target issues of local importance, and nationalconsistency in constituents measured, sampling approaches, and spatial and temporal resolution to allow for comparisons among study units. The occurrence and distribution assessment serves as a basis for designing field activities to evaluate long-term changes in water-quality conditions and studies of source, transport, fate, and effects.
Assessments of long-term trends and changes in selected water-quality characteristics will be designed based on the results of the retrospective analyses, reconnaissance, occurrence and distribution assessment, and the concurrent development of informat ion on the environmental framework. Temporal (for example, decadal) changes in the relations among physical, chemical, and biological factors will be interpreted in the context of changes in landscape features and human activities.
Source, transport, fate, and effects studies are conducted to test hypotheses and examine specific issues about characteristics and causes of water-quality degradation. These studies are targeted for high-priority water-quality issues for individual study units and the Nation. The accumulation of results from these studies among study units enables the linking of broad assessments of status and trends to specific causes and processes by example and inference. Source, transport, fate, and effects studies are designed by individual study units and conducted at a wide range of spatial and temporal scales.
The fish community sampling design incorporates existing information with estimates of the fish community at sites representing selected environmental settings. Collection of data on the presence and relative abundance of fish species is closely coupled with data collected on physical habitat, water chemistry, and benthic invertebrate and algal communities. The fish community sampling design is structured with respect to the analysis of retrospective data, type of sample collected, sampling reach, selection of sampling sites, and sampling season.
Retrospective data concerning fish species are especially important because of the abundance of existing information available concerning fish distributions. Collections of fish specimens and descriptions of their occurrence and distribution across North America have continued for more than 200 years (Heins and Matthews, 1987). Many specimens are available for examination in collections housed in more than 100 university and government museums throughout the United States and Canada (Collette and Lach ner, 1976). Detailed descriptions of fish species distributions have been compiled and summarized for the North American continent (Lee and others, 1980; Hocutt and Wiley, 1986) and many individual States (see references in Lee and others, 1980). Analysis of fish retrospective data allows project personnel to compile species lists, develop maps of species distributions, and identify areas within study units where little data are available.