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 health, 214(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 Sciences, 10(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 Biology, 224, 270-279.

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