Coastal wetlands play a pivotal role in the Great Lakes ecosystem. As buffer zones between the land and open waters of the Great Lakes, they perform a variety of essential functions providing both direct and indirect anthropogenic benefits. Geology, morphology and climate are the dominant variables that influence Laurentian Great Lakes wetland development. However, anthropogenic factors are the major contributors to alteration of natural wetland processes. This paper provides an overview of natural and anthropogenic factors important in Great Lakes coastal wetland development and provides statistical information describing the Great Lakes Basin. A brief description of wetlands classification and research issues is also presented.

Introduction

Coastal wetlands have important ecological, economic and social roles in the Great Lakes ecosystem. Their importance stems from their ability to act as buffer zones between the land and open waters of the Great Lakes, providing coastal protection, water purification and habitat for many species of fauna and flora. Studies of freshwater wetlands have shown that ecological integrity of these ecosystems depends on the complex interaction between various physical, chemical and biological factors.

More than 50 definitions of wetlands are used world wide. A key characteristic distinguishing the coastal wetlands of the Great Lakes from other freshwater inland wetlands is hydraulic connectivity with Great Lakes waters (Keough et al., 1999). Unlike inland wetlands, coastal wetlands are more dynamic, display a greater variety of landforms and have largely mineral sediments (Fuller et al., 1995). For the purpose of this issue a definition of Keough et al. (1999) (modified from Cowardin et al. (1979)), was used to define coastal wetlands of the Great Lakes as: ‘lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface or the land is covered by shallow water. Wetlands must have one or more of the following attributes: 1) at least periodically land supports predominantly hydrophytes; 2) the substrate is predominantly undrained hydric soil; and 3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season of the year. Wetlands may be considered to extend lakeward to the water depth of two meters, using the historic low and high water levels, or the greatest extent of wetland vegetation. Hydrologic connection with one of the Great Lakes may extend upstream along rivers since exchanges caused by seiches and longer-period lake-level fluctuations influence riverine wetlands. Wetlands under substantial hydrologic influence from Great Lakes waters may be considered coastal wetlands.

Great Lakes wetlands vary in their size, character and flora (Albert, 2000). Their development is a function of complex interactions of hydrogeologic and climatic factors. In addition to these natural factors, economic forces (land-use/cover activities) play an important role in their development. Anthropogenic activities such as lake level regulation, urban development, agriculture and a variety of other activities have had a profound effect on the natural dynamics of wetlands. To better protect the health and to assure the sustainability of Great Lakes coastal wetlands a thorough understanding of natural physical factors (conditions and processes (Keough et al., 1999)) is needed. This knowledge is particularly important for establishment of ‘reference’ conditions (Karr et al., 1986; Karr, 1991; Lyons, 1992; Keough et al., 1999; Wilcox et al., 2002), a concept introduced for an overall assessment of ecosystem conditions, against which the degradation of wetlands can be measured. Of physical features, bedrock and surficial geology, topography, hydrology and climate are important factors influencing Great Lakes wetlands ecology. This paper presents an overview of the physical characteristics of the Great Lakes system that include the natural factors important in the Great Lakes coastal wetlands formation and provides basic information on the environmental parameters and land use within the basin.

