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Great Lakes Coastal Wetlands: Abiotic and Floristic
Characterization
Great Lakes coastal
wetlands occur along the Great Lakes shoreline proper and in portions of
tributary rivers and streams that are directly affected by Great Lakes
water regimes. These wetlands form a transition between the Great
Lakes and adjacent terrestrial uplands, and are influenced by both.
Over the past two decades, a wealth of detailed information has been
generated on these systems, ranging from a comprehensive inventory of
wetlands along the U.S. Great Lakes shoreline (Herdendorf et al.
1981a-1981f), to detailed studies on the hydrology, sediment history,
water chemistry, and flora of one or a few sites (e.g. Krieger 1989).
To date, however, there has been little emphasis on regional variability
within these wetlands and how that variability relates to environmental
parameters.
This study addresses that
gap, by providing a regional perspective on the abiotic variability and
associated vegetation characteristics of herbaceous coastal wetlands of
the Great Lakes shoreline. This study is based on field sampling
conducted in over 110 coastal wetlands in Minnesota, Wisconsin, Michigan,
Ohio, Pennsylvania, and New York by Michigan Natural Features Inventory (MNFI)
between 1987 and 1994 (Albert et al. 1987, 1988, 1989; Minc 1996a, 1996b,
1997a, 1997b, 1997c). The goal of MNFI's project was to identify
vegetatively distinct wetland types, and to develop a wetland
classification for the Great Lakes coastline that links floristic
variability to controlling abiotic factors.
Floristic variability
among coastal wetlands can be characterized in terms of specific
vegetative zones. Moving from deeper water to the shore, typical
zonation includes (1) the Submergent Marsh containing submergent and/or
floating vegetation; (2) Emergent Marsh characterized by shallow water or
saturated soils, and typically dominated by bulrushes, cat-tails, and
other emergent species, but also containing submergent and/or floating
vegetation; and (3) a narrow but diverse Shoreline or Strand Zone, at or
just above the water line where seasonal water-level fluctuations and
waves cause erosion, usually dominated by annual herbs. Inland from
the water's edge, additional zones can be identified: (4) the Herbaceous
or Wet Meadow zone characterized by saturated or periodically flooded
soils, and dominated by sedges, grasses, and other herbs; and (5) the
Shrub Swamp and (6) Swamp Forest zones, both characterized by periodic
standing water, and dominated by woody species adapted to a variety of
flooding regimes. Not all zones are present or well-developed in
every wetland.
In order to provide
baseline data on both the biotic and abiotic components of selected
coastal wetlands, sampling transects were established to include the full
range of vegetation zones present, extending from the upland boundary
lakeward to water depths of approximately 2 m. At 10-20 m intervals
along the transect, the cover value for each species present was recorded,
along with data on substrate, organics, and water depth. Species
cover values were then summed by vegetation zone and their mean cover
values computed, in order to generalize species behavior throughout the
zone as a whole. These data were then integrated with regional
information on bedrock characteristics, glacial landform, soils,
topography, shoreline configuration, and water clarity.
The present study
concentrates on patterns of species co-occurrence and distribution in
three wetland zones (emergent marsh, herbaceous/wet meadow, and shrub
swamp), and the results were combined into a final, synthetic
classification of Great Lakes coastal wetlands. Data for these
analyses were taken along 130 transects representing 102 marshes sampled
in the field (Fig. 1).1 The
vegetative analyses had two specific goals: first, to define groups of
transects with similar species composition that represent distinct wetland
types; and second, to clarify the site factors controlling species
distribution and hence the occurrence of the different wetland types.
The identification of
distinctive patterns of species co-occurrence relied heavily on TWINSPAN
or Two-Way Indicator Species Analysis, a polythetic divisive method of
classification of samples based on the differential occurrence or
abundance of one or more indicator species (Hill 1973, 1979). This
analysis results in an ordered two-way table that groups similar samples
together and similar species together, thereby displaying the significant
divisions within the data set. Site factors determining species
distributions were assessed through a variety of methods. Key
aspects of spatial or geographic patterning were identified through
regional plots of species density. Species preferences for specific
types of substrate and shoreline configuration were clarified through
maps, tables, descriptive statistics, and bar plots, where appropriate.
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1. Transects
excluded from the study typically had high levels of human disturbance.
