Data for selected biological metrics were used to divide the 14 wadeable stream sites into four groups (table 32) to investigate relations between bioassessment data and site characteristic and water-quality data. Metrics from each trophic level (algae, benthic invertebrates, and fish) were selected based on their known high sensitivities to water quality; metrics were selected (either alone or in unison with others) to yield the most complete picture of water quality at each trophic level. To avoid potential bias toward any one metric or trophic level, data were standardized (ranked, lowest number indicating best water quality) and summarized at each trophic level, and the average of these ranks across trophic levels yielded aggregate bioassessment rankings. Sites were then divided into four groups based on the quartile ranges of the aggregate rankings. For the remainder of this discussion, sites will be referred to in terms of their quartile number: quartile 1 contained sites where bioassessment data indicated the least-degraded water quality among those sampled, and quartile 4 contained sites that indicated the most-degraded water quality. Quartiles contained the following stream sites:

• Quartile 1: Milwaukee River near Cedarburg, Milwaukee River at Milwaukee, Jewel Creek, and Menomonee River at Menomonee Falls
• Quartile 2: Willow Creek, Root River near Franklin, and Root River at Grange Avenue
  
• Quartile 3: Menomonee River at Wauwatosa, Oak Creek, and Little Menomonee River
  
• Quartile 4: Honey Creek, Underwood Creek, Lincoln Creek, and Kinnickinnic River
  

Table 32. Average trophic-level rankings and aggregate bioassessment rankings for Phase II stream sites in the Milwaukee Metropolitan Sewerage District planning area, Wis.

[IBI, Index of Biotic Integrity; EPT, Ephemeroptera, Plecoptera, and Trichoptera; HBI, Hilsenhoff Biotic Index; fill color indicates quartile of ranking (quartile 1, blue; quartile 2, light blue; quartile 3, light orange; quartile 4, orange; each column is considered independently)]

   ¹ Averaged trophic-level rankings included only fish IBI scores.
   ² Averaged trophic-level rankings included Shannon index of diversity scores, percent of EPT taxa, and HBI-10 scores.
   ³ Averaged trophic-level rankings included percent of most-sensitive diatoms and percent of sensitive diatoms.

Aggregate bioassessment rankings, however, are averages of the three trophic-level rankings, and as such, results at individual trophic levels may vary. For example, although Root River at Grange Avenue had an aggregate ranking that placed it into quartile 2, results for macroinvertebrate metrics alone would have placed it into quartile 4. Site characteristics (in this case, drainage area and land use) and selected water- and sediment-quality constituent results were summarized based on the four bioassessment quartiles (table 33) to determine if there were relations with the aggregate bioassessment rankings.

return to top

Table 33. Summarized results of Phase II constituents, grouped by aggregate bioassessment ranking in the Milwaukee Metropolitan Sewerage District planning area, Wis.

[mg/L, milligram per liter; µg/L, microgram per liter; col/100 mL, colonies per 100 milliliters; MPN/100 mL, most probable number per 100 milliliters; plaques/100mL, plaques per 100 milliliters; DF, detection frequency; mg/kg, milligram per kilogram; µg/g, microgram per gram; PCB, polychlorinated biphenyl; PAH, polycyclic aromatic hydrocarbon; EDC, known or suspected endocrine-disruptor; <, less than. Values in parentheses indicate the range of possible median concentrations for constituents where the median is an average calculated from a concentration above and a concentration below the reporting level. Fill color indicates relative value of each constituent and increases in value from blue, to light blue, to light orange, to orange; where there is only one mid-range value (that is, where there is a single value, or two identical values), fill color is determined by the difference from the minimum and maximum values: light blue if closest to minimum value and light orange if closest to maximum value.]

Biological metrics often correlate with site characteristics such as land use and drainage area, with higher-quality biological communities and increased diversity as urban land use decreases and drainage area increases (Vannote and others, 1980; Wang and others, 2001; Paul and Meyer, 2001). Sites in quartile 1⁶ had the lowest mean percent urban land use and the largest mean drainage area, while sites in quartile 4 had the highest mean percent urban land use and the smallest mean drainage area (table 33). Though mean site characteristics for quartiles 2 and 3 did fall between these two extremes, sites in quartile 2 had generally more urban land use and smaller drainage areas than those in quartile 3.

