Wetland

Description

Ontario's freshwater wetlands form a continuum of ecosystems united by one defining feature: water at or near the soil surface for all or much of the growing season. They range from treed swamps dominated by Silver Maple, Green Ash, and Black Ash to vast open cattail marshes, sedge meadows, and peat-accumulating fens and bogs. Together they cover approximately 25% of Ontario's land area — an extraordinary proportion that makes the province one of the most wetland-rich jurisdictions in the world.

The ecological land classification system recognizes several wetland ecosite types, distinguished by hydrology, nutrient status, and dominant vegetation structure. Swamps are forested or tall-shrub wetlands on mineral or shallow organic soils, with at least 25% tree or tall shrub cover. Marshes are herbaceous wetlands dominated by emergent vegetation — cattails, bulrushes, and reeds — in standing water through much of the year. Sedge meadows (wet meadows) are graminoid-dominated wetlands on saturated mineral soils, seasonally flooded but typically drying by late summer. Fens are peatlands influenced by mineral-rich groundwater, supporting sedges, rushes, and calciphilic mosses. Bogs are ombrotrophic peatlands, receiving water and nutrients exclusively from precipitation, producing acidic, nutrient-poor conditions dominated by Sphagnum mosses and ericaceous shrubs.

Southern Ontario has lost over 70% of its pre-settlement wetlands to drainage, agriculture, and urban development. The remaining wetlands are disproportionately important — they filter pollutants from surface and groundwater, attenuate floods, store more carbon per unit area than any other terrestrial ecosystem, and provide habitat for an estimated 40% of Ontario's species at risk.

Physical Characteristics

• Soils: Hydric — saturated long enough to develop anaerobic conditions. Swamps: mineral loams and clays, often alluvial, with dark organic-rich surface horizons. Marshes: mineral or shallow organic substrates, nutrient-rich from sediment deposition. Sedge meadows: saturated mineral soils, often calcareous. Fens: moderately to well-decomposed peat over mineral substrate, groundwater-influenced. Bogs: deep, poorly decomposed fibric peat (Sphagnum), rainwater-fed, extremely nutrient-poor.
• Moisture: Wet. Permanent saturation in marshes, swamps, and bogs. Seasonal flooding with summer drawdown in sedge meadows and fens. The hydroperiod — the seasonal pattern of water level — is the primary ecological driver, determining which plant species can persist.
• pH: Broad range. Circumneutral to alkaline (pH 6.0-8.0) in calcareous fens and groundwater-fed marshes. Acidic (pH 3.5-5.5) in bogs and Sphagnum-dominated swamps. Many wetland plants are pH generalists; others are strict calciphiles or calcifuges that define the community.
• Disturbance regime: Flooding and beaver activity are natural disturbance vectors. Fire is rare in most wetlands but can occur during drought years in drained fens and bogs, resetting succession. The primary anthropogenic disturbances are drainage, nutrient loading from agricultural runoff, invasive species (Common Reed, Purple Loosestrife, Glossy Buckthorn), and altered hydrology from roads and development.

Characteristic Vegetation

Wetland vegetation is structured by water depth, duration of flooding, and nutrient availability:

• Canopy (swamps only): Silver Maple (Acer saccharinum), Green Ash (Fraxinus pennsylvanica), Black Ash (Fraxinus nigra), Swamp White Oak (Quercus bicolor), Eastern White Cedar (Thuja occidentalis), Tamarack (Larix laricina) in fens and bogs
• Shrub layer (swamps and fen margins): Speckled Alder (Alnus incana), Red-osier Dogwood (Cornus sericea), Winterberry (Ilex verticillata), various willows (Salix spp.), Sweet Gale (Myrica gale), Leatherleaf (Chamaedaphne calyculata) in bogs
• Emergent marsh vegetation: Broad-leaved Cattail (Typha latifolia), Narrow-leaved Cattail (Typha angustifolia), Hard-stem Bulrush (Schoenoplectus acutus), Pickerelweed (Pontederia cordata), Arrowhead (Sagittaria latifolia)
• Sedge meadow / ground layer: Tussock Sedge (Carex stricta), Lakebank Sedge (Carex lacustris), Palm Sedge (Carex muskingumensis), Graceful Sedge (Carex gracillima), Canada Bluejoint (Calamagrostis canadensis), Swamp Milkweed (Asclepias incarnata), Bottle Gentian (Gentiana andrewsii), Dense Blazing Star (Liatris spicata), Marsh Marigold (Caltha palustris), Blue Flag Iris (Iris versicolor)
• Bog mat: Sphagnum mosses, Labrador Tea (Rhododendron groenlandicum), Bog Rosemary (Andromeda polifolia), Cranberry (Vaccinium macrocarpon, V. oxycoccos), Pitcher Plant (Sarracenia purpurea), Sundew (Drosera rotundifolia)

Characteristic Fauna

Wetlands support exceptional faunal diversity, including a disproportionate share of species at risk:

• Birds: Great Blue Heron, American Bittern (Threatened), Least Bittern (Threatened), Virginia Rail, Sora, Common Gallinule, Pied-billed Grebe, Wood Duck, Mallard, Blue-winged Teal, Marsh Wren, Swamp Sparrow, Red-winged Blackbird, Yellow Warbler (Setophaga petechia), Cedar Waxwing (Bombycilla cedrorum), American Goldfinch (Spinus tristis)
• Mammals: Beaver (Castor canadensis) — keystone wetland engineer, Muskrat, River Otter, Mink, Moose in boreal wetlands
• Amphibians and Reptiles: Northern Leopard Frog, Green Frog, Spring Peeper, Gray Treefrog, American Toad, Eastern Newt, Snapping Turtle (Special Concern), Painted Turtle, Blanding's Turtle (Threatened), Spotted Turtle (Endangered), Eastern Ribbonsnake (Special Concern)
• Insects: High diversity of Odonata (dragonflies and damselflies), wetland-specialist Lepidoptera, and aquatic Diptera (mosquitoes, midges, crane flies) that form the base of the vertebrate food web. Bumble bees including Bombus impatiens forage on marsh wildflowers.

Ontario Distribution

Wetlands occur province-wide, with distinctive wetland types associated with different ecological regions:

• Southern Ontario (Carolinian and Great Lakes-St. Lawrence): Remnant swamps and marshes within agricultural landscapes. Significant complexes include Point Pelee Marsh, Long Point Marshes, Cootes Paradise (Royal Botanical Gardens), Luther Marsh, Minesing Swamp, and the Lake St. Clair wetlands — one of the largest freshwater delta systems in the Great Lakes
• Canadian Shield: Extensive peatlands (fens and bogs) across the Boreal Forest region, including the Hudson Bay Lowlands — the third-largest wetland complex on Earth, covering over 370,000 km² of patterned fen and bog
• Lake Simcoe-Rideau: Sedge meadows and calcareous fens along the Niagara Escarpment and in the Bruce Peninsula, supporting rare orchid communities and disjunct calciphilic flora
• Eastern Ontario: Alfred Bog, Mer Bleue Bog (Ottawa), and extensive alvar-associated wetlands in the Napanee Plain