6.4 Lakes, wetlands, and their ecological importance

Lakes and wetlands are crucial components of freshwater systems, supporting diverse ecosystems and providing essential services. These water bodies form through various geological processes, developing unique characteristics that influence their ecology and function within the hydrologic cycle.

From regulating water quality to supporting biodiversity, lakes and wetlands play vital roles in Earth's ecosystems. They offer habitat for countless species, help control floods, and provide recreational opportunities, making their conservation critical for both environmental and human well-being.

Lake Characteristics and Processes

Lake Formation and Structure

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• Lakes form in depressions on Earth's surface that fill with water from precipitation, runoff, groundwater seepage, or melting glaciers
• Can be created by various geological processes such as glaciation (Great Lakes), tectonic activity (rift lakes like Lake Baikal), volcanic activity (crater lakes), or fluvial processes (oxbow lakes)
• Lake basins are typically divided into littoral zone (shallow, near-shore area), limnetic zone (open water area), and profundal zone (deep, bottom waters)
• Littoral zone supports rooted aquatic plants, diverse invertebrate communities, and fish spawning areas due to ample light penetration
• Limnetic zone is the main area of primary production by phytoplankton, supporting zooplankton and fish populations
• Profundal zone is typically cold, dark, and nutrient-rich, supporting decomposers and bottom-dwelling organisms

Lake Stratification and Mixing

• Many lakes develop thermal stratification during summer months, with warm, less dense water (epilimnion) overlying cooler, denser water (hypolimnion), separated by a thermocline
• Stratification affects the distribution of dissolved oxygen, nutrients, and organisms throughout the water column
• In fall, surface waters cool and become denser, causing the lake to mix (turnover) and redistribute nutrients and oxygen
• In winter, some lakes develop inverse stratification, with colder, less dense water near the surface and warmer, denser water at the bottom
• Dimictic lakes mix twice per year (spring and fall), while monomictic lakes mix once (usually in winter), and polymictic lakes mix frequently throughout the year
• Meromictic lakes have a permanently stratified bottom layer (monimolimnion) that does not mix with the upper layers

Eutrophication and Water Quality

• Eutrophication is the process of nutrient enrichment in a lake, leading to increased primary production and potentially harmful algal blooms
• Can occur naturally over long time scales or be accelerated by human activities such as agricultural runoff, sewage discharge, or urban development
• Excess nutrients, particularly phosphorus and nitrogen, stimulate the growth of phytoplankton and aquatic plants
• As organic matter from algal blooms decomposes, it consumes dissolved oxygen, leading to hypoxic or anoxic conditions in the bottom waters
• Anoxia can cause fish kills, release of nutrients and toxins from sediments, and shifts in species composition towards more tolerant organisms
• Cultural eutrophication is a major threat to water quality and ecosystem health in many lakes worldwide (Lake Erie, Lake Taihu)

Wetland Ecology

Wetland Types and Characteristics

• Wetlands are transitional ecosystems between terrestrial and aquatic environments, characterized by waterlogged soils, hydrophytic vegetation, and hydric soils
• Can be classified based on hydrology, vegetation, and substrate into categories such as marshes, swamps, bogs, and fens
• Marshes are dominated by herbaceous plants like cattails and rushes, with standing water for much of the growing season (Everglades)
• Swamps are forested wetlands with trees adapted to periodic flooding, such as cypress and mangroves (Okefenokee Swamp)
• Bogs are acidic, nutrient-poor wetlands dominated by sphagnum moss and shrubs, with little groundwater input (peatlands of Canada and Siberia)
• Fens are similar to bogs but receive groundwater inputs, resulting in less acidic conditions and greater plant diversity (prairie potholes)

