Report for the Lake Powell National Recreation Area
Utah State University
chemical gradient, zooplankton, chlorophyll
From April 21 st to April 25th, 1995 students from Utah State University's FW 462 class sampled the trophic gradient in Lake Powell between Bullfrog (Mile 99) and Hite Marinas (Mile 144). We sampled at 3-12 stations along the gradient, depending on the parameter measured. The purpose of the trip was three-fold. First, the primary objective of the class was to provide field and laboratory experiences in aquatic ecology for the students. Secondly, we wished to provide data on the trophic environment that the endangered razorback sucker larvae from the Colorado River would encounter when the entered the reservoir. Thirdly, the limnological and fisheries data collected in this, and subsequent years, may provide a useful long-term data set for the parties responsible for managing Lake Powell. Single, or groups of students were responsible for sampling, analyzing, and writing up specific topic areas. During the investigation, runoff from the Colorado River was still low, and associated trophic enhancement was limited to the upper reaches of Lake Powell. Secchi depths were < 1 m between river mile (RM) 142 and 126, but increased to 12 m at mile 99 near Bullfrog Marina. Conductivity measurements indicated that a chemocline was present at 25 m near RM 92. Total phosphorus levels were near 140 ug/L at RM 144 close to the inflow, but decreased rapidly, and reached concentrations below our level of detection (2 ug/L) by RM 125. Chlorophyll a levels near the inflow were 4 ug/L, but decreased to < 1 ug/L near RM 99. A nutrient addition bioassay conducted on water collected near Bullfrog indicated that phosphorus was limiting algal growth. Crustacean zooplankton densities were < 2/L near the river inflow, peaked at over 20/L at RM 22, and then declined to <10/L by RM 99. Zooplankton biomass, however, was maximal at RM 92, and progressively declined up the reservoir. Daphnia and calanoid copepods dominated the zooplankton. Benthic invertebrate abundance in both the littoral and profundal zones was extremely low, with biomasses varying from 0.08 to 3.5 g dry weightlm2. At most stations oligochaets or chironomids dominated the fauna. Volatile organic residue in the profundal sediments was maximal (5.5%) at RM 99 and decreased near the Colorado River inflow. Hydroacoustic estimates of fish indicated a strong gradient in abundance. At RM 93 densities of small targets (2-5 cm) were only 0.9 x 10-6/m 3 but increased to 540 x 1 0-6/m3 at RM 136. Larger targets (6-187 cm) showed a similar gradient, with 4.9 x 10-6/m 3 at RM 93 and 180 x 10-6/m 3 at RM 136. Gillnet catches of fish did not show a distinct trend along the gradient, probably because of high variances between replicates.
Wurtsbaugh, Wayne A. and Steinhart, G., "The Trophic Gradient in Lake Powell" (1995). Watershed Sciences Faculty Publications. Paper 522.