Biogeochemistry of Tidal Freshwater Wetlands

Abstract

The biogeochemistry of tidal freshwater wetlands is unique in the coastal landscape because of their position at the limit of tidal influence, where processes are influenced by both tidal flushing and the chemical milieu of freshwater. In this chapter, we describe the cycling of carbon, nitrogen, phosphorus, and silicon in tidal freshwater wetlands and how these cycles are affected by major biotic (e.g., plant and microbial communities) and abiotic drivers (e.g., organic matter chemistry, environmental conditions). Carbon cycling is a fundamental driver of biogeochemical transformations in ecosystems. Primary production largely establishes the upper limit of heterotrophic activity, whereas decomposition drives the release or retention of nitrogen and other nutrients in organic matter. The quantity and lability of organic carbon plays an important role in regulating microbial transformations such as denitrification. Much of the study of the nitrogen and phosphorus cycles in tidal freshwater wetlands has focused on the role of these ecosystems in improving water quality (e.g., through denitrification and trapping of particulate phosphorus) and controlling wetland primary productivity, with additional research addressing the transformations between organic and inorganic forms that can affect nutrient availability and soil storage. The open hydrology of tidal freshwater wetlands means that transformations that occur within wetland soils can influence adjacent riverine and estuarine systems. For example, tidal freshwater wetlands are an important location for the transformation of silicon from biogenic to dissolved forms, increasing the availability of silicon to aquatic primary producers. As environmental conditions change, the biogeochemistry of tidal freshwater wetlands is likely to be altered, leading to shifts in how these ecosystems perform valuable ecosystem functions (e.g., carbon storage, nutrient removal) and affect global climate.