Amy E. Myrbo (Research Assistant), Limnological Research Center, Department of Geology and Geophysics, University of Minnesota,  has allowed us to put this presentation on our website.  This presentation has yet to be published,  if you would like to copy or reproduce this presentation in any way please contact Amy Myrbo at amyrbo@umn.udu or (612) 626-7889.

 

Executive Summary 

Lakes in urban settings receive major inputs of nutrients and salt from the lawns and roads that surround them.  Increased biological productivity and salinization of lake bottom waters cause degradation of water quality in these valuable municipal resources.  Monitoring efforts provide water quality information with high temporal resolution; however, due to logistical and budgetary constraints, monitoring rarely captures more than a few years’ data; hence, these snapshots may not accurately represent the high interannual variability of physical and chemical parameters in lakes.

This study uses sediment cores and certain water chemistry analyses to generate a high-resolution record of historic and pre-European changes in the chemistry and productivity of Lake McCarrons, Roseville, Minnesota, with the goal of placing recent (1985-2001) monitoring data in a long-term context.  Geochemical and isotopic parameters recorded in lake sedimentary components over time are used as proxies for processes and changes in the water column; a minimal number of water analyses are used to calibrate the system with respect to the isotopic proxies and to answer specific questions germane to the sediment study.  

Results to date indicate that Lake McCarrons has undergone cycles of increased biological productivity in this century, and that productivity is as high now as it has been at any time during this period.  Sediment data also suggest that the degree of bottom-water anoxia has increased over the past 40 years; this observation is further supported by water chemistry data indicating that the lake does not fully mix at fall and spring overturn, behavior which would feed back and support the development of greater anoxia.  Sedimentation rates increase dramatically in the early 20^th century as land-use changes add detrital material and nutrients to the lake; since about 1970, sedimentation rates have stabilized although eutrophication has continued.  There is a strong relationship in the sedimentary record between moisture balance, lake stratification, and indices of biological productivity; the record of changes in the 20^th century also demonstrate much higher variability than seen in the pre-human impact period.  We hope that this information is useful to lake managers making decisions on remediatory measures with reference to the “natural” state of the lake.

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