Geographic setting

The Laurentian Great Lakes lie like inland seas halfway across the North American continent, stretching more than 1600 km from West to East (Figure 1). They are situated in a lowland corridor nearly halfway between the equator and the North Pole (Edsall, 1998). Because of the magnitude of the region which they cover, it is not surprising that the landscape of the Great Lakes Basin is highly variable in geology, climate, topography, vegetation and land use (Detenbeck et al., 1999). There are five adjoining temperate lakes, Superior, Michigan, Huron, Erie and Ontario, with a combined surface area of 245,759 km2 (Table 1). Together with their connecting channels, they are the world's largest freshwater ecosystem containing about one-fifth of the world's surface freshwater (about 23,000 km3). The lakes drain a watershed of 521,830 km2 (Fuller et al., 1995) through two outflows, St. Lawrence River at the East and Chicago Sanitary and Ship Canal at the West. Outflow from the Great Lakes is relatively small (less than 1% per year of their volume), with 90 m3 s−1 flowing through the Chicago Canal and 6,940 m3 s−1 via St. Lawrence River (R. Moulton, Environment Canada, Burlington, ON, Canada, pers. comm.), which conveys the waters of Lake Ontario into the Atlantic Ocean, 1670 km away. Regulation of these outflows in addition to natural factors such as over-lake precipitation and runoff from the surrounding land (Yee et al., 1993) contribute to changes in water levels in the lakes. The largest and the deepest of the five lakes is Lake Superior, with the average depth of 147 m and maximum depth of 405 m (Table 1). The outflow from Lake Superior enters Lake Huron through St. Mary's River, a 97-km waterway that is the northernmost part of the connecting channel system. Lake Huron also receives water from Lake Michigan through the Straits of Mackinac. Because these two lakes are at the same elevation (Table 1), they are sometimes referred to as one lake hydrologically (Yee et al., 1993). Lake Huron and Lake Erie are connected by a 143-km long channel consisting of St. Clair and Detroit rivers, and Lake St. Clair. Lake Erie, with the average depth of 19 m and the maximum depth of 64 m, is the shallowest of all the Laurentian Great Lakes. Lake Erie is linked to Lake Ontario by a 60 km stretch of Niagara River, with an average discharge of 5,700 m3 s−1 and the man-made Welland Canal. Lake Ontario, the last in the chain of the Great Lakes, is the smallest of all five lakes, with a surface area of 19,000 km2.

Figure 1.

Map of North America, showing the location of the Great Lakes. The insets show (a) the Great Lakes region outlining the Great Lakes-St. Lawrence basin, and international, state and provincial boundaries; (b) outline of Precambrian igneous and metamorphic rock of the Canadian Shield and Paleozoic sedimentary rock of the southern Great Lakes region. Adapted from Environment Canada (2002).

Figure 1.

Map of North America, showing the location of the Great Lakes. The insets show (a) the Great Lakes region outlining the Great Lakes-St. Lawrence basin, and international, state and provincial boundaries; (b) outline of Precambrian igneous and metamorphic rock of the Canadian Shield and Paleozoic sedimentary rock of the southern Great Lakes region. Adapted from Environment Canada (2002).

Physical features of the Great Lakes (adapted and revised from U.S. EPA).

Table 1.
Physical features of the Great Lakes (adapted and revised from U.S. EPA).
 Superior Michigan Huron Erie Ontario Totals 
Elevation (m) 183 176 176 173 74  
Average Depth (m) 147 85 59 19 86  
Maximum Depth (m) 406 282 229 64 244  
Volume (km) 12,100 4,920 3,540 484 1,640 22,684 
Water Area (km282,100 57,800 59,600 25,700 18,960 244,160 
Land Drainage Area (km2127,700 118,000 134,100 78,000 64,030 521,830 
Total Area (km2209,800 175,800 193,700 103,700 82,990 765,990 
Shoreline Length (km) 4,385 2,633 6,157 1,402 1,146 17,017 
Retention Time (y) 191 99 22 2.6  
 Superior Michigan Huron Erie Ontario Totals 
Elevation (m) 183 176 176 173 74  
Average Depth (m) 147 85 59 19 86  
Maximum Depth (m) 406 282 229 64 244  
Volume (km) 12,100 4,920 3,540 484 1,640 22,684 
Water Area (km282,100 57,800 59,600 25,700 18,960 244,160 
Land Drainage Area (km2127,700 118,000 134,100 78,000 64,030 521,830 
Total Area (km2209,800 175,800 193,700 103,700 82,990 765,990 
Shoreline Length (km) 4,385 2,633 6,157 1,402 1,146 17,017 
Retention Time (y) 191 99 22 2.6  

The Great Lakes are bordered by the states of New York, Pennsylvania, Ohio, Michigan, Indiana, Illinois, Wisconsin and Minnesota and the Province of Ontario (Figure 1a). Except for Lake Michigan, which is entirely within the United States, waters and the shoreline of the Great Lakes are shared jurisdictions of Canada and the United States.

