10.3 Barrier islands and coastal lagoons
6 min read•july 30, 2024
Barrier islands and coastal lagoons are dynamic coastal features shaped by waves, tides, and sediment transport. These landforms protect mainland coasts, provide crucial habitats, and respond to sea-level changes. Understanding their formation and evolution is key to coastal management.
Human activities significantly impact these systems through development, engineering, and pollution. Climate change further threatens their stability. Balancing human needs with natural processes is crucial for preserving these vital coastal environments and their ecological and economic benefits.
Formation and Morphology of Barrier Islands
Barrier Island Characteristics and Formation
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- Barrier islands form elongated, offshore landforms composed primarily of sand and sediment running parallel to the mainland coast
- , , and sea-level fluctuations over geological time scales contribute to formation
- Barrier island chains extend for hundreds of kilometers along coastlines (Gulf Coast of the United States)
- Individual islands vary in size and shape influenced by local conditions (wind patterns, wave energy)
- Width and elevation of barrier islands depend on factors such as:
- Wave energy
- Tidal range
- Sediment availability
- Formation process involves:
- Sediment accumulation from longshore currents
- Wave action shaping the sediment into an elongated form
- Vegetation establishment stabilizing the structure
Coastal Lagoon Features
- Coastal lagoons form shallow bodies of water separated from the open ocean by barrier islands or spits
- Limited water exchange with the ocean characterizes lagoon environments
- Lagoon formation occurs when:
- Barrier islands or spits develop, enclosing a body of water
- Sea level rises, flooding low-lying coastal areas behind barriers
- Lagoon water properties often differ from the open ocean (salinity, temperature, nutrient content)
- Ecological importance of lagoons includes:
- Nursery grounds for marine species
- Habitat for unique plant and animal communities
Barrier Island Morphology Components
- Ocean-facing beaches shape the seaward side of barrier islands
- Dunes form wind-blown sand accumulations providing elevation and storm protection
- Washover fans develop from storm surge deposits on the island's landward side
- Back-barrier marshes or tidal flats occupy the area between the island and mainland
- Inlets create dynamic channels connecting lagoons to the open ocean, allowing for:
- Water exchange
- Sediment transport
- Marine organism movement
- Morphological features evolve over time due to:
- Storms and hurricanes
- Sea-level changes
- Human interventions
Sea-Level Changes and Sediment Supply
Sea-Level Influence on Barrier Island Development
- Holocene epoch sea-level rise played a crucial role in forming many modern barrier island systems
- Transgressive barrier island model describes landward migration during sea-level rise:
- of seaward side
- of sediment to landward side
- Gradual landward movement of entire island
- Regressive barrier island model explains seaward growth during sea-level fall:
- Exposure of previously submerged areas
- Seaward progradation of beach ridges
- Expansion of island area
- Barrier islands may migrate landward through rollover process preserving form if sediment supply suffices
- Sea-level fall can lead to barrier island progradation seaward, expanding area with adequate sediment
Sediment Supply Dynamics
- Sediment sources crucial for barrier island maintenance and growth include:
- Rivers (Mississippi River delta)
- Coastal erosion (cliff erosion along coastlines)
- Offshore deposits (continental shelf sediments)
- Balance between sea-level change rates and sediment supply determines barrier island and lagoon evolution:
- Growth
- Shrinkage
- Position maintenance
- Variations in sediment supply along a coastline result in:
- Differential rates of barrier island development
- Varying degrees of lagoon infilling
- Sediment budget factors affecting barrier island stability:
- Longshore transport rates
- Cross-shore transport
- Aeolian (wind-driven) sand movement
Coastal System Responses to Environmental Changes
- Barrier islands respond to sea-level rise through various mechanisms:
- Vertical accretion (keeping pace with rising water levels)
- Landward migration (rollover process)
- Drowning in place (insufficient sediment supply)
- Sediment supply fluctuations impact coastal morphology:
- Increased supply leads to island growth and lagoon infilling
- Decreased supply results in erosion and potential barrier breaching
- Climate change affects barrier island-lagoon systems through:
- Accelerated sea-level rise
- Changes in storm frequency and intensity
- Alterations in sediment delivery from watersheds
Ecological Importance and Vulnerability of Barrier Islands
Biodiversity and Habitat Significance
- Barrier islands and lagoons provide critical habitats for diverse species:
- Migratory birds (piping plovers, least terns)
- Fish (flounder, red drum)
- Shellfish (oysters, clams)
- Dynamic nature of barrier islands creates a habitat mosaic supporting high biodiversity:
- Maritime forests
- Dune systems
- Inter-tidal zones
- Back-barrier marshes
- Lagoons support extensive ecosystems:
- Seagrass beds (eelgrass, turtle grass)
