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Abstract

While intertidal flats only make up less than 10% of the world's coastlines, they are among the most productive continental shelf ecosystems. They are important sites of recycling for both terrestrially and marine-derived organic matter and nutrients and also support high rates of primary productivity. These tidal flats are located between the spring high- and low-tide marks, lack rooted vegetation, and span a range of composition from mudbanks to coarse sand flats. They are found in sheltered bays, estuaries, and coasts that are protected by barrier islands. Biogeochemical processes in tidal flat sediments are regulated by the dynamic interaction of microbial reactions and water and sediment transport processes. Temporal and spatial heterogeneity and strong forcing by tidal and wind dynamics generate extreme biogeochemical variability in intertidal flats and distinguish these habitats from subtidal coastal sediments. Here, we synthesize the last several decades of research on biogeochemical patterns and processes in tidal flat sediments and place them within their geological and hydrodynamic contexts. We first define and describe the processes that control water and solute exchange between different types of intertidal flats and the overlying or adjacent seawater. We go on to discuss how organic matter is broken down or altered in tidal flat sediments. Finally, we discuss microbial nitrogen, phosphorus, and silica cycling within tidal flat sediments and the influence of these processes on water column nutrient availability.