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University of Nevada, Las Vegas


Phytoplankton species composition and community size structure were studied in four warm-monomictic Colorado River reservoirs; lakes Powell, Mead, Mohave, and Havasu from March 1981 to February 1982. Sampling was done at approximately monthly intervals from several stations in each reservoir. The Utermohl technique was used to enumerate phytoplankton. The phytoplankton assemblage was divided into the following six size classes using microscopic techniques; netplankton (>64 um), and nannoplankton (>5, 5-11, 12-21, 22-44, and 45-64 um).

Total phytoplankton biomass and community size structure were different among these four reservoirs with considerable spatial and temporal variation present. Average reservoir-wide areal weighted biomass was similar in lakes Powell, Mohave, and Havasu (0.8-0.9 g/m3) while biomass in Lake Mead was lower (0.3 g/m3). Based on maximum and mean annual phytoplankton biomass, all four reservoirs are classified as oligotrophic.

Highest station biomass was measured near the inflows; the Colorado River at Hite (Lake Powell), Eldorado Canyon (Lake Mohave), and upper Lake Havasu; the San Juan River at Zahn Bay (Lake Powell); and Las Vegas Wash at Middle Las Vegas Bay (Lake Mead).

Phytoplankton size structure was similar in lakes Powell and Mead where netplankton (>64 um) was the main component, contributing 37 and 42 percent of the total annual biomass, respectively. The most common species in this size class were, Synedra ulna (Nitz.) Ehr., Fragilaria crotonensis (Edw.) Kitton, and Ceratium hirundinella (Mueller) Schrank. Nannoplankton were more common in lakes Mohave and Havasu where the 22-44 um size component made up 45 and 37 percent of total biomass, respectively. The most important species in this size class were Cryptomonas erosa Ehr., Peri dim'urn spp., and Anomoeoneis vitrea.

Biomass of cells <21 um were also important in lakes Mohave and>Havasu, contributing one third of total annual biomass. Several small flagellates were numerous in all four reservoirs. Rhodomonas minuta var. nannoplanctica Skuja, Katablepharis ovalis Skuja, and Chrysochromulina parva Lackey were observed in nearly every sample.

Nutrient levels were generally highest near the inflows, however, total phosphorus concentrations were low in all four reservoirs. Average values were 0.009 mg/1 in Lake Powell, 0.011 mg/1 in Lake Mead, and 0.012 mg/1 in lakes Mohave and Havasu. Ortho-phosphorus (P04-P) was extremely low at all locations. Average concentrations ranged from 0.003 to 0.004 mg/1 in each reservoir. Average total nitrogen concentrations were 0.429 mg/1 in Lake Powell, 0.364 mg/1 in Lake Mead, 0.346 mg/1 in Lake Mohave, and 0.337 mg/1 in Lake Havasu.

Physical characteristics are different among these reservoirs but most similar in Powell-Mead and Mohave-Havasu. The former two reservoirs are characterized by greater mean and maximum d depth, surface area, volume, and longer hydraulic rentention time. Nutrients, inflow-outflow and physical characteristics appear to be most important in regulating the phytoplankton biomass size structure and species composition in these reservoirs.


Aquatic biology; Aquatic ecology; Aquatic organisms; Freshwater phytoplankton; Lake Havasu (Ariz.); Lake Mead (Ariz. and Nev.); Lake Mohave (Ariz.); Lake Powell (Utah and Ariz.); Nannoplankton; Phytoplankton; Zooplankton


Biology | Environmental Health and Protection | Environmental Indicators and Impact Assessment | Environmental Monitoring | Natural Resources and Conservation | Sustainability | Terrestrial and Aquatic Ecology | Water Resource Management