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Mean concentrations of POM in the tailwater of Lake Havasu (Parker Dam) was 0.89 g/m3, but this decreased to 0.75 g/ms at Headgate Rock Dam, about 30 kilometers downstream. Whether this was due to filtering and processing by benthic inverte- brates (i.e., they ingested the material) or some other factor was not determined. Phytoplankton species collected below Lake Mohave and Lake Havasu reflected the lentic conditions of the reservoirs. Investigators have found the return to lotic conditions below the dam tends to be inhospitable to reservoir- produced algae that are not suited to riverine conditions (Stober 1963 [38]; Soballe and Bachmann 1984 [391). In this study, of the lower Colorado River, zooplankton from reservoirs did not seem to survive a great distance downstream. Petts (1984 [3]) found the number of drifting zooplankton decreased rapidly downstream from the lake outlet caused by the filtering effect of bottom vegetation. Other investi- gators (Ward, 1975 [30]; Armitage, 1977 [32]; Sandlund, 1982 [40]; Herlong and Mallin, 1985 [41 ]) have reported a rapid decrease in zooplankton densities and a change in composition and size from large to small zooplankton in impounded rivers within 7 kilometers of a dam. The POM decreased between Parker and Headgate Rock Dams and began to increase 60 km downstream from Headgate Rock Dam. During phase 1, POM increased by: of the nutrient (Newbold et al., 1983 [43]). In the lower Colorado River phosphorus was often unde- tected, and when detected showed no upstream to downstream trend. Total phosphorus (TP) includes various soluble and insoluble, organic and inorganic phosphorus forms (U.S. Geological Survey, 1977 [441). While orthophosphorus (PO4-P) is commonly measured because it is immediately available for algal uptake and growth (Cole 1979 [45]), Lambou et al. (1976 [23]) points out that due to the "... high mobility and short turnover times of phosphorus within the general 'phosphorus pool,' TP is often a good approximation of bioavailable phosphorus." Because phosphorus often occurs in a particulate form, being absorbed onto soil particles or chemically bound to iron (Buckman and Brady, 1969 [461), it is susceptible to increased loading with increased stream flow. In the Colorado River system, the retention of sediment by reservoirs has been blamed for phosphorus-limited conditions and nutrient limitation in both reservoirs and the river (Gloss et al., 1981 [47]). However, during this study, nutrient levels observed along the main channel stations were similar to the levels in tailwaters. For example, TP ranged from 0.007 to 0.014 mg/L at all stations except the Palo Verde Irrigation Drain. These levels are similar to annual mean values for TP in upper Colorado River reservoirs reported by Paulson and Baker (1984 [48]). In 1981, they found mean TP levels, mg/L, to be: • 32 percent from 0.75 g/m3 at Headgate Rock Dam - Lake Powell .... 0.008 to 0.99 g/m3 at Palo Verde Diversion Dam - Lake Mead ...... .008 • 90 percent to 1.88 g/m3 at Cibola - Lake Mohave ... .012 • 14 percent to 2.15 g/m3 at Imperial Dam - Lake Havasu .... .012 Through these reaches POM were dominated by: • Detritus and diatoms in the fine size class (<25 µm) • Green algae in the large size class (>25 µm) • Macrophyte fragments in the coarse size-fraction (>505 µm). Upstream, POM was autochthonously produced due to the stream reaches bordered by Lake Havasu and the fact that POM composition from the reservoir outflow was different than the downstream POM composition. Possible sources for the increased POM downstream are: • In-stream production by autotrophs • Inputs of material from backwater marshes • Inputs from agricultural drains • Inputs from streambed and streambank erosion Generally, nutrient levels increase along a river continuum (Cummins, 1974 [42]) and nutrient cycling is directly related to downstream transport On a broader scale, Plante and Downing (1989 [49]) reported a TP median of 0.013 mg/L for 16 lakes having a wide range in geomorphology across the globe. As stated, the only significant increase in nutrient levels occurred at the Palo Verde Irrigation Drain which reflects fertilizer use. Regarding the origin of the increased POM in river reaches downstream from Lake Havasu, the nutrient levels in the main channel may be low, but not lower than in the upstream reservoirs and certainly high enough to allow some in-channel POM production. It does not appear that in-channel production by autotrophs accounts for the majority of POM increase. This conclusion is supported by results of chlorophyll a analyses that showed no correlation between the <25-µm POM and chlorophyll-indicating autotro- phic production is not the primary source of this organic material. Within these reaches, other possible sources of organic material include off- channel contributions from either backwater marshes or agricultural drains or streambed and streambank erosion. 25