Dietary contribution of the microphytobenthos to infaunal deposit feeders in an estuarine mudflat in Japan
The food sources of benthic deposit feeders were investigated at three stations in an estuarine mudflat (Idoura Lagoon, Sendai Bay, Japan) during July and August 2005, using δ13C and δ15N ratios. Sediment at the stations was characterized by low chlorophyll (chl) a content (0–1 cm depth, <4 μg cm−2) and the dominance of riverine–terrestrial materials (RTM) in the sediment organic matter (SOM) pool. Surface-deposit feeders (Macoma contabulata, Macrophthalmus japonicus, and Cyathura muromiensis) exhibited much higher δ13C values (−18.4 to −12.4‰) than did the SOM pool (<−25‰). A δ13C-based isotopic mixing model estimated that benthic diatoms comprised 45–100% (on average) of their assimilated diet, whereas RTM comprised a lesser fraction (29% maximum). The major diet of the deep-deposit feeding polychaetes Notomastus sp. and Heteromastus sp. was benthic diatoms and/or marine particulate organic matter (POM), with little RTM assimilated (39% maximum). The consumers appeared to lack specific digestive enzymes and to use detritus-derived carbon only after its transfer to the microbial biomass. The isotopic mixing model also showed that the dietary contribution of RTM increased slightly (15% maximum) in the vicinity of freshwater input, suggesting that spatial changes in RTM supply affect the dietary composition of deposit feeders. These results clearly demonstrate that deposit feeders selectively ingest and/or assimilate the more nutritious microalgal fractions in the SOM pool. Such adaptations may allow enhanced energy gain in estuarine mudflats that are rich in vascular plant detritus with low nutritive value.
KeywordsParticulate Organic Matter Sediment Organic Matter Benthic Diatom Deposit Feeder Potential Food Source
We thank Dr. K. Ito for access to the mass spectrometer at the Department of Agriculture, Tohoku University, as well as A. Muraoka for her help with analyzing the sediment samples. We also thank the editor and three anonymous referees for their critical comments on this manuscript. This study was partly supported by Grant-in-Aid for Scientific Research (C) from Japan Society for the Promotion of Science (No. 17570012).
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