Patterns of species
co-occurrence and distributions are determined by multiple abiotic
factors, including aquatic system, Great Lakes water level fluctuations,
surficial bedrock, glacial landform, climate, and land use. These
factors operate at different scales (from regional to local), and the
importance of any one factor varies markedly across the region.
A.
Aquatic System
Four major aquatic
systems, defined largely on water flow characteristics and residence time
(Sly and Busch 1992), are applicable to the Great Lakes Basin; each has a
different potential influence on associated coastal wetlands.
Lacustrine
systems are controlled directly by waters of the Great Lakes, and
involve wetlands of the Great Lakes shoreline strongly affected by
littoral (longshore) currents and storm-driven wave action.
Lacustrine habitats generally experience the greatest exposure to wind
and wave action and to ice scour, the primary agents responsible for
shore erosion and redeposition of sediments.
Connecting channels
refer to the major rivers linking the Great Lakes, including the St.
Marys, Detroit, St. Clair, Niagara, and St. Lawrence rivers.
Connecting channels are characterized by a large flow, but seasonally
stable hydrology; their shallowness and current result in earlier spring
warming and better oxygenation than in other aquatic systems. All
the connecting channels are modified, primarily to accommodate shipping.
Riverine aquatic
systems refer to smaller rivers tributary to the Great Lakes whose water
quality, flow rate, and sediment load are controlled in large part by
their individual drainages. Tributary rivers have a much lower
volume, but seasonally more variable flow than connecting channels, and
are influenced by the Great Lakes near their mouth.
Lacustrine or freshwater estuaries, formed where some tributary
rivers enter the lakes, are aquatic systems distinctive to the Great
Lakes, and represent a zone of transition from stream to lake within
which water level, sedimentation, erosion, and biological processes are
controlled by fluctuations in lake level.
B. Fluctuations in Great Lakes Water Levels
The preceding aquatic systems are influenced, to differing degrees, by
fluctuations in Great Lakes water levels. These fluctuations occur over
three temporal scales: (1) short-term fluctuations in water level caused
by persistent winds and/or differences in barometric pressure; (2)
seasonal fluctuations reflecting the annual hydrologic cycle in the Great
Lakes basin; and (3) interannual fluctuations in lake level as a result of
variable precipitation and evaporation within their drainage basins.
All of these scales contribute to the dynamic character of coastal
wetlands, although interannual fluctuations impose the greatest stress.
These extreme lake-level fluctuations can range from 3.5 to 6.5 feet
(1.3-2.5 m), and occur with no regular periodicity. In general, as water
levels rise and fall, vegetation communities experience a locational
shift: landward during high-water years, and lakeward during low-water
years. However, fluctuating lake levels effect not only a change in water
depth, but a broad range of associated stresses to which plants must
respond, including changes in water current, wave action, turbidity
(clarity or light penetration), nutrient content or availability,
alkalinity, and temperature, as well as ice scour and sediment
displacement. Since individual species display different tolerance limits
along one or more of these dimensions, species composition can also change
dramatically within a zone.
Conversely, the absence or dampening of natural lake-level fluctations
alters species composition as well. Coastal wetland systems are adapted to
and require periodic inundation. Where water-level regulation has
significantly reduced the occurrence of extreme high and low water levels,
disruption of the natural cycle favors species intolerant of water-depth
change and associated stresses, and/or excludes species requiring periodic
exposure of fertile substrates, potentially leading to a reduction of
species diversity. The dominance of cat-tails in many Lake Ontario marshes
suggests a trend toward reduced species diversity following a reduction in
the amplitude of natural water-level fluctuations (Wilcox et al. 1993).
C. Surficial Bedrock
The physical and chemical characteristics of different surficial
bedrock types can affect both wetland location and species composition.
The major bedrock distinction in the Great Lakes Basin is between igneous
and metamorphic bedrock (including granite, basalt, and rhyolite) of the
Precambrian period, and younger (Paleozoic) sedimentary bedrock (including
sandstone, shale, limestone, and dolomite). Igneous and metamorphic
bedrocks form the north shore of Lake Superior and Georgian Bay, and line
much of the St. Lawrence River; they are locally present along the
southern shore of western Lake Superior as well, where they co-occur with
younger sedimentary rock, primarily sandstone. In contrast, the softer,
sedimentary bedrock types underlie Lakes Michigan, Huron, St. Clair, Erie,
and Ontario.
