Tidal Wetlands and Estuaries

Estuaries and tidal wetlands are dynamic ecosystems that host high biological production and diversity (Bianchi 2006). They receive large amounts of dissolved and particulate carbon and nutrients from rivers and uplands and exchange materials and energy with the ocean. Estuaries and tidal wetlands are often called biogeochemical “reactors” where terrestrial materials are transformed through interactions with the land, ocean, and atmosphere. Work conducted in the past decade has clearly shown that open-water estuaries as a whole can be strong sources of carbon to the atmosphere—both carbon dioxide (CO₂) and methane (CH₄)—despite the fact that how degassing (i.e., gas emissions) rates vary in space and time in many estuaries is unknown (Borges and Abril 2011; Cai 2011). In contrast, tidal wetlands represent a small fraction of the land surface but are among the strongest long-term carbon sinks, per unit area, because of continuous organic carbon accumulation in sediments with rising sea level (Chmura et al., 2003). Estuaries are included here in the Second State of the Carbon Cycle Report (SOCCR2) but were not included in the First State of the Carbon Cycle Report’s (SOCCR1; CCSP 2007) assessment of coastal carbon cycling. Estuaries have been reviewed in recent synthesis activities, partic­ularly the Coastal CARbon Synthesis (CCARS; Benway et al., 2016). Tidal wetlands were included in the wetlands chapter of SOCCR1 but are separated from inland wetlands in this SOCCR2 assessment to reflect their unique connections to estuarine and ocean dynamics. Consistently missing from previous fieldwork and syntheses are important annual carbon exchanges (including CO₂ and CH₄ flux) across boundaries of intertidal (hereafter, wetland) and subtidal ecosystems and deeper waters (hereafter, estuarine). As subsystems of an integrated coastal mixing zone, this lack of information limits understanding of the relative roles of wetlands and estuaries in carbon cycling at the critical land-ocean margin. An updated synthesis of current knowledge and gaps in quantifying the magnitude and direction of carbon fluxes in dynamic estuarine environments is presented herein.

According to Perillo and Picollo (1995) and Pritchard (1967), estuaries are commonly defined as “semi-enclosed coastal bodies of water that extend to the effective limit of tidal influence, within which seawater entering from one or more free connections with the open sea, or any saline coastal body of water, is significantly diluted with fresh water [sic] derived from land drainage, and can sustain euryhaline biological species from either part or the whole of the life cycle.” For the purpose of this report, the landward boundary of estuarine zones is defined as the “head of tide” (i.e., the maximal boundary of tidal expression in surface water elevation) and the shoreward limit of the continental shelf (i.e., the relatively shallow sea that extends to the edge of continental crust). While island coastlines are included in the overall SOCCR2 domain (namely Hawai‘i, Puerto Rico, and the Pacific Islands), due to reliance on recent synthesis products for carbon accounting, the focus herein is exclusively on continental coastlines where stocks and fluxes have been quantified and mapped most comprehensively. Section 15.2, provides a brief historical overview of carbon flux in estuaries and tidal wetlands with an emphasis on coastal processes with global applicability. Section 15.3 compiles information on carbon fluxes of estuaries and tidal wetlands of North America in the global context and from regional perspectives. Through literature summaries and data syntheses, Section 15.4 provides new estimates of selected fluxes and stocks in tidal wetlands and estuaries of North America. Section 15.5 discusses new and relevant coastal carbon observations through indicators, trends, and feedbacks, and Section 15.6 reports on management and decisions associated with societal drivers and impacts within the carbon cycle context. Finally, Section 15.7 provides a synthesis that summarizes conclusions, gaps in knowledge, and near-future outlooks.