Lake Erie hypoxia spatial and temporal dynamics present challenges for assessing progress toward water quality goals

Seasonal hypolimnetic hypoxia has been documented in Lake Erie’s central basin since the 1950s. Ship-based surveys to monitor hypoxia have been conducted since the 1980s, but they occur at a relatively low frequency and focus on the deeper areas of the central basin. To better document the seasonal development of stratification and the consequent occurrence of hypoxia, we deployed eight moorings, in both nearshore-shallow areas and offshore-deep areas of the central basin, equipped with temperature and oxygen sensors at multiple depths, that recorded temperature and oxygen concentrations every 10 min. Results from 2017 to 2019 reveal that hypoxia occurs as early as July in the shallower areas west of, and around the southern perimeter of the central basin, but does not occur until August or September in the deeper central basin. Hypoxia is intermittent in the shallower perimeter areas; whereas in the deeper areas, hypoxia can persist into October, often progressing to anoxia. The intra- and inter-annual differences in the spatial and temporal extent of hypoxia indicate that an extensive monitoring program will be necessary to more accurately assess progress toward reducing the extent of hypoxia pursuant to the lake ecosystem objectives of the 2012 Great Lakes Water Quality Agreement.