The physical structure of bedrock type limits the distribution
of coastal wetlands at a regional scale. The rugged Lake Superior
shoreline of sandstone, igneous, and metamorphic rock lacks the shallow
protected waters and fine-textured substrates that support broad coastal
wetlands; coastal wetlands exist only behind protective barrier beaches or
are localized at stream mouths. In contrast, the horizontally-deposited
marine and near-shore sedimentary rock which underlies Lakes Michigan,
Huron, St. Clair, Erie, and Ontario, provides broad zones of shallow water
and fine-textured substrates for marsh development.
Where bedrock is at or near the surface, bedrock chemistry
affects wetland species composition. Soils derived from much of the
Precambrian crystalline bedrock are generally acid and favor the
development of poor fen or bog communities. In contrast, soils derived
from marine deposits, including shale and marine limestone, dolomite, and
evaporites, are typically more calcareous (less acid), nutrient- and
moisture-rich loams and clays; where these bedrock types are at or near
the surface, their alkalinity creates the preferred habitat for
calciphitic species.
D. Glacial Landform
Today, glacial landforms, in combination with recent longshore
transport processes, create the prevalent physiographic features along
much of the Great Lakes shoreline. Their characteristic differences in
substrate, soils, slope, and drainage conditions largely determine both
natural shoreline configuration and sediment composition. These, in turn,
generate distinctive contexts for wetland development that vary in their
exposure and resilience to lake stresses, and in their floristic
composition.
The major morphometric or site types represented in the wetland systems
inventoried for this study are presented in Table 1 (see also
Plates 1-4).
Among freshwater systems, many of these geomorphic features are unique to
the Great Lakes coasts, and are typically overlooked in national wetland
classification schemes. However, the importance of these features for
classifying Great Lakes coastal wetlands is clear. As Herdendorf et al.
(1981a:110) state it, "the occurrence, distribution, and diversity of
[Great Lakes] coastal wetlands is, in part, determined by the morphology
of the coast. Perhaps in no other geographic environment is the
relationship between landforms and vegetation so evident".
Several of these morphometric types can co-occur; other types are
gradational. The site types are not mutually exclusive categories, rather
they illustrate how the convergence of landform and lake create and
influence wetland and aquatic habitats. Further, since the floristic
diversity of a wetland is dependent on the diversity of wetland habitats,
the variety of morphometric types represented is significant for
understanding the vegetational characteristics of a site.
E. Climate
Regional patterns of climatic variability within the Great Lakes Basin
are largely determined by latitude, with the modifying influence of the
lakes (i.e. lake effect) operating at a more local level (Derecki 1976;
Eichenlaub et al. 1990). The strong latitudinal gradient, from southern
Lake Erie to northern Lake Superior, creates marked differences in length
of growing season and annual input of solar energy across the region.
These differences, in turn, are reflected in the regional distributions of
a number of species common to Great Lakes wetlands.
While most aquatic macrophytes are widely distributed, species with
known southern affinities make their appearance, as do those of the boreal
forest. Lake Erie wetlands, for example, are rich in southern marsh
species at the northern edge of their range which rarely occur along the
other Great Lakes; a southern wet-prairie floristic element is apparent
there as well (Stuckey 1989; Keddy and Reznicek 1985, 1986). Both of these
southern floras differ significantly from the complex of boreal, subarctic,
and arctic species found in the northern portions of Lakes Huron,
Michigan, and Superior. Other species common to Great Lakes coastal
wetlands reveal regional concentrations corresponding to a North-South
gradient; these are species with wide distributions which achieve a
greater density toward either the northern or southern edge of the Great
Lakes Basin (Minc 1997c).
F. Human Land Use and Anthropogenic Stress
Differences in land use -- whether urban, agricultural, or forested
-- create regional differences in the extent and quality of wetlands, as
well as in their species composition. To a large extent, land use is a
composite variable reflecting climate, physiography, and soils. The tension
zone, a rough climatic boundary separating the forested north from the
more agricultural south, closely follows regional differences in summer
mean daily air temperature. Urban development, in contrast, reflects the
early location of good harbors and the distribution of natural resources
such as timber and mineral ores.
