6.4 Lakes, wetlands, and their ecological importance

Lake Characteristics and Processes

Lake Formation and Structure

Lakes form in depressions on Earth's surface that fill with water from precipitation, runoff, groundwater seepage, or melting glaciers. The geological process that creates the depression determines the lake's shape, depth, and long-term behavior.

Several formation mechanisms produce distinct lake types:

• Glacial lakes form when glaciers carve basins or leave behind moraines that trap water (the Great Lakes formed this way during the last ice age)
• Tectonic lakes form along fault lines or rift zones where the crust pulls apart, creating deep, narrow basins (Lake Baikal, the world's deepest lake at ~1,642 m, sits in an active rift zone)
• Volcanic lakes fill calderas or craters after eruptions (Crater Lake, Oregon)
• Oxbow lakes form when a river meander gets cut off from the main channel through fluvial processes

Lake basins are divided into three main zones based on depth and light availability:

• Littoral zone: the shallow, near-shore area where light reaches the bottom. This supports rooted aquatic plants, diverse invertebrate communities, and fish spawning habitat.
• Limnetic zone: the open-water area where phytoplankton drive most of the lake's primary production, supporting zooplankton and fish populations.
• Profundal zone: the deep, dark bottom waters. Cold and nutrient-rich, this zone is dominated by decomposers and bottom-dwelling organisms.

Lake Stratification and Mixing

During summer, many lakes develop thermal stratification, where the water column separates into distinct layers based on temperature and density. The warm, less dense surface layer is the epilimnion, the cold, dense bottom layer is the hypolimnion, and the zone of rapid temperature change between them is the thermocline.

This layering matters because it controls where dissolved oxygen and nutrients end up. The epilimnion stays oxygenated through contact with the atmosphere, while the hypolimnion can become oxygen-depleted as decomposition consumes what's available. Nutrients released from bottom sediments get trapped below the thermocline.

In fall, surface waters cool and become denser until they match the temperature of deeper water. At that point, wind can mix the entire water column. This event, called turnover, redistributes oxygen downward and nutrients upward, fueling a pulse of productivity. Some lakes also experience a spring turnover after ice melts.

Lakes are classified by how often they mix:

• Dimictic lakes mix twice per year (spring and fall), typical of temperate regions
• Monomictic lakes mix once, usually in winter (common in warm climates)
• Polymictic lakes are shallow enough to mix frequently throughout the year
• Meromictic lakes have a permanently stratified bottom layer (the monimolimnion) that never mixes with upper waters, often due to dissolved salts increasing density at depth

In winter, some lakes develop inverse stratification: ice and near-freezing water (at ~0°C) float on top of slightly warmer, denser water (at ~4°C, where freshwater reaches maximum density). This is why lakes freeze from the top down, allowing aquatic life to survive beneath the ice.

Eutrophication and Water Quality

Eutrophication is the process of nutrient enrichment in a lake that leads to increased primary production. It can happen naturally over thousands of years as sediments and nutrients slowly accumulate, but human activities accelerate it dramatically. This accelerated version, called cultural eutrophication, is one of the biggest threats to lake ecosystems worldwide.

The main culprits are excess phosphorus and nitrogen entering lakes from agricultural runoff, sewage discharge, and urban stormwater. Here's the chain of events:

1. Excess nutrients wash into the lake
2. Phytoplankton and algae populations explode, sometimes forming dense algal blooms (some of which produce toxins harmful to humans and wildlife)
3. When the bloom dies, bacteria decompose the massive amount of organic matter
4. Decomposition consumes dissolved oxygen, creating hypoxic (low oxygen) or anoxic (no oxygen) conditions in bottom waters
5. Anoxia triggers fish kills, releases additional nutrients and toxins from sediments, and shifts species composition toward pollution-tolerant organisms

Lake Erie and Lake Taihu (China) are well-known examples of lakes severely affected by cultural eutrophication. In both cases, agricultural nutrient loading has driven recurring harmful algal blooms that threaten drinking water supplies and fisheries.

Wetland Ecology

Wetland Types and Characteristics

Wetlands are transitional ecosystems between terrestrial and aquatic environments. They're defined by three features: waterlogged soils (at least periodically), hydrophytic vegetation (plants adapted to wet conditions), and hydric soils (soils that develop under saturated conditions).

The four major wetland types differ in their hydrology, vegetation, and chemistry:

• Marshes have standing water for much of the growing season and are dominated by herbaceous plants like cattails and rushes. The Florida Everglades is the most famous example.
• Swamps are forested wetlands with trees adapted to periodic flooding, such as bald cypress and mangroves. The Okefenokee Swamp in Georgia is a classic example.
• Bogs are acidic, nutrient-poor wetlands dominated by sphagnum moss. They receive water almost entirely from precipitation, with very little groundwater input. The vast peatlands of Canada and Siberia are bog systems.
• Fens resemble bogs but receive groundwater inputs, which makes them less acidic and more nutrient-rich, supporting greater plant diversity. Prairie potholes in the northern Great Plains function as fen-like systems.

