environmental setting, water quality, and ecological indicators of

area and agricultural land-use intensity, and (2) sites
were grouped statistically after data collection using
canonical correspondence analysis (CCA) and classification
(cluster analysis) techniques on surface-water
quality, habitat, and aquatic invertebrate data. Aquatic
invertebrate communities were used as ecological indicators
of surface-water quality and habitat conditions at
these sites. The CCA identified four significant environmental
variables (instream cover score, percentage of
open canopy, concentrations of dissolved oxygen, and
maximum concentrations of dissolved fipronil) that
described the distribution of aquatic invertebrate communities
among ecological data-collection sites. Results
from the CCA were used in a cluster analysis to identify
four site groups that had similar water quality, habitat,
and aquatic invertebrate characteristics. Environmental
variables and biological metrics within the study-design
(a priori, before sampling) and CCA-assigned (posteriori,
after sampling) site groups were compared.
Median values of 17 water-quality variables were
lowest at sites in the northern part of the study area,
where less than 45 percent of a drainage area is used for
rice cultivation. Median values of 11 water-quality variables
were highest at sites in the southeastern part of the
study area, where the percentage of a drainage area used
for rice cultivation varies. Median values of turbidity,
and concentrations of total ammonia plus organic nitrogen,
nitrate, total phosphorus, and dissolved fipronil,
were highest at sites in the north-central part of the study
area. Possible explanations for the differences in water
quality among ecological data-collection sites may be
the differences in (1) general soil composition and drainage
characteristics, and (2) percentage of land used for
agriculture in these basins.
Habitat characteristics including channel size and
morphology, water clarity, open canopy, and substrate
differed between streams in the northern and southern
parts of the study area. Stream habitat ratings were based
on the total of 10 habitat parameter scores, using the
Rapid Bioassessment Protocols habitat characterization.
Scores increase as habitat quality increases. Ratings
were suboptimal (102-154) to optimal (155-200) for 16
of the 19 ecological data-collection sites. Three sites
were rated marginal (49-101). Differences in channel
size, bank stability, and pool substrate may account for
some differences in aquatic invertebrate communities
between site groups distinguished by agricultural intensity.
Organisms tolerant of turbidity, organic enrichment,
and low dissolved-oxygen concentrations were common
in the study area and dominated the aquatic invertebrate
community. Metrics for aquatic invertebrate communities
were significantly different (p ≤ 0.05) among agri-
2
cultural land-use intensity site groups and CCA site
groups in (1) percentage of noninsects, (2) abundance of
midge taxa, (3) abundance of feeding groups, and (4)
number of tolerant organisms. Dominance and diversity
metrics were significantly different (p ≤ 0.05) among
CCA site groups. In this report, the maximum concentration
of dissolved fipronil was the only significant environmental
variable related to consistent decreases in
relative abundance of many species, notably midges.
Low species abundance in this report was associated with
lower concentrations of fipronil degradation products
than of the parent compound fipronil.
INTRODUCTION
The National Water-Quality Assessment
(NAWQA) Program is a long-term program of the U.S.
Geological Survey (USGS) designed to describe the status
and trends in the quality of the Nation’s surface- and
ground-water resources and to provide an understanding
of the natural and human factors that can affect the quality
of these resources (Gilliom and others, 1995). The
program is interdisciplinary and integrates biological,
chemical, and physical data to assess the Nation’s water
quality at local, regional, and national scales. Assessing
the quality of water in every part of the Nation would not
be practical; therefore, NAWQA Program studies are
conducted in a set of areas called study units. The Acadian-Pontchartrain
(ACAD) is one such Study Unit, and
consists of all or parts of 39 parishes in southern Louisiana
and 5 counties in southwestern Mississippi (Demas
and others, 1999). The 26,000-mi 2 ACAD Study Unit
includes the Mermentau River Basin, a distinctive agricultural
area in southwestern Louisiana. Water-quality
and ecological data are needed to describe interactions
between ground- and surface-water components of the
hydrologic cycle and to determine the effects of agricultural
land use on water quality in the Mermentau River
Basin.
Purpose and Scope
This report characterizes ground-water and surfacewater
quality and ecological indicators (aquatic invertebrate
communities and habitat conditions) for selected
sites in the Mermentau River Basin. Bed-sediment quality
also is characterized at selected sites. The environmental
setting, including agricultural, industrial, wateravailability
and use, and hydrologic characteristics of the
Mermentau River Basin are described. This report (1)
describes the occurrence and distribution of selected
nutrients, major inorganic ions, trace elements, and pesticides
in ground water and surface water, suspended sed-