Isotopic evidence for phytoplankton as a major food source for macrobenthos on an intertidal sandflat in Ariake Sound, Japan

Abstract

To delineate a soft-shore trophic structure, macrobenthos was collected from an intertidal sandflat at the mouth of the Shirakawa River in the central part of Ariake Sound, Kyushu, Japan, on 3 occasions in 2003 (January, March, and June to July during the rainy season), and the isotopic compositions of animals and their potential food sources were analyzed. Food sources of animals were assessed based on the diet -tissue isotopic fractionation (δ 13 C = 0.6 to 2.0 ‰, δ 15 N = 3.4 to 3.9 ‰) of 3 main constituent species (the bivalves Mactra veneriformis and Ruditapes philippinarum and the ghost shrimp Nihonotrypaea japonica), which had been determined by an earlier laboratory experiment. The results revealed that (1) consumers were classified into 3 groups; a group consisting of 51 species including 4 main species (the above-mentioned species and the mud shrimp Upogebia major) that derived nutrition mainly from coastal phytoplankton, one consisting of 7 species that incorporated more 13 C-enriched material (e.g. benthic microalgae) and a group consisting of 4 species that incorporated more 13 C-reduced material (e.g. riverine organic matter); (2) the phytoplankton-based trophic structure was composed of 3 trophic levels; (3) M. veneriformis juveniles collected during the rainy season appeared to incorporate riverine organic matter; and (4) reeds from the flood plain, detrital terrestrial plant material, sewage effluents and seaweeds were not food sources for most consumers. We concluded that the high phytoplankton biomass and scarcity of benthic microalgae on this sandflat account for its phytoplankton-based trophic structure.KEY WORDS: Stable isotopes · Estuary · Intertidal sandflat · Trophic structure · Phytoplankton · Benthic microalgae Resale or republication not permitted without written consent of the publisher

Distribution of gastropods in a tidal flat in association with digestive enzyme activities

“…However, all of the gastropods examined showed detectable activities of mannanase and laminarinase. In accordance with previous findings, our results suggest that gastropods ingest algae and benthic diatoms entering the estuary (see, for example, Kang et al 2003;Kanaya et al 2005;Yokoyama et al 2005).…”

“…(1) Cerithidea cingulata, B. multiformis, and B. attramentaria have lower abilities to digest cellulose than does Cerithidea rhizophorarum, and therefore cannot settle inside reed beds, which contain abundant reed-derived carbohydrates that are difficult to degrade. Thus, as previously reported, these three gastropod species feed mainly on benthic diatoms or seaweed in tidal flats (Kang et al 2003;Kanaya et al 2005;Yokoyama et al 2005). (2) In order to degrade plant-derived carbohydrates from reeds, Cerithidea rhizophorarum secretes cellulase and xylanase more efficiently than do Cerithidea cingulata, B. multiformis, and B. attramentaria.…”

“…The results of recent stable isotopic studies suggest that benthic animals in estuaries preferentially feed on benthic diatoms or microphytobenthos (MPB; Kang et al 2003;Kanaya et al 2005;Yokoyama et al 2005). Most benthic animals are unable directly to assimilate organic matter derived from land plants, because they lack the enzymes required to digest cellulose, which is one of the primary components of land plants (Kristens 1972).…”

Dietary responses of tidal flat macrobenthos to reduction of benthic microalgae: a test for potential use of allochthonous organic matter

“…However, the contribution of benthic microalgae to macrobenthos diets is likely site-and species-dependent (Doi et al 2005, Sakamaki et al unpubl. data); other studies have reported that macrobenthos nutritionally depended upon marine phytoplankton (Yokoyama et al 2005) or riverorigin OM (Kasai & Nakata 2005). To date, it is unclear which factors control the relative importance of autochthonous and allochthonous OM in estuarine tidal flat food webs.…”

Trophic relationships in early spring along the Okhotsk coast of Hokkaido, Japan, as traced by stable carbon and nitrogen isotopes

“…The surfaces of terrestrial plant debris, seagrass and macroalgae samples were cleaned by brushing to remove the epiphyte components, and then the samples were washed with 1.2 N HCl, rinsed with distilled water, dried at 60°C, and ground into powder. In processing of the organic matter in seabed sediment, the upper 2 cm of the sediment that was dried at 60°C and ground was then soaked in 1.2 N HCl overnight to remove carbonate (Yokoyama et al 2005), dried, and ground again. The pelagic invertebrate samples were sorted and identified to the lowest taxon possible under a dissecting microscope and kept frozen.…”