The Warm Water IBI consists of twelve metrics, and is used on the vast majority of Iowa streams. A separate BMIBI-Cold Water index is currently under development.
The twelve BM-IBI metrics quantify various attributes of the benthic macroinvertebrate community that relate to taxa richness, community balance, pollution tolerance, and trophic (feeding) guild composition. The metrics vary in how they are quantified (i.e. integer, proportion, real number); therefore, the ranges of possible values are not equivalent. In order to construct a multi-metric index in which each metric is assigned equal weigh, it is first necessary to convert the raw metric values into a standard scoring range. The procedures described by Hughes et. al (1998) were used to convert the metric values into scores ranging from 0 – 10.
The final IBI score is calculated by summing the scores of the 12 metrics and multiplying the result by 0.8333. This keeps the IBI on a 0-100 scale.
Taxa richness metrics:
1. Multi-habitat Taxa Richness (MHTR). In Iowa’s warmwater streams, benthic macroinvertebrate taxa richness decreases with decreasing water quality and habitat complexity. The highest levels of taxa richness are generally found in streams that have good water quality and a diversity of benthic habitats (e.g. detritus, heterogeneous sediments, root mats, streamside vegetation, woody debris). Conversely, low taxa richness is found in streams that have extreme flow fluctuations, monotonous habitat characteristics, and poor water quality.
The MHTR metric represents the total number of benthic macroinvertebrate taxa collected by handpicking organisms from all types of benthic habitat found in the sampling reach. As stream size increases from headwater stream to middle order stream, the optimum level of benthic macroinvertebrate taxa richness generally increases and then levels off (Figure ).
2. Standard-habitat Taxa Richness (SHTR). The SHTR metric represents the total number of taxa identified in a single, 100-organism standard-habitat subsample. There are two types of standard-habitat samples: (1) coarse rock substrates in riffle/shallow run habitat; (2) artificial wood-plate substrates in shallow run habitat (streams lacking riffles). In healthy streams, wood or rock substrates situated in flowing water support a good abundance and diversity of benthic macroinvertebrates. Iowa’s wadeable streams can support twenty or more taxa in a relatively small area (~ 0.1 m). As water quality declines, the benthic macroinvertebrate community becomes simplified and fewer taxa are supported.
3. Multi-habitat EPT richness (MHEPT). EPT taxa are benthic macroinvertebrates that belong to the pollution-sensitive aquatic insect orders: Ephemeroptera, Plecoptera, and Trichoptera. Pollution sensitivities of EPT taxa range from extremely sensitive to moderately tolerant. Many EPT taxa are adversely impacted by toxic contaminants, such as heavy metals and insecticides. High quality streams support relatively high numbers of EPT taxa. As stream quality declines, the number of EPT taxa also declines. The MHEPT metric represents the total number of EPT taxa collected by handpicking organisms from various types of benthic habitat in the sampling reach. MHEPT has a broad range of response to varying water quality and habitat conditions.
4. Standard-habitat EPT richness (SHEPT). This metric represents the number of EPT taxa identified from an 100-organism subsample of the standard habitats described above. Many EPT taxa have a strong affinity for coarse substrates situated in flowing water. In healthy streams, relatively high numbers of EPT taxa are expected to colonize this type of habitat. A reduction or absence of EPT taxa suggests a water quality problem since habitat is not limited. An unusally low number of EPT taxa might also indicate the food resource base is unbalanced and favors organisms of a particular functional feeding group (e.g., collector-filterer organisms) to the exclusion of other organisms.
Note: It might seem unnecessary to measure taxa richness and EPT richness metrics from both multi-habitat and standard-habitat samples. However, there is an important difference in the scale of measurement which ensures the metrics are not redundant and contributes to a stronger biological assessment. Multi-habitat taxa richness metrics reflect water quality, as well as habitat availability and suitability at the stream reach scale. Standard habitat samples are designed more to reflect water quality differences alone since the habitat sampled is standardized across sites.
When both types of samples are included, there are several possible assessment outcomes. A healthy stream with good water quality and benthic habitat diversity will ordinarily support high total numbers of taxa and EPT taxa in both the standard habitat and multi-habitat samples. Conversely, a stream with poor habitat and poor water quality will yield relatively few taxa in both types of samples. In streams where water quality is acceptable but benthic habitat is limited, taxa richness might be relatively high in the standard-habitat sample, but relatively low in the multi-habitat sample.
5. Multi-habitat Sensitive Taxa Richness (MHSTR). The number of sensitive taxa declines as stream water quality declines. For the purposes of this metric, sensitive taxa are defined as taxa that have a biotic index tolerance value of three or less on the Hilsenhoff scale from 0 (no organic enrichment) –10 (severe organic pollution). This group includes the most pollution-sensitive of the EPT taxa, as well as several non-EPT taxa. With increasing nutrient availability and organic enrichment, sensitive benthic macroinvertebrate taxa are replaced by more tolerant, facultative organisms.