Both urban and agricultural development have resulted in severe
degradation and loss of coastal marshes through pollution, land
management, and ecosystem alteration:
 Urban
development
-
Armoring of the shoreline and dredging of channels
to create harbors has resulted in marsh elimination.
-
Dumping of waste materials such as sawdust and
sewage, and a wide variety of chemicals has mechanically and
chemically altered the shallow-water marsh environment, increasing
turbidity, reducing oxygen concentrations, and altering the pH
-
Shipping traffic has mechanically eroded shoreline
vegetation. ·Water-level control of the Great Lakes and connecting
rivers has altered natural wetland dynamics.
Agriculture
- Drainage has eliminated large areas of marshes and coastal
wetlands.
- Sedimentation has greatly increased turbidity, eliminating
submergent species requiring clear water.
- Nutrient loading has locally reduced oxygen levels, prompted algal
blooms, and led to the dominance of high-nutrient tolerant species
such as cat-tails.
- Heavy agricultural sedimentation has led to the deposition of rich
organic mud in the wet meadows and along the shoreline, favoring the
dominance of early successional species.
- Introduction of exotic plants has altered macrophyte species
composition.
The preceding abiotic variables (including aquatic system, water level
fluctuations, surficial bedrock, glacial landform, climate, and land use)
combine to determine the distribution, as well as the morphology, species
composition, and quality, of Great Lakes coastal wetlands. The final,
synthetic classification of Great Lakes coastal wetlands (based on both
abiotic and vegetation analyses) identified nine groups, each with
distinctive floral characteristics and a restricted geographic
distribution (Table 2). Vegetation zonation and key species (i.e. species
showing a preferential distribution relative to each group) are discussed
below.
(1) Lake Superior Poor
Fen.
This
group contains most of the wetlands sampled along the Lake Superior
shoreline. These wetlands occupy sheltered sites, including barrier-beach
lagoons, estuaries, and tributary river deltas, since marshes cannot
develop along unprotected stretches of Lake Superior's harsh shoreline.
These sites are characterized by fairly acidic, sandy soils and an extreme
northern climate. As a result, organic decomposition is retarded and deep
organic soils develop.
Characteristic vegetation includes northern poor fen in the herbaceous
zone grading into poor shrub fen at the inland wetland periphery; the poor
fen is typically the most extensive zone within Lake Superior wetlands.
Species showing strong preferences for this habitat include Sphagna
spp., the forbs Sarracenia purpurea (pitcher-plant), Menyanthes
trifoliata (buckbean), Rhynchospora alba (beak-rush), Triadenum
fraseri (marsh St. John's-wort), Pogonia ophioglossoides (rose
pogonia), and the shrubs Chamaedaphne calyculata (leatherleaf), Andromeda
glaucophylla (bog rosemary), Myrica gale (sweet gale), Vaccinium
macrocarpon (large cranberry) and V. oxycoccus (small
cranberry). Continuity in species composition for northern poor fen is
strong across a considerable range of lake levels (Minc 1997b), although
extreme high lake levels reduce the abundance of some characteristic
species, including Sphagnum spp. and Chamaedaphne calyculata.
The emergent zone, typically only a narrow fringe, contains species
associated with clear, well-aerated waters, including a low-density mix of
Eleocharis smallii (spike-rush), Sparganium fluctuans (bur-reed),
and the bulrush Scirpus subterminalis. Common floating-leaved
species include Nuphar variegata (yellow pond-lily), Brasenia
schreberi (water shield), and Megalodonta beckii
(water-marigold), while the pondweed Potamageton gramineus is the
most frequently encountered submergent species.
(2) Northern Rich Fen.
This group
comprises coastal sites concentrated near the Straits of Mackinac and
located on marly substrates. Most of these sites occupy embayments of the
open, sandy shoreline where limestone bedrock or cobble is at or near the
surface. These sites have calcareous soils (with a pH as high as 8.2),
resulting either from calcareous substrates, water flow off adjacent
limestone bedrock or limestone-rich till, or algal precipitation of
calcium carbonate in the relatively warm, carbonate saturated waters. The
result is the formation of distinctive "marly flats" and an
associated complex of calciphile plant species.
