Movements of the hypolimnion into shallow regions appear to be relatively frequent events in many large lakes. For example, Effler et al. (2004) documented an 11-hour upwelling event in Onondaga Lake, New York, in September 2002 that resulted in dissolved oxygen concentrations <1 mg/L in surface waters at the windward end of the lake, and they estimated that at least 14 such events occurred within the interval 1990-2002. Similarly, it is known that the hypolimnion of Lake Erie’s central basin is dynamic, from the time it forms in June or July until it dissipates in September or October (Bartish 1987, Royer et al 1987). The dynamics are three-dimensional. Hypolimnion thickness varies as a result of seasonal erosion of the metalimnion and internal seiches and upwelling events following forcing events such as wind storms and differential barometric pressure (Bedford 1992). Horizontal movements or expansions of the hypolimnion into areas usually occupied by the metalimnion or epilimnion have been much less frequently observed (Bartish 1987).

There are few data that demonstrate the horizontal dynamics of the hypolimnion, although buoys deployed since 2003 or earlier by Ohio Sea Grant, Univ. of Windsor, and GLERL at several points in the central basin now provide data on time of hypolimnion establishment, progressive change in thickness and temperature profile, and vertical oscillations at those points. For example, GLERL Buoy No. 5 (Figure 1) collects temperature and turbidity data at 1, 5, and 10 m above the lake bottom (11 m(http://www:glerl.noaa.gov/res/Programs/erie/pgs/moorings.html). Our project will enhance the information provided by the buoys by increasing the spatial resolution of the data during selected periods, especially in marginal areas of great biological importance that cannot be extrapolated readily from the buoy data. The role of DO in controlling the structure (species composition, distribution, abundance, demography) and biomass of the benthic and benthic-pelagic food webs is unknown because high-resolution spatial and temporal data on DO concentrations near the edges of the central basin hypolimnion are lacking both presently and historically.

This Sea Grant development project helps us begin to address two hypotheses. Hypothesis 1: That the distribution and demography of major zoobenthic species (round gobies and other benthic fishes, dreissenids, mayflies, midges) near the margins of the central basin are a function of transitory (hours-long to days-long) shifts in the position of oxygen-depleted hypolimnion waters. Hypothesis 2: That high-resolution, frequent observation of horizontal hypolimnion dynamics coupled with observation of differences in zoobenthic community structure (composition, demography, and abundance) will explain the general absence of oxygen-sensitive zoobenthic species from vast areas of the central basin that have not heretofore been shown to experience hypoxia/anoxia.

A historical basis for Hypothesis 1 exists in data gathered by the National Center for Water Quality Research at Heidelberg College in 1978 and 1979 (unpublished) in which DO measurements taken on three consecutive days at some central basin stations as part of USEPA’s Lake Erie Intensive Study varied from well oxygenated to anoxic (<1 mg/L). Furthermore, Bartish (1987) reported an intrusion of central basin hypolimnion water far into Pigeon Bay of the western basin. He suggested that similar intrusions may occur one to two times per year.

Daily 24-27 July, 16-18 and 22 August, and 6 September 2006, we recorded vertical profiles of DO and temperature at several stations along the two transects. In a few instances, DO and temperature were recorded only within 1 m of the lake bottom. We traversed each transect to obtain a “snapshot” of the shoreward extent and thickness of the hypolimnion (indicated by DO and temperature) with the primary expectation of capturing the position of the lateral boundary of the hypolimnion. We selected specific stations on each date successively in part on the basis of the vertical profiles obtained at preceding stations that day. On several dates we were able to collect a second set of data at several stations about 3-3.5 hours following the initial readings.

DO meters included a YSI Model 58 augmented by YSI Model 55, YSI Model 550A, and YSI Model 95 meters. The four meters were calibrated against a certified thermometer and modified-Winkler titrations. The Model 58 and two other meters were within factory specifications for precision and accuracy; therefore, the field data were not adjusted prior to analysis. The few data collected by the fourth meter, which was outside specifications for DO, were not included in our analyses.

The region of hypoxia followed the contour of the lake bottom, being shallower in nearshore waters and deeper further offshore. At a given site, the thickness of hypoxic water varied by as much as 1.5 m over a few hours. DO concentrations <1 mg/L were found briefly as shallow as 10 m in July. DO <1 mg/L was generally restricted to depths >16 m during our July sampling interval and >14.5 m during the August interval. We conclude that (1) hypoxic events of sufficient duration to impact invertebrate and fish communities probably occur frequently in nearshore waters of the central basin, and (2) elevated oxygen demand of sediments beneath shallow inshore waters is sufficiently great to result in hypoxia or anoxia near the sediment-water interface even at depths above the hypolimnion.

James Ryan, a Heidelberg College undergraduate, and Anne Stearns, on the staff of the National Center for Water Quality Research at Heidelberg College, along with Bill Edwards and Mike Bur (co-PI) of the USGS Lake Erie Biological Station, assisted in data collection and analysis.

The following presentation will be made at the Fiftieth Conference on Great Lakes Research of the International Association for Great Lakes Research in May 2007: KRIEGER, K.A.¹, BUR, M.T.², and STEARNS, A.M.¹, ¹National Center for Water Quality Research, Heidelberg College, Tiffin, OH, 44883; ²U.S. Geological Survey, Lake Erie Biological Station, Sandusky, OH, 44870. Nearshore Dynamics of Hypoxia in Central Lake Erie.