Multiscale relationships between Great Lakes nearshore fish communities and anthropogenic shoreline factors

The multi-scale nature of streams, rivers, and inland lakes is well documented, although relationships between the ecology of Great Lakes nearshore areas and shoreline processes are generally poorly described. Given the high levels of development pressure currently exerted on Great Lakes shorelines, we sought to determine whether patterns exist between measures of shoreline development quantified at multiple spatial scales and adjacent fish community measures. We expected that fish measures for nearshore areas immediately adjacent to intact versus modified shorelines would differ as a result of the greater buffering capacity of the intact shorelines. Further, we expected anthropogenic shoreline factors to act cumulatively in combination with prevailing currents to influence fish communities in downdrift nearshore areas. Our results indicated that a few shallow water and nearshore fish community measures exhibited significant patterns that may be attributable to immediately adjacent shoreline characteristics. In addition, several fish measures were related to urban-residential land uses and shore structure numbers of updrift shoreline areas, suggesting that cumulative anthropogenic factors operating over larger spatial scales also influence local fish communities. Based on these results, we argue that there is critical need for multi-scale management strategies for shorelines that address the potential for both local and cumulative, larger-scale environmental impacts relative to local nearshore biota.     

Lake Ontario’s nearshore zooplankton: Community composition changes and comparisons to the offshore

In large lake systems the nearshore habitat is an intermediate zone between the shoreline and offshore, is an important nursery for larval fish, and is highlighted as an area in need of research in the Laurentian Great Lakes. In this study, we used two long-term monitoring programs to characterize the nearshore zooplankton community composition using seasonal data (May – October) and to compare the nearshore and offshore zooplankton community composition changes over time (1998 – 2019) to determine if the changes were synchronized. In the nearshore, we found the highest zooplankton biomass during the late summer/early fall (August 27th – Oct 6th), compared to mid-summer (July 1st – Aug 26th) and late spring (May 20th – June 30th). In the summer, the nearshore zooplankton community was dominated by cladocerans while copepods dominated the offshore community. From 1998 to 2019, both nearshore and offshore copepods shifted from a cyclopoid to a calanoid-dominated state, but the details of this change were different. For example, taxon-specific analysis revealed that despite reduced cyclopoids in both habitats, Mesocyclops edax increased in the nearshore. Additionally, taxon-specific analysis suggested the changes occurred an average of three years earlier in the nearshore. Using Analysis of Similarity, the nearshore and offshore summer zooplankton community compositions became increasingly distinct over time. Results from this study highlight the uniqueness of the nearshore in large lake systems, the importance of seasonal and long-term monitoring, and the potential of the nearshore as an early indicator of offshore changes.     

Cumulative Habitat Impacts of Nearshore Engineering

A multi-disciplinary, multi-institutional research team evaluated a broad range of physical and biological characteristics at six Great Lakes nearshore sites in order to develop and test a conceptual modeling framework to assess linkages between bluff erosion, sediment supply, coastal processes, and biological utilization of nearshore and coastal habitats. The sites were chosen to represent a broad range of hydrogeomorphic conditions, with the objective of assessing the response of these nearshore systems to anthropogenic modifications and coastal change. As a result of this 2-year field effort, new methods and integrated approaches were developed to characterize, map, and assess the dynamic nature of the nearshore zone (area generally less than 10 m water depth). Thus, these data provide an initial quantitative assessment of nearshore change. In addition, our data indicate that shoreline modifications have led to cumulative impacts that have irreversibly modified Great Lakes nearshore coastal habitats and the processes that create and maintain them. Of special note is our observation that altered nearshore substrate dynamics resulting from shoreline modifications may enhance the colonization success of lithophilic aquatic invasive species in nearshore areas of the Great Lakes. Continued development of the shoreline may exacerbate changes in Great Lakes nearshore food-web structures and ecosystem services. Further study and monitoring of these phenomena are needed, and our work suggests that a holistic, multidisciplinary approach is necessary to develop effective management strategies to address these and other issues affecting nearshore areas of the Great Lakes.     