Hydrophytic Vegetation and Adaptations

• Hydrophytes are plants adapted to growing in water or waterlogged soils, with specialized structures and physiological adaptations
• Many hydrophytes have aerenchyma tissue, which contains air spaces that allow oxygen transport to roots in anoxic soils
• Some plants have adventitious roots that grow from stems or leaves to access oxygen above the water surface (mangroves, bald cypress)
• Floating plants like water lilies and duckweed have leaves with large air spaces for buoyancy and gas exchange
• Submerged plants like pondweeds and coontail have thin, dissected leaves to reduce drag and increase surface area for nutrient uptake
• Emergent plants like cattails and bulrushes have tall, rigid stems to withstand water currents and wind

Biogeochemical Cycling in Wetlands

• Wetlands play a critical role in the cycling of nutrients, carbon, and other elements between the atmosphere, land, and water
• Anaerobic conditions in wetland soils promote slow decomposition and accumulation of organic matter, making them important carbon sinks
• Wetland plants and microbes facilitate the removal of excess nutrients like nitrogen and phosphorus from water through uptake and transformation
• Denitrification by anaerobic bacteria converts nitrate to atmospheric nitrogen gas, helping to reduce nitrogen loads in downstream waters
• Sulfate reduction in salt marshes produces hydrogen sulfide, which can bind with heavy metals and reduce their toxicity
• Methane production by methanogenic archaea in wetland soils is a significant source of atmospheric greenhouse gases
• Wetland soils also act as sinks for pollutants like heavy metals, pesticides, and hydrocarbons, helping to improve water quality

Habitat Diversity and Ecosystem Interactions

• Wetlands support a wide range of plant and animal species adapted to the unique hydrologic and soil conditions
• Provide critical habitat for many threatened and endangered species, such as the whooping crane, wood stork, and bog turtle
• Serve as important breeding, nesting, and feeding grounds for migratory birds, fish, and invertebrates (Chesapeake Bay, Prairie Pothole Region)
• Contribute to the productivity of adjacent ecosystems by exporting organic matter, nutrients, and organisms
• Act as nursery areas for commercially important fish and shellfish species, supporting coastal fisheries and economies
• Provide connectivity between aquatic and terrestrial habitats, facilitating the movement of species and gene flow
• Influence local and regional climate through evapotranspiration, carbon sequestration, and temperature moderation

Ecosystem Services of Lakes and Wetlands

Regulating and Supporting Services

• Lakes and wetlands provide numerous ecosystem services that benefit human well-being and support ecological processes
• Water purification is a key service, as wetlands filter pollutants, sediments, and excess nutrients from surface water and groundwater
• Flood control is another important service, as lakes and wetlands store and slowly release water, reducing the intensity of downstream flooding
• Shoreline stabilization by wetland vegetation helps to reduce erosion and protect against storm surges and sea-level rise
• Climate regulation through carbon sequestration in lake sediments and wetland soils helps to mitigate greenhouse gas emissions
• Nutrient cycling and transformation in lakes and wetlands supports primary production and maintains ecosystem productivity
• Groundwater recharge is facilitated by some lakes and wetlands, helping to maintain water supplies for human use and ecosystem needs

Provisioning and Cultural Services

• Lakes and wetlands provide valuable provisioning services such as fresh water for drinking, irrigation, and industrial use
• Many communities rely on lakes and wetlands for food production, including fish, shellfish, waterfowl, and aquatic plants (rice, cranberries)
• Wetland plants like reeds, sedges, and willows are used for construction materials, handicrafts, and biofuels
• Peat from bogs is harvested for use as a fuel source and horticultural substrate in some regions (Ireland, Russia)
• Lakes and wetlands also offer important cultural services, such as recreation, tourism, and aesthetic appreciation
• Activities like fishing, boating, birdwatching, and hunting generate significant economic benefits for local communities (Finger Lakes, Okavango Delta)
• Many lakes and wetlands have cultural and spiritual significance for indigenous peoples, serving as sacred sites and traditional use areas
• Provide opportunities for scientific research, environmental education, and nature-based learning experiences