Valuing Carbon Stocks across a Tropical Lagoon after Accounting for Black and Inorganic Carbon: Bulk Density Proxies for Monitoring

Abstract

Managing seagrass and mangrove can be enhanced through carbon valued payment incentives schemes. Success will depend on the accuracy and extent of the carbon stock mitigation and accessible methods of monitoring and marking changes. In a relatively closed socioecological Southeast Asian lagoon we estimated the value of total organic carbon stocks (TOC) of both seagrass and mangroves. Mitigation corrections were also made for black carbon (BC) and calcareous inorganic carbon equivalents (PIC equiv ), and their sediment dry bulk density (DBD) tested as a cost effective means of both estimating those stock concepts and possible impacts outside their parameter confidence intervals.Overall, seagrass and mangroves TOC densities across the lower lagoon ranged from 15.3±4.3 and 124.3±21.1 Mg C ha -1 respectively, 175.2±46.9 and 103.2±19.0 Mg C ha -1 for seagrass and 355.0±24.8 and 350.3±35.2 Mg C ha -1 ) for mangroves across the two upper lagoon branches.Only mangrove biomass made significant additional contributions ranging from 178.5±62.3 to 120.7±94.8 Mg C ha -1 for lower and upper regions respectively. The difference between the lagoons total seagrass and mangroves TOC stocks (5.98±0.69 and 390±33.22 GgC respectively) was further amplified by the lagoons' larger mangrove area. When corrected for BC and PIC equiv , the carbon stock mitigation was only reduced by a moderate 14.2%. Across the lagoon the sedimentary DBD showed strong (R 2 = 0.85, P < 0.001) to moderate (R 2 = 0.67, P < 0.001) linear correlations with seagrass and mangrove [TOC] respectively, moderate correlations with seagrass [PIC] (R 2 = 0.6, P < 0.001), but an invariant and relatively constant response to mangrove [PIC] (2.7 kg m -3 ± 0.07). Valuations as CO 2 e was worth on average 0.

A Re-evaluation of Wetland Carbon Sink Mitigation Concepts and Measurements: A Diagenetic Solution

“…A seagrass sediment dominated by relatively recalcitrant mangrove detritus and moderate fractions of BC (Chuan et al 2020). The sites were also selected to bookend expected rates of supply and retention of organic carbon to their sediments (Li et al 2018;Gallagher et al 2020). The mangrove was situated near the entrance of its lagoon's marine tidal delta region, a region of relatively high rates of exchange with coastal waters.…”

“…Nevertheless, a mangrove biomass for a similar niche and species assemblage close to the entrance of its lagoon (Gallagher et al 2020), suggests an additional mitigation service of around 891 gC m −2 yr −1 , should the mangrove be consumed or burnt over 20 years. This is in contrast to the largely insignificant contribution of seagrass biomass relative to its meadow's total organic carbon stock (Gallagher et al 2020). Together, there appears to be a surprising convergence between the mitigation potential of these two disparate tropical ecosystems (i.e.…”

“…Although, it must be said that the convergence was enhanced by the relative positions within their lagoons. The mangrove forest is located close to the entrance where carbon biomass is just over half of the upper lagoon counterparts (Gallagher et al 2020). In contrast, the seagrass meadow is located in the upper and muddier parts of its lagoon, where sedimentary carbon stock is more than 10 times greater than their lower lagoon counterparts (Gallagher et al 2020).…”