The calciphiles Chara sp. (muskgrass) and Eleocharis
rostellata (spike-rush) frequently dominate the emergent zones, along
with Scirpus acutus (hardstem bulrush); overall species diversity
is low. The herbaceous zone -- the most distinctive and diagnostic zone --
is consistently a northern rich fen. Calamagrostis canadensis
(blue-joint grass) can dominate, but the calciphiles Carex viridula
(sedge) and Lobelia kalmii (Kalm's lobelia) are key species for
this group. Other species indicative of rich fen along the Great Lakes
shoreline include Cladium mariscoides (twig-rush), Potentilla
anserina (silverweed), Panicum lindheimeri (panic grass), Triglochin
maritimum (common bog arrow-grass), and Hypericum kalmianum (Kalm's
St. John's-wort). Common woody species of the associated rich shrub fen
include Myrica gale and Potentilla fruticosa (shrubby
cinquefoil), while Larix laricina (larch) and Salix pedicellaris
(bog willow) are consistently present, but in low numbers. This
characteristic suite of calciphiles make the Northern Rich Fen type
readily recognizable across a range of lake-level fluctuations (Minc
1997b).
(3) Northern Great Lakes
Marsh.
This group includes all marshes along the St. Marys River, as well as
circumneutral sites of Lake Superior and northern Lakes Michigan-Huron; it
is the largest group of Great Lakes wetlands sampled. Marshes of this type
occur on a diversity of glacial landforms and substrates, including clay
lakeplain, sand lakeplain, and sandy ground moraine. Sites types vary:
Lake Superior northern marshes typically inhabit open water and stream
margins within estuarine sites, often within a much larger poor fen; those
of northern Lakes Michigan-Huron are typically found in relatively
protected coastal embayments. The largest group of sites, however, is the
channel-side wetlands and embayments along the St. Marys River.
The open emergent zone features low densities of Scirpus acutus
(hardstem bulrush) and Eleocharis smallii (spike-rush), along with Scirpus
subterminalis (bulrush), Equisetum fluviatile (water
horsetail), Najas flexilis (slender naiad), and Sparganium
eurycarpum (common bur-reed). The submergent pondweeds Potamageton
gramineus and P. natans are common occurrences in this zone as
well. The herbaceous zone is consistently a northern wet meadow dominated
by Calamagrostis canadensis (blue-joint grass), and the sedges Carex
stricta and C. lacustris; key forbs include Campanula
aparinoides (marsh bell-flower) and Potentilla palustris (marsh
cinquefoil).
A narrow band of northern shrub meadow, often only 10-20 meters wide,
borders many of these wetlands. Shrubby species strongly preferential to
this zone include Spiraea alba (meadowsweet) and Salix
petiolaris (meadow willow), but other wide-spread woody species are
found here as well, primarily Alnus rugosa (speckled alder) and Myrica
gale; Calamagrostis canadensis, Carex stricta, C.
lacustris, and Potentilla palustris remain dominant as in the
adjacent wet meadow zone.
(4) Green Bay Disturbed Marsh.
This
Lake Michigan group contains a small number of relatively well-protected
sites, including deltaic channels, estuarine channels, and sheltered
sand-spit embayments, primarily within Green Bay, WI. These sites are
located near the tension zone and display both northern and
southern vegetation characteristics. In general, the emergent zones of
these sites contain a more southern flora high in floating species, while
their wet meadow zones span the north-south division. These sites also
share a highly disturbed habitat. The adjacent flat, poorly drained clay
lakeplain has been intensively farmed with row crops, and waters of Green
Bay are generally characterized as quite turbid, owing both to erosion
from agricultural activities and to industrial and urban pollution.
Emergent zone dominants are species associated with quiet,
nutrient-rich waters, and typically more abundant in the southern Great
Lakes. Key species include Ceratophyllum demersum (coontail), Elodea
canadensis (common waterweed), Lemna minor (small duckweed),
and Spirodela polyrhiza (great duckweed), along with the widespread
Nymphaea odorata (sweet-scented waterlily) and Sagittaria
latifolia (common arrowhead). The herbaceous zone is a wet meadow in
which Calamagrostis canadensis is clearly the dominant, while both Carex
stricta and C. lacustris are present in low levels. Wet meadow
species more characteristic of the south include Impatiens capensis
(spotted touch-me-not) and Typha angustifolia (narrow-leaved
cat-tail), as well as the exotics Lythrum salicaria (purple
loosestrife), Phragmites australis (giant bulrush), and Phalaris
arundinacea (reed canary grass). A distinct shrub zone was seldom
encountered in sampling transects, due to heavy disturbance in the upland
portion of the landscape.
