MEASURING THE FLOW OF ORGANIC MATTER IN A MANGROVE ESTUARY USING STABLE ISOTOPES OF CARBON AND SULFUR
Hsieh, H.-L.1*; Yango, H.-H.2; Chen, C.-P.1,2 & Chen, Y.-K.3
1Institute of Zoology, Academia Sinica, Taipei, Nankang, Taiwan, 115, ROC
2Institute of Fisheries Science, National Taiwan University
3Institute of Geology, National Taiwan University, Taiwan, Republic of China
In the mangrove estuary of the Tanshui River in northern Taiwan, the components of the detrital food chain, including the producers, a deposit feeder, a suspension feeder and sediments, were examined at three sites using stable isotopes of carbon and sulfur to reveal the flow of organic matter. The producers exhibited a wide range of values of 13C and the values were distinguishable from one another. The fresh leaves of the vascular plant Kandelia candel had a 13C as -26.2, indicating that it is a C3 plant. K. candel had a 34S value of 5.6. The 13C level of particulate organic matter (POM) of sea water (water collected at flood tides) was -23.40.97 (6), and that of river water (water collected at ebb tides) was -24.6 0.42 (5), whereas benthic microalgae had a 13C level of -19.41.24 (2). The 13C and 34S levels of sediments ranged from -26.1 to -24.6 and 5.0 to 9.1, respectively, suggesting that the carbon source of sediments comes mainly from the river-water POM and the vascular plants. The isotopic values did not significantly differ according to the site (3 sites) or the depth (2 depths), but did differ according to particle size. The13C value was -25.9 0.43 (18) and 34S was 8.7 0.2 (6) in sizes > 62 m, whereas the13C value was -24.8 0.28 (18) and 34S was 6.9 1.07 (6) in sizes < 62 m. These results indicate that particle size is a key factor in harboring different sources of organic matter. In polychaetes, the deposit feeder (the nereid Neathes glandicincta), and the suspension feeder (the sabellid Laonome albicingillum) had 13C values ranging from -21.4 to -20.8., and from -22.2 to -21.9, respectively; whereas 34S values ranged from 8.6 to 13.8, and 10.2 to 11.7, respectively. The nereids exhibited a greater variation in the sources of their detrital organic matter than the sabellids. Such a discrepancy may be attributed to their different feeding modes. The present study suggests that the detritus consumed by the two studied polychaete species are probably from either the river-water born POM, the benthic microalgae, or the silts and clays or some combination thereof. In conclusion, the flow of organic matter in the mangrove estuary is so complicated that multiple stable isotopes and other molecular tracers are needed to determine the pathways of detrital transportation in benthic communities.