Seasonal Variations in Dissolved Carbon Inventory and Fluxes in a Mangrove‐Dominated Estuary

Abstract

Two experiments were performed during the wet and dry seasons to quantify dissolved carbon dynamics and fluxes in the Shark River, a tidal estuary flowing through the largest contiguous mangrove forest in North America (Everglades National Park, Florida, USA). During these experiments, between 80% and 87% of the total dissolved carbon pool consisted of inorganic carbon (DIC). Carbon inputs from mangroves to the estuary were slightly higher during the wet season, whereas alkalinity inputs were comparable during the two experiments. Longitudinal dissolved carbon fluxes to the coastal ocean were slightly higher during the wet season (13.53 ± 0.76 × 10 5 mol day −1 during the wet and 11.70 ± 0.32 × 10 5 mol day −1 during the dry), whereas longitudinal alkalinity flux was comparable during both experiments (10.64 ± 0.74 in the wet vs. 9.88 ± 0.30 × 10 5 mol day −1 during the dry season). Overall, DIC production in surface water, porewater, and groundwater was dominated by oxic mineralization of mangrove-derived organic matter and carbonate dissolution. Carbonate dissolution was the most important alkalinity production process in the system. The experiments show that regardless of the season and hydro-climatic conditions, Shark River receives large inputs of dissolved carbon from the upstream marsh, mangroves, and carbonate dissolution, and that per area, it exports a greater amount of dissolved carbon than many other mangrove-dominated estuaries in the world.

Global Trends in Air‐Water CO2 Exchange Over Seagrass Meadows Revealed by Atmospheric Eddy Covariance

“…Because these factors act synchronously and are often correlated with each other, it is impossible to attribute the global trend of decreasing magnitude and range in FCO 2 with a single “tidal” factor. For example, tidal mixing may interact with allochthonous factors, driving net CO 2 release via DIC “outwelling” (Polsenaere et al., 2012; Volta et al., 2020), or enhancing CO 2 uptake through coastal wetland alkalinity export (Akhand et al., 2020; Cai et al., 1999; Santos et al., 2015). In these cases, blue carbon assessments should be careful to avoid “double‐counting” carbon that was produced or consumed in tidally connected systems.…”

“…For the other sites with complete seasonal data, net CO 2 uptake was ∼100% (EES) to over 800% (FU) of global average CBR. At these sites, the presence of tidal forcing brings into question if or how this CO 2 uptake should be incorporated into blue carbon budgets, as it is likely driven by allochthonous factors like lateral DIC or alkalinity exchange (Akhand et al., 2020; Volta et al., 2020).…”

Carbon and alkalinity outwelling across the groundwater‐creek‐shelf continuum off Amazonian mangroves

“…The slopes imply that the production of DIC and TA can be a result of different biogeochemical reactions, especially at the estuary and shelf scales due to sources other than mangroves. Sulfate reduction seems to represent the main organic carbon mineralization pathway within the mangrove (groundwater and tidal creek samples) as observed in other mangrove systems (Sippo et al 2016;Twilley et al 2019;Reithmaier et al 2020). However, other processes, such as calcium carbonate dissolution, might represent an important source of alkalinity in mangroves and on the shelf (Macreadie et al 2017;Volta et al 2020;Saderne et al 2021).…”

“…Future studies investigating multiple scales and vertical gradients are needed to provide additional insight into drivers of outwelling and decrease uncertainties of global estimates. Carbon isotopes can also reveal the relative contribution of different sources organic matter (Taillardat et al 2018;Volta et al 2020;Kwon et al 2021). Combining radioactive and stable isotopes tracers will help to build complete carbon budgets for this region and elsewhere.…”

“…Mangroves have one of the highest aerial carbon burial rates due to high primary production and complex root systems that allow organic matter deposition and long-term storage in the anoxic soil layers (Alongi 2020;Jennerjahn 2020). Part of the mangrove soil carbon might be released to the atmosphere after microbial decomposition and exported to ocean as particulate and dissolved matter (Bouillon et al 2008;Volta et al 2020). Outwelling is the lateral or horizontal transport of suspended and dissolved material at the land-ocean interface (Santos et al 2021).…”