Recent increases in sediment and nutrient accumulation in Bear Lake, Utah/Idaho, USA Joseph M. Smoak 1, * & Peter W. Swarzenski 2 1 Environmental Science, University of South Florida, 140 7th Avenue South, Davis Hall 258, St. Petersburg, FL 33701, U.S.A. 2 US Geological Survey, 600 Fourth Street South, St. Petersburg, FL 33701, U.S.A. (*Author for correspondence: Tel.: +1-727-553-4078, Fax: +1-727-553-4526, E-mail: smoak@stpt.usf.edu) Received 15 October 2003; in revised form 28 January 2004; accepted 9 February 2004 Key words: paleolimnology, sediments, 210 Pb dating, geochemistry, Utah, Idaho Abstract This study examines historical changes in sediment and nutrient accumulation rates in Bear Lake along the northeastern Utah/Idaho border, USA. Two sediment cores were dated by measuring excess 210 Pb activities and applying the constant rate of supply (CRS) dating model. Historical rates of bulk sediment accumu- lation were calculated based on the ages within the sediment cores. Bulk sediment accumulation rates increased throughout the last 100 years. According to the CRS model, bulk sediment accumulation rates were <25 mg cm )2 year )1 prior to 1935. Between 1935 and 1980, bulk sediment accumulation rates in- creased to approximately 40 mg cm )2 year )1 . This increase in sediment accumulation probably resulted from the re-connection of Bear River to Bear Lake. Bulk sediment accumulation rates accelerated again after 1980. Accumulation rates of total phosphorus (TP), total nitrogen (TN), total inorganic carbon (TIC), and total organic carbon (TOC) were calculated by multiplying bulk sediment accumulation rates times the concentrations of these nutrients in the sediment. Accumulation rates of TP, TN, TIC, and TOC increased as a consequence of increased bulk sediment accumulation rates after the re-connection of Bear River with Bear Lake. Introduction Human activities can accelerate sediment and nutrient loading to lakes and rivers. Alterations in sediment and nutrient loading can result from anthropogenic disturbances such as urban devel- opment, road and railroad construction, oil and gas exploration, agriculture, and hydrologic changes. Limnological data rarely document the full effect of human activities on water quality because they are generally collected over a time span that is too short to record pre-disturbance conditions. The sediments of a lake, however can preserve the environmental history of a drainage basin, and provide valuable information about the lake’s response to external influences (O’Sullivan, 1979). The paleolimnologic record can document pre-disturbance conditions and the influence of external changes, and can be useful for making predictions about how future external changes might alter the lake (Brenner et al., 1993). Establishing a geochronology is essential to examining the timing of changes recorded in sed- iment records. Because the past 100 years is the period of interest for this study, 210 Pb was selected as the most appropriate tracer for establishing geochronology. Krishnaswami et al. (1971) first applied 210 Pb to sediment chronologies in lakes, and the technique has been used widely in paleo- limnology to determine recent (i.e., approximately Hydrobiologia 525: 175–184, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 175