SEDIMENTARY AND HISTORICAL CONTEXT OF EUTROPHICATION AND REMEDIATION AS RECORDED IN THE SEDIMENTS OF URBAN LAKE MCCARRONS (ROSEVILLE, MINNESOTA)

Geochemical analysis of the sediments of a small urban lake in east-central Minnesota shows that most water-quality and watershed indicators are approaching prehistoric (pre-1850s) values after an excursion to anomalous levels during the 1920s through 1950s. This information provides historical perspective to lake managers planning remedial measures with reference to the “natural” state of the lake, especially in regard to the perception that water quality has declined only in recent decades (i.e., in the latest 20th century). The Lake McCarrons shoreline was settled by dairy farmers in the 1850s, was the site of a popular public bathing beach beginning in the 1920s, and has become progressively more densely packed with residential and commercial development up to the present day. The lake is now eutrophic and oligomictic, with oxygen-depleted bottom waters seasonally enriched in dissolved solids relative to surface waters. Lake sediments are annually laminated (varved) for the past ~90 years, providing an internal chronology for the study and permiting sampling and analysis of annual to subannual layers. The sedimentary record of Lake McCarrons shows a strong response to agricultural and urban development in the basin, and only a minor response to remediation efforts in the mid-1980s to 1990s. Cyclic trends in oxygen stable-isotopic ratios are coherent with the instrumental record of aridity in the region; significant hydrological modification of the lake is indicated by a shift from high covariance to non-correlation between δ¹⁸O and δ¹³C in carbonates beginning at the time of initial European impact. Indices of terrestrial inputs and algal productivity clearly show evidence of a lake that was at its most heavily impacted during the mid-20th century, and which has progressively improved since the 1960s. The timing of this improvement is consistent with the implementation of residential sewage treatment and other pollution controls in the metropolitan area.