Rainbow Darters Showcase Harmful Impacts of Wastewater Treatment Plant Discharge

Figure 1: A male Rainbow Darter in full spawning color captured in Indiana. Photo courtesy of Jarret Maurer (@Indianaspeciesfishing)

What is a Rainbow Darter?

The Rainbow Darter (Etheostoma caeruleum) is a small, fantastically colored freshwater fish that can be found throughout the state of Ohio and much of the Midwest. One of the most common darters in the state, they can be found living in riffles, which are the shallow rocky stretches of a river or stream. While the females are a somewhat muted color, the vibrant reds and blues the males boast throughout the breeding season are enjoyed by many naturalists who sweep the riffles with nets to find them (Figure 1). While darters like the Rainbow Darter are enjoyed by many people as a sight to behold, they also serve an important role in monitoring the health and quality of our waterways. They are considered to be sensitive to water pollution, so it’s important to listen to what they are saying about the health of our water by monitoring their populations (Simon and Evans, 2017). One water pollution source that darters can tell us about is wastewater treatment plants.

What does it have to do with Wastewater Plants?

Ever wonder what happens to all the water we use before it goes back into our rivers and oceans? In cities, that water often runs through a wastewater treatment plant that filters and treats the water to remove much of the harmful substances in it. While our city, state, and national regulations try to limit the impact wastewater has on the ecosystem by setting quality standards for the treated water that is released, pollutants like nitrogen, phosphorus, or less studied pollutants like pharmaceuticals (trace amounts of medicines in water) still have the potential to harm the ecosystem so monitoring of these sensitive fish is necessary to help us be more conscious of the impacts we have on our country’s waterways (Deblonde et al 2011). Researchers in Southwestern Ontario working on the Grand River are doing just that.

They collected Rainbow Darters from locations above and below wastewater treatment plant effluents and found some noteworthy differences. First they found that male and female Rainbow Darters collected downstream from the treated effluent water  had higher oxygen consumption rates (Mehdi et al 2018). You might remember aerobic respiration from your biology courses in school. That’s the process where organisms like people (or a fish in this instance) take the oxygen they breath with their lungs or gills and use it to breakdown the food they eat in order to produce the energy they need to live their lives. The issue with Rainbow Darters having increased oxygen consumption rates has to do with where they live. The air we breath is made up of around 21% oxygen whereas the water that runs through our darter’s gills has around 1% the amount of oxygen dissolved in it for aquatic organisms to use.

The second observation these researchers studied was that the gills the darters use to breath had a different morphology or structure depending on where they were found. They observed that fish found below the treatment water had damaged or modified gills when compared to fish found above the wastewater treatment plants (Hodgson et al 2020). While the fish were still in good health, the damaged gills would still be less effective at collecting oxygen as less of the gills are exposed to water for collecting oxygen (Hodgson et al 2020). Less effective gill structures, in combination with a higher oxygen consumption rate, demonstrates a real threat for those darters living downstream from wastewater plants.

What does this mean?

These two studies suggest that Rainbow Darters living downstream from wastewater plants could suffer if water quality worsens as they may not be able to breathe enough oxygen to survive if water quality continues to degrade. While these studies primarily focused on Rainbow Darters, there are dozens of other species of darters that can be monitored to assess water quality. By monitoring darter populations above and below wastewater treatment sites, we can better understand what pollutants we are releasing and what impacts they have on our most sensitive fish species. If we monitor pollution sensitive species like darters, we can catch harmful pollutants early so that we have a chance to treat wastewater more effectively before the larger waterway is impacted. This also has implications for humans as the health impacts of newer pollutants like pharmaceuticals in people are relatively unknown and protecting fish from these and other pollutants is important as many people catch and keep fish from our rivers to eat, which would expose them to these pollutants.

References:

Deblonde T, Cossu-Leguille C, Hartemann P (2011). Emerging pollutants in wastewater: a review of the literature. International journal of hygiene and environmental health214(6), 442-448.

Hodgson R, Bragg L, Dhiyebi HA, Servos MR, Craig PM (2020). Impacts on Metabolism and Gill Physiology of Darter Species (Etheostoma spp.) That Are Attributed to Wastewater Effluent in the Grand River. Applied Sciences10(23), 8364.