Owing to the relatively flat topography, fluctuations in Lake
Michigan's water level considerably alter the size of these coastal
wetlands as well as their species composition (Harris et al. 1977).
Receding high waters expose substantial portions of sandy beach and open
mud flats, which are quickly colonized by dense stands of Scirpus
validus (softstem bulrush), Bidens cernuus (nodding
bur-marigold), and one or more species of Polygonum (smartweed).
Over a period of several years, these colonizing species decline and are
replaced by a sedge meadow consisting primarily of Carex spp. and Calamagrostis
canadensis (Harris et al. 1981).
(5) Lake Michigan
Lacustrine Estuaries.
This group consists of barred lacustrine estuaries of western Lower
Michigan, generally south of the tension zone. All of the major rivers
along this stretch have lacustrine estuaries at their mouths. Most are
partially to largely barred by longshore sand transport, and many have
artificially maintained channels to Lake Michigan. These estuarine systems
can extend for a considerable distance inland, where the rivers occupy
linear floodplains cut into surrounding glacial moraines and sand
lakeplain. Sites of this group are well protected from wind and wave
action, owing to their long, narrow configuration and partial separation
from Lake Michigan. This protection results in deep accumulations of
organic deposits (mucks and peats) throughout the emergent and herbaceous
vegetation zones; open stream channels are generally shallow and nutrient
rich, owing to the input of fine sediments and the presence of deep
underlying organic substrates.
In the emergent zone, Nuphar advena (yellow pond-lily) and Peltandra
virginica (arrow-arum) are characteristic of these muck soils, while
the large cover values for the floating species Ceratophyllum demersum
and the duckweeds Spirodela polyrhiza, Lemna trisulca, and L.
minor reflect relatively protected waters with a high nutrient
content. Nymphaea odorata has a wide-spread distribution, but can
form particularly dense beds in these protected sites.
The herbaceous zone conforms to the southern wet meadow type. Calamagrostis
canadensis is a frequent dominant, but key southern species include Impatiens
capensis, Rorippa palustris (yellow cress), Polygonum lapathifolium
(nodding smartweed), and Leersia oryzoides (cut grass). The
southern shrub swamp is characterized by a mix of Alnus rugosa, Cornus
stolonifera (red-osier dogwood), and Fraxinus pennsylvanica
(red ash), but many of the above-mentioned herbaceous species occur
throughout. In addition, Osmunda regalis (royal fern) is common
here.
Vegetative response to changing lake levels in these estuarine sites
appears to be largely a function of site morphometry (Minc 1997b). Steep
confining banks appear to limit shallow water habitats during high water
periods such that shallow water vegetation is largely eliminated. As a
result, high-water years are marked by a decrease in edge communities and
shallow water emergents, and a concomitant increase in floating-leaved
communities and associated duckweeds.
(6) Saginaw Bay Lakeplain Marsh.
This group contains most sites from Saginaw Bay. Formed by a flat glacial
lakeplain that slopes gently into Lake Huron, Saginaw Bay is very shallow
with a thin veneer of sand over clay; the Wildfowl Bay Islands are
extensive sand spits formed over local exposures of limestone bedrock.
Wetland site types range from protected sand-spit embayments to open
coastal embayments.
Wetlands in this group contain a mix of northern and southern species;
this dual affinity may reflect the location of the climatic tension zone
across Saginaw Bay. In addition, most sites contain ample floristic
evidence of surrounding intensive agricultural land-use.
The emergent zone is generally a northern emergent marsh containing the
key northern species Scirpus acutus and Eleocharis smallii,
although not in great densities. However, excessive sedimentation appears
to have excluded many submergent species typically found within a northern
emergent marsh, including most pondweeds, which are generally intolerant
of turbidity. Both Scirpus americanus (three-square bulrush) and
the more southerly S. validus are frequently present as well, while
Typha angustifolia and Najas flexilis are common
co-dominants; floating species (such as the duckweeds) are only minimally
present. Along more open stretches of the bay, Scirpus americanus
can form a narrow, but dense fringe, apparently due to its greater
tolerance of extreme wave action.