- Salt marshes (Spartina alterniflora)
- These coastal ecosystems serve as:
- Highly productive environments
- Important carbon sinks (blue carbon storage)
- Nursery grounds for commercially important species
Coastal Protection and Ecosystem Services
- Barrier islands and lagoons act as natural buffers protecting mainland coasts from:
- Storm surges
- Wave energy
- Erosion
- These systems provide various ecosystem services:
- Water filtration
- Nutrient cycling
- Flood mitigation
- Recreational opportunities (fishing, birdwatching)
- Economic value of barrier island-lagoon systems includes:
- Tourism revenue
- Commercial fisheries support
- Natural disaster risk reduction
Vulnerability to Environmental Changes
- Barrier island and lagoon ecosystems face high vulnerability to climate change impacts:
- Accelerated sea-level rise threatens to outpace vertical accretion
- Increased storm intensity may cause more frequent overwash and breaching
- Changes in temperature and precipitation patterns affect species distributions
- These systems show sensitivity to changes in sediment supply altered by:
- Dam construction on rivers
- Coastal armoring
- activities
- Limited elevation of barrier islands increases susceptibility to:
- Overwash during extreme weather events
- Breaching, potentially altering lagoon hydrology and ecology
- Climate change-induced shifts in species ranges may lead to:
- New predator-prey dynamics
- Changes in vegetation communities
- Altered ecosystem functioning
Human Impacts on Coastal Environments
Coastal Development Effects
- on barrier islands disrupts natural sand transport processes through:
- Construction of buildings and infrastructure
- Removal of native vegetation
- Alteration of dune systems
- Human activities limit the ability of barrier systems to migrate in response to sea-level changes:
- Hard structures (seawalls, revetments) prevent landward movement
- Urbanization reduces available space for natural coastal processes
- projects:
- Temporarily maintain shorelines for recreation and property protection
- May alter natural sediment dynamics
- Impact local ecosystems (burial of benthic organisms)
Coastal Engineering Interventions
- Construction of coastal structures affects barrier island systems:
- Jetties and groins interrupt longshore sediment transport
- Lead to downdrift erosion and changes in barrier island morphology
- Dredging of inlets for navigation purposes:
- Modifies tidal prisms
- Alters sediment transport patterns
- Affects barrier island stability and lagoon water quality
- Efforts to stabilize barrier islands through hard engineering structures often result in:
- Exacerbated erosion in adjacent areas
- Altered wave refraction patterns
- Reduced sediment supply to neighboring beaches
Anthropogenic Stressors on Coastal Ecosystems
- Groundwater extraction on barrier islands accelerates land subsidence:
- Compounds effects of sea-level rise
- Increases flooding risk
- Alters freshwater-saltwater interface
- Pollution and nutrient runoff from human activities degrade lagoon water quality:
- Leads to eutrophication (algal blooms, hypoxia)
- Results in loss of biodiversity
- Impacts commercially important species
- Coastal habitat destruction and fragmentation due to development:
- Reduces available nesting sites for sea turtles and shorebirds
- Disrupts migration corridors for various species
- Alters the natural disturbance regime essential for some coastal ecosystems
Key Terms to Review (18)
Barrier island: A barrier island is a coastal landform that is a long, narrow strip of sand or sediment that runs parallel to the mainland, often separated by a lagoon or bay. These islands are formed by the accumulation of sediment through wave action and currents, serving as protective barriers against storms and high waves for the coastal areas they shield. Barrier islands play a crucial role in coastal ecosystems and provide habitat for diverse wildlife, as well as recreational areas for humans.
Beach nourishment: Beach nourishment is the process of adding sand or sediment to an eroding beach to restore it to a wider and more stable condition. This technique is often used as a coastal management strategy to combat erosion, protect coastal infrastructure, and enhance recreational areas. By replenishing the beach with new sediment, it can also help support the ecosystem and maintain habitat for various species.
Coastal development: Coastal development refers to the processes and activities that shape and modify coastal areas, often through human intervention, including construction, land use changes, and environmental management. This term encompasses various structures and features that arise from both natural processes and human actions, impacting ecosystems, sediment dynamics, and local communities. Understanding coastal development is crucial for managing resources and mitigating hazards in coastal zones.
Coastal lagoon: A coastal lagoon is a shallow body of water separated from a larger body of water, typically the ocean, by a barrier such as sandbars or barrier islands. These ecosystems are characterized by brackish water, which is a mix of fresh and saltwater, and they often serve as vital habitats for a variety of wildlife, including fish, birds, and invertebrates. Coastal lagoons are important for their role in coastal protection and as nurseries for marine life.
