Summary: After integrating all the above mitigation services, many of the subaquatic organic process variables are reduced to manageable and measurable terms for a robust methodology.
This article has both argued and largely demonstrated which vectors should be considered valuable as wetland mitigation services for the emerging blue carbon offset market. It argues that lateral export of DIC and TA should not be classified as additional sequestration services. However, excluding a known service of situ storage of POC and DOC, will not overly underestimate or disadvantage a carbon offset methodology. Furthermore, the importance of carbon sink offsets from the remineralisation of non–refractory allochthonous carbon needs to be assessed within the framework of a previously unrecognised mitigation service. Specifically, attention is needed to evaluate how much of that non-refractory allochthonous carbon is saved from coastal remineralisation.
However, the challenge is to develop a methodology that can account for the above additional allochthonous carbon vectors in a manner that continues to be resource- and technically feasible. In its simplest form, consider a floating line seaweed farm that supports a non-vegetated bottom. Here, allochthonous mitigation services become irrelevant. The ability to trap, and preserve allochthonous carbon within similar sediments is expected to be near identical for the site before and after the farm is established. As a result, the differences in sequestration, as ΔNEP, with and without the seaweed farm (Eq. (2)) are equivalent to their differences in sedimentary carbon accumulation rates, ΔCA, irrespective of recalcitrant allochthonous and allochthonous carbon respiratory subsidies (Eq. (3)).
$$\:\varDelta\:NEP=\left(CW-{L}_{0y}-{C}_{r}\right)-\left(CB-{L}_{0y}-{C}_{R}\right)=CW-CB\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:$$
2
$$\:\varDelta\:CA=\left(CW+{L}_{r}+{C}_{R}\right)-\left(CB+\:{L}_{0y}+\:{C}_{R}\:\right)=CW-CB\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:$$
3
Where CW and CB are the accumulation of the wetlands and baseline scenario systems autochthonous carbon, respectively; L0y represents the accumulation of the labile or non-refractory carbon deposits at time zero; and CR represents the rate of allochthonous recalcitrant carbon accumulated.
However, a vegetated wetland will trap and deposit organic carbon more efficiently than a non–vegetated baseline scenario ecosystem 49. Under these circumstances ∆NEP and ∆CA will be expected to diverge, given the non-equivalence of their component terms between the rate of deposition of the wetlands and baseline scenarios allochthonous labile and recalcitrant carbon, WL0y and BL0y and WCR and BCR respectively, (Eq. (4)).
$$\:\varDelta\:NEP=\left(CW-{WL}_{0y}-{WC}_{r}\right)-\left(CB-{BL}_{0y}-{BC}_{R}\right)\ne\:CW-CB$$
4
Nevertheless, it can be shown that a number of terms are eliminated when the allochthonous mitigation service between the wetland and the baseline scenario is added to the full component parts of ∆NEP (see Eq. (5) and then equations (6) and (7). Allochthonous mitigation service (AM) for a wetland that will be likely replaced by baseline scenario of a non–vegetated ecosystem that traps a smaller proportion of the allochthonous inputs and, like the wetland, (Fig. 5) also remineralises some fraction of that trapped material, leaving behind their remains WLr and BLr, respectively:
The total mitigation service (TM) can be written as ΔNEP + AM when separated into all the non–refractory organic source components (Eq. (6), where WCa and BCa are the autochthonous carbon accumulation rates for the wetland and the baseline scenarios and WL0y and BL0y, their respective initial deposition rates for different allochthonous trapping efficiencies respectively. A wetland canopy system with a different allochthonous carbon trapping efficiency than its baseline counterpart 49. Note that allochthonous recalcitrants forms are outside the carbon loop and should be measured and subtracted from the equations (IPCC39).
$$\:TM=\left(WCa-\left(W{L}_{0y}-WLr\right)\right)-\left(BCa-\left(B{L}_{0y}-BLr\right)\right)+(WLr-BLr)$$
6
This then simplifies (Eq. (7)) by solving for CW-CB as the differences between the wetland and baseline scenario total non-refractory carbon accumulation rates, and (WL0y-WLr)-(BL0y-BLr) as their respective difference in the rates of non-refractory allochthonous remineralization.
$$\:TM=(CW-CB)+\left((W{L}_{0y}-WLr\right)-(B{L}_{0y}-BLr))$$
7
This arithmetic proof also indicates how and where the previously uncritical “ideal” notion that differences in carbon accumulation is the one mitigation service required for restoration diverges when the full cohort of services is considered. That is to say the differences in the rates of non–refractory allochthonous remineralisation, once the allochthonous recalcitrant components has been removed from the assessment. It also shows that it is not necessary to measure individual sequestration rates for the wetland and baseline scenario, when the objective is carbon mitigation. Furthermore, it should be noted from Eq. (7) that all of the above variables, including allochthonous recalcitrant forms can be directly measured in ways that are both technically and resource-friendly. In short, TOC and allochthonous recalcitrant forms can be measured gravimetrically after combustion 38, rates of initial deposition of organic carbon can be tackled with well–designed flat traps, and burial rates can be estimated with horizon sand markers or poles 56. The only caveat is the additional expertise in disentangling allochthonous (Eq. (7)) from autochthonous deposition and a model to account for submersion on the air sea flux of CO2. However, this may not be an issue as a project proposal is often a collaboration between professional organisations, local communities and/or government bodies 57.