John C. Holz

Effects of Phosphorus Reduction on Water Quality: Comparison of Alum-Treated and Untreated Portions of a Hypereutrophic Lake

ABSTRACT The effects of a reduction in total phosphorus concentration on the water quality and plankton community structure in a 86-ha hypereutrophic sandpit lake with high internal phosphorus loading were assessed by dosing an isolated 4.6-ha section of the lake with 34,065 L (dose = 10 mg Al·L−1) of liquid aluminum sulfate (alum). During the three summers following treatment, hypolimnetic total dissolved phosphorus, epilimnetic total phosphorus, and eplimnetic total nitrogen were decreased by 97%, 74%, and 36%, respectively, in the treated section. Secchi depth was 134% greater in the treated area. Alum treatment also increased the volume of usable fish habitat by 22%, as the depth of the 3.0-mg/l dissolved oxygen isocline was 52% deeper in the treated portion than in the untreated portion. Total phytoplankton biovolume decreased by 40% and chl a concentration by 65% in the treated area. Although cyanophytes continued to dominate in die treated area, there was a shift in relative abundance from cyanophy...

Phytoplankton community response to reservoir aging, 1968–92

The effects of reservoir aging on the phytoplankton community of amidwestern U.S. reservoir constructed in 1965 (Pawnee Reservoir) werestudied by comparing algal biovolume and species composition from April 1992through November 1992 to surveys conducted in 1968–73 and 1990. Meansummer total phosphorus, nitrate-nitrogen, Secchi disk depth, totalsuspended solids, chlorophyll a, and phytoplankton species composition datacharacterized Pawnee Reservoir during 1968–69 as a high nutrient,relatively clear water environment. Phytoplankton biomass was relativelylow, consisting mainly of cyanophytes and non-flagellated chlorophytes.During 1970–73, water clarity was poor, total suspended solids werehigh, and total phosphorus was lower, but was still greater than 100 µgl−1. The 1970–73 phytoplankton biomass was high and wasdominated by cyanophytes. Mean summer total phosphorus remained >100µg l−1, water clarity remained poor, but phytoplanktonbiomass decreased significantly during 1990–92. The dramatic drop inchlorophyll a and low mean volatile suspended solids indicated thatinorganic suspended sediments, rather than phytoplankton, accounted for themajority of the turbidity in 1990-92. In addition to lower phytoplanktonbiomass, community composition shifted away from buoyancy-regulatingcyanophytes toward flagellated chlorophytes. These data suggest that asreservoirs located in agricultural watersheds age, (1) inorganic suspendedsediments have a significant effect on the light environment as well asphytoplankton biomass and species composition, (2) the control ofphytoplankton biomass and species composition shifts away from nutrients tolight and suspended sediments, and (3) there is a 1–2 year lag in theresponse of phytoplankton biomass to maximum nutrient loading during thetrophic upsurge period. Thus, sedimentation has been shown to be a primarydeterminant of plankton and benthic macroinvertebrate community compositionas Pawnee Reservoir aged.

Evaluating climatic and non-climatic influences on ion chemistry in natural and man-made lakes of Nebraska, USA

Conductivity and major ion chemistry data were analyzed for a suite of Nebraska (USA) natural lakes, reservoirs, sand pits, and barrow pits to evaluate the magnitude of climatic versus non-climatic influence on ionic concentration and composition. In both natural lakes and sand and barrow pits, conductivity is positively related to longitude and reflects decreasing effective moisture from east to west. Reservoirs showed no relationship between lake conductivity and location, probably because the reservoirs are very strongly influenced by groundwater and surface water inflow and have shorter residence times relative to the other lake types. At smaller spatial scales, conductivity among natural lakes is variable. Lakes that are at low elevation within a groundwater flow system were fresh, because of substantial input of fresh groundwater. In contrast, lakes at high elevation exhibited a wide range of conductivity, probably because of differences in the degree of connection to groundwater and surface to volume ratio impacts on evaporation rates. Differences also were evident among natural lakes in terms of their response to seasonal changes in precipitation. Sub-saline and saline lakes showed more seasonal variation in conductivity than freshwater lakes, and lakes in the more arid part of the state showed larger responses to precipitation change than those in areas to the east that receive higher precipitation.

ADCP Water Velocity Measurements in a Hydropower Tailwater Impoundment: A Case Study

An acoustic Doppler current profiler (ACDP) was used to characterize water velocities and flow patterns in a hydropower tailwater impoundment in western Nebraska. The impoundment, a 100 hectare L-shaped lake formed by a diversion dam located 3000 meters downstream of the hydropower dam, regulates flow into a supply canal. A trout fishery in the tailwater lake has recently experienced stresses, likely caused by some combination of low dissolved oxygen and/or toxicity (e.g., from ammonia or hydrogen sulfide). In addition, macrophytes attached to the lake bed in shallow areas may be exacerbating the stresses by restricting water flow and causing “stagnant” areas. Finally the mode of operation of the hydropower facility may contribute to the fishery stress by discharging low-DO water in the evening when macrophyte respiration is further reducing DO. The hydropower facility operation regime, in conjunction with fairly constant outflows through the supply canal, causes the water level in the tailwater lake to fluctuate more than one meter per day, resulting in complex flows within the lake. To help characterize water quality dynamics (e.g., dissolved oxygen, toxic constituents) in the tailwater lake, and to support physical and computer models of the lake, water velocities were measured throughout the tailwater lake using an ADCP. The ADCP was mounted on the bow of an outboard boat. Transects were made with the boat at numerous locations on the lake. Transects were made during both filling and draining cycles and were designed to capture the effects of the hydropower operation, the lake bathymetry, and the macrophytes on flow patterns. Measured velocities ranged from greater than 4 m/sec to less than 0.05 m/sec. Overall, the ADCP worked well for collecting the water velocity data needed for this study. Difficulties identified with the use of the ADCP in this study include loss of velocity and bottom-tracking data in areas with dense macrophytes, poor ADCP performance on transects that had drastically varying depths, and difficulty in collecting near-shore velocity data. The physical and computer models make use of the ADCP data to characterize circulation patterns in the lake and to predict concentrations of dissolved oxygen (and other constituents) under various remedial and operational scenarios for the lake.