MEPS 255:303-309 (2003)  -  doi:10.3354/meps255303

ABSTRACT: A stable isotope study was conducted to identify the sources of nutrition for the semelid bivalve Theora lubrica living in estuarine subtidal sediments. Along an estuarine gradient in Gokasho Bay, central Japan, δ¹³C and δ¹⁵N were determined both in tissues of T. lubrica and its potential food sources, i.e. particulate organic matter (POM) from riverine and marine sites, sedimentary organic matter (SOM) and microalgae on an intertidal mudflat. Differences existed among δ¹³C and δ¹⁵N in riverine POM (yearly mean value: -26.5” and 0.5‰, respectively), marine POM (-20.4‰, 6.3‰) and benthic microalgae (-14.7‰, 4.9‰). SOM exhibited a gradual isotopic enrichment both in ¹³C and ¹⁵N from the estuarine station adjacent to the mudflats to the more seaward stations, indicating a progressive mixing of terrestrial and marine sources. However, the isotopic composition of T. lubrica did not follow this trend: as the sampling station was shifted seaward, δ¹⁵N increased from 7.5 to 9.4‰, whereas δ¹³C decreased from -16.0 to -17.8‰. These values cannot be due to the utilization of terrestrial organic matter carried by the river inflow. The observed spatial variation in the δ¹³C values suggest that T. lubrica has 2 dominant food sources, i.e. benthic microalgae (mainly diatoms) and marine POM (mainly coastal phytoplankton), and that in subtidal areas near the mudflats benthic microalgae were an important dietary component.

KEY WORDS: stable isotope · Benthic microalgae · Macrobenthos · Estuary · Theora lubrica