10.3 Barrier islands and coastal lagoons

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
• Longshore drift, wave action, and sea-level fluctuations over geological time scales contribute to barrier island 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:
  1. Sediment accumulation from longshore currents
  2. Wave action shaping the sediment into an elongated form
  3. 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:
  1. Barrier islands or spits develop, enclosing a body of water
  2. 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:
  1. Erosion of seaward side
  2. Overwash of sediment to landward side
  3. Gradual landward movement of entire island
• Regressive barrier island model explains seaward growth during sea-level fall:
  1. Exposure of previously submerged areas
  2. Seaward progradation of beach ridges
  3. 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:
  1. Vertical accretion (keeping pace with rising water levels)
  2. Landward migration (rollover process)
  3. 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
  • Dredging 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

• Coastal development 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
• Beach nourishment 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