Limnological monitoring on the UMRS, 1993 1996: LTRMP Onalaska Field Station Soballe, D. M., J. R. Fischer, L. A. Hodge-Richardson, and T. L. Clemment. 2002. Limnological monitoring on the Upper Mississippi River System, 1993 1996: Long Term Resource Monitoring Program Onalaska Field Station. U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin, October 2002. LTRMP 2002-P003. 20 pp. + Appendixes A-F (NTIS# PB2003-101578) ABSTRACT Since 1988, the Long Term Resource Monitoring Program (LTRMP) staff have performed basic limnological field measurements in the Upper Mississippi River System. The period of this report (1993 96) includes a major revision of the LTRMP sampling design in 1993 that added randomization, broader spatial coverage, and increased monitoring of tributaries and locations that allow monitoring of material transport. Personnel from the LTRMP Onalaska (Wisconsin) Field Station made about 1,300 visits to fixed limnological sampling sites and 2,100 visits to stratified random sites in the vicinity of Navigation Pool 8 from 1993 through 1996. An extended, late-season flood in summer 1993 coincided with some of the highest nutrient and lowest dissolved oxygen (DO) concentrations recorded during the report period. Decreases in nutrient concentrations (particularly nitrogen) and increases in water clarity occurred over the 4-year period. Midday DO concentrations were above 5 mg/L throughout most of the year and across most of Pool 8, with localized exceptions during periods of snow cover. Pool 8 and its watershed are a net source of sediment to the Mississippi River; main channel suspended solids concentrations were greater downstream than upstream, and concentrations in the tributaries were typically higher than in the main channel. Strong seasonal patterns were evident for temperature, DO, Secchi transparency, turbidity, total and volatile suspended solids, silicate, chlorophyll a, and total phosphorus. Spatial variability (among strata) of most measurements generally increased during lower discharge. Within the individual strata, the pattern of variability could be generalized as follows: isolated backwater > contiguous backwater > impounded > side channel > main channel. KEYWORDS Annual report, limnology, LTRMP, Mississippi River, water quality