Integrated Carbon Budget Models for the Everglades Terrestrial-Coastal-Oceanic Gradient: Current Status and Needs for Inter-Site Comparisons

Abstract

Recent studies suggest that coastal ecosystems can bury significantly more C than tropical forests, indicating that continued coastal development and exposure to sea level rise and storms will have global biogeochemical consequences. The Florida Coastal Everglades Long Term Ecological Research (FCE LTER) site provides an excellent subtropical system for examining carbon (C) balance because of its exposure to historical changes in freshwater distribution and sea level rise and its history of significant long-term carbon-cycling studies. FCE LTER scientists used net ecosystem C balance and net ecosystem exchange data to estimate C budgets for riverine mangrove, freshwater marsh, and seagrass meadows, providing insights into the magnitude of C accumulation and lateral aquatic C transport. Rates of net C production in the riverine mangrove forest exceeded those reported for many tropical systems, including terrestrial forests, but there are considerable uncertainties around those estimates due to the high potential for gain and loss of C through aquatic fluxes. C production was approximately balanced between gain and loss in Everglades marshes; however, the contribution of periphyton increases uncertainty in these estimates. Moreover, while the approaches used for these initial estimates were informative, a resolved approach for addressing areas of uncertainty is critically needed for coastal wetland ecosystems. Once resolved, these C balance estimates, in conjunction with an understanding of drivers and key ecosystem feedbacks, can inform cross-system studies of ecosystem response to long-term changes in climate, hydrologic management, and other land use along coastlines

Constraining Marsh Carbon Budgets Using Long‐Term C Burial and Contemporary Atmospheric CO2 Fluxes

“…In tidal wetlands, NEE may exceed these burial rates because the marsh is known to export carbon to the estuary via porewater drainage and tidal flushing (Raymond & Hopkinson, ; Vallino et al, ). As expected, our measured NEE indicates a larger net C uptake than the burial rate, and this difference may be used as first‐order estimate of lateral C export (Troxler et al, ). At our site this would suggest a bulk lateral export on the order of 69 g C m −2 a −1 .…”

Increasing Rates of Carbon Burial in Southwest Florida Coastal Wetlands

“…Evidence of the allochthonous delivery mechanism is supported by increased NC‐mineral sediment deposition, which showed significant acceleration in cores SMg1, SMg2, and NMg3 (Figure ). If the OC would otherwise have been exported from the mangroves to the marine environment (Bouillon et al, ; Troxler et al, ), then the increase in burial rates at these sites represents a net increase in the regional carbon sink that was not present a century ago. If the OC is simply being redistributed from one wetland location to another, then OC burial increases at these sites occur at the expense of OC burial decreases and erosion at other sites.…”

“…Evidence of the allochthonous delivery mechanism is supported by increased NC-mineral sediment deposition, which showed significant acceleration in cores SMg1, SMg2, and NMg3 (Figure 10). If the OC would otherwise have been exported from the mangroves to the marine environment (Bouillon et al, 2008;Troxler et al, 2013), then the increase in 10.1029/2019JG005349…”

“…Constant, linear rates of burial contribute a majority of the records used to account for global average burial rates (Chmura et al, , and references therein; Breithaupt et al, , and references therein; Ouyang & Lee, , and references therein). These averages are a necessary part of carbon budgeting because of interest in the long‐term flux of components such as productivity, respiration, and aqueous export (i.e., Bouillon et al, ; Troxler et al, ). However, reliance only on multidecadal or centennial averages may give a misleading impression of burial rates as a static or stable flux.…”

Understanding Coastal Carbon Cycling by Linking Top‐Down and Bottom‐Up Approaches

“…NECB has become a central theme and rallying point for collaborations aimed at understanding the complexities of C cycling in coastal systems [e.g., Troxler et al, 2013] and has helped to identify several practical challenges. For example, how can components of the C budget be synchronized across space and time?…”