The herbaceous zone is typically a southern wet meadow with a high
percentage of early successional and disturbance species. Typical
colonizing species include Bidens cernuus, Impatiens capensis, Rorippa
palustris, Scirpus validus, and Polygonum lapathifolium; common
exotics include Lythrum salicaria, Phragmites australis, Phalaris
arundinacea, and Polygonum persicaria (lady's thumb). The
absence of a distinct shrub swamp zone for this group may reflect the
intensity of land-use in this area, in which fertile lacustrine soils are
farmed as close to Great Lakes coastal wetlands as possible.
In contrast to most other wetland types, the marshes of the Saginaw Bay
lakeplain experience significant changes in the abundance and ubiquity of
dominant species relative to lake-level fluctuations, particularly in the
herbaceous zone (Minc 1997b). The gentle slope of the lakeplain, combined
with the heavy deposition of fine sediments, creates the potential for
extensive, fertile mud flats following dry-down and favors colonizing
species adapted to the cyclical exposure of this habitat (including Rorippa
palustris, Bidens cernuus, Polygonum lapathifolium, and Scirpus
validus). In contrast, when lake levels remain at or below the mean,
succession to a more typical wet meadow takes place, with an increased
dominance of Calamagrostis canadensis and Carex stricta and
a dramatic drop in the cover values of the colonizing species.
(7) Lake Erie-St. Clair Lakeplain Marsh.
This group includes all marsh sites from the glacial lakeplain of Lake St.
Clair and western Lake Erie. Although the lakeplain formerly supported
extensive marsh and wet prairie communities, the predominant remaining
wetlands are the lacustrine estuaries formed at the mouths of rivers
drowned by the post-glacial rise in lake level. However, even the
remaining marshes reflect high levels of agricultural disturbance
characteristic of the fertile, flat lakeplain soils, along with heavy
manipulation of the shoreline through diking and rip-rap.
All of the wetlands occupy fairly protected sites (estuaries,
barrier-beach lagoons, or sand-spit swales); in addition, the Lake Erie
sites enjoy the most moderate climate of the Great Lakes region. As a
result, their emergent marshes feature a relatively southern flora, while
herbaceous zones are typically a southern wet meadow with a high
proportion of disturbance species.
Common species of the emergent zone include the floating duckweeds (Lemna
minor and Spirodela polyrhiza), and the canopy-forming
submergents Ceratophyllum demersum and Elodea canadensis,
characteristic of quiet, turbid waters. The southern species Nuphar
advena is common, while Nelumbo lutea (American lotus) attains
very high densities at selected sites. Sagittaria latifolia, Scirpus
validus, Typha angustifolia, and T. x glauca (hybrid cat-tail)
are common edge species.
Herbaceous zones are a southern wet meadow dominated by Calamagrostis
canadensis, along with Phalaris arundinacea, Typha
angustifolia, and Polygonum lapathifolium. The standard suite
of early successional species (Bidens cernuus, Impatiens capensis,
Rorippa palustris) and common exotics (Lythrum salicaria and
Phragmites australis) are present as well. As in the case for Saginaw
Bay, the absence of a distinct shrub swamp zone often reflects the
intensity of land-use in this area, in which fertile lacustrine soils are
farmed as close to coastal wetlands as possible.
The St. Clair River delta is loosely joined with the St. Clair-Erie
lakeplain group; however, the St. Clair "flats" is a unique site
in the Great Lakes, and its vegetation differs significantly from sites of
Saginaw Bay to the north and Lake Erie to the south. Emergent zone
vegetation is more typical of northern, open marshes, perhaps owing to the
flow of the river. Common early successional and exotic species were only
minimally present in large portions of the wet meadow.
(8) Lake Ontario Lagoon Marshes.
Wetland sites in eastern Lake Ontario make up this group; all but one are
barrier-beach lagoons. These sites share a similar protected site type, a
lake-level regime characterized by a dampening of natural extreme
fluctuations, and an associated set of distinctive species in the
emergent, herbaceous, and shrubby zones.