Temporal shifts in the biodiversity of nearshore small fishes in Lake Simcoe

Nearshore small fish species represent a large proportion of fish biodiversity in Lake Simcoe, a large inland lake in southern Ontario, Canada. Over the past 30 years, Lake Simcoe has experienced several changes to its aquatic habitat, benthic invertebrate communities and predatory fish populations. This study compared samples of the nearshore small fish community in three geographic areas of Lake Simcoe. Fish community data were grouped into two time periods: a contemporary period (2007–2009) and a historical period (1982–1995). The fish community was compared across time periods for each area to assess if observed ecological changes had an impact on the small fish community. Species richness significantly declined between time periods in two areas (Cook's Bay and the southeast shoreline), the number of individuals captured declined between time periods in one area of the lake (Kempenfelt Bay) and Simpson's diversity index declined between time periods in one area of the lake (southeast shoreline). There were no significant differences in the Shannon–Weiner evenness index between time periods in any of the study areas. Additional analyses of intra- and inter-annual variation in fish sampling results generally supported the findings that shifts in the fish community occurred between time periods. Overall, this study suggests that the nearshore small fish biodiversity of Lake Simcoe has shifted over time but these shifts are not clearly related to recent increases in water clarity, macrophyte growth and nearshore benthic invertebrate densities.     

Monitoring shallow benthic fish assemblages in the Laurentian Great Lakes using baited photoquadrats: Enhancing traditional fisheries monitoring methods

Photoquadrats and underwater video surveys are standard non-destructive monitoring methods in marine ecosystems and are becoming more common in freshwater systems. The high water clarity found in most of the Laurentian Great Lakes make them ideally suited for photoquadrat sampling. We compared the effectiveness of baited photoquadrats to monitor benthic fish communities of shallow, littoral habitats of northern Lake Michigan. We compared our results with baited minnow traps, a technique commonly used to monitor benthic fish communities in freshwater ecosystems. Photoquadrats baited with lake trout (Salvelinus namaycush) eggs, the most effective attractant, proved to be an efficient tool for sampling round goby (Neogobius melanostomus) relative abundance and resulted in higher round goby catches than in photoquadrats baited with a commercially available fish attractant or unbaited photoqudrats. This method allowed sites to be surveyed rapidly (requiring <1?h per site), and replicate samples produced data with low variability. In contrast, baited minnow traps produced highly variable catch per unit effort (CPUE) irrespective of soak time. Photoquadrat methods appear to be an improvement over traditional sampling with minnow traps for round goby and may also be better for other nearshore benthic fishes.     

An Evaluation of Effects of Groundwater Exchange on Nearshore Habitats and Water Quality of Western Lake Erie

Historically, the high potentiometric surface of groundwater in the Silurian/Devonian carbonate aquifer in Monroe County, MI resulted in discharge of highly mineralized, SO₄-rich groundwater to the Lake Erie shoreline near both Erie State Game Area (ESGA) and Pointe Mouillee State Game Area (PMSGA). Recently, regional groundwater levels near PMSGA have been drawn down as much as 45 m below lake level in apparent response to quarry dewatering. From August to November of 2003, we conducted preliminary studies of groundwater flow dynamics and chemistry, shallow lake water chemistry, and fish and invertebrate communities at both sites. Consistent with regional observations, groundwater flow direction in the nearshore at ESGA was upward, or toward Lake Erie, and shallow nearshore groundwater chemistry was influenced by regional groundwater chemistry. In contrast, at PMSGA, the groundwater flow potential was downward and lake water, influenced by quarry discharge seeping downward into nearshore sediments, produced a different lake and shallow groundwater chemistry than at ESGA. Although the invertebrate and young fish community was similar at the two sites, taxonomic groups tolerant of degraded water quality were more prevalent at PMSGA. Sensitive taxa were more prevalent at ESGA. We propose a conceptual model, based on well-described models of groundwater/seawater interaction along coastal margins, to describe the interconnection among geologic, hydrologic, chemical, and biological processes in the different nearshore habitats of Lake Erie, and we identify processes that warrant further detailed study in the Great Lakes.     

Sustainable fisheries management in the Great Lakes: Scientific and operational challenges

Fisheries managers seek to sustain Great Lakes' fish populations in a large, complex lake‐watershed ecosystem responding to often competing issues: non‐indigenous species, resource allocation and environmental quality. Within the past 200 years, human activity has caused dramatic changes in the character of this ecosystem. Before the 1900s, the offshore fish communities in each of the Great Lakes were dominated by the piscivorous lake trout and burbot. The current fish fauna of the Great Lakes' basin includes 179 species representing 29 families in 18 orders and two classes of fish. Twenty‐five non‐indigenous fish species have established populations in the Great Lakes' ecosystem. Sustainable management of Great Lakes' fisheries depends on social, economic and ecological factors. Hundreds of millions of dollars are spent annually to protect and preserve Great Lakes' fisheries and their associated ecosystems. Management of Great Lakes' fisheries on a species‐by‐species basis is pointless. Recreational fishing provides larger economic benefits on the Great Lakes, compared to commercial fisheries. Further, quota management, even when practiced at levels well below maximum sustainable yield, does not lead to stable fish communities. Management will be constrained more by ecological reality than by economic forces, but ultimately a managed system comprised of both indigenous and non‐indigenous fishes is a logical objective.     

