Why Milwaukee's Streams Are Struggling: Ten Years of Algae, Insects and Fish

The Milwaukee River at Cedarburg in 2013 — the healthiest site in the study. Ben Young, USGS.

The study

In 2004 the U.S. Geological Survey began sampling 14 wadable streams in urban or urbanizing watersheds around Milwaukee, Wisconsin, with the Milwaukee Metropolitan Sewerage District (MMSD). The aim: judge the health of the streams' aquatic life to guide watershed management, and find which physical and chemical stressors were hurting it. This summary covers 2004–2013; the full report and the data are online.

Figure 1. The 14 sites, sampled every three years since 2004. USGS.

Why three kinds of life

Each group has different tolerances, mobility and life spans, so together they give the fullest picture:

• Algae live days to weeks, so they show short-term change. They are the base of the food web and help process nitrogen and phosphorus; too many nutrients often show as a lot of algae.
• Invertebrates live months to years and mostly stay put, so they reflect that spot's conditions.
• Fish live longest and roam furthest, so they sum up longer exposure over a wider area, such as a whole watershed.

Urban stressors include runoff from impervious surfaces, straightened and armored channels, cleared streamside trees, and chemicals from sewage, road salt and pesticides. Untangling them takes several lines of evidence, and long-term records — ten years or more.

How they did it

• Every three years, in late summer or early fall, crews sampled a stretch ("reach") of each stream with standard USGS methods (no habitat data in 2007).
• Algae were scraped from rocks in riffles, and invertebrates netted by disturbing the rocks; at one site with no rocks, both came from woody snags.
• Fish were caught by electrofishing the reach — plus seining where that worked poorly, such as deep holes — then identified, counted and released alive.
• Stressors came from streamgage flows, land cover, habitat mapped by GIS and on the ground, the microhabitat where invertebrates were taken, and water chemistry, mostly MMSD samples averaged for each sampling year.
• Six measures went into a combined ranking:
• • algae: the percentage of the most pollution-tolerant diatoms and of sensitive diatoms;
  • invertebrates: the Shannon diversity index, the share of mayflies, stoneflies and caddisflies (EPT), and the Hilsenhoff Biotic Index (HBI-10);
  • fish: the Wisconsin Index of Biotic Integrity (IBI).

Each group's measures were ranked and averaged, then combined into one ranking per site, and the sites split into four groups by 2013 rank.

Lincoln Creek, 2013 — among the most degraded sites. Nic Buer, USGS.

What changed, 2004 to 2013

Figure 2. (A) Pollution-sensitive diatoms; (B) invertebrate HBI ratings from fall samples; (C) fish IBI ratings, 2004–13. USGS.

• Algae: sites in the central metro area had significantly fewer pollution-sensitive diatoms than outlying ones, and most sites had more pollution-tolerant diatoms in 2013 than in 2004 — signs of worsening water.
• Invertebrates: 2013 HBI ratings ran from "fairly poor" to "good." Against 2004 they were the same at 7 sites, worse at 5 and better at 2, and most sites had fewer kinds of invertebrates.
• Fish: most sites rated "fair" to "very poor" in 2013. Ratings were the same at 8 sites, worse at 2 (one had no fish) and better at 4. Pollution-tolerant fish were at least half the catch everywhere, though their share fell by at least 10 percent at four sites. The worry: at more than half the streams, the number of fish caught was nearly 50 percent lower than in 2004; it rose at 3 sites and held steady at 2.

The combined ranking

Lower numbers mean healthier; groups run from 1 (least degraded, below the 25th percentile) to 4 (most degraded, above the 75th).

Rankings for 2013 unless marked. Table 1 of the fact sheet, USGS.

• Most sites showed some degradation.
• Worse since 2004: the Menomonee River at Wauwatosa and the Root River at Greenfield. Better: Oak Creek and Honey Creek.
• The Menomonee gets worse downstream as its watershed grows more urban (Menomonee Falls → Wauwatosa); the Root improves downstream as it grows less urban (Greenfield → Franklin).
• The groups didn't always agree at a site, which is why using all three gave a sturdier verdict than any one alone.

The concrete-lined Kinnickinnic River at 11th Street, 2013 — the most degraded site. Ben Young, USGS.

What's hurting them

Local, urban physical and chemical conditions went with degraded communities. Three stressors lined up with poorer health in all three groups:

• Flashy flows: higher peak streamflow, with its floods and scour, a common result of urban runoff. Peak flow rose with the share of developed land — in the watershed and within 30 meters of each bank — with impervious surface, fecal contamination and chloride.
• Sewage: more fecal contamination (high fecal coliform counts). It is a nutrient source for algae, and can cause oxygen stress for invertebrates and fish.
• Urban land: more developed land in the watershed and along the stream.

A fourth hit invertebrates and fish: chloride, washed in from road salt. Higher average chloride went with worse communities, and annual averages topped the EPA's chronic criterion for aquatic life (230 milligrams per liter) in one or more years at every site in the two most degraded groups.

Figure 3. Invertebrate health against (A) mean chloride, with the EPA criterion marked, and (B) peak flow per square mile of drainage area. USGS.

The bottom line

• Over the decade, some streams and some groups got better, some worse, and many held steady in fair or poor condition.
• The key stressors — flashy runoff, untreated sewage and road-salt chloride — all carry an urban signature: developed land and impervious surface, and they rise together.
• These stressors must come down before stream life can recover. Fixing local habitat alone won't be enough.
• The variation over time and between groups shows why monitoring for more than ten years, with several groups of organisms, matters.

Sources

Based on Physical and Chemical Stressors on Algal, Invertebrate, and Fish Communities in 14 Milwaukee Area Streams, 2004–2013, USGS Fact Sheet 2020–3051, by Barbara C. Scudder Eikenberry, Michelle A. Nott, Jana S. Stewart, Daniel J. Sullivan, David A. Alvarez, Amanda H. Bell and Faith A. Fitzpatrick, prepared with the Milwaukee Metropolitan Sewerage District, U.S. Geological Survey; a work of the United States government in the public domain. Figures, photographs and Table 1 are taken from the fact sheet's PDF.