Wetland Types

Why This Matters

Wetlands sit at the intersection of terrestrial and aquatic systems, making them one of the most dynamic topics in limnology. You're being tested on more than just definitions. Exams want you to understand hydrology (where the water comes from), biogeochemistry (how nutrients and organic matter cycle), and ecological function (what services each wetland provides). These concepts show up repeatedly in questions about carbon sequestration, nutrient cycling, habitat provision, and water quality regulation.

Don't just memorize that bogs are acidic and marshes have grasses. Know why each wetland type develops its characteristic features and how water source, chemistry, and vegetation interact to create distinct ecosystems. When you can explain the mechanism behind each wetland's formation and function, you'll handle both multiple-choice and free-response questions with confidence.

---

Peat-Forming Wetlands: Ombrotrophic vs. Minerotrophic Systems

The key distinction here is water source and nutrient availability. Ombrotrophic wetlands receive water primarily from precipitation, while minerotrophic wetlands receive groundwater inputs rich in dissolved minerals. This single factor determines acidity, plant communities, and decomposition rates.

Bogs

• Ombrotrophic hydrology means bogs receive water almost exclusively from precipitation, cutting them off from mineral-rich groundwater and creating extremely nutrient-poor conditions.
• Low pH (3.5–4.5) results from organic acids released by sphagnum moss, which dominates these systems and actively acidifies its environment through cation exchange. Sphagnum releases hydrogen ions while absorbing cations like calcium and magnesium from the surrounding water, driving pH down.
• Slow decomposition in cold, acidic, anaerobic conditions makes bogs exceptional carbon sinks, storing roughly 30% of global soil carbon despite covering only about 3% of land area.

Fens

• Minerotrophic hydrology means fens receive groundwater inputs that deliver dissolved minerals like calcium and magnesium, supporting higher nutrient availability than bogs.
• Higher pH (5.5–7.5) allows for greater plant diversity, including sedges (Carex spp.), grasses, and wildflowers that cannot tolerate bog acidity. The mineral-buffered water neutralizes organic acids that would otherwise accumulate.
• Faster decomposition rates mean fens accumulate peat more slowly than bogs but support more complex food webs and greater biodiversity.

Peatlands

• Umbrella category encompassing any wetland that accumulates peat, which is partially decomposed organic matter at least 30–40 cm deep.
• Classification depends on chemistry. Peatlands fed by rain become bogs; those fed by groundwater become fens. This is a clean example of how hydrology drives ecosystem type.
• Global climate significance is enormous. Peatlands store approximately 500 gigatons of carbon, making their drainage or degradation a major source of $CO_2$ emissions.

---

Freshwater Wetlands: Herbaceous vs. Woody Vegetation

The dominant vegetation type, herbaceous plants versus woody plants, reflects differences in hydroperiod (the duration and timing of flooding) and soil saturation patterns. This distinction matters because it shapes habitat structure and the ecosystem services each wetland provides.

Marshes

• Herbaceous vegetation including cattails (Typha), bulrushes (Schoenoplectus), and sedges dominates because persistent standing water prevents tree establishment. Tree seedlings simply can't survive continuous inundation.
• High primary productivity supports dense wildlife populations, making marshes critical stopover sites for migratory waterfowl along major flyways.
• Water filtration capacity stems from dense root systems and associated microbial communities that remove nitrogen, phosphorus, and suspended sediments from inflowing water. This is why constructed treatment wetlands are typically designed to mimic marsh conditions.

Swamps

• Woody vegetation (trees and shrubs) tolerates seasonal flooding but requires periodic drawdown for seedling establishment and root aeration. Species like bald cypress (Taxodium distichum) and tupelo (Nyssa) have specialized adaptations such as buttressed trunks and "knees" for gas exchange.
• Forested structure creates vertical habitat complexity, supporting diverse communities of fish, reptiles, mammals, and cavity-nesting birds.
• Carbon storage occurs in both living biomass and saturated soils, with cypress and tupelo swamps among the most productive forested wetlands in North America.