Great Lakes wetlands are interspersed along the shoreline of all Laurentian Great Lakes. In total, there are about 1500 coastal wetlands (Herdendorf, 2004) along the 17,017 km (Table 1) of the Great Lakes shoreline. It has been estimated that they cover an area of 1,730 km2 (Herdendorf, 2004). Lake Michigan has the greatest number and the largest area of coastal wetlands followed by Lake Superior.

Geology

The watershed of the Great Lakes extends over three physiographic (landform) provinces: Canadian Shield, Central Lowlands and St. Lawrence Lowlands. Three types of bedrock, namely igneous, metamorphic and sedimentary rock underlie the area around the Great Lakes. This bedrock is a result of processes occurring in the area of the Great Lakes basin during the early geologic history of the North American continent. The foundation of the basin was laid down about 3 billion years ago during the Precambrian Era. This was an era of intense volcanic activity and great stresses when early sedimentary and volcanic rocks were folded and heated, resulting in formation of metamorphic rocks which together with the igneous rocks form the Canadian Shield. The igneous and metamorphic bedrock of Canadian Shield form the northern and northwestern portion of the basin (Figure 1). The hard erosion-resistant granitic rocks of the shield continue southward beneath the Paleozoic sedimentary rocks, where they form the ‘basement’ structure of the southern and eastern portion of the basin (Fuller et al., 1995). The sedimentary rocks were deposited during the Paleozoic Era (185–520 million years ago), when most of central North America was flooded repeatedly by warm and shallow seas. During this time, lime silts, clays, sand and salts were deposited and subsequently consolidated into limestone, shales, sandstone, halite and gypsum. These softer sedimentary deposits are more physically and chemically erodable than the granitic rocks of the Precambrian Shield in the northern and north-western part of the basin. The differences in the geology of the watershed are reflected in the water and sediment chemistry in each of the Great Lakes and without a doubt influence the establishment and development of coastal wetlands.

During the Pleistocene Epoch of the Cenozoic Era (most of the last million years), the continental glaciers repeatedly advanced over the Great Lakes region from the north. The glaciers which were up to 2,000 m thick scoured the surface of the earth and reworked the preglacial deposits, as they advanced forward. These processes substantially modified the preglacial drainage system (Sly and Thomas, 1974). The preglacial river valleys, deepened and enlarged by ice scour formed the basins for the Great Lakes. Millennia later, the climate became warmer and the glacier retreated, leaving behind large volumes of meltwater. Because the land was depressed from the large weight of ice sheet, glacier lakes, precursors of the present Great Lakes, formed. These lakes were much larger than current Great Lakes. The cycle of advancement and retreat of ice sheet was repeated several times. Four major glacial stages (Wisconsinan, Illinoian, Kansan and Nebraskan) and three interglacials (Sangamonian, Yarmouthian, and Aftonian) are recognized in North America. The current configuration of the basin was left behind by the most recent (Wisconsinan) glaciation about 10,000 years ago. After the last retreat of glaciers, coastal processes have redistributed the glacial deposits and eroded bedrock headlands modifying the coastline and creating suitable conditions for wetlands. The tills and parent soils distributed through the region by the ice sheet weathered differently to produce soils of varying fertility and moisture-holding capacity (Edsall, 1998). In the north, where the terrain is dominated by granitic bedrock, a thin layer of acidic soils developed. In the southern areas, deeper soils, containing glacial deposits of clays, silts, and sands developed. The calcareous soils, derived from limestone and other sedimentary rock, were generally more suitable for colonization by plants than the soils produced from sandstone and granitic bedrock.

The type of bedrock affects the weathering process and so governs the concentrations of key constituents in water, influencing the water chemistry of the lakes. For instance, Lake Superior is largely surrounded by granitic bedrock that is much less alkaline and contains fewer ions (lower alkalinity and specific conductance) than Lakes Michigan or Erie, which are surrounded by sedimentary bedrock and glacial deposits (Keuogh et al., 1999). Moreover, the type of bedrock, topography and coastal processes affect the coastal erosion and substrate development and together with precipitation patterns play an important role in wetland evolution.

