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Boundary dynamics of the hypoxic hyplimnion of Lake Erie's central basin

"The distributions and productivity of benthic invertebrates (quality  and availability of fish food) in Lake Erie's central basin may be  highly influenced by summer hypoxia.  Data on dissolved oxygen (DO)  depletion in shallow waters has been lacking.  We predicted episodic,  short-term (days-long or hours-long) intrusions of hypoxic (<2mg/L)  hypolimnion water into nearshore areas.  We corded DO concentrations  once or twice a day during intervals of 3 to 4 days in July and August  2  6 along depth gradients (7.4m-16.9m) near Lorain, Ohio.  The region  of hypoxia followed the contour of the lake bottom, being shallower in  nearshore waters and deeper further offshore.  At given site, the  thickness of hypoxic water varied by 1.5m over a few hours.  DO  concentrations <1mg/L were found briefly as shallow as 1 m in July.   DO <mg/L was generally restricted to depth >16m during our July  sampling interval.    The Project concluded that: 1) hypoxic events of sufficient duration  to impact invetebrate and fish communities probably occur frequently  in nearshore waters of the central basin, and 2) elevated oxygen  demand of sediments beneath shallow inshore wataers is sufficiently  great to result in hypoxia or anoxia near the sediment-water interface  even at depths above the hypolimnion.    The above findings should correlatewith the structure of the benthic  invertebrate communities both within and beyond the observed lateral  excursions of the hypolimnion and should help to explain the  distributions of oxygen-sensitive taxa such as Hexagenia spp.   (burrowing mayflies) and Dreissena spp. (zebra and quagga musses)".   

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Annex 17
Effects of varying lake levels and relation to pollution sources, fate and effects

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