Spatial and temporal dynamics of near-bottom dissolved oxygen in the central basin of Lake Erie

Abstract

Abstract Eutrophication continues in Lake Erie and low oxygen concentration remains a concern in the central basin of the lake. Summertime dissolved oxygen concentrations can be hypoxic (low dissolved oxygen) and anoxic (dissolved oxygen <1 mg l-1) in the hypolimnion. We examined the spatial and temporal patterns of hypoxia in the central basin along a ∼ 26 km west-east station transect in the western portion of the central basin (depth gradient from 11.4 m in the west to 20 m in the east). Water column properties were monitored using moored instruments (dissolved oxygen, temperature, turbidity, Chlorophyll a fluorescence) and instrument profiling during cruises in 2008 and 2009. Hypoxia was examined using a biologically relevant value of 40% dissolved oxygen saturation (i.e. ∼4 mg l-1) and 25% dissolved oxygen saturation (i.e. ∼2.5 mg l-1) as used by government agencies. Our goals were to determine the onset and location of hypoxia, as well as the frequency of hypoxic events. We observed differences in the spatial and temporal patterns between the two years, related potentially to different water levels, stronger winds, and a smaller hypolimnion and lower thermocline depth in 2009. Near-bottom hypoxia occurred in the east at the end of June and extended westward by the end of July 2008 using 40% saturation; in early July to mid-August using 25% saturation. The onset of hypoxia (40% sat) occurred earlier in the west in 2009 but was similar to 2008 using 25% saturation. Hypoxia was not static, rather there were a total of ∼100 events of both levels of hypoxia, which were of different duration, noted across the transect in both years. Both the frequency and duration of hypoxic events (> 1 min) were higher and longer in 2008, perhaps related to water circulation and the resuspension of bottom sediment by synoptic-scale storms, which coincided with low oxygen events. Understanding the spatial and temporal patterns of hypoxia provides insight into their effects on habitat quality as well as biogeochemical processes in benthic and hypolimnetic environments in Lake Erie.