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STREAM SPECIES QUALITY INDEX |
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What question does this index answer?How healthy are the assemblages of aquatic species in the streams of each watershed? Why is this important for biology?Species assemblages are a reflection of their environment, landscape features, land use history, and connectivity among habitats. Landscape and aquatic alterations can degrade conditions that support aquatic species. The presence of expected macroinvertebrate and fish species indicates a healthy aquatic system, capable of supporting a diverse community of organisms. A reduced number of species indicates a decline in habitat condition, loss of connection between suitable habitats, or some other change in the aquatic ecosystem. A large number of species tolerant of altered habitat conditions also signals a decline in stream condition. |
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| CREATING THE INDEX | INDEX RESULTS | SUPPORTING SCIENCE | NEXT STEPS |
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Species assemblages are a reflection of their environment, landscape features, land use history, and connectivity among habitats. Landscape and aquatic alterations can degrade conditions that support aquatic species. The presence of expected macroinvertebrate and fish species indicates a healthy aquatic system, capable of supporting a diverse community of organisms. A reduced number of species indicates a decline in habitat condition, loss of connection between suitable habitats, or some other change in the aquatic ecosystem. A large number of species tolerant of altered habitat conditions also signals a decline in stream condition.
The macroinvertebrate and fish “observed/expected” approach, known as the O/E ratio (Hawkins et al. 2000) was adopted. This approach is based on RIVPACS (River Invertebrate Prediction and Classification System) a statistical model used to predict the expected species at a site based on reference site conditions (Wright 1995). The O/E approach is widely used across the world (Armitage et al. 1983; Norris 1995; Wright 1995; Marchant et al. 1997; Moss et al. 1999) and more recently in the western United States (Hawkins et al. 2000). Species used in the calculations were from stream surveys conducted from the Indicators of Biotic Integrity (IBI) stream survey data of the Minnesota Pollution Control Agency (MPCA).
Site counts of observed fish and macroinvertebrate species are compared to expected species counts. The scores typically range from 0 (no expected species present) to 1 (all expected species present). Scores may exceed one when more species are observed than were expected at a site. Each site in each watershed was given an O/E score. If that score is +/- 1 standard deviation (SD) of values observed at reference sites, that site was considered as meeting the reference condition. If a site had score that was greater than +/- 1 SD from the reference condition, the site was considered in an altered condition. The percentage of sites found to be altered (greater than 1 SD from the expected condition) was used to generate the score.
For watersheds with fewer than 3 sample sites, O/E scores from adjacent watershed(s) in the same river network were averaged to create a larger sample size.
A third metric was calculated using the MN DNR Statewide Mussel Survey data from 1999-2010. The percent of species found live at each survey site was calculated by dividing the number of live species by the number of live species and dead shell species. This “percent live mussel species” value was calculated for each survey site. The average value for all survey sites in each watershed was then calculated.
The percent of scores meeting reference conditions for fish and for aquatic macroinvertebrates were averaged with the percent live mussel species into one value for a combined Stream Quality Index score.
The percentage of sites meeting reference conditions was used to directly rank and score the results.
The values for: Fish ranged from 22 to 100.
Aquatic invertebrates ranged from 22 to 100.
Live mussel species ranged from 13 to 100.
The combined index values ranged from 23 to 100.
The Stream Species Quality values were ranked by generating equal intervals between 0 and 100; where 100 indicates all expected species are found at a site, i.e., the O/E ratio was > or = 1 or the live/total species ratio was:
0-20 = none to very few expected species present
20-40 = few expected species present
40-60 = some expected species present
60-80 = many expected species present
80-100 = most to all expected species present
The combined Stream Species Quality scores ranged from 22 to 100; however, most scores indicated many expected species were present across the state, but the watersheds with most expected species were widely distributed across the state. There were interesting contrasts in the pattern among taxonomic groups. The lowest scores for fish were generally in watersheds on the western border of the state. The lowest scores for macroinvertebrates were in watersheds in north central to northwestern Minnesota. Finally, the lowest scores for mussels were in the southern third of the state.
Overall, this index indicates that the observed number of stream species is near the number of species expected. However, each species group tells a somewhat different story. Macroinvertebrates and mussels are sensitive to channel modification, siltation, and changes in hydrology. Macroinvertebrates and mussels primarily live on or in the streambed, and they can move to a limited extent. Increased sediment inputs and siltation may extirpate local populations, and low dispersal allows these low numbers to persist. The lowest scores for mussels were found in watersheds with significant agricultural land use in the southern part of the state with some low scores in the Red River Valley. In contrast to mussels, the lowest scores for macroinvertebrates were distributed across the central part of the state. Fish have slightly lower scores across the middle of the state, including portions of the Upper Minnesota, Upper Mississippi, and Upper Red River drainages. The lowest scores for each species group are widely dispersed and when averaged across the state the scores indicate moderate to minor impairment. More investigation is needed to find the drivers for the disparity among species groups.
The introduction of contaminants, temperature changes, sediment, nutrients and other inputs into the aquatic environment have a direct impact on the ability of aquatic organisms to survive and thrive. The loss of more sensitive (intolerant) aquatic species is often the direct result of water quality degradation.
Many aquatic species need a variety of habitat types to complete their life cycle. Aquatic systems fragmented by dams, zones of poor water quality, levees, ditches, low flows, and other barriers to movement often eliminate access to essential habitats and result in the loss of aquatic species diversity.
