Blue Carbon and Microplastic Sequestration in Coastal Habitats of the Philippines

Climate change and human development pose significant threats to coastal ecosystems, and an understanding of the history of these ecosystems is necessary to assess potential climate mitigation and coastal resiliency strategies. The high-resolution histories in coastal vegetated habitats, enabled by high sedimentation rates, record long-term sequestration of below-ground carbon as well as the history of pollutants such as microplastics. Rates of sequestration, in addition to bulk storage estimates, are needed to inform future management decisions. This study documents organic carbon and microplastic concentrations and accumulation rates from five distinct sites in the southern Philippines. The carbon and plastic records are then evaluated based on their correlations to their sedimentary environments and satellite-derived spatial models of land-use. Cores (n=13) were dated with 210Pb alpha spectroscopy to determine sediment accumulation rates. Carbon accumulation rates ranged from 91-342 g C/m2/yr in mangroves (n=10), and 58-195 g C/m2/yr (n=3) in seagrasses. Microplastic concentrations and burial rates (n=6) were observed to increase exponentially though time, with surface burial rates ranging from 2100-6578 particles/m2yr. Patterns in sedimentation rates and habitat structure suggest autochthonous input of sedimentary organic carbon is important while allochthonous supply from the marine environment is reflected by the microplastic measurements. Substantial inter-site variation was observed, and rates of sequestration were not found to follow the same trends as the below-ground stocks, highlighting the need to understand the local sedimentary environments of coastal ecosystems.