2016 Salt Lake County Watershed Symposium

Utah Lake Eutrophication: Role of Sediment-Water Column Interactions
Utah Lake is the largest natural freshwater lake in the western United States in terms of surface area with a maximum length and width of 24 and 13 miles, and has a surface area of roughly 375 km2 (145 square miles). It is a shallow lake with an average depth of approximately 9-10 feet during normal reservoir operating conditions with a storage capacity of 902,400 acre feet (Psomas & SWCA, 2007). The lake is managed as a reservoir to have a minimum surface elevation of 4,489 feet above sea level to maintain a consistent shoreline.

Utah Lake is considered hypereutrophic in terms of trophic status and experiences extreme algal blooms in the late summer and fall throughout the lake (Psomas & SWCA, 2007). Low dissolved oxygen, or anoxic, events have not been observed in Utah Lake and this is attributed to the shallow lake being well mixed and wind induced reaeration. There is an increasing concern about the role of nutrients, especially phosphorus, in contributing to Utah Lake water quality. This has led to a lots of debate about sources and sinks of phosphorus in Utah Lake among the local community. Phosphorus is a tricky nutrient whose fate is primarily pH dependent. In this EPA and UDWQ funded research, we closely looked at P in the sediments and looked at its speciation, sediment minerology and potential of release under changed DO and pH conditions.

More recently, we have collected five more sediment cores from the Utah Lake and are in the process of evaluating parameters which will be useful for Utah Lake modeling efforts let by the University of Utah and Utah DWQ. Water column samples collected show the correlation between nutrients, chlorophyll a, and cyanotoxin with water quality and lake bottom sediments. Majority of phosphorus in Utah Lake sediments is calcite bound which can only be immobilized under low pH conditions. We also developed an in-house toxin measurement method based on high performance liquid chromatography. The samples collected and analyzed did not show toxin concentrations above world health organization’s prescribed limits. The sediments were a source of dissolved N and P while the water column was a sink, and once again this was associated with plankton bioassimilation. The sediments in Utah Lake proper were similar in terms of %TS and roughly 37% of the VS were organic carbon, and are composed primarily of carbonate and clay minerals which are easily resuspended. The high carbonate content, elevated ambient pH, and alkalinity provide a pathway for abiotic P precipitation. In-situ sediment oxygen demand measurements revealed that the Utah Lake water column is responsible for the majority of the ambient oxygen demand, not SOD, and was associated with phytoplankton respiration and decay.