Combined Carbon Isotope and C/N Ratios as Indicators of Source and Fate of Organic Matter in a Poorly Flushed, Tropical Estuary: Hunts Bay, Kingston Harbour, Jamaica

Abstract

Stable carbon isotopes and C/N ratios of particulate organic matter (POM) in suspended solids, surficial sediments and sediment cores were used to define the spatial and temporal variability of POM in a poorly flushed, urbanized, eutrophic tropical estuary (Hunts Bay, Kingston Harbour, Jamaica). C/N variation in the sediment surface POM is a function of initial suspended particulate organic matter (SPOM) (or other POM) type and the alteration of C/N ratios in the water column or on the sediment surface. The δ13CSPOM(−20 to −25‰) values suggest that this material is a mixture of: (1)in situphytoplankton organic matter; (2) terrestrial river-borne SPOM; (3) terrestrial river-borne bottom sediment POM; and (4) sewage. Downcore variation in organic carbon content, C/N and δ13Cis attributed mainly to change in the supply rate and type of organic matter. In the NE of Hunts Bay, down core variation in sedimentology and geochemistry are consistent with a change from fully marine to freshwater runoff-dominated sedimentation with increasing organic matter input from sewage in recent times. Despite large overlaps in the C/N and δ13Corg‘end-members’ pollutant POM, in this case sewage, was the only source which could account for the amount of POM deposited, the surface sediment C/N and δ13Corgvalues and the trajectories of evolution in C/N andδ13Corgvalues in cores. The data show that the combinedδ13C and C/N successfully identifies the source, fate and history of POM even in a poorly-mixed estuary.