It's All in the Mud - the Use of Sediment Geochemistry to Estimate Contemporary Water Quality in Lakes

Lake ecosystems around the world are subject to multiple anthropogenic pressures leading to increased eutrophication and degraded ecosystems. The understanding of lake health at national scales is constrained by limited data, however, the increased long-term monitoring required to address such data deficiencies would be impractical, expensive and subject to significant time lags. More efficient methods of assessing contemporary water quality are needed. Lake sediments and the flux of constituents across the sediment-water interface in response to various biogeochemical cycles can result in the surficial sediments providing a time-integrated record of environmental conditions in the lake. As such, sediment geochemistry may offer a valuable and efficient indicator of contemporary lake water quality. To assess the potential of sediment indicators of water quality, geochemistry was analysed in surficial sediment samples (0-2 cm) collected from 101 lakes across New Zealand for which long-term water quality monitoring data was available. The selected lakes spanned various gradients, including lake type, trophic state, depth, latitude, altitude, catchment land use and sediment geochemistry. Linear modelling was undertaken to predict trophic state from sediment geochemistry and lake physiographic data separately and then from the two datasets combined. The combined model proved to be the strongest predictor of contemporary water quality (R-2 = 0.80) and is referred to here as the Sediment Geochemistry Trophic Model (SGTM). This model was then used to predict the trophic level in 76 unmonitored lakes. The sediment geochemistry analyses conducted for the development of the SGTM are routinely conducted for lake management projects and, aside from their predictive power for trophic levels, provide useful insights into nutrient cycling and lake restoration planning. The relationship between lake water quality and sediment geochemistry is complex, however, the SGTM offers a highly efficient method for assessing the state and drivers of contemporary water quality in unmonitored lakes.