The viability of aquatic species is closely related to changes in the hydrologic flow regime and runoff from the surrounding terrestrial landscape. Many species are sensitive to sediment loads, temperature fluctuations, and the timing and magnitude of discharge. Aquatic species assemblages have been found to be closely linked to hydrologic alteration. Diminished flows were the primary predictors of biological integrity for fish and macroinvertebrates communities at more than 200 sites across the United States and the likelihood of biological impairment was related to the severity of diminished streamflows.
The geologic setting coupled with land cover alteration influence erosion rates, vulnerability to climate extremes, and groundwater contamination. These changes potentially impact the presence and viability of aquatic species.
This index is well supported, resting on a well-developed theoretical foundation, with strong theoretical support in the technical processes of generating the base data and index. There are a broad set of studies connecting changes in ecological, chemical, and physical characteristics of water bodies that are measured with the O/E approach, across the world (Armitage et al. 1983; Wright 1995; Norris 1995; Wright 1995; Marchant et al. 1997; Moss et al. 1999) and more recently used in the western United Sates (Hawkins et al. 2000).
Previous studies have found various relationships between the percent agricultural use and forest harvesting in a watershed and fish assemblages. IBI scores, an alternative assessment approach used by the MPCA, decline with an increase in the percent agricultural land (between 35 and 70%), especially in riparian areas (Roth et al. 1996, Wang et al. 1997, Lyons 2006). Deforestation in riparian areas may also result in a reduction in the number of species or a change in species assemblage to more tolerant species (Jones et al. 1999).
Although fish reflect changes in land use in a watershed, macroinvertebrates may be more sensitive to human-induced changes at local scales. Macroinvertebrates respond to stream conditions relating to discharge and channel characteristics, as well as contaminants (Yagow et al. 2006, Brazner et al. 2007, Riva et al. 2008, Stepenuck et al. 2008). Wang and Kanehl (2003) found a shift from intolerant macroinvertebrate species, such as Ephemeroptera, Plecoptera, and Trichoptera (EPT), to more tolerant species and lower community richness with increasing urbanization.
The number of mussel species was highly variable across the state. Mussels are relatively immobile and live in close contact with both the sediments and the water column, thus along with macroinvertebrates, reflect impacts at a local scale. Mussels may have always been rare in some areas in Minnesota, such as the northeast, thus there may be uncertainty whether mussels are declining, and may be classified as “rare”.
Agriculture, forest harvesting, and urbanization alter the hydrology in a watershed resulting in increased sediment, nutrient and contaminant delivery to streams, and an increase in water flow, which can result in streambank and streambed erosion. The loss of more sensitive (intolerant) aquatic species is often the direct result of water quality degradation (Davies and Jackson 2006).
Confidence in the metric is reasonably high. The data coverage and quality is high, and the metric is based on a studied set of calculations. Data for this index were collected by the MPCA for over 1,500 sites between 1996 and 2006. The Stream Species Quality Index was calculated for 705 sites where both fish and macroinvertebrates were collected in the same year at the same sites. Thus, there are extensive data available to calculate O/E scores. However, the O/E score only indicates whether an expected species was present, but does not depend on the abundance of species present at a site or whether a species is rare at a site. Rare taxa may have more specialized habitat requirements and exhibit higher sensitivity to environmental disturbances, and serve as early indicators to changes in water quality degradation or of the loss of particular habitat types (Cao et al., 1998; Cao and Williams, 1999).
In addition, Dolph et al. (2011) found that fish and macroinvertebrate assemblages indicated a concordant response in 17 of 25 watersheds. Both fish and macroinvertebrates are included in the index because the US Environmental Protection Agency recommends that two different taxonomic groups be sampled, and Rule 7050 in Minnesota State Statues requires that the MPCA evaluate two taxonomic groups to assess aquatic systems.
The index gains strength because the fish and macroinvertebrate taxa provide information about watershed conditions at a different scale. Fish are mobile and provide an indication of watershed conditions at a broad scale, whereas macroinvertebrates are less mobile and reflect conditions at a reach scale. Taken together, these two taxonomic groups provide a comprehensive overview of conditions in a watershed.
Although fish and macroinvertebrates have been assessed across the state, some watersheds have fewer assessments than others. Most assessments have been conducted in small streams within the 81 watersheds, because small streams represent 80-90% of stream miles within watersheds (Leopold et al. 1964) and most are “wadeable” by biologists. Thus, the condition of the larger rivers is less well represented. These data are collected from small stream reaches (less than 500 m in length) that may reflect conditions in a reach, which may or may not represent the conditions in the entire stream. The MPCA has developed a disturbance index comprised of eight variables designed to quantify the overall degree of human disturbance acting upon a stream site. Streams were designated as reference sites if disturbance scores fell within the upper 25% of scores for their ecoregion. The O/E approach predicts presence or absence of species based on prediction for sites that are classified as least impaired (reference sites). Because levels and types of human disturbance vary across Minnesota, reference sites were chosen to reflect the best obtainable condition in each ecoregion, rather than a single standard that could be applied to the entire state. Thus, a least impacted site does not necessarily represent an unaltered site. For example, a “least impacted” site with the highest score in a watershed could be in an area with extensive agriculture or in a suburban area with suitable habitat for a limited number of organisms. There can be a range in the quality of the reference sites that underlie the scoring, resulting in low expected values, and in turn inflating all scores, which may explain relatively higher values than expected in agricultural regions.
The process and selection of reference sites could be evaluated to limit sites to those that are least impacted. Additional data to bolster the assessments in watersheds with incomplete sampling would strengthen the assessment of Stream Species Quality.
