By Hannah Foster
If you take a cup of water from a pond, stream, or even a dirty drainage ditch and look at it closely, you’ll find that it is teaming with miniscule life. Some of the organisms you’re nearly certain to spot are water mites, a highly diverse group of tiny arthropods (most less than three millimeters) that can be found in just about every freshwater aquatic ecosystem. The adults of many species are quite colorful, often sporting striking patterns, making them appear like swimming jewels. In spite of their prevalence (and their elegance), water mites are poorly studied. In fact, a high proportion of species have not even been described. This might lead one to assume that water mites aren’t terribly important, but that couldn’t be further from the truth! According to Ray Fisher, a taxonomist at the University of Arkansas (one of only a few American researchers who specialize in aquatic mites), not only are water mites critical players in their ecosystems, but they could also be one of the best indicators of water quality.
Because water quality is so important, it is critical that we have methods of assessing it. There are a number of ways that organizations, researchers and agencies like the United States Environmental Protection Agency (EPA) test water quality, which include taking the temperature of the water, testing the pH, water clarity, and level of dissolved oxygen, as well as determining the levels of nutrients and toxic substances. Another way is to examine bioindicators, which are sort of like a canary in a coalmine — they are living organisms that indicate the quality of their environment by their presence or absence.
Why Use Bioindicators?
The main reason to use bioindicators is that chemical analysis of water provides only a snapshot of the quality of a body of water. A single sample from a stream provides information solely about that particular section of the stream on that particular day. In order to get an accurate idea of the stream’s quality, you would need to take dozens of samples in all parts of the stream. After that, you would need to come back year-round, since water quality can change due to weather or seasons. But bioindicators can be sensitive to low levels of pollutants or small changes in the chemical characteristics of a body of water. Even if a pollutant is released into the water just once a year, a single sample of the organisms in the water can inform researchers that something is wrong. Another major benefit of studying bioindicators is that sampling for bioindicators is much more cost-effective than testing physicochemical characteristics. Although it does cost something to have an expert identify the organisms in a water sample, this cost is nominal compared to the price of chemical tests.
What organisms are used as bioindicators?
The main challenge of using bioindicators to assess water quality is finding the right bioindicators. Good bioindicators for aquatic ecosystems should be sensitive to water quality, have limited mobility (i.e., do not travel long distances), and should be easy to catch and identify. Fish, other vertebrates, and even many large insects are generally not good indicators of water quality because they are mobile animals and can flee poor-quality water. Fish are also difficult to catch, so it may be impossible to generate a large enough sample size from fish species to come to any meaningful conclusions. The most widely used bioindicators are benthic macroinvertebrates, most of which are aquatic insects.
Largely recognized as the best bioindicators among aquatic insects is an abundant and diverse group in the family Chironomidae, the non-biting midges. Chironomids are fantastically diverse and contain a large range of members that can only survive in high-quality water, as well as a range of members that can tolerate low quality, and then there are many species that fall somewhere in between. Furthermore, while the adults have wings, they generally don’t fly very far to lay eggs and are essentially restricted to a given habitat.
However, chironomids come with some problems. While they can usually be easily identified to family level, identifying them to genus or species level can be difficult or even impossible. An accurate identification is necessary since some aquatic insect species live in poor-quality habitats and others live in high-quality habitats.
So if chironomids can’t practically be utilized as bioindicators, using something that depends on them for survival may be the next best thing. This is where water mites come into the picture. Water mites are predators of aquatic invertebrates , and many researchers believe they could be excellent indicators of water quality.
Why use water mites as bioindicators?
Although water mites utilize nearly all aquatic insects as hosts, water mite diversity coincides closely with one particular group of insects. The largest branch of this group of insects is none other than Chironomidae. As it turns out, the bulk of water mite species — even across several superfamilies — utilize adult chironomids as hosts for their larvae. Research suggests that wherever you find high chironomid diversity, you will also find high levels of water-mite diversity, and that cleaner water means higher diversity for both organisms. However, chironomids and water mites are so poorly understood that the relationship of these two organisms and the effect of the environment on them are not entirely understood. It may be that high chironomid diversity in clean water provides more hosts or more of the right hosts for water mites, leading to high water mite diversity. It could also be that higher water quality merely offers better habitat for both groups. At present, researchers are uncertain.
Regardless of why water mite diversity is correlated with higher water quality, the practical reality is that you can tell by the assemblage of water mites in a given habitat what the water quality is. Unlike chironomids, water mites can be remarkably easy to identify with even scant training. This makes them a potentially powerful tool in assessing water quality.
Why aren’t U.S. researchers jumping on the water mite bandwagon?
Studies in Europe have already shown that water mites are excellent predictors of water quality. However, the use of water mites in the U.S. has not been explored. Why? The answer is two-fold. First, established sampling protocols were designed for insects, so most mites slip right through the majority of commonly-used nets. This could be solved easily by changing the sampling protocols. Second, there are no user-friendly tools for identification. The latter is slowly changing, thanks to people like Ray Fisher who are working to describe unknown species of mites and make water mite identification accessible to a wider group of researchers. Hopefully, with the help of these researchers, water mites can become a standard tool in water quality assessment in order to help solve the global problem of poor water quality.
Hannah Foster is a PhD student in molecular and cellular biology at Harvard University and a freelance writer. She studies protein biochemistry in microbes, and enjoys writing about science and non-science alike. You can follow her on Twitter at @Foster_HR and read her blog about boxing as it pertains to life at theblowbyblow.com. She is also a frequent contributor to Harvard Science in the News Flash and to The Bitter Empire.