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1 ' 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 crops 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. t The rivers of the upper Colorado River basin are turbid and contain large expanses of sand substrate. Production of phytoplankton and zooplankton that form the basis for a food pyramid are extremely low in these rivers (Grabowski and Hiebert 1989; Cooper and Severn 1994 a, b, c, and d; Mabey and Shiozawa 1993). High turbidity obstructs the penetration of sunlight that is needed for phytoplankton production. Backwaters and embayments along the main river ' channels and flooded bottomlands in off-channel areas provide favorable conditions for phytoplankton production. Sediments deposited in these areas where the water velocity is decreased provide nutrients and sunlight penetrates the clearer water that allows phytoplankton to flourish as primary producers and t to stimulate production of the food chain. Low velocity off-channel habitats become warmer than the riverine environment in the upper basin (Kaeding and Osmundson 1988; Osmundson and Kaeding 1989). The combination of nutrients, ' sunlight penetration of the water column, and warmer water temperatures in low velocity habitats provide the best conditions for phytoplankton production in the upper basin. ' Importance of Low Velocity Habitats to Zooplankton Production. The importance of low velocity habitats to the production of zooplankton for fish in riverine environments has not been documented very well. Most riverine studies ' have concentrated on macroinvertebrates occurring in the drift (Waters 1969). Mabey and Shiozawa (1993) reported that the most comprehensive studies have been made of the plankton communities in the Orinoco River, Venezuela. Mean densities ' of cladocerans and copepods (the most abundant taxa) were 421 organisms per liter in the Laguna la Orsinera. Welcomme (1989) summarized zooplankton densities in floodplains in a range of 270 to 10,000 organisms per liter. Mabey and Shiozawa (1993) documented zooplankton densities in the middle Green River as 0.3 to 1.3 ' organisms per liter, 1.5 to 7.1 in the Ouray backwater, 63.4 at Intersection Wash (another backwater), and 206 to 690 in Old Charlie Wash (Woods Bottom) on the Ouray National Wildlife Refuge, located downstream from Vernal, Utah. Grabowski and Hiebert (1989) reported 0 to 20 planktonic crustaceans (cladocerans and copepods) per liter in the middle Green River channel and 0.02 to 17 organisms per liter in backwaters in 1987 and 1988. In an open water bottomland habitat ' of the Moab Slough on the Colorado River near Moab, Utah, the density of planktonic crustaceans (cladocerans and copepods) averaged about 36 organisms per liter in the summer of 1993 (Cooper and Severn 1994a). Cooper and Severn reported ' 11 1