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

The spatial relationships and linkage of the detrital flows among the water column, the sediment and the oyster Crassostrea gigas cultured in the wate r column were examined by using stable carbon isotopes (delta(13)C) in a tr opical shallow lagoon from October 1996 to June 1997. The lagoon is located in southwestern Taiwan and is isolated from the sea by sand barriers excep t at two tidal inlets. It receives freshwater mainly from two rivers. A tot al of 12 stations were set up along three transect lines, each running acro ss the lagoon from riverine to tidal inlet localities. The delta(13)C value s of the water-column POM exhibited a marked sea-river gradient, with value s depleted from a high of -21.7 parts per thousand at seaward stations to a low of -28.2 parts per thousand at riverine stations; those in the sedimen tary POM (< 62 mu m grain size) also revealed this trend, but to a lesser e xtent. Oysters of two known ages, 6 months old ("old oysters") and newly se ttled individuals ("young oysters"), were transplanted from one station to each of the remaining stations, while some were left at the original statio n. Values of delta(13)C in the muscle of transplanted oysters changed in pa rallel with the sea-river gradient of delta(13)C in POM (decreasing from -1 6.0 to -18.5 parts per thousand in old oysters and from -16.8 to -21.9 part s per thousand in young ones). The spatial sea-river gradient of the oyster 's delta(13)C is related not only to the distance between the site that the oyster inhabits and sea or riverine environment, but also to the tidal flo w pattern that surrounds its feeding place. Although the delta(13)C value o f the sedimentary POM was correlated with that of the water-column POM, the delta(13)C value of the oyster tissue was significantly correlated with th at 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 sedime ntary POM.