Evaluation of macroinvertebrate assemblages in Idaho rivers using multimetric and multivariate techniques, 1996-98

Macroinvertebrate assemblages and environmental
variables were evaluated as part of the Idaho
statewide surface-water quality monitoring program
during 1996–98. Two assessment approaches were
used to evaluate the macroinvertebrate data collected
from Idaho rivers—biological metrics and
multivariate statistical analyses. A total of 247 macroinvertebrate
taxa were identified in semiquantitative
riffle habitat (richest targeted habitat; RTH)
and qualitative multiple habitat (QMH) samples,
which were collected from 40 sampling sites. Riffles
supported most of the taxa collected at all sites.
One hundred and eighty-four taxa (74 percent of
total taxa) were identified in the RTH samples.
Taxa considered abundant in RTH samples included
Oligochaeta,
Baetis tricaudatus
,
Hydropsyche
,
Simuliidae, Chironomidae pupae,
Cricotopus
,
Eukiefferiella
, and
Orthocladius
complex. Comparisons
of RTH and QMH sample types indicated
little difference in various metrics evaluated; either
sample type could be used to evaluate biological
condition. Fourteen coldwater taxa were collected
during this study at 12 sampling sites, representing
only about 6 percent of all taxa collected and a frequency
of occurrence of 30 percent for all sites. An
evaluation of the Idaho Department of Environmental
Quality invertebrate river index (IRI) identified
statistically significant differences between highand
low-quality sites, providing evidence that the index
can successfully discriminate impairment. IRI
scores for all sampling sites identified 25 percent
of the sites with poor biotic condition and 68 percent
with good biotic condition. Maximum temperatures
at 62 percent of all sampling sites exceeded
Idaho’s instantaneous coldwater temperature
criteria of 22
°
C. No correspondence was evident
between ecoregion percentages upstream
from each site and macroinvertebrate assemblages.
Multivariate analyses of RTH samples identified
various environmental variables operating at different
spatial scales that affect the macroinvertebrate
assemblages in Idaho rivers. Six environmental
variables—percent forested land, percent
agricultural land, urban land, maximum water
temperature, percent substrate fines, and stream
gradient—were significant in describing variance
in the macroinvertebrate assemblages. Two distinct
groups of sites and associated taxa were identified:
one represented high-gradient, coldwater,
forested and rangeland sites, and the other represented
sites influenced by human disturbance,
indicated by increased percent substrate fines and
increased water temperatures typically associated
with agricultural and (or) urban land uses.