No abstract

Recent studies have produced a new understanding of the hydrological history of North America's Great Lakes, showing that water levels fell several meters below lake basin outlets during an early postglacial dry climate in the Holocene (younger than 10,000 radiocarbon years, or about 11,500 calibrated or calendar years before present (B.P.)). Water levels in the Huron basin, for example, fell more than 20 meters below the basin overflow outlet between about 7900 and 7500 radiocarbon (about 8770–8290 calibrated) years B.P. Outlet rivers, including the Niagara River, presently falling 99 meters from Lake Erie to Lake Ontario (and hence Niagara Falls), ran dry. This newly recognized phase of low lake levels in a dry climate provides a case study for evaluating the sensitivity of the Great Lakes to current and future climate change.

Abstract. Zebra mussels (Dreissena) have expanded rapidly throughout most of the Laurentian Great I~akes since their inadvertent release in 1986. These exotic molluscs now occur in great numbers on the bottom of western Lake Erie where they are found increasingly in deeper areas of the basin (average depth: 10 m), on sott, muddy substrates. '[-his study is aimed at quantifying the density and the distribution patterns of mussel colonization in the basin as a first step in investigating the effect on sediment properties of such an abrupt change in benthic community structure. Underwater video imager T and diver-collected samples taken from representative o~shore areas (seven sites) in western Lake Erie showed colonization levels of up to 20,000 live mussels per m in soft sediments (adult.,; with shells '10 mm comprised 47 %). Digital side-scan sonar records confirmed that colonization patterns were not random, but showed distinctive spatial signatures ranging from 30-m-long parallel stripes, to large ovate masses. Broad irregular mats were found in association with bard bottoms (bedrock, boulders, or wrecks and large debris). Mussel densities were averaged from the sites, assuming consistent relationships with substrate type ,and were combined with digitized percentage &areal coverage of major bottom types in western Lake Erie. This resulted in the first population figure of 10 ~3 in the basin. "['his figure includes molluscs of all sizes 9 0.84 mm.

When Lake Iroquois drained between 11.7 and 11.4 ka BP, lake level in the Ontario basin fell from a high of more than 40 m above present lake level to a minimum close to the then-existing sea level, which was approximately 40 m below present sea level. Since that time, lake level has been rising at an exponentially decreasing rate in the western portion of the basin as a result of postglacial and neotectonic uplift of the outlet near Kingston, at the eastern end. The published lake level history has been combined with other less well-known parameters (the post-Iroquois regional topography, erosion – deposition rates, and distribution of resistive shore materials) to reconstruct the evolution of the western Lake Ontario shoreline. Borehole, long piston core, and other subsurface data sources, primarily from the western portion of the lake near Hamilton Harbour, provide most of the physical constraints. Time references were provided by radiocarbon dates on shallow-water organics in the subsurface sediments. A computer program was designed to calculate and contour the changing elevations of the rebounding post-Iroquois topographic surface, allowing the time-dependent water-plane elevation to be superimposed. Semiquantitative allowance was made for differential erosion and deposition along the advancing shoreline. The reconstruction provides a perspective on past and future shoreline evolution in the basin and possibly on the location of potentially commercial offshore deposits of aggregate.

An analytical method to determine faecal sterols was developed and applied to the analysis of samples including pig manure, sewage treatment plant sludge and combined sewer overflow effluent. Compounds including coprostanol (5β-cholestan-3β-ol), epicoprostanol (5β-cholestan-3α-ol), cholesterol (cholestan-5-en-3β-ol) and dihydrocholesterol (cholestanol, 5α-cholestan-3β-ol) were quantified in these source samples and their relative ratios calculated to investigate their potential application as source tracers. A mean coprostanol:epicoprostanol ratio of 7.6 ± 1.17 was calculated for pig manure samples from three Ontario livestock operations. This ratio was much lower (approximately 1.8) for sewage treatment plant sludge while the ratio for a combined sewer overflow effluent was very high (approximately 70). This approach, with the addition of physical measurements (e.g., current velocities) and meteorological data, may assist in determination of the influence of shore-based activities, including sewage treatment and livestock operations, on aquatic systems.