environmental setting, water quality, and ecological indicators of

ank stability at Tortue and the reduced riparian vegetative
zone at Grand Marais also contributed to lower RBP
habitat scores for these sites.
Typically, organisms considered tolerant of turbidity,
organic enrichment, and low dissolved-oxygen concentrations
were common in the entire study area and
dominated the overall aquatic invertebrate community.
Sensitive taxa such as Ephemeroptera (mayflies), Plecoptera
(stoneflies), and Trichoptera (caddisflies) (EPT)
were rare at all sites in the Mermentau River Basin and
were only abundant at the Whisky site in the Calcasieu
River Basin. Tolerant taxa abundant throughout the Mermentau
River Basin included Diptera (true flies), especially
the family Chironomidae (midges); Tubificida
(aquatic worms); the tolerant Ephemeroptera family
Caenidae (caenids); and the Gastropoda (snails, limpets)
family Physidae (pouch snails).
A total of 22,678 aquatic invertebrates were collected
at ecological data-collection sites, including
95 species, 80 genera, and 43 families. Taxon groups of
Diptera were the most common (40 species), followed by
Coleoptera (beetles; 14 species), Trichoptera (7 species),
Ephemeroptera (6 species), and Gastropoda (5 species).
Greater than 80 percent of all sampled aquatic invertebrates
were accounted for by the orders Diptera (46 percent),
Amphipoda (scuds, sideswimmers; 18 percent),
Tubificida (7 percent), Ephemeroptera (6 percent), and
Gastropoda (5 percent) (app. 8).
Comparison of Site Groups
The study design, based on drainage area and agricultural
land-use intensity, was used to determine natural
and human-related influences on surface-water quality at
19 ecological data-collection sites. Aquatic invertebrate
communities were used as ecological indicators of water
quality and habitat conditions at these sites. Through
CCA, four significant (p ≤ 0.05) environmental variables
(instream cover score, percentage of open canopy, and
concentrations of dissolved oxygen and maximum dissolved
fipronil) were identified that described the distribution
of aquatic invertebrate communities among
ecological data-collection sites. Cluster analysis of CCA
site scores from the first two axes identified four site
groups which separated ecological data-collection sites
geographically within the study. Environmental variables
and aquatic invertebrate metrics within the studydesign
(a priori, before sampling) and CCA (posteriori,
after sampling) site groups were compared (fig. 7).
30
Study-Design Site Groups
The study was designed to determine significant
differences in chemical, biological, and physical habitat
indicators of surface-water quality among (1) basin site
groups (large and small), and (2) intensity site groups
(high and low agricultural intensity). Low-intensity sites
were used to characterize natural chemical, biological,
and physical habitat conditions. Site groups were compared
to evaluate aquatic invertebrate relations to environmental
variables.
Stream Habitat Characteristics
Significant statistical differences (p ≤ 0.05) in
stream habitat characteristics occurred between basin
and intensity site groups (table 8). Bank erosion scores
and bankfull and wetted channel widths differed between
basin site groups, whereas the bank stability index and
pool substrate differed between intensity site groups.
Differences in bank characteristics, channel width, and
pool substrate may account for some differences in
aquatic invertebrate communities among site groups.
Generally, median bank erosion scores and median
measurements of bankfull and wetted channel widths
were greatest at sites with large drainage areas. Median
bank erosion scores at large-basin sites were 2.5 times
the scores at small-basin sites. Median bankfull and wetted
channel widths at large-basin sites were at least two
times the median at small-basin sites.
Generally, the median bank stability index and pool
substrate characterization score were lowest at all site
groups with high agricultural intensity. The bank stability
index (Fitzpatrick and others, 1998) was used to
assess stream bank structure among site groups and
included bank measurements of angle, vegetative cover,
height, and substrate. A higher bank stability index indicated
a less stable bank. Eighty percent of the sites were
classified as having unstable banks. High-intensity sites
had greater bank stability (smaller median bank stability
index) than low-intensity sites. Differences in the bank
stability index between intensity site groups were attributed
to differences in bank height. As bank height
increased the bank stability index increased. The median
pool substrate characterization score at low-intensity
sites was more than 1.5 times the score at high-intensity
sites. The high-intensity sites had mud- or clay-dominated
pool substrate containing few or no root mats and
little or no submerged vegetation. Pool substrate at lowintensity
sites was fine sand mixed with mud or clay with
some root mats and submerged vegetation.