Two distinct shoreline areas contain barrier-beach lagoons. Along the
southern shore of Lake Ontario, the lake truncates a field of N-S oriented
drumlins; here, low barrier beaches (generally less than 3 m high) across
intervening embayments have created a series of shallow lagoons, including
the East Bay, Black Creek, and Sterling Creek sites. Similarly, along the
eastern end of Lake Ontario, predominant wind and water currents have led
to the accumulation of sands, creating a low shoreline characterized by
numerous embayments with barrier beaches and sand dunes rising up to 30 m
above the lake. The barrier beaches create a string of shallow lagoons and
wetlands, including Deer Creek, Cranberry Pond, South Colwell Pond, and
Lakeview Pond.
The emergent zones of this type feature very high densities of the
canopy-forming submergent species, Ceratophyllum demersum and Elodea
canadensis, along with the duckweeds Spirodela polyrhiza and Lemna
trisulca. Nuphar advena and Nymphaea odorata are also
common. All of these reflect the well-protected and nutrient-rich waters
of the lagoons, although Lemna trisulca may be associated with
cold, spring-fed streams. High densities of this last species are
distinctive to the Lake Ontario and St. Lawrence sites, as is the
prevalence of Potamogeton zosteriformis (flat-stemmed pondweed).
The herbaceous zone is a wet meadow in which Typha angustifolia
typically dominates, along with Calamagrostis canadensis and Thelypteris
palustris (marsh fern). Cat-tail is particularly sensitive to
flooding; its dominance in Lake Ontario corresponds historically to the
recent period of lake-level regulation. In contrast, species adapted to
the cyclical exposure of shoreline mud flats are poorly represented in
these sites.
The shrubby zones divide into two distinct types. Buttonbush thicket
features a mix of Decodon verticillata (swamp loosestrife) and Cephalanthus
occidentalis (buttonbush), along with Alnus rugosa; Thelypteris
palustris and Peltandra virginica dominate mucky openings
within the thickets. In contrast, poor shrub fen was encountered in areas
of low water flow behind barriers, typically distant from the active
stream channel. Here, poor fen shrubs (Chamaedaphne calyculata, Myrica
gale, Vaccinium macrocarpon, and Andromeda glaucophylla)
dominate, while Sphagna spp. and Sarracenia purpurea attain
high cover values in the groundcover.
(9) St. Lawrence River Estuaries.
This group contains wetland sites along the upper reaches of the St.
Lawrence River where the river is strongly influenced by Lake Ontario.
This stretch features numerous islands and bedrock knobs on the adjacent
mainland shore which are the surface expression of the Frontenac Arch,
where overlying limestone and sandstone formations have been removed by
glacial scouring to reveal the irregular surface of the underlying
Precambrian rock. The exposed bedrock is mostly pink, massive rock of
granitic composition; most of the exposures are smoothly rounded by
scouring ice.
The St. Lawrence wetland sites are typically estuarine. Small streams
or rivers occupy apparent pre-glacial valleys cut through the rounded
bedrock knobs and ridges which have been partially filled in by outwash
and alluvial deposits to form fairly broad, flat basins. Extensive
wetlands (up to 1 km wide) line the lower reaches of the streams for
several kilometers inland as they flow through the basins; a narrow delta
has formed at the mouth of some basins. The valley of Crooked Creek is one
of the best examples of wetland development along this stretch of the St.
Lawrence River (Herdendorf et al. 1981a:53), while those of nearby
Chippewa and Cranberry creeks are also of considerable importance to fish
and wildlife (Geis and Kee 1977).
As in the preceding group, the emergent zone is characterized by high
densities of floating species, including Utricularia vulgaris
(great bladderwort) and the duckweeds Lemna trisulca and Spirodela
polyrhiza, along with the canopy-forming submergent species, Ceratophyllum
demersum and Elodea canadensis. Other submergents preferential
to this group include Potamogeton zosteriformis, P. friesii (Fries's
pondweed), and Zizania aquatica (wild rice). The exotic Hydrocharis
morsus-ranae (frog's bit) is abundant. The herbaceous zone is a broad
wet meadow zone with deep organic soils (often > 4 m), featuring Typha
angustifolia along with Calamagrostis canadensis and Thelypteris
palustris. Again, the dominance of cat-tail may reflect the reduction
of natural lake-level fluctuations. However, Impatiens capensis, a
species adapted to the cyclical exposure of shoreline mud flats is well
represented in this group of sites.
The assignment of Great Lakes coastal wetland sites to the preceding
groups is summarized in Table 2. For maps showing site locations, see
Figures 2-6.
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