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I I I I I I I I I I I I I I I I I I I ppm on July 19 to a low of 0.8 ppm on August 4. The low concentration appeared to be only a transitory condition as water column invertebrate survival during this period was not significantly reduced (Cooper and Severn 1994b). The similarity in distribution patterns of the factors studied may be related to the agricultural practices on the terrace east of the wetlands. The area directly east of the main wetland does not have a center pivot sprinkler. However, center pivots are present on the terrace to the north and the south. Thus, salt and selenium distributions may related to the presence of center pivots and their irrigation patterns. CONCEPTUAL ECOLOGICAL MODEL Generally, the input of terrestrial organic matter provides the basis for energy in headwater streams (Vannote et ale 1980). Large rivers, such as those of the upper basin, depend upon fine particulate organic matter from upstream reaches for basic productivity (Vannote et ale 1980; Sedell and Richey 1989). Although detailed ecological studies of the various habitats in the upper basin have not been made, inferences can be drawn from the published literature. A. Relation of Nutrients. Sunlight Penetration. and Warm Water Temperatures to Phytoplankton Production. Phytoplankton productivity provides the basis for development of a food web. Phytoplankton production and standing crops increase in concert with increases in annual input of nutrients regardless of latitude. Carbon, nitrogen, and phosphorus are key elements for phytoplankton production. Phosphorus is perhaps most limiting in north temperate and subarctic climates (Schindler 1978). Nitrogen is the most abundant element in the atmosphere and is generally not limiting. Also, abundant carbon dioxide in the atmosphere provides the necessary carbon. Therefore, phytoplankton production and standing crop in north temperate freshwaters is generally proportional to the phosphorus input. Particulate phosphorus, either chemically desorbed or actively mobilized by microbiota, is not readily available' in rivers with a high sediment load because most of the phosphorus is bound to the sediments (Ellis and Stanford 1988). Watts and Lamarra (1983) determined that between 21% and 49% of the total phosphorus in Colorado River water at the bridge upstream from Moab, Utah in September and October 1978 was bioavailable with most of the extractable element in the form of calcium-bound phosphorus. Therefore, Watts and Lamarra (1983) concluded that algae production was not nutrient limited in this reach of the Colorado River but that primary production was inversely related to the turbidity of the riverine environment. 27