Dredging: Dredging is the process of removing sediment and debris from the bottom of water bodies, such as rivers, lakes, and coastal areas, often using specialized equipment. This activity is vital for maintaining navigable waterways, enhancing habitats, and managing sediment transport, while also having significant environmental impacts.
E. P. Odum: E. P. Odum was an influential ecologist known for his foundational work in ecosystem ecology, emphasizing the interconnectedness of organisms and their environments. His concepts laid the groundwork for understanding energy flow and nutrient cycling within ecosystems, which are essential for studying environments like barrier islands and coastal lagoons.
Erosion: Erosion is the process by which soil, rock, and other surface materials are worn away and removed from their original location by natural forces such as water, wind, ice, or gravity. This process is essential in shaping landscapes and influencing sediment transport, which connects various components of the Earth's surface system.
Longshore drift: Longshore drift is the process by which sediment is transported along the coastline, driven by the angle of wave approach and the resulting movement of water and sand. This natural phenomenon plays a crucial role in shaping coastal landforms, affecting both erosional and depositional features while influencing coastal management practices.
Marine ecosystem: A marine ecosystem is a complex community of organisms, including plants, animals, and microorganisms, that interact with each other and their environment in oceanic and coastal areas. These ecosystems can vary greatly in terms of their biodiversity, physical characteristics, and the ecological roles that different species play. They are essential for supporting life on Earth, influencing climate patterns, and providing resources for human use.
Overwash: Overwash is a geological process where sediment, usually sand, is transported over the top of a coastal barrier or dune during storm events, typically as a result of high waves and strong winds. This process contributes to the dynamic nature of coastal environments, particularly barrier islands, as it helps maintain their shape and structure by redistributing sediment. Overwash plays a crucial role in forming and altering coastal lagoons as well, influencing their ecological health and resilience against storms.
Restoration Ecology: Restoration ecology is the scientific study and practice of renewing and restoring degraded, damaged, or destroyed ecosystems and habitats to their natural state. It focuses on understanding ecological processes and the interplay between organisms and their environment, aiming to bring back biodiversity, ecosystem functionality, and resilience. This field is crucial for addressing issues related to land use changes and for managing coastal ecosystems, including barrier islands and lagoons, where human activities can significantly alter natural landscapes.
Robert A. Morton: Robert A. Morton is a notable geologist and coastal researcher recognized for his contributions to understanding barrier islands and coastal lagoons. His work emphasizes the importance of sediment dynamics, coastal processes, and the evolution of these unique landforms, linking them to broader environmental and ecological contexts.
Salinity Gradient: A salinity gradient refers to the variation in salt concentration within a body of water, often occurring in coastal environments where freshwater meets saltwater. This gradient plays a crucial role in shaping ecological dynamics, influencing the distribution of organisms and the physical properties of the water, such as density and buoyancy.
Salt marsh: A salt marsh is a coastal ecosystem characterized by salt-tolerant plants, found in the intertidal zones between land and salty or brackish waters. These ecosystems play a crucial role in coastal protection, nutrient cycling, and providing habitats for a variety of wildlife, including birds and fish. Salt marshes are typically located in sheltered areas such as behind barrier islands or along the edges of estuaries, making them an essential feature in the context of coastal landforms.
Sediment deposition: Sediment deposition is the process by which sediment settles out of a transporting medium, such as water or wind, and accumulates in a new location. This process is crucial for shaping landscapes and forming various geographical features, as it allows materials to build up over time, influencing ecosystems and coastal dynamics.
Submergence: Submergence refers to the process where land is gradually submerged under water due to rising sea levels or geological processes. This phenomenon can significantly influence coastal environments, leading to the formation of features like barrier islands and coastal lagoons, which are shaped by sediment deposition and erosion in response to changing water levels.
Tidal influence: Tidal influence refers to the effect that ocean tides have on coastal environments, particularly in shaping landforms and influencing ecosystems. This phenomenon is critical for the development and maintenance of barrier islands and coastal lagoons, as tidal actions can lead to sediment deposition and erosion, thereby affecting the structure and health of these coastal features.
Wave action: Wave action refers to the energy and movement generated by waves as they break upon the shoreline, influencing coastal processes such as erosion, sediment transport, and the formation of landforms. This dynamic interaction between waves and the coastal environment plays a crucial role in shaping coastlines, redistributing sediments, and contributing to the development of various coastal features.