Linking Intertidal and Subtidal Food Webs: Consumer-Mediated Transport of Intertidal Benthic Microalgal Carbon

We examined stable carbon and nitrogen isotope ratios for a large variety of consumers in intertidal and subtidal habitats, and their potential primary food sources [i.e., microphytobenthos (MPB), phytoplankton, and Phragmites australis] in a coastal bay system, Yeoja Bay of Korea, to test the hypothesis that the transfer of intertidal MPB-derived organic carbon to the subtidal food web can be mediated by motile consumers. Compared to a narrow δ¹³C range (−18 to −16‰) of offshore consumers, a broad δ¹³C range (−18 to −12‰) of both intertidal and subtidal consumers indicated that ¹³C-enriched sources of organic matter are an important trophic source to coastal consumers. In the intertidal areas, δ¹³C of most consumers overlapped with or was ¹³C-enriched relative to MPB. Despite the scarcity of MPB in the subtidal, highly motile consumers in subtidal habitat had nearly identical δ¹³C range with many intertidal foragers (including crustaceans and fish), overlapping with the range of MPB. In contrast, δ¹³C values of many sedentary benthic invertebrates in the subtidal areas were similar to those of offshore consumers and more ¹³C-depleted than motile foragers, indicating high dependence on phytoplankton-derived carbon. The isotopic mixing model calculation confirms that the majority of motile consumers and also some of subtidal sedentary ones depend on intertidal MPB for more than a half of their tissue carbon. Finally, although further quantitative estimates are needed, these results suggest that direct foraging by motile consumers on intertidal areas, and thereby biological transport of MPB-derived organic carbon to the subtidal areas, may provide important trophic connection between intertidal production and the nearshore shallow subtidal food webs.