Hypersaline tidal flats as important “blue carbon” systems: a case study from three ecosystems

Dylan R Brown,Mario L G Soares,Wilson Machado,Roberta B Peixoto,Luciana M Sanders,Alexander Pérez,Glenda C Barroso,Isaac R Santos,Damien T Maher,James Z Sippo,Humberto Marotta,Christian J Sanders,Stephen Conrad,Alex Enrich-Prast,Joseph M Smoak

doi:10.5194/bg-18-2527-2021

Abstract

Abstract. Hypersaline tidal flats (HTFs) are coastal ecosystems with freshwater deficits often occurring in arid or semi-arid regions near mangrove supratidal zones with no major fluvial contributions. Here, we estimate that organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) were buried at rates averaging 21 (±6), 1.7 (±0.3) and 1.4 (±0.3) gm-2yr-1, respectively, during the previous century in three contrasting HTF systems, one in Brazil (eutrophic) and two in Australia (oligotrophic). Although these rates are lower than those from nearby mangrove, saltmarsh and seagrass systems, the importance of HTFs as sinks for OC, TN and TP may be significant given their extensive coverage. Despite the measured short-term variability between net air–saltpan CO2 influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal CO2 flux measurements, the OC sedimentary profiles over several decades suggest efficient OC burial at all sites. Indeed, the stable isotopes of OC and TN (δ13C and δ15N) along with C:N ratios show that microphytobenthos are the major source of the buried OC in these HTFs. Our findings highlight a previously unquantified carbon as well as a nutrient sink and suggest that coastal HTF ecosystems could be included in the emerging blue carbon framework.

Highlights

Hypersaline tidal flats (HTFs), supratidal flats, saltpans, sabkhas and salt flats are all terms used to define the shallow coastal ecosystems on the upper fringe of fluviomarine plains in estuaries showing freshwater deficits (Ridd and Stieglitz, 2002; Albuquerque et al, 2014)
Most of the parameters remained relatively constant throughout the sediment profiles, with no clear vertical trends in grain size, Organic carbon (OC), total nitrogen (TN) or Total phosphorous (TP) (Fig. 3)
Sand content was generally < 20 %, and OC, TN and TP contents ranged from 0.09 % to 1.40 %, 0.01 % to 0.16 % and 0.02 % to 0.12 %, respectively, across all sites and depth intervals (Fig. 3)

Summary

Introduction

Hypersaline tidal flats (HTFs), supratidal flats, saltpans, sabkhas and salt flats are all terms used to define the shallow coastal ecosystems on the upper fringe of fluviomarine plains in estuaries showing freshwater deficits (Ridd and Stieglitz, 2002; Albuquerque et al, 2014). These environments are generally located in an intermediary position between mangrove forests or saltmarshes and the terrestrial environment and are common in many tropical arid, and to a lesser extent non-arid, intertidal zones. Evaporation, the flat topography and pronounced hydraulic deficit results in hypersaline conditions with salinity as high as 5 times that of seawater (Ridd and Stieglitz, 2002; Shen et al, 2018)

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Discussion

Conclusion