Climate

The mid-continent location of the Laurentian Great Lakes exposes them to a considerable range of climatic conditions. The climate of the region ranges from the subarctic in the north to humid continental warm in the south (Edsall, 1998). There are three factors that influence weather in the Great Lakes basin: air masses from other regions, location of the basin within a large continental land mass, and the moderating influence of the lakes themselves (Fuller et al., 1995). The variable weather of the Great Lakes region is a result of alternating flows of warm, humid air from the Gulf of Mexico and cold dry air from the Arctic (Fuller et al., 1995). In the summer, the tropical air masses from the Gulf of Mexico are most influential in the southern part of the basin, while northern region generally receives cool dry air from the northwest. In the winter, cold dry arctic air is moderated by the warmer waters of the Great Lakes, while incursions of air masses from the Gulf of Mexico are much less frequent. The lakes are slower to warm and cool than the land, hence they act as heat sinks in the summer and heat source in the winter (Edsall, 1998). This phenomenon and the large size of the lakes are responsible for their moderating effect on the regional climate by cooling summers and tempering winters. However, a marked north-south climatic gradient is evident and has a major impact on the regional ecology. In addition to their moderating effects on climate, the lakes also influence the precipitation patterns by increasing the amount of rain and snow in the region. Air masses moving across the lakes pick up moisture from the lake surface and drop it at the lee side. In the winter, these regions receive heavy snowfalls and the areas are known as snowbelts. The mean annual frost-free period, a general measure of the growing season varies between 60 days at the higher elevations in the north and 160 days in lakeshore areas in the south, with more than 200 days for Lake Erie Islands (Goddard, 1998). The climate, including precipitation patterns, is another major factor that influences the wetlands evolution, including their plant assemblages.

Land use

It has been shown that land use in the watershed significantly impacts water and sediment quality of Great Lakes wetlands (Crosbie and Chow-Fraser, 1999). Therefore, information on the land use in the Great Lakes basin is of paramount importance for understanding of wetland functions and associated structural changes.

Land use patterns in the watershed surrounding the Great Lakes are diverse. About 55% of the land in the Great Lakes region is forested (Sea Grant Michigan and Michigan State University Extension, 2000). It is predominantly the northern part of the watershed that is covered by forests. Coniferous forests typically cover the northern part of the watershed, while hardwood forests may be found in the south. Grasslands and prairies covered the ground in the southern part of the watershed until settlement brought about major changes to the landscape. Much of the land, in this part of the basin was converted to agriculture. Intensive agriculture in the Great Lakes basin contributes about 25% and 7% to Canadian and U.S. production, respectively. Because the Great Lakes basin ecology and land use are so diverse, it was suggested that the present and historic wetland coverage is best viewed through reference to ecoregions (Omernik and Gallant, 1988; Detenbeck et al., 1999). Ten ecoregions were independently defined in each Canada and United States. Their names change at the border, as they were independently defined by each of the two countries (Detenbeck et al., 1999).

Great Lakes are a backyard for a total of 33 million people, of which 9.2 million reside in Canada (Government of Canada, 2002) and 23.8 million in the U.S. (Fuller et al., 1995). The human population in the basin increased sharply after settlement of the region in 1850. In 1990, there were more than 95 metropolitan-sized communities, larger than 50,000 inhabitants, in the Great Lakes basin. Except for the Toronto (ON) metropolitan area, there are relatively few centres with the large populations along the northern shorelines, whereas there are numerous large population centres with associated intense urban and industrial development along the southern shorelines. Major U.S, cities along the Great Lakes shorelines include Buffalo, NY, Erie, PA, Cleveland, OH, Detroit, MI, Chicago, IL, Milwaukee, WI, and Duluth, MN. Urban sprawl is a serious concern to Great Lakes environmentalists. According to recent Great Lakes Basin statistics (Sea Grant Michigan and Michigan State University Extension, 2000) 26.5 and 18.6% of U.S. and Canadian shoreline use, respectively is residential, 6.7 and 2.7% commercial/industrial, and 1.5 and 8.2% is agricultural. The remaining 65.3 and 70.5% of U.S. and Canadian shoreline use includes transportation and communication, recreation, wetlands, forestry, grassland and other unknown uses.

