ASSESSING GROUNDWATER INPUTS, OUTPUTS, AND THROUGHFLOW IN LAKES USING DIVERSITY INDICES AND BIOGEOCHEMISTRY OF OSTRACODES

A vexing issue in interpreting paleohydrological lacustrine sediment records is assessment of groundwater contributions as well as groundwater leakage. For many lakes under seasonally variable effective moisture regimes, groundwater flux is vital in order to maintain “healthy” water chemistry. For paleolimnologists who study ostracodes, groundwater flux is important for maintaining a chemistry that is favorable for valve preservation, as well as contributing to the biogenic sediment load which ultimately provides the bulk of the record. Another important (related) influence is water residence time. Lake sediment records with lacuna and turbated intervals may arise from little groundwater input and/or short water residence. Increasing groundwater inputs and water residence ostensibly results in better-preserved sediment records that potentially archive climatic information, other variables being equal (such as bathymetry, hydraulic closure, watershed processes, etc.). The other side of the spectrum is near dominant groundwater input to the lake’s moisture budget together with long residence time; such conditions dampen the effects of climatic variability, resulting in monotonous sediment and biogeochemical records.

Crystal Lake, Illinois, has the only lake record in NE Illinois to date that has yielded a continuous record of ostracodes from the Oldest Dryas (about 16,000 cal yr BP) to the present. A working hypothesis of the Crystal Lake record (discussed with Rick Forester in several lengthy, insightful correspondences) was identifying intervals in which groundwater was important (resulting in relatively low Shannon-Wiener diversity indices and relatively negative δ¹⁸O and δ¹³C values) and when groundwater input was less influential (high SW, positive stable isotope values). Once these issues were resolved, then the question arose: which mode is due to greater effective moisture or less? In the case of modern Crystal Lake and its ca. 6-month water residence time, the geohydrology dictates positive correspondence of effective moisture with groundwater input and throughflow. Species ratios also serve to highlight certain relationships. For Crystal Lake, low values of Candona ohioensis vs. Limnocythere varia suggest greater distance from shore.