Settling flux and origin of particulate organic carbon in a macro-tidal semi-enclosed embayment: Luoyuan Bay, Southeast China coast

Coastal embayments play a vital role in the global carbon cycle either as sources of organic matter to open sea or as carbon sinks due to the accumulation of organic matter in sediments. This paper describes a study of Luoyuan Bay, a typical semi-enclosed embayment with a total area of approximately 227 km2 in a strong tidal environment. The analysed results indicate that the particulate organic carbon (POC) concentration in suspended particulate matter (SPM) varies from 0.57 mg/L to 1.33 mg/L at the bottom layer (0.5 MAB, meters above bed) and from 0.54 mg/L to 1.25 mg/L at the surface layer (0.5 MBS, meters below surface). The δ13C‰ ranges from −25.52‰ to −23.54‰ and exhibits different variations at the surface and bottom layers in spring and neap tides. The POC content in deposited particulate matter (DPM) varies from 0.62% to 2.95%, increasing from spring to neap tide, and the δ13C and C/N molar ratio are −25.29‰ ∼ −21.41‰ and 4.18–8.53, respectively. The settling fluxes of POC obtained by sediment trap decrease from 2.25 g/m2·tide during the spring tide to 0.55 g/m2·tide during the neap tide with a mean value of 1.41 g/m2·tide during the observation, whereas the settling flux of SPM decreases from 456.76 g/m2·tide during the spring tide to 37.12 g/m2·tide during the neap tide. Combining the δ13C and C/N molar ratio, three end-members are recognized, i.e., freshwater algae and phytoplankton, marine algae and phytoplankton, and kelp-derived detritus. The mean contribution to POC from these three sources is 57.2%, 31.8% and 11.0% in SPM, and 39.9%, 35.0% and 25.1% in DPM, respectively. The POC from freshwater algae, phytoplankton and kelp-derived detritus is controlled by sediment dynamic processes.