Mehdi H, Dickson  H, Bragg LM, Servos MR, Craig PM (2018). Impacts of wastewater treatment plant effluent on energetics and stress response of rainbow darter (Etheostoma caeruleum) in the Grand River watershed. Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology224, 270-279.

Simon TP, Evans, NT (2017). Environmental quality assessment using stream fishes. In Methods in stream ecology (pp. 319-334). Academic Press.

It’s a colorful world – let’s keep it that way.

In many animals, the eyes have developed to be able to discriminate different colors and levels of brightness to gain an efficient way of obtaining accurate information about their biotic and abiotic environments. Animals use visual cues (e.g. color patterns, movement detection, etc.) to find appropriate mates, to find food, or to avoid predation. The study of how visual systems evolve and function to meet the ecological needs of animals is known as visual ecology (Cronin et al., 2014). Coloration is extremely important for the discretion of species, along with determining characteristics about the other organisms, like fitness.

For example, a female cardinal evaluating a male in sunlight needs to discriminate the cardinal male from other species, and the green background foliage to evaluate the color and quality of his plumage using color vision (Cronin et al., 2014). Ornamental traits are understood to be reliable indicators of an individual’s condition (Zahavi, 1975). In birds, females choose brightly colored males because it can help determine if offspring will have heritable characteristics which may help future generations survive (Hamilton & Zuk, 1982). But can humans alter this coloration through societal activities? Perhaps by altering the environment, coloration can also be altered.

One way humans can alter the environment is through the introduction of chemicals. Chemical pollution can have negative effects on the development, physiology, behavior, reproductive success, and survival of wildlife (Lifshitz & St Clair, 2016). The expression (or lack of expression) of bright ornamental traits reveal an honest indicator of an organisms encountered environmental stress, such as living in human-induced pollutants.

An extensive review by Lifshitz & St. Clair (2016) puts together how chemical pollution can alter the ornamentation of animals. Many of these chemicals reduce the carotenoid (orange and red) and increase the melanin (brown and black) pigmentation found in wildlife. For example in birds, male red-legged partridges (Alectoris rufa) exposed to the herbicide Diquat increases the area of black plumage, and reduces the red coloration of their beaks and eye rings. Thiram fungicides and imidacloprid insecticides also reduces the red coloration of A. rufa’s eye rings.

A. rufa

Aroclor (a PCB) reduces the carotenoids in the facial skin of male American kestrels (Falco sparverius).

F. sparverius

Additionally, metals like lead, cadmium, zinc, and copper increased the area of black breast stripes while lowering the carotenoid coloration of breast feathers in great tits (Parus major).

 

P. major

 

Monitoring the coloration of animals has enormous potential to be a non-invasive tool for detecting subtle and early effects of pollution long before they can be seen as population level effects. Species which exhibit carotenoid-based ornamentation can be particularly promising as a highly responsive indicator of pollution (Lifshitz and St. Clair, 2016). The adoption of regulations informed by coloration and sensory ecology is needed to mitigate the effects of human-induced environmental change and is vital for behavioral ecology and conservation (Lifshitz and St. Clair, 2016; Lim et al., 2008).

References

Cronin TW, Johnsen S, Marshall NJ, Warrant EJ (2014). Visual ecology. Princeton University Press.

Hamilton WD, Zuk M (1982) Heritable true fitness and bright birds: a role for parasites? Science 218: 384–387.

Lifshitz N, St. Clair CC (2016). Coloured ornamental traits could be effective and non-invasive indicators

of pollution exposure for wildlife. Conserv Physiol 4

Lim MLM, Sodhi NS, Endler JA (2008). Conservation with sense. Science 319:281-281.

Zahavi A (1975) Mate selection—a selection for a handicap. J Theor Biol 53: 205–214.

 

Photo credits

Cardinal diagram – Cronin TW, Johnsen S, Marshall NJ, Warrant EJ, 2014. Visual ecology. Princeton University Press.

 A. rufa – https://www.juzaphoto.com/life.php?l=en&s=alectoris_rufa

F. sparverius – https://commons.wikimedia.org/wiki/File:Falco_sparverius_-Oregon_Zoo,_Portland,_Oregon,_USA_-male-8a.jpg

P. major – http://www.flickriver.com/photos/diniscortes/sets/72157604962242130/