Reading Stream Health from Aquatic Invertebrates: New Tools for Alaska’s Parklands

Woodchopper Creek in Yukon-Charley Rivers National Preserve from the air, showing the effects of extensive dredge mining (image from the National Park Service’s article).

Alaska’s national parks are often pictured as vast tracts of unspoiled wilderness, but more than a century of mining, mainly for gold, has disturbed many streams in interior Alaska. This article from Alaska Park Science, by Trey Simmons of the National Park Service, Trip Armstrong of the Bureau of Land Management and Charles P. Hawkins of Utah State University, explains how the insects and other invertebrates living in a stream can show how healthy it is, and describes a project to build such tools for eastern Alaska.

The problem: a century of mining

Mining took place in all three interior Alaska parks, Denali, Wrangell-St. Elias and Yukon-Charley Rivers, and in much of the surrounding area. It largely ended in the parks in the 1980s, though some activity continues, and it goes on elsewhere. In some places hydraulic dredge mining drastically altered whole valleys; in others the impacts were smaller and more local. Mining can bring habitat simplification, the removal of riparian vegetation, channelization, altered hydrology, excessive sedimentation, acidification and higher levels of toxic compounds such as heavy metals, and these can change biodiversity, food web structure, productivity and nutrient cycling in streams. Roads, logging, urbanization and agriculture can have similar effects.

Hydraulic mining in the Yukon River Basin (U.S. Geological Survey).

Being able to measure such effects matters for several reasons. The National Park Service is mandated to protect, and if necessary improve, park resources, including streams and the organisms that depend on them. The Clean Water Act of 1972 aims to restore and maintain the chemical, physical and biological integrity of the nation’s waters, and it requires states to report on the condition of their waterbodies, decide whether they meet their designated uses and plan to improve those found impaired. The same measurements would help rank disturbed sites for restoration and show later whether restoration worked.

Why animals can tell you more than a water sample

Water quality was once judged chiefly by chemistry, such as whether nutrients or toxic compounds exceeded safe levels. Starting in the 1980s, practitioners moved toward a more holistic approach that also looks at biological and physical integrity, as the Clean Water Act originally spelled out. The idea behind methods now called bioassessment is that the biological community reflects overall ecological integrity: when the living parts of an ecosystem are healthy, the physical and chemical parts are in good condition too. The community is always present, so it integrates conditions over time and can reflect brief events, such as pulses of toxic compounds released during high flows, that periodic sampling can miss. It also responds to low-level chronic stress, to stressors that are hard to measure and to several stressors at once, and it integrates conditions over space, upstream and in the land around a site.

Examples of aquatic invertebrates commonly found in streams: a mountain midge, a flat-headed mayfly, a giant stonefly (or salmonfly) and a tube-case caddisfly (U.S. Geological Survey/Joe Giersch).

Most bioassessments rely on aquatic macroinvertebrates, for several reasons. They are found in every stream and river, however small or extreme, so the method can be used anywhere. They are abundant and diverse, so they are fairly easy and cheap to collect and analyze: as many as 70 unique taxa can be present at one Alaska stream site, at densities as high as 20,000 individuals per square meter. That diversity makes the community sensitive to many stressors. They live no more than a year or two, so they reflect the current state of the stream, and they are relatively sessile, so they respond to local conditions, unlike salmon, which may be responding to changes in the ocean.

Two ways to score a stream

Both main approaches use the reference condition approach: the invertebrate community in the streams being evaluated is compared with that of unimpaired reference sites, with no or minimal human activity, which are fairly easy to find in Alaska. Because communities vary naturally among streams, reference sites have to be classified properly so that each assessed site is compared with the right subset.

The first approach, the O/E index, uses the environmental setting (for example elevation, mean annual precipitation and basin area) and the invertebrates found at many reference sites to predict which native taxa should live at a site if it were undisturbed. O/E is the ratio of the expected native taxa actually observed (O) to the number expected (E), a direct measure of how complete the native community is. An O/E of 0.5 means 50% of native taxa have been lost, and a value near 1.0 means the community is largely intact. It is site-specific and standardized, so a value means the same everywhere.

The second approach builds an Index of Biological Integrity (IBI), also called a Multi-Metric Index. It starts from metrics describing ecologically important features of the community that should change with degradation, such as overall taxa richness and the percentages of sensitive mayfly, stonefly and caddisfly taxa, predatory taxa and pollution-tolerant taxa. The metrics that best tell reference sites from disturbed sites are rescaled and combined into one score: high at reference sites, low where disturbance has impaired integrity. The authors call it the most widely applied bioassessment method in the world.

With either index, a site whose score is statistically close to the reference range is judged equivalent to reference, and one that differs statistically is judged impaired, with classes running from no impairment to severe impairment. The U.S. Environmental Protection Agency uses both methods in its national assessments of streams, though Alaska has not yet been included.

The Alaska project

When the article was written in 2021, the National Park Service was working with the Bureau of Land Management and Utah State University to apply both indices to streams across eastern Alaska, since each can perform differently and together they give two complementary assessments. Earlier attempts in Alaska were limited in geographic scope and used few sites, and the authors say theirs will be the most comprehensive and large-scale bioassessment effort in Alaska. They are using macroinvertebrate and environmental data from 299 streams, collected by the NPS Central Alaska Network, the Alaska Department of Environmental Conservation for an EPA pilot study, the BLM, and the U.S. Geological Survey for two NPS-funded water quality studies, plus data collected in 2018 at a few historically mined streams in the Kantishna Hills of Denali National Park and Preserve. Most of the streams are reference sites, largely unaffected by human activity, while a subset are potentially disturbed, most of them by historic mining. Applying the indices to the disturbed sites will quantify how far the disturbance has changed their ecological integrity, which can support a defensible ranking of streams for restoration and a measure of whether restoration succeeds. The indices should also let managers and the state quantify the effects of other disturbances, such as roads and urbanization.

Sources

• National Park Service, Using Aquatic Invertebrates to Measure the Health of Stream Ecosystems: New Bioassessment Tools for Alaska's Parklands, Alaska Park Science 20(1), by Trey Simmons (National Park Service), Trip Armstrong (Bureau of Land Management) and Charles P. Hawkins (Utah State University), with the photograph of one author at work and the map of sampling sites left out; rewritten in hubnx's own words.