Diversity of benthic organic matter flows through polychaetes and crabs in a mangrove estuary: delta C-13 and delta S-34 signals

Estuaries are biologically diverse systems, especially in the spatial relationships between macrobenthos and their food sources. In a mangrove estuary in northern Taiwan, spatial patterns of organic matter flows from primary producers and detritus derived from vascular plants to macrofaunal polychaetes and crabs were examined using stable isotopes of carbon and sulfur. Sampling was conducted mainly from August 1996 to May 1997 on 2 mud flats, I on a tidal creek (Creek site) and the other on a tidal riverbank (River site). The VC and VS ranges at the River site were broader than those at the Creek site (delta(13)C: -28.3 to -13.6 parts per thousand vs -28.3 to -19.3 parts per thousand, delta(34)S: 8.1 to 15.9 parts per thousand VS 7.1 to 13.1 parts per thousand). This is due to the presence of a C4 plant, the cogon grass Imperata cylindrica at the River site. Although the mangrove Kandelia candel was the largest source of carbon at both study sites, our isotopic analyses showed that it was not as important a source of nutrition to consumers as particulate organic matter (POM), benthic microalgae, and cogon grass. When their VC and OS values were plotted against each other, the 4 crab and 2 polychaete species were differentiated into several distinguishable assemblages which correspond to sites where they reside. The conspecific nereids and fiddler crabs utilize different food sources when inhabiting different sites. Stable isotopic analyses revealed 3 kinds of feeding interactions at the Creek site. Along with detritivory (which uses POM) and herbivory (which uses microalgae), carnivory occurs between the grapsid crab Helice formosensis and its sabellid prey. Thus, trophic interactions are more complicated and diverse at the Creek than at the River site where detritivory (using POM plus detritus of cogon grass) is prevalent. A wider range of delta(13)C and delta(34)S values in a food web does not necessarily mean that the macrobenthos have a greater food variety, nor can one interpret how diverse an estuarine food web is. Instead, the diversity of an estuarine food web is characterized by a series of factors related to spatial differences in flow regimes, the availability and relative contribution of food sources, the opportunistic dietary choices of consumers, and consequently, spatially varying trophic interactions.