News & Highlights

Plant Communities of Ontario: Upper Great Lakes Coastal Meadow Marshes

While the calcareous coastal meadow marshes of the lower Great Lakes have been documented to some extent (e.g. Charlton and Hilts 1988, Bakowsky 1995), the coastal meadow marshes of the upper Great Lakes are comparatively less well known, although recently studied in the southern portion of their range (Jalava et al. 2005) . This article provides background information on this habitat, based on published information and fieldwork conducted by NHIC staff.

These meadow marshes develop on flat to gently sloping expanses of wet, infertile acidic sand, overlying igneous and metamorphic bedrock. They tend to accumulate in bays, where offshore currents become slowed and the suspended sands settle, or rivermouths which empty sandy sediments into the deeper lake waters.

In Ontario, northern or acidic coastal meadow marshes occur from Port Severn in southeastern Georgian Bay northward along the shores and islands of Georgian Bay, the North Channel and St. Mary’s River. They also occur along the eastern shores of Lake Superior, although they certainly decrease in extent and frequency along the northern and western shores. The distribution and extent of meadow marshes in Lake Superior may be related in part to sediments transported by longshore currents, which move north and eastward from Pukaskwa National Park, and southward from Batchawana Bay (Beletsky et al. 1999).

In southeastern Georgian Bay, in the vicinity of the Paleozoic contact and areas of calcareous glacial drift, these meadows tend to differ in that their flora is richer, with more calcicolous (growing or living in soil rich in lime) affinities. It is likely that the Georgian Bay meadows receive some deposits of calcareous sand, since Lake Huron currents flow through the channels between Manitoulin Island and the Bruce Peninsula, then move north, east and west within the Georgian Bay basin. Currents also flow northward into the North Channel at the western end of Manitoulin Island (Beletsky et al. 1999).

Some of the best-developed and most extensive sites include the meadow marshes on the deltaic islands at the mouth of the Mississagi River on the North Channel, and the wet beaches both north and east of Sault Ste. Marie, along the shores of Goulais Bay and Lake George (which is a bay along the St. Mary River where it meets the North Channel) respectively. Meadow marshes are more prevalent in shoreline bays and spits along the shores of Georgian Bay, and also occur with some frequency along protected bays and spits on the mainland of the North Channel. Meadow marshes become rarer on the shores of Lake Superior, where they occur primarily along the northern and eastern shores of the lake.

Coastal meadow marshes are usually situated in the interface between the shallow lake waters and sandy beach ridges, with their extent often determined by the sharpness of the vertical gradient. Where the gradient is low, they are often more extensive.

Meadow marshes are dynamic environments. Since they can experience fluctuating lake levels of as much as 1-2 m between years (Great Lakes Commission 1986), their size and extent can change on an annual basis, increasing in extent during low water years, and shrinking during periods of high water. Additionally, high water years kill woody plants which become established in low water years, which is a factor in maintaining the open conditions. In addition to fluctuating water levels, meadow marshes are also subjected to wave energy from these large lakes. This washes away organic accumulations, and deposits new layers of sand. Although impacted by this deposition, these coastal meadow marshes contain extremely dense seedbanks, in places more than 104 m-2, which is an order of magnitude higher than densities reported from prairie marshes. These seedbanks allow the vegetation to replace itself during low water periods (Keddy and Reznicek 1986).

Ice-push or scour is another factor maintaining open conditions. As the ice melts in the spring and open water appears, large masses of floating ice can be moved by both ordinary and storm winds, which can run aground and scrape the shores of these meadow marshes (Scott 1926).

This high-energy environment may account for the relative paucity of Atlantic Coastal Plain plant species along the southeastern Georgian Bay shoreline. Many of the shorelines of inland lakes adjacent to the Georgian Bay coast support numerous plant species whose range is disjunct from their primary range along the Atlantic Coast. These species depend on fluctuating water levels to maintain periodic open sandy shorelines, and during low water years, sprout abundantly from newly exposed seedbanks. They flower, and set seed again, waiting until the next low water year before they can sprout again (Keddy and Reznicek 1982, Reznicek 1994). It may be that the shore- line wave energy is too high in southeastern Georgian Bay for Atlantic Coastal Plain species seed banks to persist (W. Crins, Ontario Parks, pers. comm.).