Seventy years of food-web change in South Bay,Lake Huron

Though aquatic ecosystems (and the Laurentian Great Lakes in particular) have faced many stressors over the past century, including fisheries collapses and species invasions, rarely are data available to evaluate the long-term impacts of these stressors on food web structure. Stable isotopes of fish scales from the 1940s to the 2010s in South Bay, Lake Huron were used to quantify trophic position and resource utilization for fishes from offshore (alewife, cisco, lake trout, lake whitefish, rainbow smelt) and nearshore (rock bass, smallmouth bass, white sucker, yellow perch) habitats, providing one of the longest continuous characterizations of food webs in the Laurentian Great Lakes. Mean δ¹⁵N and δ¹³C values for each species were compared across twenty-year time periods. Using directional statistics, no significant community-wide changes were detected between time periods from 1947 to 1999. In contrast, a significant change was detected between 1980-1999 and 2000–2017, with all species showing increased reliance on nearshore resources. The increase in nearshore resource reliance for lake whitefish between these time periods was the greatest in magnitude compared with any other species between any two adjacent time periods. Besides lake whitefish, the increased reliance on nearshore resources was more pronounced for nearshore compared to offshore species. The timing of these shifts coincided with the invasion of dreissenid mussels and round goby, and declines in offshore productivity and prey densities. These results show the unprecedented magnitude of recent food-web change in Lake Huron after 50 years of relative stability.     

The influence of light,substrate, and fish on the habitat preferences of the invasive bloody red shrimp,Hemimysis anomala

The invasive bloody red shrimp, Hemimysis anomala, is a novel organism in the Laurentian Great Lakes region that utilizes benthic and open-water habitat. Hemimysis is predicted to impact nearshore fish communities in the Northeastern USA where its range is expanding, either negatively through predation of shared zooplankton prey or positively as high-calorie prey. In this experimental study, we examined the factors influencing Hemimysis’ benthic habitat selection, vertical distribution, and susceptibility to fish predation. In the presence of fish cues, Hemimysis preferred cobble over other benthic substrates (Dreissena mussels, pebble, or sand) regardless of light conditions; in dark conditions without a fish present, Hemimysis preferred open waters with sand habitat. Light and fish cues also interacted to influence the vertical distribution of Hemimysis, with the majority of mysids selecting depths that minimized perceived cumulative risk. The mean feeding rates of young-of-year (YOY) alewife (Alosa pseudoharengus), adult round goby (Neogobius melanostomus), YOY yellow perch (Perca flavescens), adult pumpkinseed sunfish (Lepomis gibbosus), and YOY lake trout (Salvelinus namaycush) varied among species, prey densities, and substrate (range = 0.77–57 mysids/fish/h). In general, feeding rates were highest for alewife, a non-native species in the Great Lakes basin, and in refuge-free conditions for all species, except for non-native round goby, which fed at similar rates regardless of prey refuge availability. Collectively, our results suggest that fish feeding success is contingent upon the interaction of light and Hemimysis refuge availability due to behavioral modifications of Hemimysis in the presence of fish and adverse light conditions.     

Lake Champlain 2010: A summary of recent research and monitoring initiatives

Lake Champlain shares a geological history with the Great Lakes and, as part of the St. Lawrence drainage, also shares biological and ecological similarities. The complex bathymetry and extensive shoreline provide a variety of lacustrine habitats, from deep oligotrophic areas to shallow bays that are highly eutrophic. The large basin:lake ratio (19:1) makes Lake Champlain vulnerable to impacts associated with land use, and in some parts of the lake these impacts are further exacerbated by limited water exchange among lake segments due to both natural and anthropogenic barriers. Research in Lake Champlain and the surrounding basin has expanded considerably since the 1970s, with a particularly dramatic increase since the early 1990s. This special issue of the Journal of Great Lakes Research brings together 16 reports from recent research and monitoring efforts in Lake Champlain. The papers cover a variety of topics but primarily focus on lake hydrodynamics; historical and recent chemical changes in the lake; phosphorus loading; recent changes in populations of phytoplankton, zooplankton, and fishes; impacts of invasive species; recreational use; and the challenges of management decision-making in a lake that falls within the legal jurisdictions of two U.S. states, one Canadian province, two national governments, and the International Joint Commission. The papers provide not only evaluations of progress on some critical management issues but also valuable reference points for future research.     

