labs_title

Record-Breaking Lake Erie Hypoxia during 2012 Drought

Y. Zhou, A.M. Michalak, D. Beletsky, Y.R. Rao, and R.P. Richards

Hypoxia (a.k.a. dead zones) occurs when dissolved oxygen levels at the bottom of a water body drop below a critical threshold, typically 2 mg/l. In this work, we show that the 2012 drought that covered much of the Great Plains resulted in the largest ever observed hypoxic event in Lake Erie. Although both hypoxia and harmful algal blooms (HABs) are driven by excess nutrients entering the lake, we also show that 2011, a year with a record-breaking HAB, likely experienced only a relatively mild hypoxic extent. Overall, we find that the severity of hypoxia in Lake Erie in a given year is explained in large part by tributary discharge, soluble reactive phosphorus loading, wind stress, and wind direction.


Figure: Annual seasonally averaged hypoxic extent from 1985 to 2012. Observed hypoxic extent is in blue with 95% confidence intervals. Model of interannual variability for years with insufficient observations is in orange, with bars indicating the 95% prediction intervals for individual years.

Abstract

Hypoxia has been observed in the central basin of Lake Erie for decades. To understand the impact of various controlling factors, we analyze a record of hypoxic extents for Lake Erie for 1985–2012 and develop a parsimonious model of their interannual variability. We find that the 2012 North American drought and accompanying low tributary discharge was associated with a record-breaking hypoxic event in Lake Erie, whereas a record-setting harmful algal bloom in 2011 was likely associated with only mild hypoxia. River discharge and the timing of nutrient input therefore impact western basin bloom growth and central basin oxygen demand in distinct ways that merit further investigation. Overall, April to June tributary discharge, May to July soluble reactive phosphorus loading, July wind stress, and June northwesterly wind duration explain 82% of the interannual variability of hypoxia, and discharge alone explains 39%, indicating that meteorological factors need to be considered in the development of nutrient management strategies, especially as both extreme precipitation events and droughts become more frequent under a changing climate.

Zhou, Y., A.M. Michalak, D. Beletsky, Y.R. Rao, R.P. Richards (2015) "Record-Breaking Lake Erie Hypoxia during 2012 Drought", Environmental Science & Technology, 49 (2), 800-807, doi:10.1021/es503981n.