Spatial and Temporal Trends in Lake Erie Hypoxia, 1987-2007
Y. Zhou, D.R. Obenour, D. Scavia, T.H. Johengen and A.M. Michalak
We develop novel methods that are able to provide a solid basis for determining the most important factors for characterizing the variability of bottom water dissolved oxygen (DO) concentrations in Lake Erie. We use this understanding to quantify DO concentrations and to derive probabilistic estimates of hypoxic extent (area with low DO) for August and September of 1987 to 2007, with the ultimate goal of reducing hypoxia and improving water quality. This work provides a set of geostatistical tools that are easily transferable to other regions, with ongoing work applying these approaches in the Chesapeake Bay and the Gulf of Mexico.
Hypoxic conditions, defined as dissolved oxygen (DO) concentrations below 2 mg/l, are a regular summertime occurrence in Lake Erie, but the spatial extent has been poorly understood due to sparse sampling. We use geostatistical kriging and conditional realizations to provide quantitative estimates of the extent of hypoxia in the central basin of Lake Erie for August and September of 1987 to 2007, along with their associated uncertainties. The applied geostatistical approach combines the limited in situ DO measurements with auxiliary data selected using the Bayesian Information Criterion. Bathymetry and longitude are found to be highly significant in explaining the spatial distribution of DO, while satellite observations of sea surface temperature and satellite chlorophyll are not. The hypoxic extent was generally lowest in the mid-1990s, with the late 1980s (1987, 1988) and the 2000s (2003, 2005) experiencing the largest hypoxic zones. A simple exponential relationship based on the squared average measured bottom DO explains 97% of the estimated variability in the hypoxic extent. The change in the observed maximum extent between August and September is found to be sensitive to the corresponding variability in the hypolimnion thickness.
Zhou, Y., D.R. Obenour, D. Scavia, T.H. Johengen, A.M. Michalak (2013) “Spatial and temporal trends in Lake Erie hypoxia, 1987-2007”, Environmental Science & Technology, 47, 899-905, dx.doi.org/10.102/es303401b.