Inorganic and Black Carbon Hotspots Constrain Blue Carbon Mitigation Services Across Tropical Seagrass and Temperate Tidal Marshes

John Barry Gallagher,Vishnu Prahalad,John Aalders

doi:10.1007/s13157-021-01460-3

Abstract

Total organic carbon (TOC) sediment stocks as a CO2 mitigation service require exclusion of allochthonous black (BC) and particulate inorganic carbon corrected for water–atmospheric equilibrium (PICeq). For the first time, we address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots that represent two different blue carbon ecosystems of Malaysia and Australia. Seagrass TOC stocks were similar to the salt marshes with soil depths < 1 m (59.3 ± 11.3 and 74.9 ± 18.9 MgC ha− 1, CI 95 % respectively). Both ecosystems showed larger BC constraints than did their pristine counterparts. However, the seagrass meadows’ mitigation services were largely constrained by both higher BC/TOC and PICeq/TOC fractions (38.0 % ± 6.6 and 43.4 % ± 5.9 %, CI 95 %) and salt marshes around a third (22 % ± 10.2 and 6.0 % ± 3.1 % CI 95 %). The results provide useful data from underrepresented regions, and, reiterates the need to consider both BC and PIC for more reliable blue carbon mitigation assessments to ensure that greenhouse gas emitters do not exceed the ecosystems’ capacity.

Highlights

Blue carbon refers to the carbon sequestered and stored in salt marsh, mangrove, and seagrass beds (Nellemann, Corcoran, & M., 2009)
We address this bias for a temperate salt marsh and a coastal tropical seagrass in BC hotspots that represent two different blue carbon ecosystems of Malaysia and Australia
It was found that allochthonous BC was likely to be a major contributor to the Total organic carbon (TOC) sedimentary stocks for coastal seagrass meadows that occupy BC hotspots, such as Middle Bank (Peninsula Malaysia)

Summary

Introduction

Blue carbon refers to the carbon sequestered and stored in salt marsh, mangrove, and seagrass beds (Nellemann, Corcoran, & M., 2009). Traditional assessments have largely failed to remove allochthonous organic recalcitrants from the equation (Gallagher 2014; Gallagher 2015; Gallagher 2017; Chew and Gallagher 2018; Chuan et al 2020; Gallagher et al 2020 ) These stable organic forms are not produced by the blue carbon ecosystem; deposition within the ecosystems’ sediments does not afford additional protection from remineralization. Their presence is not a measurable storage or sequestration service in the mitigation of greenhouse gas emissions. While the argument is unequivocal and recognised by the IPCC as an important blue carbon constraint (Bindoff et al 2019), data are scarce on their contributions to blue carbon ecosystems

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Conclusion