Influence of river plumes on the distribution and composition of nearshore Lake Michigan fishes

River plumes form in coastal areas where tributaries mix with their receiving waters. Plume waters are enriched with terrestrial-derived nutrients from their watersheds creating hotspots of biological productivity. The biological importance of plumes scales with the size and persistence of the plume; therefore, large, persistent plumes are more important than small, transient plumes. To date, most studies of plumes have focused on assimilation of terrestrial-derived energy by aquatic species or lower-level food web effects, primarily in marine systems. Few studies have described fish communities near plume habitats and compared them to non-plume areas, especially for the numerous small plumes in the Laurentian Great Lakes. Here we demonstrate that small plumes in the main basin of Lake Michigan enhance local primary productivity and influence distribution and abundance of nearshore Great Lakes fishes. We found that plume fish communities were relatively depauperate and did not support higher biological diversity of fishes compared to non-plume areas. However, individual species including rainbow smelt Osmerus mordax, spottail shiner Notropis hudsonius, and white sucker Catostomus commersonii were more abundant around plumes. Our results demonstrate that small plumes in the main basin of Lake Michigan support highly localized hotspots of biological productivity and fish abundance, primarily within 2?km of river mouths.     

Seasonal patterns in hydrochemical mixing in three Great Lakes rivermouth ecosystems

Rivermouth ecosystems in the Laurentian Great Lakes represent complex hydrologic mixing zones where lake and river water combine to form biologically productive areas that are functionally similar to marine estuaries. As urban, industrial, shipping, and recreational centers, rivermouths are the focus of human interactions with the Great Lakes and, likewise, may represent critical habitat for larval fish and other biota. The hydrology and related geomorphology in these deltaic systems form the basis for ecosystem processes and wetland habitat structure but are poorly understood. To this end, we examined hydrogeomorphic structure and lake-tributary mixing in three rivermouths of intermediate size using water chemistry, stable isotopes, and current profiling over a five-month period. In rivermouths of this size, the maximum depth of the rivermouth ecosystem influenced water mixing, with temperature-related, density-dependent wedging and layering that isolated lake water below river water occurring in deeper systems. The inherent size of the rivermouth ecosystem, local geomorphology, and human modifications such as shoreline armoring and dredging influenced mixing by altering the propensity for density differences to occur. The improved scientific understanding and framework for characterizing hydrogeomorphic processes in Great Lakes rivermouths across a disturbance gradient is useful for conservation, management, restoration, and protection of critical habitats needed by native species.     

Round Goby and Mottled Sculpin Predation on Lake Trout Eggs and Fry: Field Predictions from Laboratory Experiments

The accidental introduction of round gobies (Neogobius melanostomus) into the North American Great Lakes has raised concerns about their potential impacts on local fauna. Gobies have similar habitat and spawning requirements to mottled sculpins (Cottus bairdi) and slimy sculpins (C. cognatus), and may already be displacing sculpins where the ranges of the species overlap. Like sculpins, gobies are capable of penetrating interstitial spaces to acquire food, and therefore may become predators of interstitially incubating lake trout eggs. Laboratory experiments were conducted to compare egg consumption rates and critical size (the minimum size at which a fish was capable of ingesting an egg) between round gobies and mottled sculpins. Predation by both species on lake trout eggs and fry was also examined in two grades of substrate (cobble and gravel). Mottled sculpins consumed larger numbers of eggs than round gobies of similar size, and were capable of ingesting eggs at smaller sizes than gobies. Both gobies and sculpins had lower foraging success on smaller substrates (gravel) than on cobble. Gobies are currently present at higher densities than sculpins in areas where they are established in the Great Lakes. The similar predation of lake trout eggs by round gobies and mottled sculpin and high densities the goby has achieved at some Great Lakes sites leads to the prediction that the round goby may negatively affect lake trout reproduction and therefore rehabilitation.     

