Dominance of land-derived organic matter in highly productive Bandon Bay, Thailand

Bandon Bay is Thailand's productive area for mariculture of many economically important species including oysters and clams. High primary productivity in this shallow bay leads to organic matter (OM) production in the water column and its export to underlying sediments and adjacent areas. However, the understanding of the source and fate of OM in Bandon Bay is still scarce. To address this knowledge gap, we investigated organic carbon (OC) and total nitrogen (TN) elemental contents, C/N elemental ratios, and stable carbon (δ¹³C) and nitrogen isotopic (δ¹⁵N) compositions in surface sediments, sediment cores, and suspended particulate matter (SPM) collected from Bandon Bay and surrounding river systems during the wet season in 2019. δ¹³C values in bay sediments indicate a major contribution of from terrigenous sources. During low tide, river-associated high-δ¹⁵N input of SPM from freshwater phytoplankton input to the bay was evident. The OM source apportionment was further made using a four-end-member mixing model showing more than 70% of sediment organic matter to be of terrestrial origin (freshwater phytoplankton and land soils) most likely brought to the bay via Tapi-Phumduang River. In addition, tidal cycles, and prevailing circulation in the bay cause mixing of OM in the bay with marine OM from the Gulf of Thailand. All these processes work together to bring about dispersal, dilution, and transformation of OM in Bandon Bay, which maintain high organic content in the sediment and supply nutrients essential for high shellfish production in Bandon Bay.