Citation:

Smucker, N., J. Beaulieu, C. Nietch, AND J. Young. Increasingly severe cyanobacterial blooms and deep water hypoxia coincide with warming water temperatures in reservoirs. GLOBAL CHANGE BIOLOGY. Blackwell Publishing, Malden, MA, 27(11):2507-2519, (2021). https://doi.org/10.1111/gcb.15618

Impact/Purpose:

Cyanobacteria blooms are expected to worsen around the world and increasingly threaten drinking water sources, biodiversity, and ecosystem services. Many blooms often are dominated by species capable of producing toxins that can cause acute and chronic human health effects. However, little is known about what their rates of change could be and if these expected changes already are occurring, because historical monitoring of cyanobacteria is limited. This research documented increases in cyanobacteria cell densities from 1987-2015 in 20 reservoirs located in the Midwest and Western Appalachian regions of the United States. Cell densities began increasing around 2003 and commonly posed high relative probabilities of human health risks since 2009. These increases were associated with warming water temperatures, earlier stratification, and more severe and longer durations of deep water hypoxia. These results are evidence that the expected increases in cyanobacteria blooms associated with warming temperatures are already occurring, are likely to continue expanding geographically if warming trends continue, and possibly will remain recurrent problems once established. That is, unless vulnerable lakes and reservoirs are protected and management strategies are implemented to address underlying causes leading to ecosystem degradation and socioeconomic risks posed by cyanobacteria blooms and toxins.

Description:

Cyanobacterial blooms are expected to intensify and become more widespread with climate change and sustained nutrient pollution, subsequently increasing threats to lentic ecosystems, water quality, and human health. However, little is known about their rates of change because long‐term monitoring data are rare, except for some well‐studied individual lakes, which typically are large and broadly dispersed geographically. Using monitoring data spanning 1987–2018 for 20 temperate reservoirs located in the USA, we found that cyanobacteria cell densities mostly posed low to moderate human health risks until 2003–2005, after which cell densities rapidly increased. Increases were greatest in reservoirs with extensive agriculture in their watersheds, but even those with mostly forested watersheds experienced increases. Since 2009, cell densities posing high human health risks have become frequent with 75% of yearly observations exceeding 100,000 cells mL–1, including 53% of observations from reservoirs with mostly forested watersheds. These increases coincided with progressively earlier and longer summer warming of surface waters, evidence of earlier onset of stratification, lengthening durations of deep‐water hypoxia, and warming deep waters in non‐stratifying reservoirs. Among years, higher cell densities in stratifying reservoirs were associated with greater summer precipitation, warmer June surface water temperatures, and higher total Kjeldahl nitrogen concentrations. These trends are evidence that expected increases in cyanobacterial blooms already are occurring as changing climate conditions in some regions increasingly favor their proliferation. Consequently, their negative effects on ecosystems, human health, and socioeconomic wellbeing could increase and expand if warming trends and nutrient pollution continue.

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