In the past, the fragile nature of this vast ecosystem was not recognized and the lakes were mistreated for economic gains. Over the past 200 years, human activities altered and stressed the Great Lakes, affecting some regions more than others. Nearshore areas and coastal wetlands were the first to show the impacts of human development, as they are the zones of intense and often conflicting human use. There was a significant loss of wetland habitat resulting from urbanization and residential or recreational development on the U.S. side and conversion to agricultural land on both U.S. and Canadian sides. The loss of wetlands through reclamation and alteration coincided with major changes in fisheries and chemistry of the Great Lakes (Krieger et al., 1992). The Great Lakes and their wetlands experienced increased sediment and nutrient loads, resulting from intensive anthropogenic activities in the watershed. This resulted in increased water turbidity and changes in trophic status. Increased nutrient concentrations have lead to increased productivity of the ecosystems and changes in plant species.

Wetlands classification

As mentioned earlier, both natural and anthropogenic factors play important roles in coastal wetlands development. To define different types of wetlands and to differentiate between their structural and functional characteristics, a development of wetland classification system was essential. Several classifications of wetlands have been developed and used since early 1900 (Cowardin et al., 1979; Brinson, 1993; Zoltai and Vitt, 1995; Warner and Rubec, 1997; Mitsch and Gosselink, 2000). Classification based on the forcing functions or driving variables offered a promising approach (Mitsch, 1992). The natural and human-induced forcing functions which have major influence on coastal wetlands include (Mitsch, 1992): 1) watershed inflows, such as varying hydrologic, nutrient and toxic loadings from watersheds; 2) shoreline sediment dynamics, including changes in hydrologic, chemical and biological connection between wetlands and the Great Lakes resulting from sediment movement; 3) water level fluctuation in the Great Lakes, both seasonal and annual; 4) periodic seiches and ‘wind tides’; and 5) artificial impoundment construction and water level manipulation by humans.

A hydrogeomorphic wetland classification system used for assessing physical, chemical and biological functions of wetlands has proven to be particularly useful for comparing the level of functional integrity of wetlands (Mitsch and Gosselink, 2000). The three components of this classification system are geomorphology (topographic location of wetland in the surrounding landscape), water source (precipitation, surface or near-surface flow and groundwater discharge) and hydrodynamics (direction and strength of water movement within a wetland). Based upon this system Wilcox (1995) grouped coastal wetlands into following classes: open shoreline, shallow sloping beach, unrestricted bay, barrier beach, lake-connected, restricted riverine, river delta, and diked wetlands.

Classification was further simplified by Keough et al. (1999) who classified the Great Lakes coastal wetlands based on their physical and hydrological characteristics into three broad categories: open, drowned river mouth/flooded delta, and protected wetlands. A variety of hydrologic, chemical and biological features characterize each of the three categories. Open coast wetlands have a direct surface connection to the lake and can be, therefore, influenced by hydraulic processes such as variety of currents and ice push. Because of the hydraulic stress, their bottom substrate is largely inorganic, ranging from clay to gravel or even exposed bedrock. Drowned river mouth and flooded delta wetlands are subjected to both coastal and riverine processes. They occupy flooded river valleys (or cap deltas), while they have a direct surface water connection with the lake. Protected wetlands are isolated from most direct hydraulic processes occurring in the lake. They are most often found behind a sand barrier, therefore, a high rate of sediment supply (largely due to littoral sand accretion) is essential for their formation. Because they are somewhat isolated from the hydraulic processes (currents, etc.) thick organic substrate may accumulate in these wetlands. The three categories listed here represent the end members only (Keough et al., 1999). A continuum exists between these end members, between which most of the coastal wetlands migrate.

