Environmental Changes Associated with Organic Matter Loading Deduced from Sedimentary Deposits of a Coastal Lagoon in Boracay Island, Philippines

Tourist arrivals in Boracay increased to more than a million since 2012. Sustainable development planning and management, however, lag behind the occupation of the island and use of coastal resources. Beyond the carrying capacity of the island, anthropogenic activities infringe not just the coastal beaches but also the biogeochemical fabric of the wetlands. Laguna de Boracay, a lagoon carved from the dissolution of limestone, is a turbid swamp, surrounded by residential houses and commercial establishments. Solid wastes and sewage are disposed into the wetland. Recent land reclamation is also prominent in satellite images. The effluent flows into a narrow mangrove forest where a 31cm-sediment core was collected, logged, sub-sampled every 1 cm, and analyzed. Fifteen priority samples were decarbonated, pulverized and homogenized for bulk stable carbon and nitrogen isotope analyses. All deposits consist of siliciclastic and primarily carbonate fragments (i.e. shells, corals, foraminifera, calcareous algae residue) with coarsening upward sub-layers indicative of shoaling in the depositional environment. The top of the core are recent deposits of littoral coastal area where organic carbon is low, high nutrient input (enriched 𝛿¹⁵N). Mid-core sediments were mainly shallow marine with higher organic carbon, sulfidic and with slightly less enriched 𝛿¹⁵N. The bottom part is consistent with a nearshore environment with significant influence of terrestrial/ mangrove deposits (high C/N, more depleted 𝛿¹³C). In conclusion, the sediment deposits preserved chemical and isotopic signals consistent with a shift of the surrounding environment with increased population and deforestation in the surrounding areas and sewage input into the lagoon. Further analysis of organic compounds and their compound-specific isotope analyses, metal composition, as well as age-dating of the sediments will constrain better the biogeochemical implications of increased organic loading.