2006-10-26_AgendaPacket - Laserfiche WebLink

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~- --- _ _ - ~=_~L~,~ IJ~u LI. ~L ~ _ _-I October 24, 2006 Barr Engineering Company 4700 West 77th Street • Minneapolis, MN 55435-4803 Phone: 952-832-2600 • Fax: 952-832-2601 • www. barr.com An EEO Employer Minneapolis, MN • Hibbing, MN • Duluth, MN • Ann Arbor, MI • Jefferson City, MO Board of Managers Grass Lake Watershed Management Organization c/o Mark J. Maloney, P.E. Public Works Directory/City Engineer City of Shoreview 4600 Victoria Street North Shoreview, MN 55126 Re: Lake Owasso Water Quality Management Study Attached to this letter are Barr Engineering Co.'s proposed Work Scope and Cost Estimate for conducting aDiagnostic-Feasibility Study of Lake Owasso Water Quality Problems and Alternative Remedial Measures. These documents have been updated since your last meeting to respond to Managers' requests for additional investigations into the importance of seasonally anoxic lake sediments and reduced zooplankton. grazing of algae on lake water quality conditions. The need to wldertake this project can clearly be seen in plots of data for the three primary lake water quality parameters-summer-average total phosphorus and chlorophyll a concentrations, and corresponding Secchi disc transparencies. All three parameters exhibit a distinct degrading trend over the past 8 years. This trend is especially worrisome since it comes after an 8-year period of improving water quality that followed implementation of watershed runoff best management practices recommended in a 1991 Lake Management Alternative Report. The work we are proposing to complete will focus on determining the causes of the recently observed degrading water quality and on prescribing feasible remedial measures to restore good water quality. One theory about the cause of recently degraded Lake Owasso water quality relates to reduced grazing of algae by zooplankton due to their loss of refuge from predation in aquatic plant beds. (An increase in the numbers of planktivorous fsh could also be to blame.) To assist us in investigating this possibility, Barr Engineering has secured agreement with Dr. Joseph Shapiro, Emeritus Professor of Limnology-University of Minnesota, that he will assess available data on community characteristics (species diversity and abundance) of plankton, fish, and aquatic plants in Lake Owasso, and make recommendations about feasible remedial management measures. Another theory advanced to explain the recent decline in the Owasso water quality relates to increased recycle of phosphorus from seasonally anoxic lake sediments into overlying lake waters during late-summer periods, when thermal stratification of the water column begins to break down. To investigate this possibility, our revised work scope includes collection and analysis of sediment core samples for concentrations of potentially mobile phosphorus, which may be released into the lake water. The foregoing two theories about the cause(s) of recently degraded lake water quality presume that increased external loads of phosphorus to Lake Owasso have not increased as a result of the very few watershed land use changes that have occurred since 1990. This presumption will be either confirmed ODMA\PCDOCS\DOCS\247725\1