On behalf of the Great Lakes Wetlands Consortium, Albert et al. (2003) developed a classification scheme using three specific systems (Lacustrine, Riverine, and Barrier–Protected) based on their dominant hydrologic source and current hydrologic connectivity to the lake. Lacustrine systems are further broken down into Open Lacustrine and Protected Lacustrine. Riverine systems are further categorized into Drowned River Mouth, Connecting Channels, and Deltas. Barrier-Protected systems include subcategories of Barrier Beach Lagoon and Swale Complexes.

Wetland research: Problems and issues

Although coastal wetlands are vitally important for an overall health of the Great Lakes ecosystem, their numbers continue to decline. More than two-thirds of the Great Lakes wetlands have already been lost and many remaining wetlands are threatened by development, drainage or pollution (Fuller et al., 1995). What does the future hold for the coastal wetlands of the Great Lakes? Considering their importance in the Great Lakes ecosystems and with mounting pressures to develop the remaining wetlands, there is an increasing need for their protection and rehabilitation. To this end, government agencies, academia and non-governmental groups, including community groups and private sector, are working together in an effort to conserve the existing, and rehabilitate the degraded, coastal wetlands of the Great Lakes. A major initiative is underway to develop procedures for assessment of the basin-wide health and function of the Great Lakes wetlands, against which progress in their conservation and restoration can be measured. Similarly as for the other Great Lakes ecosystems, the International Joint Commission (1991) has proposed a practical framework for applying chemical, physical (non-biological), and biological (flora/fauna) indicators that can be used to measure the integrity of the wetland ecosystems. An indicator can be defined as a value that reflects the current conditions of an environmental function of the ecosystem and provides a reliable way to detect the trends (both spatial and temporal). Monitoring is, therefore, fundamental to success of indicator development and reporting process (International Joint Commission, 2002). This obviously requires an increased support for the indicator initiative. While the successful use of indicators to assess the state of the Great Lakes ecosystem has been shown at the State of the Lakes Ecosystem Conferences (SOLEC), hosted by the U.S. Environmental Protection Agency and Environment Canada, it has also been shown that 33 of 80 selected indicators discussed in the SOLEC 2001 Report were supported by very limited data (International Joint Commission 2002). This shortcoming prompted the Commission to caution SOLEC organizers that a balance needs to be struck between the number of indicators and the effort required to monitor them. Limited data on large number of indicators reduces their usefulness.

In 2000, the Great Lakes Coastal Wetlands Consortium was formed by a cooperative agreement between the U.S. EPA and Great Lakes Commission. The consortium included over 40 participating organizations representing both USA and Canada spanning federal, state and provincial agencies as well as academic institution and non-governmental organizations. The Consortium's purpose was to design and implement a long-term monitoring program that can be used to monitor coastal wetlands under the Great Lakes Water Quality Agreement. The Consortium recommended an index-of-biotic-integrity (IBI) approach using vegetation, macroinvertebrates, fish, amphibians and birds in conjunction with physical and chemical parameters to be used for basin-wide monitoring. Although the use of the wetland IBI was previously attempted on different ecoregions (Chow-Frazer and Albert, 1998; Burton et al., 1999; Wilcox et al., 2002), the Consortium's monitoring program using IBI is the first basin-wide monitoring program that will assist in partitioning of natural and anthropogenic disturbances. Presently, several pilot studies, investigating the variability and usefulness of SOLEC indicators (http://www.glc.org/wetlands/), are supported by the Consortium.

To conserve and better protect the Great Lakes coastal wetlands, cooperation between all levels of government and the public is essential to avert the mistakes of the past and prevent any further degradation and loss of Great Lakes wetlands. This is best accomplished when public consultations that include residents, private organizations, industry and governments are part of the decision making process.

Acknowledgments

This is an NWRI Contribution No. 04-313 and contribution 1275 of the U.S. Geological Survey, Great Lakes Science Center. We thank the two reviewers for their valuable comments and suggestions on the manuscript.

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