An exploratory investigation of the landscape-lake interface: Land cover controls over consumer N and C isotopic composition in Lake Michigan rivermouths

Rivermouth ecosystems are areas where tributary waters mix with lentic near-shore waters and provide habitat for many Laurentian Great Lakes fish and wildlife species. Rivermouths are the interface between terrestrial activities that influence rivers and the ecologically important nearshore. Stable isotopes of nitrogen (N) and carbon (C) in consumers were measured from a range of rivermouths systems to address two questions: 1) What is the effect of rivermouth ecosystems and land cover on the isotopic composition of N available to rivermouth consumers? 2) Are rivermouth consumers composed of lake-like or river-like C? For question 1, data suggest that strong relationships between watershed agriculture and consumer N are weakened or eliminated at the rivermouth, in favor of stronger relationships between consumer N and depositional areas that may favor denitrification. For question 2, despite apparently large riverine inputs, consumers only occasionally appear river-like. More often consumers seem to incorporate large amounts of C from either the nearshore or primary production within the rivermouth itself. Rivermouths appear to be active C and N processing environments, thus necessitating their explicit incorporation into models estimating nearshore loading and possibly contributing to their importance to Great Lakes biota.     

Response of fish assemblages to restoration of rapids habitat in a Great Lakes connecting channel

Rapids habitats are critical spawning and nursery grounds for multiple Laurentian Great Lakes fishes of ecological importance such as lake sturgeon, walleye, and salmonids. However, river modifications have destroyed important rapids habitat in connecting channels by modifying flow profiles and removing large quantities of cobble and gravel that are preferred spawning substrates of several fish species. The conversion of rapids habitat to slow moving waters has altered fish assemblages and decreased the spawning success of lithophilic species. The St. Marys River is a Great Lakes connecting channel in which the majority of rapids habitat has been lost. However, rapids habitat was restored at the Little Rapids in 2016 to recover important spawning habitat in this river. During the restoration, flow and substrate were recovered to rapids habitat. We sampled the fish community (pre- and post-restoration), focusing on age-0 fishes in order to characterize the response of the fish assemblage to the restoration, particularly for species of importance (e.g. lake whitefish, walleye, Atlantic salmon). Following restoration, we observed a 40% increase in age-0 fish catch per unit effort, increased presence of rare species, and a shift in assemblage structure of age-0 fishes (higher relative abundance of Salmonidae, Cottidae, and Gasterosteidae). We also observed a “transition” period in 2017, in which the assemblage was markedly different from the pre- and post-restoration assemblages and was dominated by Catostomidae. Responses from target species were mixed, with increased Atlantic salmon abundance, first documented presence of walleye and no presence of lake sturgeon or Coregoninae.     

Nearshore fish assemblage dynamics in southern Lake Michigan: 1984–2016

Many coastal ecosystems, including those of the Laurentian Great Lakes, suffer from various natural and anthropogenic stressors. Given that multiple stressors often concomitantly impact ecosystems, it may be difficult to disentangle which stressors are most influential. Upper trophic level communities, such as fish assemblages, can provide insights to the influence of diverse stressors as they may integrate cumulative effects over the long-term and also reflect responses of lower trophic levels. We used multivariate analyses and assemblage indices to investigate long-term (1984–2016) patterns in a nearshore fish assemblage indexed via annual trawling in the Indiana waters of southern Lake Michigan. Based on observations from other regions of the Great Lakes, we expected that oligotrophication, due to reduced nutrient loading and filtering by invasive mussels, would have a strong influence on the fish assemblage. However, we were unsure if the very nearshore fish assemblage would track observed decreased production patterns in offshore Lake Michigan or if observed increased primary production in the very nearshore would affect the fish assemblage. Consistent with the former expectation, overall abundance and richness of the assemblage declined over time. However, contrary to observations in other regions there was no overall evidence of species tolerant to more eutrophic conditions being replaced by more sensitive species. Moreover, there was limited evidence of the fish assemblage shifting towards species more tolerant of warm water, as might be expected with climate change. While increased numbers of invasive species added species to the system, overall species richness and native species richness declined.     

Influence of Nearshore Till Lithology on Lateral Variations in Coastline Recession Rate Along Southeastern Lake Michigan

Retreat of coastal bluffs around margins of the Great Lakes is a continuing process. Despite the positive correlation which exists between lake level and recession rates, considerable lateral variation in rates is typical of many coastal areas, while causes of this spatial variation are not well understood. Detailed examination of a 10 km segment of Lake Michigan shore near the town of Glenn, Michigan, suggests that lateral variations in the lithology of Pleistocene drift are directly correlative with spatial variations in recession rates.High recession rates and concave shorelines occur along segments composed of either outwash sand or sandy till which contains gravel up to 2 cm in diameter as the coarest clast size. Conversely, low rates and the occurrence of convex shoreline segments are related to exposures of bouldery till. The exact nature of the relationship between bouldery till and the low recession rates is ambiguous. Features observed along this area suggest two non-mutally exclusive possibilities: 1) during erosion, bouldery till develops flat gently sloping wave-cut benches in the upper shore face; and 2) erosion of this coarse till results in the development of upper shoreface surfaces covered with large boulders. Both features may serve to attenuate wave energy in the nearshore zone. In either case, lateral varition in the recession rates near Glenn is greatly influenced by the lithology of glacial drift exposed in bluff faces and the nearshore areas.     

