environmental setting, water quality, and ecological indicators of

invertebrate species in relation to environmental variables.
Communities at large-basin sites tended to vary
along the dissolved-oxygen and open-canopy gradient,
whereas communities at small-basin sites tended to vary
along the instream-cover and fipronil-concentration gradient.
Water-Quality Characteristics
Significant statistical differences (p ≤ 0.05) in
water-quality characteristics occurred among CCA site
groups (table 11). Median values were smallest for 17
water-quality variables at group A sites and largest for
11 water-quality variables at group B sites. Median values
of turbidity, and concentrations of total ammonia
37
plus organic nitrogen, nitrate, total phosphorus, and dissolved
fipronil, were largest at group C sites. Group D
sites were characterized by midrange concentrations for
all water-quality variables except concentrations of dissolved
oxygen, which were lowest.
Possible explanations for the differences in water
quality among site groups include the differences in (1)
general soil composition and drainage characteristics
(Touchet and others, 1974), and (2) the percentage of
land used for agriculture in these basins. Drainage areas
for sites in group A are low in agricultural intensity and
contain pine uplands and alluvial terraces. Soils are
acidic and loamy with moderate sand content and are
moderately well-drained. Drainage areas for group B
Table 11. Median water-quality variables for canonical correspondence analysis (CCA) site groups in
southwestern Louisiana, 2001
[Boldface values indicate significant differences (p ≤ 0.05) among site groups, based on Kruskal-Wallis test; for each variable, values preceded by
the same superscript letter are not significantly different from each other. n, number of sites; mg/L, milligrams per liter; µS/cm, microsiemens per
centimeter at 25 degrees Celsius; NTU, nephelometric turbidity units;