Defining stream integrity using biological indicators

Abstract

Biological indicators may offer the most comprehensive and accurate means to assess the integrity of streams and rivers, as changes in a biological community represent an integrated response to all environmental stressors present in an ecosystem. Biological indicators are typically designed to quantify and=or summarize important aspects of either ecosystem structure – the types and abundance of organisms found in a given habitat – or ecosystem function – rates and patterns of ecological processes such as primary and secondary production, nutrient cycling, and decomposition of organic matter. Increasingly, resource managers use such indicators to assess whether surface waters fulfill the requirements of their designated uses under the Clean Water Act. Despite the recognition that biological indicators can aid management decisions, critical questions remain regarding the best way to design, apply, and interpret them. In this dissertation, I used a suite of statistical and empirical approaches to evaluate the design and application of several different biological indicators of stream condition in various contexts and scales across Minnesota. Specifically, I used a bootstrap approach, together with a database of fish, macroinvertebrate, and environmental data collected by the Minnesota Pollution Control Agency (MPCA) between 1996 and 2006 from approximately 1500 stream sites across Minnesota, to quantify variability associated with an Index of Biological Integrity (IBI) developed by MPCA for fish communities in streams of two Minnesota river basins. I placed this variability into a management context by comparing it to impairment thresholds used in water quality determinations for Minnesota streams. I used the same MPCA dataset to develop predictive taxa richness models for fish and macroinvertebrates as additional indicators of the biological integrity of Minnesota streams, and evaluated these models for sensitivity and precision. I further determined whether fish and macroinvertebrate assemblages exhibited significant community concordance, and whether significantly concordant communities yielded equivalent indications of stream integrity at three nested spatial scales (statewide, ecoregion and catchment) in Minnesota. Finally, I used data from the MPCA database to evaluate relationships between selected environmental variables and the composition of fish and macroinvertebrate assemblages at all three spatial scales. I collected a second dataset of macroinvertebrate samples over the course of one year (2010) from three agricultural streams in southern Minnesota to evaluate relationships between structural and functional indicators of stream condition in response to a common stream conservation practice (i.e., reach-scale restoration). Specifically, I examined whether reach-scale restoration in disturbed agricultural streams in southern Minnesota was associated with changes in (1) macroinvertebrate taxa richness, (2) seasonal variability in macroinvertebrate community composition, and (3) secondary production (i.e., macroinvertebrate biomass over time). SUMMARY OF FINDINGS: 1. I found that 95% confidence intervals for IBIs scored on a 0-100 point scale ranged as high as 40 points. However, on average, 90% of IBI scores calculated from bootstrap replicate samples for a given stream site yielded the same impairment status as the original IBI score. I suggest that sampling variability in IBI scores is related to both the number of fish and the number of rare taxa in a field collection. A comparison of the effects of different scoring methods on IBI variability indicates that a continuous scoring method may reduce the amount of bias in IBI scores. 2. Predictive taxa-loss models for fish and macroinvertebrates both distinguished reference from non-reference sites. Predictive models for fish assemblages were less sensitive and precise than models for invertebrate assemblages, likely because of a relatively low number of common fish taxa. Significant concordance between fish and invertebrate communities occurred at the statewide scale as well as in six of seven ecoregions and 17 of 21 major catchments examined. However, concordance was not consistently indicative of significant relationships between rates of fish and invertebrate taxa loss at those same scales. Fish and invertebrate communities were largely associated with different environmental variables, although the composition of both communities was strongly correlated with stream size across all three scales. 3. I found no difference in macroinvertebrate taxa richness between restored and unrestored reaches of agricultural streams in southern Minnesota. However, both compositional similarity and secondary production were higher in restored reaches relative to unrestored reaches, suggesting that reach-scale restoration may have ecological effects beyond influences on diversity. These findings highlight the added complexity conveyed by a consideration of functional, as well as structural, indicators of stream condition. Higher productivity in the restored reaches was due largely to the disproportionate success of a small number of dominant taxa. Secondary production estimates were considerably lower than those reported for other similar-sized prairie streams; these low values may be indicative of stressful conditions for biotic life in the study streams.