This winter has been unseasonably warm in many parts of the United States. Mangum, Okla., saw a high of 99 degrees on Feb. 11. Warm winters don’t necessarily translate into warm summers, but summertime temperatures have been increasingly warm due to climate change. One side effect of warm summer weather is the occurrence of toxic algae blooms. Toxic algae blooms, often called harmful algae blooms (HABs), aren’t only caused by warm weather but also other environmental factors such as changes in salinity, changes to nutrient levels (particularly phosphorous or nitrogen), and increased atmospheric CO2. HABs can include both algae and cyanobacteria, since cyanobacteria was known as blue-green algae until genetic tests revealed that the species were bacteria, not true algae.
Warm water temperatures can increase algal and cyanobacterial growth through the creation of a competitive advantage for toxin-producing species, some of which are quite heat-tolerant. Warm surface waters also provide the opportunity for stratification and decreased water viscosity so that cyanobacteria and dinoflagellates can either float or swim toward the upper layers of water with little resistance, forming a bloom as they do so. As the heat of summer causes water to evaporate from lakes, reservoir salinity levels can increase, which can, in turn, stress toxin-producing algae and cyanobacteria, causing them to produce more toxins. Many algae and cyanobacteria are, of course, photosynthetic organisms that take in CO2 as part of photosynthesis, so as CO2 levels rise in the atmosphere, algae and cyanobacteria are able to conduct photosynthesis at a higher rate. Higher rates of photosynthesis increase growth, leading to yet another cause of bloom formation.
Last year’s HABs caused a multitude of news stories—particularly a bloom in Florida that led to a large portion of the coastline being covered in blue-green sludge. A bloom in the Mormon Reservoir in Idaho produced toxin levels in the water high enough to cause serious liver damage. Microcystin, the toxin responsible for the Mormon Reservoir problems, is most often produced by Microcystis cyanobacteria, and it can persist in bodies of water for weeks. Boiling water does not remove the toxin. Filtration is one option, but even low levels are harmful, particularly in young children. Ideally, preventing the formation of Microcystis blooms is the best option, but it is difficult to change farming and ranching practices so that less phosphorous and nitrogen run into bodies of water. Mitigating climate change will take long-term policy changes before we see any decrease in temperatures or CO2 levels.
So what’s to be done? Ecological engineering may be one option. A paper recently published by Yin et al. (2017) explores the potential of using fish, particularly silver carp, to adjust the Microcystis levels in a lake undergoing eutrophication and Microcystis bloom problems. Silver carp are native to Asia and are filter feeders, generally feeding on phytoplankton. They have recently become invasive in North America, however, proving to be a serious problem for many waterways, including the Mississippi River. You may have seen some of the viral videos where fish jump out of the river and smack boaters and water skiers in the face—those are silver carp. Yin et al. (2017) claim that silver carp were effective in a low concentration for controlling Microcystis by lowering total phosphorous levels, lowering chlorophyll-a levels (an indicator of Microcystis concentration), and increasing water clarity. They also discussed the phosphorous decline as one potential reason for the prevention of further blooms. It is unclear, however, what exactly the mechanism is that causes the low concentration of silver carp to be effective.
Other researchers have found evidence indicating that ingestion of Microcystis by silver carp might actually increase the photosynthesis efficiency of Microcystis (Gavel et al. 2004), therefore making a bloom worse instead of better. There are few treatments for HABs, however, and research into control and prevention needs to increase rapidly since the occurrence seems to increasing due to climate change and nutrient runoff into bodies of water and waterways. Silver carp have established themselves in the midwestern United States so, while not ideal, perhaps researchers should take the opportunity to collect data on the effect the fish have on HABs in the area so we can make more informed decisions about controlling future blooms.
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Jessie Myers, Ph.D.