Past and Current Periphyton Monitoring in Lake Tahoe Scott H. Hackley, Brant C. Allen and John E. Reuter Tahoe Research Group, University of California, Davis      Among the first visible evidence of the changing water quality of Lake Tahoe, was the increased amount of attached algae (called periphyton) observed along the shoreline in the 1960Ős. While observations by C.R. Goldman in the late 1950Ős indicated that rocks along shore showed only slight amounts of periphyton growth, in the spring of 1967 significant periphyton growth along shore was apparent. This increase coincided with the period of rapid growth and development in the basin during the 1960's and could be attributed to an increased nutrient loading from the surrounding watershed via stream and ground waters. The Tahoe Research Group has studied many aspects related to the biology and distribution of periphyton in Lake Tahoe since the 1970Ős. These studies included: characterization of species in the nearshore; assessment of physical, chemical and biological factors affecting growth; assessment of importance of nitrogen fixation and nitrogen metabolism in the periphyton; and seasonal and spatial variations in growth patterns. Periphyton growth has been monitored at consistent sites during several of these studies (various years during 1978-92) providing an important body of data for comparative purposes.      In March of 2000, the Tahoe Research Group reinitiated regular monitoring of periphyton abundance around the lake for the Lake Tahoe Interagency Monitoring Program (LTIMP). As part of this program, the TRG is monitoring algae growth at ten stations (five each in CA and Nev.), nine of which have historical data on periphyton amounts. Samples of natural periphyton are collected directly from rocks at 0.5-meter depths, approximately monthly during the peak growth season (Jan.- June) and less frequently July-Dec. Chlorophyll a content of the samples is measured in the lab to estimate the amount of living algae (biomass) present. The figure below shows results for peak periphyton biomass measured during March - June 2000 at each of the sites. The data for this period show that peak periphyton biomass was highest for the northwest monitoring stations (PI= Pineland, TC= Tahoe City, DL= Dollar Pt.). Ongoing monitoring, together with historical data on periphyton growth hopefully will allow discernment of growth trends for this algal community which is particularly apparent to a largely shorebound populace. Since periphyton is (1) a good indicator of site-specific nutrient input, (2) a signature of ongoing cultural eutrophication, and (3) interferes with the beneficial uses of the lake, we have suggested that an Environmental Threshold for this attached algae be considered.