the temperature resilient cyanobacteria to outcompete algae. As global warming continues, cyanobacterial blooms will become more frequent, more intense, and increasingly difficult for health and environmental officials to monitor and manage.
Figure: Concentration of cyanobacteria observed in each reservoir from 1987 to 2018. The line represents the average cyanobacteria concentration observed. Each circle represents the maximum concentration of cyanobacteria at each reservoir for a given year. The colors represent the different levels of human health risk (gray, low risk; yellow, moderate risk; red, high risk).
Reference:
Smucker, Nathan J., et al. "Increasingly severe cyanobacterial blooms and deep water hypoxia coincide with warming water temperatures in reservoirs." Global Change Biology (2021).
Comments