Mangroves as a Source of Greenhouse Gases to the Atmosphere and Alkalinity and Dissolved Carbon to the Coastal Ocean: A Case Study From the Everglades National Park, Florida

Abstract

Most research evaluating the potential of mangroves as a sink for atmospheric carbon has focused on carbon burial in sediments. However, the few studies that have quantified lateral exchange of carbon and alkalinity indicate that the dissolved carbon and alkalinity export may be several-fold more important than burial. This study aims to investigate rates and drivers of alkalinity, dissolved carbon, and greenhouse gas fluxes of the mangrove-dominated Shark River estuary located in the Everglades National Park in Florida, USA. Spatial surveys and 29-hr time series were conducted to assess total alkalinity (TAlk), organic alkalinity (OAlk), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2 O) dynamics. Dissolved carbon and greenhouse gas concentrations were coupled to porewater input, which was examined using radon-222. Shark River was a source of CO 2 (92 mmol/m 2 /day), CH 4 (56 μmol/m 2 /day), and N 2 O (2 μmol/m 2 /day) to the atmosphere. Dissolved carbon export (DIC = 142 mmol/m 2 /day, DOC = 39 mmol/m 2 /day, normalized to mangrove area) was several-fold higher than previously reported carbon burial rates in the study area (~28 mmol/m 2 /day). The majority of the DIC was exported as TAlk (97 mmol/m 2 /day), which remains dissolved in the ocean for millennia and, therefore, represents a long-term sink for atmospheric carbon. By integrating our results with previous studies, we argue that alkalinity, dissolved carbon, and greenhouse gas fluxes should be considered in future blue carbon budgets. Plain Language Summary Protecting mangroves can help us deal with one of the biggest challenges of our time: Climate change. Mangroves remove carbon dioxidethe gas that is making the world hotterfrom the air around us and store it in their surrounding soils. Many scientists have studied how much carbon is trapped in mangrove soils, and the newest studies say that some of that trapped carbon is flushed out to the coastal ocean. We wanted to find out for ourselves so we went to Everglades National Park, which protects the biggest mangrove forest in North America. We cruised along the Shark River estuary and spent day and night on the boat to uncover this carbon mystery. We found that the estuary loses greenhouse gasses to the atmosphere, but much less compared to what it loses to the Gulf of Mexico. We could also confirm that the amount of carbon lost was much greater than what is trapped in soils. We encourage other researchers to investigate dissolved carbon export in other mangroves in order to find out how much mangroves can alleviate the impacts of climate change.

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).…”

Alkalinity Production Coupled to Pyrite Formation Represents an Unaccounted Blue Carbon Sink

“…Export values have been retrieved from Reithmaier et al. (2020b) for the Everglades and from Sippo et al. (2016) for the remaining sites.…”

“…Reithmaier et al (2020b).b Sippo et al (2016).c Brown et al (2018).d Maher et al (2015).e Reithmaier et al (2020a). f Lee and Patterson (2002).…”

“…A linear regression is shown for Barwon Heads, Jacobs Well, Hinchinbrook Island, and Darwin. Export values have been retrieved fromReithmaier et al (2020b) for the Everglades and fromSippo et al (2016) for the remaining sites.…”

Dissolved and particulate carbon export from a tropical mangrove‐dominated riverine system

“…of $ 15-20 Pg C on global area basis; Donato et al 2011). Lines of evidence have revealed that mangroves can export a significant amount of dissolved organic and inorganic carbon (DOC, DIC) and particulate organic carbon (POC) to the sea (Dittmar et al 2006;Ray et al 2020;Reithmaier et al 2020). Mangrove pore water-derived C flux is equivalent to $ 30% of global river inputs (Chen et al 2018).…”