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General introduction

King County uses Puget Lowland Benthic Index of Biotic Integrity (B-IBI) scores to evaluate stream health. The index is composed of ten metrics that measure various aspects of benthic macroinvertebrate  diversity and community structure. Each metric was included in the B-IBI because it reflects some aspect of the benthic macroinvertebrate community that responds to water quality or habitat conditions. Many of the metrics relate to taxa richness, or the number of unique types of benthic macroinvertebrates present in a sample. Other metrics account for the habits and feeding ecology of taxa known to be sensitive to degradation. The ten metric scores are added to produce the overall B-IBI score that ranges from a low of 0 to a high of 100. Collectively, these ten metrics and the overall B-IBI score help characterize how the stream community has responded to conditions at the sampling site and in the surrounding watershed over time.

King County manages a monitoring program web site and a publicly available database - the Puget Sound Stream Benthos (PSSB) database. The PSSB contains B-IBI scores, as well as the individual sample data describing the number and types of taxa present in a sample. General descriptions of the B-IBI scoring system are provided below (a more detailed description is available on the PSSB web site).

-->>View King County's B-IBI data.<<--

Metric descriptions

Below are descriptions of the ten B-IBI metrics tested and developed for lowland streams in the Pacific Northwest.

Total taxa  richness. The number (richness) of different types of macroinvertebrates (taxa). The biodiversity of a stream declines as flow regimes are altered, habitat is lost, chemicals are introduced, energy cycles are disrupted, and non-native taxa invade. Total taxa richness represents the number of unique types of benthic macroinvertebrates collected from a stream site, including mayflies, caddisflies, stoneflies, true flies, clams, snails, and worms.

Mayfly (Ephemeroptera) taxa richness. Mayfly richness declines in response to most types of human disturbance (for example, stream channelization, forest removal, increase in impervious surfaces). Many mayfly taxa cling to rocks and graze on algae. Many types of mayflies are particularly sensitive to chemical pollution and excessive fine sediment. 

Stonefly (Plecoptera) taxa richness. Stoneflies are typically the first taxa to disappear from a stream as human disturbance increases. Many stonefly taxa are predators that stalk their prey and hide around and between rocks. Hiding places between rocks are lost as sediment washes into a stream. Many stoneflies are shredders and feed on leaf litter that drops from the overhanging tree canopy. Most stoneflies, like salmonids, require cool water temperatures and high levels of oxygen to complete their life cycles.

Caddisfly (Trichoptera) taxa richness. Different caddisfly taxa feed in a variety of ways: some spin nets to trap food, others collect or scrape food (algae) from exposed rocks, and others are predators. Many caddisfly taxa build gravel or wood cases to protect themselves. Even though they are very diverse in habit, caddisfly taxa richness declines steadily with increasing human disturbance.

Intolerant taxa richness.  Benthic macroinvertebrates identified as intolerant are the most sensitive taxa; they represent approximately 15% of the taxa present in the region. These organisms are the first to disappear from streams as human disturbance increases.

Clinger taxa richness. Taxa defined as clingers have physical adaptations that allow them to hold onto smooth substrates in fast water. These organisms typically occupy the open areas between rocks and cobble along the bottom of the stream. Thus, they are particularly sensitive to fine sediments that can fill these spaces and eliminate the availability and variety of these small habitats. 

Long-lived (semi-voltine) taxa richness. While most benthic macroinvertebrates in the Pacific Northwest live about one-year, long-lived taxa require two or more years to complete their life cycle. Thus, they are more likely to be exposed to episodic disturbance events like floods or pulses of contaminants.

Percent tolerant. Tolerant benthic macroinvertebrates are present at most stream sites, but as disturbance increases and habitat is degraded, they represent an increasingly large percentage of organisms present. Benthic macroinvertebrates  designated as tolerant represent the 15% most tolerant taxa in a region. In a sense, they occupy the opposite end of the spectrum from intolerant taxa.

Percent predator. Predator taxa represent the peak of the food web and depend on a reliable source of other benthic macroinvertebrates that they can eat. The percentage of predator taxa provides a measure of the trophic complexity supported by a site. Less disturbed sites support a greater diversity of prey items and a variety of habitats in which to find them.

Percent dominance (3 taxa). As diversity declines, a few taxa come to dominate the organisms present at a site. Opportunistic taxa  that are less particular about where they live replace taxa that require special foods or more specific types of physical habitat. Dominance is calculated by adding the number of individuals represented by the three most abundant taxa and dividing by the total number of individuals collected in the sample.


For questions about information on this page, please contact Jenée Colton, Lead, Toxicology and Contaminant Assessment Group or Kate Macneale, Environmental Scientist.