The Midwest Stream Quality Assessment: How Farms and Cities Affect Streams

A Midwestern stream running through farmland. Peter Van Metre, USGS.

Healthy streams, and the fish and other life in them, add to our quality of life. But the Midwest's landscape has been transformed: row crops and pavement replaced grassland and woodland, streams were straightened, and wetlands and fields drained. Runoff from farms and cities carries sediment and chemicals into streams.

• The questions: what is the chemical, physical and biological condition of Midwestern streams? Which stressors harm their communities, where do they come from, and how could we reduce them?
• The study: in 2013 the USGS ran the Midwest Stream Quality Assessment (MSQA) with the EPA's National Rivers and Streams Assessment, sampling 100 streams chosen to represent the region's watershed types.
• The method: biological condition was judged from the number and diversity of fish, algae and invertebrates; changes in habitat and chemistry — the stressors — were measured and linked to the landscape and to biology with mathematical models.

Sites sampled from May to August 2013, over land cover, and how the study links landscape, stressors and biology. USGS.

Life in the streams

Algae

Algae sit at the base of the food chain and react quickly to change, making them sensitive indicators. The MSQA sampled diatoms, which usually cling to rocks and wood.

• Healthiest: streams with rock and gravel beds, cool water and little herbicide.
• Fewer sensitive diatoms: where beds held more fine silt and sand, water was warmer, and triazine herbicides such as atrazine were higher.

Algae growing on streambed rocks. Peter Van Metre, USGS.

Invertebrates

Insects such as mayflies and caddisflies, snails and clams, worms, and crustaceans like amphipods and crayfish are both predators and food for fish and birds.

• Habitat: too much fine sediment, poor streamside condition, stream velocity and channel shape — all shaped by straightening and nearby development — went with losses of sensitive insects.
• Chemistry: ammonia and pesticides, especially the pyrethroid insecticide bifenthrin, were also linked to degraded communities.

A mayfly larva, one of the invertebrates sampled. Alan Cressler, USGS.

Fish

Fish eat invertebrates, other fish, algae and plants, feed wildlife, and support sport fishing worth over $15 billion a year in the MSQA states (American Sportfishing Association). The MSQA found 135 species, the green sunfish most common.

• Top stressor: total nitrogen — species declined as nitrogen rose.
• Fine sediment smothering the bed cost species that need gravel or cobble to spawn.
• Pesticides and other contaminants didn't appear to be harming fish in most streams.

Fish are stunned with an electroshocker and netted for identification, then released. Peter Van Metre, USGS.

Habitat alteration

Losing the streamside

The riparian zone — the land beside a stream — shelters and feeds wildlife, shades the water and blocks runoff. Straightening and encroaching farms and cities have disturbed it along many Midwestern streams, harming invertebrates and algae. Yet an intact, forested riparian zone can keep a stream healthy even in a heavily farmed watershed:

Sediment

Too much fine sediment buries the gravel and cobble many algae, invertebrates and fish need, and clouds the water, blocking the light plants use. It was tied to degraded algae, invertebrates and fish. On average, streambank and channel erosion supplied about two-thirds (64%) of the fine sediment and soil erosion about one-third (36%) — useful to know when choosing where to act.

Streambank erosion supplies about twice as much sediment as soil erosion. Peter Van Metre, USGS.

Nutrients

Streams richer in nitrogen and phosphorus — from fertilizer and animal waste — more often had degraded communities. Excess nutrients feed algae growth that can bring oxygen depletion, fish kills and toxic blooms.

• Fertilizer: nitrate rose with the share of a watershed planted in corn. In the 36 streams with less than 20 percent corn, mean nitrate was only 1.2 milligrams per liter. Nitrate makes up most of the total nitrogen — the top stressor for fish.
• Weather: in the 2012 drought, nitrogen built up in soils as crops grew poorly and little rain washed it out. Heavy rain in 2013 flushed it, so nitrate in many streams was higher than normal during sampling.

More corn, more nitrate. USGS.

Contaminants

Pesticides in water

• Found: 184 of the 228 pesticides analyzed turned up at least once. Over the 3-month sampling period, each site averaged 54 pesticides and each water sample 25.
• Toxicity: benchmarks for harm were exceeded more often for invertebrates and algae than for fish. Chronic or acute effects on invertebrates were indicated in 55 percent of streams, and 12 percent of those had at least one potentially lethal short-term spike in insecticide.
• Farm vs. city: agricultural streams carried more herbicides, so their water was predicted to be most toxic to algae; urban streams carried more insecticides, most toxic to invertebrates.
• Culprits: the neonicotinoid imidacloprid, pyrethroid insecticides, triazine herbicides and the pesticide toxicity index (a measure of mixture toxicity) were significantly tied to harm to algae and invertebrates.

Share of streams where pesticides exceeded a toxicity benchmark:

Contaminants in sediment

Sediment was tested for pesticides, PCBs, PAHs and metals, and sediment from 99 of the 100 streams was tested in the lab for toxicity to two invertebrate species.

• Mostly clean: concentrations were below benchmarks protecting aquatic life and lab toxicity was rare — partly because eroded bank soil diluted the bed sediment. Where toxicity appeared, it was tied to bifenthrin.
• Urban streams: significantly more metals, PCBs, PAHs and several pesticides (including chlordane and DDE) than farm streams.

Sediment in urban streams, such as Wolf Creek in Cincinnati, Ohio, is often contaminated. Peter Van Metre, USGS.

Bifenthrin

This widely used pyrethroid collects in sediment and is highly toxic to aquatic invertebrates. In mesocosm experiments (artificial streams), effects appeared at concentrations like those in MSQA sediment:

• Low levels shifted the timing of adult insect emergence — which could disrupt reproduction and the food web on land.
• Moderate and high levels killed sensitive species such as scrapers, which graze algae off rocks; with them gone, algae grew faster.

The MSQA streams agreed: plant-eating mayflies declined as bifenthrin rose.

Artificial streams (mesocosms) used to test bifenthrin on natural invertebrate communities. Travis Schmidt, USGS.

As bifenthrin rose, scrapers declined — in the lab and in the MSQA streams. USGS.

What it adds up to

1. No pristine streams: every site had at least 28 pesticides in its water.
2. No single cause: nearly every stream and measure of biological health involved several physical and chemical stressors, in a mix unique to each site (see the site scorecards on the USGS Regional Stream Quality Assessment website) — so improving streams will take several approaches.
3. Consistent threads: habitat — especially excess fine sediment and the condition of the channel and streamside — matters to all communities, and triazine herbicides and pyrethroid insecticides are hurting algae and invertebrates.
4. Double benefits: tackling one stressor, for example by preserving or restoring riparian buffers, may reduce others too.

Sources

Based on The Midwest Stream Quality Assessment — Influences of Human Activities on Streams, by Peter C. Van Metre, Barbara J. Mahler, Daren Carlisle and James Coles, USGS Fact Sheet 2017–3087 (April 2018), U.S. Geological Survey (DOI), summarizing Garrett and others (2017), Munn and others (2018), Waite and Van Metre (2017), Meador and Frey (2018), Nowell and others (2018), Gellis and others (2017), Mahler and others (2017), Moran and others (2017), Rogers and others (2016), Shoda and others (2016) and Van Metre and others (2016, 2017); a work of the United States government in the public domain. The figures and photographs come from the fact sheet's PDF, which also puts its scrambled fish and conclusions passages back in order; a cover photograph and diagram photographs under Creative Commons licences, a satellite image with third-party imagery, and a fish illustration are left out.