Riparian Wetlands

• Fluvial connectivity defines these wetlands. They form along stream and river corridors, with hydrology controlled by flood pulses and lateral water movement from the channel into the floodplain.
• Buffer function is critical. Riparian zones intercept agricultural runoff, trap sediments, and reduce bank erosion through root stabilization. This is why riparian buffer width is a common management target.
• Ecotone characteristics create high biodiversity as species from both aquatic and upland systems overlap in this transitional habitat.

---

Temporary and Seasonal Wetlands: Hydroperiod as a Filter

These wetlands experience predictable wet-dry cycles that act as ecological filters. Species must complete critical life stages during the wet phase or possess adaptations for surviving desiccation. This creates unique communities found nowhere else.

Vernal Pools

• Ephemeral hydroperiod means pools fill during winter/spring rains and dry completely by summer. Because they dry out, fish can't establish, creating a predator-free window for reproduction.
• Obligate species like fairy shrimp, California tiger salamanders, and spadefoot toads have evolved life cycles synchronized to this temporary flooding regime. These organisms literally cannot reproduce without vernal pool conditions.
• Resting stages (eggs, cysts, seeds) survive dry periods in the soil, allowing rapid community reassembly when pools refill. This is a key adaptation for drought tolerance and one reason vernal pool soils are ecologically valuable even when dry.

Prairie Potholes

• Glacial origin sets these apart. These depressions formed when ice blocks buried in glacial till melted, creating millions of small wetlands across the northern Great Plains. They're often called the "duck factory" of North America.
• Waterfowl production is extraordinary. Prairie potholes produce 50–80% of North American ducks despite covering less than 10% of the continent's wetland area.
• Groundwater recharge occurs as water infiltrates through pothole bottoms, while during wet years they provide flood attenuation by storing excess precipitation across the landscape.

---

Coastal and Saline Wetlands: Marine Influence

Coastal wetlands exist at the land-sea interface, where tidal flooding, salinity gradients, and wave energy create distinct zonation patterns. Salt tolerance becomes the primary filter determining which species can establish.

Mangrove Swamps

• Halophyte trees (salt-tolerant species like Rhizophora and Avicennia) dominate tropical and subtropical coastlines between approximately 25°N and 25°S latitude. These species cope with salt through exclusion at the roots, secretion through leaf glands, or accumulation in senescent leaves.
• Structural complexity from prop roots and pneumatophores (aerial roots for gas exchange in anaerobic sediments) creates protected nursery habitat for juvenile fish, shrimp, and crabs of commercial importance.
• Coastal protection is substantial. Mangrove forests reduce wave energy by 70–90% and buffer shorelines against storm surge and erosion.

Coastal Wetlands (Salt Marshes and Estuaries)

• Salinity zonation creates distinct vegetation bands. Cordgrass (Spartina alterniflora) dominates the regularly flooded low marsh, while pickleweed (Salicornia) and salt hay (Spartina patens) occupy the irregularly flooded high marsh. This zonation reflects each species' tolerance to flooding frequency and salinity.
• Tidal exchange drives nutrient cycling, with marshes exporting organic matter and detritus that fuels estuarine food webs. This "outwelling" of nutrients connects marsh productivity to offshore fisheries.
• Blue carbon ecosystems is the term for these systems because salt marshes sequester carbon at rates 2–4 times higher than terrestrial forests per unit area, storing it in anaerobic sediments for millennia.

---

Quick Reference Table

---

Self-Check Questions

1. Both bogs and fens accumulate peat, yet they support very different plant communities. What single factor best explains this difference, and how does it affect pH and nutrient availability?

2. Compare vernal pools and prairie potholes: what do they share in terms of hydroperiod, and how do their geographic origins and wildlife communities differ?

3. If a free-response question asks you to explain why mangroves and salt marshes are both considered "blue carbon" ecosystems, what mechanisms would you describe for each?

4. A wetland receives water primarily from an adjacent river during spring floods and supports a mix of trees and shrubs. Which wetland type is this, and what ecosystem services would you expect it to provide?

5. Rank the following from most acidic to least acidic and explain the hydrological reason for the pattern: fen, salt marsh, bog.