10.2 Estuarine and Coastal Biogeochemistry

Estuaries and coastal areas are dynamic zones where freshwater meets the sea. These unique environments host diverse ecosystems and play crucial roles in nutrient cycling, carbon sequestration, and pollutant transformation.

Estuarine biogeochemistry is shaped by salinity gradients, tidal influences, and land-sea interactions. These factors create distinct habitats like salt marshes and seagrass beds, which act as biogeochemical reactors, processing nutrients and organic matter in complex ways.

Estuarine and Coastal Biogeochemistry Characteristics

Biogeochemical features of estuaries

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• Salinity gradients
  • Transition from freshwater to saltwater creates dynamic mixing zone
  • Estuarine circulation patterns driven by density differences (salt wedge, partially mixed, well-mixed)
  • Salinity influences chemical reactions and biological processes (osmoregulation)
• Tidal influence
  • Periodic water level fluctuations alter habitat availability and chemistry
  • Tidal flushing and water exchange transport nutrients and organisms
  • Intertidal zones support diverse communities adapted to desiccation and submersion cycles
• Land-sea interactions
  • Terrestrial runoff delivers nutrients and organic matter to coastal waters
  • Sediment transport and deposition shape estuarine morphology and turbidity
  • Coastal erosion and accretion processes modify shorelines and affect habitat distribution
• Unique habitats
  • Salt marshes and mangrove forests act as nutrient filters and carbon sinks
  • Seagrass beds and coral reefs provide complex 3D structure for diverse ecosystems
  • Mudflats and sandy beaches support benthic communities and migratory birds

Estuaries as biogeochemical reactors

• Nutrient cycling
  • Nitrogen fixation by microbes and denitrification in anoxic sediments regulate N availability
  • Phosphorus adsorption to sediments and desorption under changing redox conditions
  • Silicon cycling in diatom-dominated systems influences phytoplankton community structure
• Organic matter processing
  • Primary production by phytoplankton and benthic algae forms basis of estuarine food webs
  • Bacterial decomposition and remineralization recycle nutrients
  • Formation of dissolved organic matter (DOM) affects water color and light penetration
• Pollutant transformation
  • Heavy metal sequestration in sediments through adsorption and precipitation
  • Biodegradation of organic pollutants (PAHs, PCBs) by specialized microbial communities
  • Bioaccumulation and biomagnification in food webs concentrate certain contaminants
• Carbon cycling
  • Blue carbon sequestration in coastal ecosystems (mangroves, salt marshes, seagrasses)
  • CO2 exchange between air and water influenced by biological activity and physical processes
  • Methane production in anaerobic sediments contributes to greenhouse gas emissions

Benthic-Pelagic Coupling and Human Impacts

Benthic-pelagic coupling in estuaries

• Benthic-pelagic coupling
  • Exchange of nutrients between sediments and water column drives productivity
  • Influence on primary production and food web dynamics varies with depth and season
  • Seasonal variations in coupling strength related to temperature and organic matter input
• Bioturbation
  • Sediment reworking by benthic organisms (polychaetes, bivalves) enhances nutrient flux
  • Enhanced oxygen penetration into sediments stimulates aerobic decomposition
  • Facilitation of organic matter decomposition through particle mixing and burrow irrigation
• Sediment resuspension
  • Wind-driven and tidal resuspension events redistribute particulate matter
  • Release of nutrients and pollutants from sediments affects water column chemistry
  • Impact on water column turbidity and light penetration influences primary production
• Benthic fluxes
  • Diffusive and advective transport of solutes across sediment-water interface
  • Role of porewater exchange in nutrient cycling, especially in permeable sediments
  • Importance of benthic microalgae in shallow systems for nutrient retention and stabilization

Human impacts on coastal biogeochemistry

• Coastal development
  • Increased nutrient loading from urban runoff leads to eutrophication
  • Alteration of natural shorelines and habitats reduces ecosystem services
  • Changes in sediment transport and deposition patterns affect coastal morphology
• Aquaculture
  • Nutrient enrichment from fish farms stimulates algal growth and oxygen depletion
  • Introduction of antibiotics and other chemicals affects microbial communities
  • Modification of local food webs and ecosystem structure through species introductions
• Eutrophication
  • Excessive algal blooms and oxygen depletion create dead zones
  • Shifts in species composition and biodiversity favor opportunistic species
  • Formation of hypoxic or anoxic zones alters biogeochemical cycling (denitrification, P release)
• Climate change impacts
  • Sea level rise and saltwater intrusion modify estuarine salinity gradients
  • Ocean acidification effects on calcifying organisms (shellfish, corals) alter community structure
  • Changes in precipitation patterns and freshwater input affect nutrient delivery and stratification