Detrital flows through the feeding pathway of the oyster ( Crassostrea gigas ) in a tropical shallow lagoon: δ 13 C signals

Abstract

The spatial relationships and linkage of the detrital flows among the water column, the sediment and the oyster Crassostrea gigas cultured in the water column were examined by using stable carbon isotopes (δ13C) in a tropical shallow lagoon from October 1996 to June 1997. The lagoon is located in southwestern Taiwan and is isolated from the sea by sand barriers except at two tidal inlets. It receives freshwater mainly from two rivers. A total of 12 stations were set up along three transect lines, each running across the lagoon from riverine to tidal inlet localities. The δ13C values of the water-column POM exhibited a marked sea–river gradient, with values depleted from a high of −21.7‰ at seaward stations to a low of −28.2‰ at riverine stations; those in the sedimentary POM (<62 μm grain size) also revealed this trend, but to a lesser extent. Oysters of two known ages, 6 months old (“old oysters”) and newly settled individuals (“young oysters”), were transplanted from one station to each of the remaining stations, while some were left at the original station. Values of δ13C in the muscle of transplanted oysters changed in parallel with the sea–river gradient of δ13C in POM (decreasing from −16.0 to −18.5‰ in old oysters and from −16.8 to −21.9‰ in young ones). The spatial sea–river gradient of the oyster's δ13C is related not only to the distance between the site that the oyster inhabits and sea or riverine environment, but also to the tidal flow pattern that surrounds its feeding place. Although the δ13C value of the sedimentary POM was correlated with that of the water-column POM, the δ13C value of the oyster tissue was significantly correlated with that of the water-column POM, but not with that of the sedimentary POM. This suggests that the oyster feeds primarily on water-column rather than sedimentary POM.