Vegetation and Floristics

A list of the plants which are characteristic of the coastal meadow marshes of the Upper Great Lakes is presented in Table 1. The table distinguishes species present in the Georgian Bay, North Channel basin, from those present in Lake Superior coastal meadow marshes. While the marshes in these two basins are not that different floristically, those in Georgian Bay tend to be more species-rich than those in Lake Superior. This may be due to a variety of factors, including temperature, growing season, pH (M.J. Oldham, pers. comm.), as well as post-glacial plant migrations.

Although Table 1 was compiled from qualitative field surveys, widespread, ubiquitous species (e.g., Field Horsetail (Equisetum arvense), Tall Goldenrod (Solidago altissima)), found in a variety of habitats in addition to coastal meadow marshes have been subjectively excluded, as have similarly, widespread wetland species such as Swamp Milkweed (Asclepias incarnata) and Water Parsnip (Sium suave).

As can be seen from the list, the flora is dominated by graminoids, particularly members of the sedge (Cyperaceae) and rush (Juncaceae) families. Prominent sedge genera include Carex, Eleocharis, and Schoenoplectus., while Juncus is a well-represented genus in the rush family.

The dominant species vary from site to site, and can change through time within a site, due to the variable conditions they experience, such as fluctuating water levels, storm events, and sediment deposition. The more common dominants include Common Three-square (Schoenoplectus pungens), Twig-rush (Cladium mariscoides), Canada Bluejoint (Calamagrostis canadensis), Greenish Sedge (Carex viridula), Elliptic Spikerush (Eleocharis elliptica), Baltic Rush (Juncus balticus), Mud Rush (J. pelocarpus), and Canadian St. John’s-wort (Hypericum canadense).

Some of the more interesting and distinctive sedges found in these coastal meadow marshes include Umbrella Sedge (Cyperus bipartitus), Low Fimbristylis (Fimbristylis autumnalis), Small-headed Beak-rush (Rhynchospora capitellata) and Brown Beak-rush (R. fusca).

A number of uncommon grass species are also present in these habitats, including One-flowered Satin Grass (Muhlenbergia uniflora) and Prairie Slough-grass (Spartina pectinata), which often forms extensive sterile clones on drier backshore portions of the meadow marshes.

There are also a number of carnivorous plant species which occur in these acid, infertile and water-logged environments. By capturing and digesting microscopic invertebrate prey, species such as Spathulate-leaved Sundew (Drosera intermedia), Horned Bladderwort (Utricularia cornuta) and Inverted Bladderwort (U. resupinata) are able to augment their nutrient intake.

A number of provincially rare species occur in these coastal meadow marshes. Globally, Stiff Yellow Flax (Linum medium var. medium, G5T3T4 S3) occurs almost entirely within Ontario, where it occurs almost exclusively in the coastal meadow marshes of southeastern Georgian Bay. Crested Arrowhead (Sagittaria cristata, G4? S3), Bedstraw (Galium brevipes, G4? S2?), Smith’s Club-rush (Schoenoplectus smithii, G5 S3), Yellow-eyed Grass (Xyris difformis, G5 S3?) are other provincially rare species also found in this part of Georgian Bay and/or the North Channel.

Status

Using conservation ranking criteria developed by NatureServe (which is standardized across the Americas), Upper Great Lakes Coastal Meadow Marshes are considered to be an S4 vegetation type, in terms of provincial conservation concern. That is, they are considered to be Apparently Secure —Uncommon but not rare; some cause for long-term concern due to declines or other factors.

Threats

This vegetation may be negatively affected by human shoreline development, including residential and commercial development and inappropriate recreational vehicle use such as ATV traffic. While no systematic surveys of invasive plant species have been undertaken in these marshes, surveys conducted by NHIC staff have not detected any significant impacts on this vegetation by invasive plant species such as Purple Loosestrife (Lythrum salicaria). Similarly, recent extensive surveys along the Georgian Bay coast have also found low percentages of introduced species in this habitat (Jalava et al. 2005). However patches of the aggressive invasive species Common Rush (Phragmites communis) have been observed in a few coastal meadow marshes in the extreme southeastern portion of the bay near Honey Harbour, where the area is fairly disturbed by high boat traffic and dense cottaging (R. Jones, pers. comm.).

References