innovate arkansas

Freshwater macroinvertebrates don't get a lot of credit; many people would have a hard time naming more than a few of these backboneless creatures, which include crayfish, snails, mollusks, aquatic worms and mayfly nymphs. Yet these animals act as the middlemen of the smorgasbord of life, serving as food for fish while also consuming algae, shredding leaves and eating other types of organic matter in the water. They perform critical functions in streams, rivers, small ponds and large lakes.

Many ways exist to measure water quality, and one of those ways involves looking at the health of populations within a stream. Biological sciences professor Michelle Evans-White and her colleagues, Debra Baker and Donald Huggins of the Central Plains Center for Bioassessment and Walter Dodds of Kansas State University, wanted to look at a possible way to assess stream health, and therefore water quality. The Environmental Protection Agency has created clean water criteria that all states must comply with - or the states can develop their own criteria, which must be scientifically defensible. Looking at macroinvertebrate biodiversity may be one way of developing such criteria.

To look at stream health, the researchers examined the biodiversity of macroinvertebrates in relation to the nutrients phosphorus and nitrogen found in the water. Many streams have accumulated excess levels of nitrogen and phosphorus due to runoff from fertilizers on lawns and fields. While it is known that these excessive phosphorus and nitrogen levels change stream communities, researchers are still examining the relative importance of various mechanisms and their relationship to biodiversity.

"Biodiversity can be important to ecosystem function," she said. "Losses in the biodiversity of these macroinvertebrates could mean losses in stream functionality." Without diverse communities of macroinvertebrate middlemen, functioning streams may founder.

Evans-White used data on the biodiversity of macroinvertebrates collected by state agencies over the past 20 years at different sites in Missouri, Kansas and Nebraska. The collectors used nets at timed intervals to scrape the bottom of the body of stream, overhanging vegetation, undercut banks, submerged tree roots and other dominant habitat types. All species collected were then identified and counted at each location.

The researchers took the biodiversity data and plotted it against a gradient of total nitrogen and phosphorous levels, which were taken from the same locations within 30 days of the biodiversity samples. They found that there is a "change point" at fairly low levels of nitrogen and phosphorous where the biodiversity plunges - and stays low. The point at which the biodiversity crashes lies at nitrogen and phosphorus levels near EPA clean water standards. But more than 70 percent of the streams surveyed had nutrient levels that were even higher - too high to sustain biodiversity among the macroinvertebrate populations.

"Communities are constrained to low levels of diversity when there is a high nutrient load," she said. "We wondered whether changes in food nutritional quality might drive the relationship."

The researchers decided to examine the nutrient content of the animals, which is an indicator of dietary nutrient demand, and its effects on the community.

"It's a little like ‘you aren't necessarily what you eat,'" Evans-White said. "An organism's food choice doesn't always match exactly what its body needs to grow. This is true for humans too. Some macroinvertebrates need more nitrogen and phosphorus in their diets to grow than others."

In other words, they looked at the organisms in terms of their ability to consume and excrete, or cycle, nitrogen and phosphorus. The thought was that as nutrient levels increased, organisms that needed lots of nitrogen and phosphorous would out-compete those that used less.

"We are seeing evidence for that on a large scale in certain macroinvertebrates," she said. Many organisms living in the water rely on low quality foods, so organisms with high phosphorus or nitrogen demands and high growth rates are kept in check under normal circumstances. As nitrogen and phosphorus levels increase and communities that thrive on these nutrients grow, they could crowd out the lower-demand organisms, resulting in lower levels of biodiversity.

However, the picture is not entirely clear; the researchers in this study did not find evidence that this high consumption scenario to be the case for all macroinvertebrate feeding groups - including those that scrape and feed upon algae.

"Their diversity is decreasing, but in this case there is no evidence in our study that this mechanism is causing the decrease. It could be multiple things," she said.

To look at other possible mechanisms causing biodiversity to crash, Evans-White plans to study "dropouts" and successful species to see what might be causing certain species to disappear. In the meantime, streams continue to decrease in biodiversity as nutrient levels rise.

"By enriching these streams we are lowering biodiversity and potentially altering the stream state," Evans-White said. The question of whether or not these streams can recover if the nutrient content is returned to normal has not yet been answered.

"Macroinvertebrate biodiversity is just one variable people are looking at," she said. "It's one part of the whole story."

(This article was originally published in the spring 2008 issue of the University of Arkansas at Little Rock's Research Frontiers.)