Proportional abundance metrics (calculated from standard-habitat samples only).
6. Percent abundance of 3-dominant taxa (P3DOM). The proportion of the total number of organisms represented by the three most-abundant taxa is an indicator of benthic macroinvertebrate community balance. P3DOM is inversely related to stream biological integrity. Healthy warmwater streams have diverse benthic macroinvertebrate assemblages in which the majority of organisms are distributed somewhat evenly among numerous taxa. As stream conditions degrade, an increasingly higher proportion of the total number of benthic macroinvertebrate organisms is represented by just a few opportunistic taxa.
7. Biotic Index (BINDX). This metric is adapted from the Hilsenhoff Biotic Index which was developed as an indicator of stream organic enrichment (Hilsenhoff 198_, 1987). The BINDX metric responds inversely to increased levels of organic waste and nutrient loading. The proportional abundance of each taxon in the sample is multiplied by its tolerance value. The products are then summed to obtain a weighted-average pollution tolerance score for the entire sample. BINDX metric values can range from 0 (no organic pollution) to 10 (severe organic pollution). However, metric values rarely exceed 6.0. To improve the metric’s sensitivity, a minimum (zero score) line was drawn at the lowest measured value (Figure _).
8. Percent abundance of EPT taxa (%EPT). In healthy streams, EPT taxa are usually abundant and often dominate stable rock or wood substrates that are situated in flowing water. EPT organisms tend to be replaced by tolerant organisms as water quality impacts or siltation problems become severe. Many EPT taxa are particularly sensitive to toxic contaminants such as ammonia, metals, and insecticides. Their absence or rare occurrence in standard habitat samples is strong evidence of a water quality problem. In Iowa streams, the %EPT metric seems to have a narrow range of response that is mostly observed in streams that experience acute or chronic water quality impacts. Concentrated animal feeding operations and wastewater discharges are common pollution sources.
9. Percent abundance of Chironomidae taxa (%CHR). Aquatic dipterans of the Chironomidae family (midges) are a normal component of healthy benthic macroinvertebrate communities. Several chironomid taxa are intolerant of pollution impacts. Other chironomids are very tolerant of pollution impacts, such as organic enrichment, sedimentation, and toxic metal loading. Ordinarily chironomids represent a relatively small proportion of the organisms in standard-habitat samples. Where significant water quality impacts occur, the abundance of tolerant and opportunistic chironomids often increases dramatically, while other benthic macroinvertebrates are eliminated or reduced in number. The %CHR metric has a relatively narrow range of response that is mostly concentrated in the lower end of the stream quality spectrum.
10. Percent abundance of Ephemeroptera taxa (%EPHM). Ephemeroptera (mayflies) are normally abundant and diverse in healthy Iowa streams. As a group, they are pollution-sensitive, and several taxa disappear quickly as stream disturbance increases. Mayflies compete with many other benthic macroinvertebrates for food resources and limited space on coarse substrates such as rocks or wood. They are often replaced by filter-feeding caddisflies at intermediate levels of organic enrichment.
11. Percent abundance of scraper organisms (%SCR). The proportion of organisms belonging to the scraper functional feeding group generally decreases as streams become more organically-enriched. streams due to changes in food resource availability and habitat suitability. The main food sources of scraper organisms are periphyton and organic matter contained in the thin bio-film that is present on coarse substrates. As streams become more enriched, filter-feeding organisms (e.g., Diptera: Simulidae; Trichoptera: Hydropsychidae) often become dominant in response to greater availability of fine particulate organic matter (FPOM). There is also a shift in the periphyton community in favor of filamentous algae, which is not efficiently utilized by scrapers as a food resource. Filamentous algae provides a good substrate for filter-feeder colonization and is a source of additional FPOM.
12. Percent abundance of dominant functional feeding group (%DFFG). This metric is a measure of the degree of balance among benthic macroinvertebrate functional feeding groups. As stream disturbance increases, the %DFFG also increases. It is based on the assumption that extreme dominance by one functional feeding group indicates the stream food web is unbalanced, probably due to an overabundance of a particular food item.
In healthy Iowa warmwater streams, most benthic macroinvertebrates occupying coarse substrates in flowing water belong in one of three functional feeding groups: (a) scrapers; (b) collector-filterers; (c) collector-gatherers. Other functional feeding groups, such as macrophyte (herbivore) piercers, predators, and shredders, are often present in much smaller numbers. As stream disturbance increases, one functional feeding group, typically collector-filterers or collector-gatherers, tends to dominate the benthic macroinvertebrate community and trophic diversity is reduced.