Benthic algae of benchmark streams in agricultural areas of eastern Wisconsin

Benthic algae were collected from 20 streams in the Western Lake Michigan Drainages by the U.S. Geological Survey in May and June of 1993 as part of the National Water-Quality Assessment program. These streams were selected to represent "benchmark" streams that were minimally affected by human activities, especially agriculture, for comparison to other streams in similar environmental settings. Streams were chosen from four relatively homogeneous units (RHU's) in agricultural areas with differing texture of surficial deposits and bedrock type.

Blue-green algae were the dominant algal cells at all but 5 of the 20 stream sites, and the most abundant species at these sites was Calothrix parietina, a nitrogen-fixer typically found in pristine streams. Most of the taxa at all sites were diatoms. The dominant diatom guilds observed were the Achnanthes spp., erect forms, and Navicula spp.

Except for three streams thought to have low productivity, the Shannon-Wiener diversity index for diatoms was high at all benchmark streams and indicated either minor stress or no stress on the diatom community. With regard to water quality, additional diatom indexes for 17 of 20 benchmark streams indicated no pollution effects and no significant siltation. All benchmark streams had good to excellent biological integrity and either minor or no impairment of aquatic life with regard to diatoms.

A variety of algal metrics and relative abundances of diatom morphological guilds correlated with basin-, segment-and reach-level habitat characteristics, including drainage area, basin drainage density, basin soil permeability, Q/Q2 (instantaneous discharge measured at time of sampling divided by the estimated 2-year flood discharge), stream length, and average width of natural riparian vegetation. Algal taxa richness decreased with higher percentages of agricultural land and lower percentages of forested land. The relative abundance of pollution-tolerant diatoms was higher in streams where the basin land was primarily agricultural as compared to forested. The Shannon- Wiener diversity index for diatoms, the percentage of diatom taxa, and the percent relative abundances of diatom cells, pollution tolerant diatoms, Achnanthes spp., erect diatom forms, nitrogen-fixing algae, and blue-green algae differed significantly among either RHU's or ecoregions. Higher abundances of pollution-sensitive diatoms and a higher pollution index indicate that water quality in sampled streams in the North Central Hardwood Forests ecoregion may be less degraded than in streams in the Southeastern Wisconsin Till Plains ecoregion. Algal taxa richness decreased as specific conductance, dissolved nitrate plus nitrite, and suspended sediment increased. This relation may indicate a negative effect of agricultural activities on the algal taxa richness of the stream. Pollution-tolerant diatoms and the pollution index increased as these and additional factors correlated with agriculture increased.

Multivariate analyses indicated multiple scales of environmental factors affect algae. Although two-way indicator species analysis (TWINSPAN), detrended correspondence analysis (DCA), and canonical correspondence analysis (CCA) generally separated sites according to RHU, only DCA ordination indicated a separation of sites according to ecoregion. Environmental variables con-elated with DCA axes 1 and 2 and therefore indicated as important explanatory factors for algal distribution and abundance were factors related to stream size, basin land use/cover, geomorphology, hydrogeology, and riparian disturbance. CCA analyses with a more limited set of environmental variables indicated that pH, average width of natural riparian vegetation (segment scale), basin land use/cover and Q/Q2 were the most important variables affecting the distribution and relative abundance of benthic algae at the 20 benchmark streams,