⁶ While the mean drainage area of quartile 1 is much larger than the rest of the quartiles, it should be noted that this quartile contains sites with drainage areas among the largest (Milwaukee at Milwaukee, 690 mi²) and the smallest (Jewel Creek, 8.16 mi²) of those sampled.

Median chloride concentrations were lowest in quartile 1, with increasing concentrations in quartiles 2, 3, and 4. Chloride concentrations indicated a positive relation with increasing urban land use (fig. 9); in particular, increasing transportation land use across the quartiles may be responsible for increases in chloride concentration.

Nutrients, chlorophyll a, and suspended sediment results exhibited a wide range of relations to bioassessment quartiles. Median concentrations of total nitrogen and nitrate generally decreased from quartile 1 to quartile 4, and may be attributed to differences in the percent of agricultural land use between the quartiles, as previous studies have shown that agricultural sites have higher concentrations of these constituents than do urban sites (Mueller and Spahr, 2006). Median concentrations of total phosphorus generally increased from quartile 1 to quartile 4, while no particular relations were observed for median chlorophyll a and suspended sediment concentrations.

Mercury constituents did not exhibit consistent relations to bioassessment quartiles. Median dissolved total mercury concentrations generally increased from quartile 1 to quartile 4, whereas median dissolved methylmercury concentrations decreased across the quartiles. Previous studies have shown that urban sites have lower methylation (conversion of mercury to methylmercury) efficiency when compared to agricultural sites (Krabbenhoft and others, 1999). Median concentrations of particulate total and methylmercury varied across the quartiles.

Median results for all indicator organisms increased from quartile 1 to quartile 4. The increasing detection frequencies (DFs) of the coliphage serogroups II and III across the quartiles indicated probable increases in human sources of fecal contamination. Percent urban land also generally increased from quartiles 1 to 4.

In contrast with indicator organisms, DFs for pathogenic organisms did not indicate a particular increase or decrease across quartiles.

DFs of wastewater compound (WWC) constituent classes and known or suspected endocrine-disrupting compounds (EDCs) generally increased across the quartiles, though there were a few exceptions. For example, the DFs for fuels varied across the first three quartiles, but all had similar results and were less than half the DF observed for quartile 4. DFs for nonprescription human drugs in quartiles 1 and 2 were similar and lower than DFs in quartiles 3 and 4, which were higher and also similar to each other. Although median bed sediment constituent concentrations did not show consistent relations across the quartiles, quartile medians observed for three constituents (zinc, total polychlorinated biphenyls (PCBs), and total phosphorus) contained one quartile with notably higher concentrations than the remaining three quartiles. Two of these constituents (zinc and total phosphorus) had the highest median concentrations in quartile 4. Previous studies have shown that zinc concentrations in street refuse are highest in areas of transportation land use; therefore, the higher concentrations observed in quartile 4 may be attributable to the higher proportion of transportation land use in this quartile (Novotny and Chesters, 1981). Other studies have found that the highest proportion of total phosphorus in urban land uses comes from lawns and streets; the higher proportion of residential and transportation land uses in quartile 4 may be responsible for these higher concentrations (Waschbusch and others, 1999). In contrast, the highest median PCB concentration was observed in quartile 1; this may be due to contamination from the Cedar Creek Superfund alternative site which is upstream of two of the four sites (Milwaukee River near Cedarburg and Milwaukee River at Milwaukee) included in that quartile. Fish-tissue samples collected from the Milwaukee River at Milwaukee also had detections of PCBs (fish-tissue samples were not collected at Milwaukee River near Cedarburg). PCB contamination at these sites has been well-documented in previous studies (Scudder and others, 1997; Steuer and others, 1999; Wisconsin Department of Natural Resources, 2006).

The imperfect relation of biological metrics to site characteristics and water-quality data is likely indicative of additional influences on the quality of biological communities. A number of influences on biological communities may assist in explaining these differences: stream flashiness (by minimizing macroinvertebrates and algae colonization); localized intermittent pollution; and toxicity from unidentified constituents. In addition, reach selection shows inherent bias toward sites where biota is available for sampling, which may not accurately represent the overall quality of the stream. Since neither biological metrics nor water-quality data provided a complete description of stream quality, the Phase II assessment of streams was strengthened by the use of both approaches to establish a holistic baseline assessment of stream quality.

return to top
continue to Potential Areas for Data Collection in Phase III