Tidal Wetlands and Estuaries

As land- and freshwater-use changes have an outsized effect on estuarine carbon dynamics, societal drivers are at the heart of future projections for coastal zone carbon cycling. Dissolved carbon inputs are thought to have increased over the past century to Atlantic and GMx estuaries through riverine delivery, largely as a result of agricultural developments (Raymond et al., 2008; Tian et al., 2016). Similarly, delivery of nutrients from agricultural or urban growth and intensification can stimulate primary production in surface waters and respiration in bottom waters, leading to hypoxia and acidification in subsurface estuarine habitats (Cai et al., 2011; Feely et al., 2010; Irby et al., 2018). These human inputs reflect potential pathways for carbon management within estuaries by state, local, or provincial agencies and stakeholders (Chan et al., 2016; Washington State Blue Ribbon Panel on Ocean Acidification 2012). One step removed from carbon are the rich biological resources that have supported human populations on North American estuaries for millennia (e.g., Jackley et al., 2016), which link carbon management to fisheries and ecosystem management processes more broadly (Cooley et al., 2015). As ocean warming and CO₂ uptake drive changes in estuarine circulation, metabolism, and biogeochemistry, myriad changes to estuarine carbon cycles are expected over both short and long timescales, with impacts ranging from direct effects on individual species of ecosystem or economic importance to indirect effects on human health and livelihoods through stimulation of disease vectors (Bednarsek et al., 2017; McCabe et al., 2016; ­Waldbusser et al., 2014). Broad thinking about societal drivers of carbon cycle change and its ecosystem impacts, as well as building effective partnerships with diverse stakeholders, will be critical to effective management of estuarine carbon cycle problems over the coming decades (DeFries and Nagendra 2017).

Coastal wetlands in temperate and tropical latitudes are a “directly or indirectly” managed landscape component, with increasing pressures from human stressors and sea level rise. Given their role in linking land, ocean, and atmospheric carbon fluxes, the increasing rate of global wetland loss and degradation is concerning. Tidal wetland areas in the United States have recently experienced relatively low rates of conversion and loss: ~0.2% per year, according to NOAA Coastal Change and Analysis Program (C-CAP) data from 1996 to 2010, with 92% of all loss occurring in Louisiana (Couvillion et al., 2017; Holmquist et al., 2018b). However, direct and indirect conversions of tidal wetlands to drained or impounded land uses continue actively along coastlines globally. In Mexico, 10% of mangrove area has been lost from 1980 to 2015, resulting in CO₂ emissions ranging from 0.4 to 1 Tg C per year (Troche-Souza et al., 2016); while GMx has more mangrove area, loss is high on the Pacific Coast due primarily to anthropogenic land-use changes.

Coastal “blue carbon” ecosystems—tidal marshes, mangroves, and estuarine sea grasses—are characterized by high areal rates of carbon sequestration, low rates of CH₄ and nitrous oxide (N₂O) emissions, and large soil carbon pools (Howard et al., 2017). Because the influence of coastal ecosystems on carbon cycles greatly exceeds their area (Najjar et al., 2018), activities that affect the conservation, degradation, or restoration of these ecosystems have implications for greenhouse gas (GHG) emissions and national GHG accounting (Kennedy et al., 2014). Loss of tidal hydrology likely shifts tidal wetlands from sinks to sources as large soil carbon reservoirs in tidal wetlands can become large sources of CO₂ emissions when disturbed (Pendleton et al., 2012), and freshwater dominance can dramatically impact CH₄ emissions (Kroeger et al., 2017). Further, nitrate pollution can dramatically impact N₂O emissions (Moseman-Valtierra et al., 2011). In 2013, the Intergovernmental Panel on Climate Change (IPCC) issued guidance on including management of seagrasses, tidal marshes, and mangroves as an anthropogenic carbon flux in national GHG inventories (Kennedy et al., 2014). Currently a number of countries, including the United States, are in the process of implementing these guidelines (U.S. EPA 2017), an action which would be a major step toward reducing uncertainties in national carbon budgets and understanding the roles played by coastal tidal wetland management in national GHG emissions. This new information includes the relatively strong long-term sink for carbon in tidal and subtidal wetland soils, relatively limited CH₄ emissions in saline wetlands, and relatively large GHG emissions associated with wetland loss. In addition to improved knowledge of tidal wetland carbon balance, inclusion of tidal wetlands in the U.S. national GHG inventory provides an opportunity for enhanced estimation of the ecosystem services these wetlands offer to coastal communities. Ongoing research on feedbacks among hydrology, geomorphology, nutrient availability, plant productivity, and microbial activity is needed to understand and manage the impacts of human activities on the GHG balance of these ecosystems.