Hydrophytic Vegetation and Adaptations

Hydrophytes are plants adapted to growing in water or waterlogged soils. The central challenge they face is getting oxygen to their roots in saturated, oxygen-poor soils. Different growth forms have evolved different solutions:

• Aerenchyma tissue is the most widespread adaptation. These are internal air channels that transport oxygen from above-water stems and leaves down to submerged roots. Most wetland plants have some form of aerenchyma.
• Adventitious roots grow from stems or trunks above the waterline to access atmospheric oxygen. Mangroves and bald cypress both use this strategy.
• Floating plants like water lilies and duckweed have leaves with large internal air spaces for buoyancy and gas exchange at the water surface.
• Submerged plants like pondweeds and coontail have thin, finely dissected leaves that reduce drag from water currents and maximize surface area for absorbing dissolved nutrients and gases.
• Emergent plants like cattails and bulrushes grow rooted in shallow water with tall, rigid stems extending above the surface to withstand currents and wind.

Biogeochemical Cycling in Wetlands

Wetlands are biogeochemical hotspots because the boundary between aerobic (oxygenated) and anaerobic (oxygen-free) conditions creates the right environment for a wide range of chemical transformations.

Carbon cycling: Anaerobic conditions in wetland soils slow decomposition dramatically, causing organic matter to accumulate rather than break down. This makes wetlands important carbon sinks. Peatlands alone store roughly twice as much carbon as all the world's forests combined. However, methanogenic archaea in these same anaerobic soils produce methane ($CH_4$), a potent greenhouse gas. Wetlands are the largest natural source of atmospheric methane, so they play a complex role in climate regulation.

Nitrogen cycling: Wetland microbes perform denitrification, converting dissolved nitrate ($NO_3^-$) into nitrogen gas ($N_2$) that escapes to the atmosphere. This process removes excess nitrogen from water before it reaches downstream rivers, lakes, and coastal areas.

Phosphorus and pollutant removal: Wetland plants and soils absorb phosphorus, heavy metals, pesticides, and hydrocarbons from water flowing through them. In salt marshes specifically, sulfate reduction produces hydrogen sulfide ($H_2S$), which binds with heavy metals and reduces their toxicity. These filtering capabilities are why wetlands are sometimes called "nature's kidneys."

Habitat Diversity and Ecosystem Interactions

Wetlands support an outsized share of biodiversity relative to their area. The mix of open water, saturated soils, and dry edges creates a mosaic of microhabitats within a small space.

• Wetlands provide critical habitat for many threatened and endangered species, including the whooping crane, wood stork, and bog turtle
• Migratory birds depend on wetlands as breeding, nesting, and stopover feeding sites. The Prairie Pothole Region of the northern Great Plains produces roughly 50-80% of North America's ducks.
• Coastal wetlands like those in Chesapeake Bay serve as nursery areas for commercially important fish and shellfish, directly supporting fisheries and local economies
• Wetlands export organic matter, nutrients, and organisms to adjacent ecosystems, boosting productivity in surrounding terrestrial and aquatic habitats
• They provide connectivity between aquatic and terrestrial systems, facilitating species movement and gene flow
• Locally, wetlands moderate temperature and humidity through evapotranspiration, influencing microclimate in surrounding areas

Ecosystem Services of Lakes and Wetlands

Regulating and Supporting Services

Lakes and wetlands provide a suite of regulating services that buffer environmental extremes and maintain ecological processes:

• Water purification: Wetlands filter pollutants, sediments, and excess nutrients from both surface water and groundwater. Constructed wetlands are now used in many places specifically for wastewater treatment.
• Flood control: Lakes and wetlands store water during heavy rainfall and release it slowly, reducing the intensity and timing of downstream flood peaks. A single acre of wetland can store roughly 1 to 1.5 million gallons of floodwater.
• Shoreline stabilization: Wetland vegetation anchors sediments, reducing erosion and buffering coastlines against storm surges and sea-level rise.
• Climate regulation: Carbon sequestration in lake sediments and wetland soils removes $CO_2$ from the atmosphere over long timescales, though methane emissions partially offset this benefit.
• Nutrient cycling: Transformation and recycling of nutrients in lakes and wetlands sustains primary production and overall ecosystem productivity.
• Groundwater recharge: Some lakes and wetlands allow water to percolate into underlying aquifers, helping maintain groundwater supplies for both human use and ecosystem needs.

Provisioning and Cultural Services

Beyond regulating functions, lakes and wetlands directly supply resources and enrich human communities:

• Freshwater supply: Lakes and wetlands provide water for drinking, irrigation, and industrial use. The Great Lakes alone hold about 21% of the world's surface freshwater.
• Food production: Many communities depend on lakes and wetlands for fish, shellfish, waterfowl, and aquatic crops like rice and cranberries.
• Raw materials: Wetland plants (reeds, sedges, willows) are harvested for construction, handicrafts, and biofuels. Peat from bogs is used as fuel and horticultural substrate in regions like Ireland and Russia.
• Recreation and tourism: Fishing, boating, birdwatching, and hunting at lakes and wetlands generate significant economic activity. The Okavango Delta in Botswana and the Finger Lakes in New York are examples where tourism is a major economic driver.
• Cultural and spiritual significance: Many lakes and wetlands hold deep importance for indigenous peoples as sacred sites and areas of traditional use.
• Education and research: These ecosystems provide valuable settings for scientific research, environmental monitoring, and nature-based learning.