Towards improving an Area of Concern: Main-channel habitat rehabilitation priorities for the Maumee River

The Maumee River watershed in the Laurentian Great Lakes Basin has been impacted by decades of pollution and habitat modification due to human settlement and development. As such, the lower 35 km of the Maumee River and several smaller adjacent watersheds comprising over 2000 km² were designated the Maumee Area of Concern (AOC) under the revised Great Lakes Water Quality Agreement in 1987. As part of pre-rehabilitation assessments in the Maumee AOC, we assessed fish and invertebrate communities in river km 24–11 of the Maumee River to identify: 1) areas that exhibit the highest biodiversity, 2) habitat characteristics associated with high biodiversity areas, 3) areas in need of protection from further degradation, and 4) areas that could feasibly be rehabilitated to increase biodiversity. Based on benthic trawl data, shallow water habitats surrounding large island complexes had the highest fish diversity and catch per unit effort (CPUE). Electrofishing displayed similar fish diversity and CPUE patterns across habitat types early in the study but yielded no discernable fish diversity or CPUE patterns towards the end of our study. Although highly variable among study sites, macroinvertebrate density was greatest in shallow water habitats <2.5 m and around large island complexes. Our results provide valuable baseline data that could act as a foundation for developing rehabilitation strategies in the lower Maumee River and for assessing the effectiveness of future aquatic habitat rehabilitation projects. In addition to increasing in-channel habitat, watershed-scale improvements of water quality might be necessary to ensure rehabilitation strategies are successful.     

Great Lakes Flood Thresholds and Impacts

Using the location, data, and water levels from flood events along the Canadian shore of the Great Lakes, flood damage thresholds were determined to identify and compare water levels at which static and storm-induced high water impact shoreline interests on several shore reaches of Lakes Erie, Huron, Ontario, and St. Clair. Spatial differences identified may be related to several factors, including: 1) nearshore bathymetries; 2) extent of residential development along low-lying shorelines; 3) degree of riparian adjustment to flooding; and 4) location relative to dominant wind or storm directions. Correlation analyses found that flood damage levels are more closely correlated to fluctuations in static levels on Lakes Ontario, Huron, and St. Clair, while flood damage levels are more closely correlated to maximum instantaneous water levels on Lake Erie. Correlation analyses of individual gauge data identified locations possibly more susceptible to storm surges. A conservative approach to determining flood damage thresholds is suggested, being based on a standard deviation below the mean of maximum instantaneous flood levels for a given gauge. The standard deviation threshold, while lower than current “critical levels” used in management, is more representative of the majority of flood damage levels than thresholds based on lowest maximum instantaneous lake levels. However, caution is urged in applying any critical level solely based on water level gauge information as Great Lakes flooding is a highly site-specific phenomenon influenced by meteorologic factors.     

Effects of Great Lakes Water Loading upon Glacial Isostatic Adjustment and Lake History

Over the last century geological studies of the ancestral Great Lakes have confirmed that the large surface load of the Laurentide ice sheet deformed the region causing tilting of ancient lake shorelines. Models of this glacial isostatic adjustment mechanism have promoted understanding of this process but have only included ice sheet loads as the source of earth deformation in the region. We describe a method, utilizing a model of glacial isostatic adjustment combined with GIS, that recreates the paleohydrology of the Great Lakes. Predictions include the extent of late glacial, postglacial, and Holocene lakes and their associated outlets and bathymetries. This predicted history of the Great Lakes is similar to that obtained from a century of detailed field studies but our method uses only the present digital elevation model, a prescribed ice sheet chronology, and an assumed earth viscoelastic rheology. Ancient lake bathymetry predictions provide an estimate of water loads associated with each lake. The effect of these lake loads upon vertical deformation of the Great Lakes region is shown to be small, less than 15 m, but not insignificant when compared to approximately 150 m of deformation forced by ice and ocean loads. Maximum lake-induced deformation is centered upon Lake Superior where water depths were greatest. Where topography is low relief, prediction of shoreline locations should include the lake loading effect as well as the ice and ocean loads.