Effects of wastewater discharges on periphyton growth in Lake Mead, Nevada-Arizona

A study of the effects of secondary-treated wastewater on periphyton growth in Lake Mead, Nevada-Arizona was conducted from September 1979 to December 1980. Periphyton ash-free dry weight, chlorophyll-a, dominant species composition, and alkaline phosphatase activity were measured on fiberglass substrates. Substrates were incubated for two to four weeks in littoral and limnetic habitats. Physical and chemical variables and phytoplankton chlorophyll-a were measured concurrently. Transparency increased with increasing distance from the discharge. Secchi depth ranged from 0.75 m at the discharge confluence (station 2) in August, to greater than 20 m at the most distant stations (stations 9 and 10) in spring. Ortho-phosphorus (PO4-P) and ammonia-nitrogen (NH3-N) concentrations also followed this trend. Average PO4-P concentrations were 37 ug/1 at station 2 and 1-2 ug/1 at stations 9 and 10. Ammonia-nitrogen concentrations averaged 84 ug/1 at station 2 and 3 ug/1 at the distant stations. Nitrite and nitrate-nitrogen concentrations were higher at the stations distant from the discharge because of the influence of the Colorado River inflow. Phytoplankton standing crop, estimated by chlorophyll-a, averaged 30 ug/1 at station 2 and reached a maximum concentration (105 ug/l) in summer. Phytoplankton standing crops decreased with increasing distance from the discharge. Periphyton production in Lake Mead followed the same spatial trend as PO4-P, NH3-N, and phytoplankton standing crop. The highest production was measured at station 2, near the discharge, and the lowest was measured at station 9. Light was found to be the primary factor and nitrogen the secondary factor controlling periphyton growth at station 2. Nutrient concentrations are reduced by phytoplankton uptake and dilution as the inflow moves toward Hoover Dam. Periphyton and phytoplankton production was reduced accordingly. Nitrogen was the most likely factor limiting periphyton growth at station 5. Nitrogen and phosphorus were alternately limiting to growth at station 8. With seasonal N:P ratios greater than 100, phosphorus was certainly a limiting growth factor at stations 9 and 10 in the upper lake basin. Phosphorus reduction by advanced wastewater treatment is being implemented to reduce phytoplankton productivity in Las Vegas Bay, Lake Mead. If light is limiting periphyton in Las Vegas Bay as evidence suggests, and there is shading of periphyton by phytoplankton, reduction in phytoplankton could result in an increase in periphyton