Abstract. As a result of the drought that began in 2000 and reservoir operations that provided a constant water supply to the lower Colorado River basin states, Lake Powell is currently at only 46% capacity. Under these conditions, the river inflows cut through sediments previously deposited in the Colorado River Delta. We hypothesize that the consequent resuspension of sediments may result in enhanced phosphorus release and metal redox cycling. In recent years, it has been observed that anoxia develops in bottom water intersecting the delta sediments, which could also result in P release due to reductive dissolution of iron oxide carrier phases in the sediments. Primary production in Lake Powell is phosphorus limited and changes in phosphorus availability could have significant ecological consequences. Suspended sediment transported by the Colorado River is the principal source of phosphorus to Lake Powell; much of this sediment load is deposited as the Colorado River flows in to the lake (i.e., the river-lake boundary). Thus previously-deposited delta sediments represent a substantial reservoir of phosphorus. Intellectual Merit. The proposed project will examine the coupling of biogeochemical and physical processes and their impacts on phosphorus and trace metal dynamics. Speciation of phosphorus in solid phases (specifically the suspended sediment and delta sediments) will be determined and related to the conditions under which phosphorus may be released from the sediments. The role of reductive dissolution of metal (e.g., iron) oxides in phosphorus release will also be examined. Broader Impact. Phosphorus dynamics exert a key control on primary production in Lake Powell. We anticipate that our results will show whether phosphorus and trace metal dynamics are altered under the current conditions where Colorado Rived inflows cut through previously-deposited delta sediments. If phosphorus dynamics are affected, patterns of primary production and the development of bottom water anoxia during the recovery from drought conditions may be significantly different from historical patterns. This SGER project will also support the graduate training of Mr. Richard Wildman, a third year graduate student in Environmental Science & Engineering at Caltech. In addition to his academic studies, Mr. Wildman has been actively engaged in outreach and curriculum development at the Environmental Charter High School in Lawndale, CA, a public school located in an underserved area of Los Angeles County. SGER Justification. Because of the current low lake level, there is a unique opportunity to examine the effects of sediment reworking on nutrient dynamics, specifically phosphorus release from sediments. Currently, previously-deposited sediments are exposed to river inflows. As the lake levels rise, these sediments will be resubmerged. Since the extreme drawdown in the 2000- 2004 drought and the above-average inflows of the 2005 run-off are creating unprecedented physical conditions in the sediment delta, we are requesting SGER funding to examine the phosphorus and trace metal dynamics in this changing system. This work is also timed to coincide with planned field sampling by the Bureau of Reclamation, which will provide logistical support for sampling.
