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What is NAWQA? What are they doing in southern Louisiana?
Studies and Surveys
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Ecological Characterization of Southern Louisiana Streams
An Accompanying Study to Fixed Water-Quality sites
The focus of the NAWQA Program in its first cycle has been the occurrence and distribution of various chemicals and compounds. A general survey of water-chemistry and ecological components gives a baseline against which NAWQA may compare future studies for the analysis of trends. For a national survey, NAWQA spreads its sampling across many study units, each contributing sampling of "basic fixed sites" for local and national assessment of water-quality status and trends. Our study unit has eight such basic fixed sites (called "fixed" because they will be sampled over many NAWQA cycles).
Descriptions of biological communities and habitat conditions are essential for an overall assessment of the status of water resources. Surveys of fish, invertebrate, and algae communities, along with characterization of aquatic habitats, are conducted in conjunction with monthly water-quality sampling. These data are used to improve understanding of relations among aquatic biological communities and the physical, chemical, and hydrologic conditions associated with different land uses or ecological regions. A nationally-consistent methodology allows comparison of these data across large areas of the country. Background and rationale for these ecological studies are described in the Ecological Studies section of the "Design of the National Water-Quality Assessment Program: Occurrence and Distribution of Water-Quality Conditions, Circular 1112".
Sites selected for ecological characterization were the Fixed Sites where we collect water samples monthly. Representative reaches are selected as areas nearby to the water-sampling sites, but sufficiently distant to avoid effects from bridges and other structures. We conducted a reduced set of characterizations at sites in the Mermentau Basin Study, which are described with the rest of that study.
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Methods and Analysis
Fish communities are sensitive to overall water quality, both short- and long-term. They are also highly dependent on availability of specific habitats and flow regimes. We collected fish by electroshocking up and down a selected reach. Data for fish species collected can be found on our Data and Publications page. Methods for characterizing fish communities are described in "Methods for Sampling Fish Communities as a Part of the National Water-Quality Assessment Program, Open-File Report 93-104".
Invertebrate communities are sensitive to chronic changes in water-quality and available habitats such as aquatic plants and woody debris. We collected invertebrates from multiple, randomly selected habitat types (such as wood, plants, mud) to describe the number of species that could be found in the area. We collected invertebrates from wood only, measuring the surface area of wood we collected to determine relative abundances of different species. Methods for sampling invertebrate communities are described in "Methods for Collecting Benthic Invertebrate Samples as Part of the National Water-Quality Assessment Program, Open-File Report 93-406". Identification of the large number of organisms collected is difficult and complex; our laboratory uses sorting, identification, and quality-assurance methods described in "Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory -- Processing, taxonomy, and quality control of benthic macroinvertebrate samples, Open-File Report 00-212".
Algae communities are sensitive to changes in nutrients and their ratios, as well as physical habitat changes such as turbidity and temperature. We collected algae from multiple, randomly selected habitat types (such as wood, other plants, floating in the water-column) to describe the number of species that could be found in the area. We collected algae from wood only, measuring the surface area of wood we collected to determine relative abundances of different species. Methods for characterizing algae communities are described in "Methods for Collecting Algal Samples as Part of the National Water-Quality Assessment Program, Open-File Report 93-409".
To understand what habitats are available for these organisms, and to characterize the larger-scale physiography and land use, we survey a random "reach", dependent on the size of the stream. A reach length for sampling purposes is defined as 20 times the average width of a stream, which is done in order to capture the full range of habitats along one set of natural meanders. Each reach is divided into 11 equidistant transects (one at each end), and microhabitats (such as woody debris and aquatic vegetation) are described at points along each transect. Morphological features, such as bank heights and widths, water velocities, bank and bed substrates, are described at each transect as well. All these data are averaged or aggregated together to give a composite view of each reach. Details of the methods for characterizing stream habitat are described in "Revised Methods for Characterizing Stream Habitat in the National Water Quality Assessment Program, Water Resources Investigation Report 98-4052".
For questions about our Fixed Site ecological surveys, please contact Dennis Demcheck [firstname.lastname@example.org], 225-298-5481.
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