As a transition zone between land and a variety of aquatic environments, wetlands share some characteristics of both, yet they have their own unique qualities belonging to neither land (terrestrial) nor water habitats.
One basic definition of a wetland is that it is where the water table — which we usually think of as being below ground — occurs at or near the surface of the land, so that the land is covered by shallow water. As a transition zone between land and aquatic habitats (such as lakes, streams, and rivers), wetlands have one or more of these characteristics:
- At least periodically, the wetland is dominated by aquatic plants (hydrophytes).
- The substrate (ground) is predominantly undrained water-saturated or water-covered soil.
- The substrate, if not technically a soil, is saturated or covered by water at some time during the growing season.
Wetland Natural Communities
Wetlands technically can occur within many other habitat types, including bottomland forests and woodlands, flatwoods, prairies, and stream edges. Many systems for classifying habitats include these as subtypes, but here we will focus on the following wetland natural communities.
Riverine wetlands are associated with rivers. They occur in a river or stream valley, between the stream channel and the valley edge. They include floodplains, blue holes, sand ponds, oxbows, and sloughs. They are flooded or inundated frequently. Most occur near wet or wet-mesic bottomland forest or wet bottomland prairie. They can vary greatly in terms of water and soil characteristics.
Riverine wetland types include marshes, swamps, and shrub swamps. In Missouri, riverine wetlands occur mainly in the lowlands and floodplains of the Bootheel, along the Missouri and Mississippi rivers, and in northern and western Missouri, especially in the region where the Grand, Chariton, Thompson, and Missouri rivers meet.
Sinkhole Pond Wetlands
Sinkhole ponds occur where the slow, dissolving action of mildly acidic groundwater has enlarged a cave tunnel so much that the cave roof collapses, causing the land above it to sink down. Sometimes, these sinkholes open directly into the cave system below, but other times, they are clogged with surface materials (soil, brush, etc.), and a pond forms.
Sinkhole pond wetlands are usually not connected to other streams or waterways. Instead, they are filled by rainwater, runoff, and/or groundwater. Where does this water go? It either evaporates, or it drains into the cave or the porous karst bedrock below.
Sinkhole pond waters are usually stagnant and hold little dissolved oxygen, and there is usually little water movement. They range from neutral to acidic and offer few available nutrients to plants and other life-forms. As a result, sinkhole ponds host many plants and animals adapted to that special environment that cannot survive in other places.
In Missouri, sinkhole pond wetland communities include pond marshes, pond swamps, and pond shrub swamps. Most of Missouri’s sinkhole pond wetlands occur in the Ozarks, in the southern half of the state, where karst geology predominates, and along the Mississippi River hills.
Groundwater Seepages: Fens and Seeps
Where the land meets the water table, groundwater seeps into the soil, keeping it constantly, or almost constantly, saturated or under water. This infiltration of water occurs in complex patterns, varying with the slope of the land and the shape and types of bedrock beneath the soil.
Because fens and seeps are closely associated with groundwater, the temperature and other characteristics of the water tend to be similar to those of underground water: cool and pH-buffered by limestone and dolomite bedrock. You might think of fens and seeps as essentially very diffuse springs.
Seeps are often quite small; most are less than 5 acres, and many are between 0.1 and 1.0 acre. Size varies with the amount of water being discharged and the local landscape and bedrock. Seeps can occur in upland areas, owing to the way groundwater pools atop a depression in the bedrock below, creating a muddy seep above it.
Fens are a special kind of seep: they are large enough to create gaps in tree canopies (because the saturated ground inhibits tree survival), and they often create an area dominated by sedges and grasses. Fens are also called swampy meadows, wet seepy meadows, or wet calcareous meadows. In Missouri, fens support plants and animals that usually are only found far north of our state — remnants of the last glacier’s retreat.
Some fens produce more vegetation than their cool, low-oxygen, fairly acidic water allows to decay. This buildup of half-decayed dead plant materials eventually becomes peat. The peat itself influences the character of the fen. In Missouri, these accumulations of quaking or soggy ground have been called bogs or shaking springs.
Other characteristics of fens include muck, an organically rich, little-oxygenated soil that is more decomposed than peat, and marl, rocklike deposits of calcium and magnesium that have been precipitated out of the hard groundwater by acidic fen conditions.
Acid seeps are another special type of groundwater seepage. Acid seeps notably occur in the Crowley’s Ridge area near the Bootheel. They host several unusual and interesting plants found nowhere else in the state.
Saline seeps, also called salt springs, mineral springs, or salt licks, occur in the northern half of Missouri, in places where nearby soils contain concentrated alkaline salts and minerals. Water percolates through buried salt deposits and is continually supplied by brackish water. As with acid seeps, saline seeps host a variety of unusual plants not found elsewhere. In this case, ones that can tolerate plenty of salt.
Springs and Spring Branches
These wetland communities have a continuous flow of mineralized groundwater through established fractures or openings such as sinkholes, aquifers, or losing streams. The amount of flow can vary, but water chemistry and temperature are relatively constant. The cool water strongly influences the types of aquatic animals that live in these habitats.
Many kinds of aquatic plants and animals live in spring and spring branch wetlands. In Missouri, limestone and dolomite springs occur mostly in the Ozarks.