We conducted experiments to determine isotope changes in the deposit-feeding chironomid larvae Chironomus acerbiphilus during feeding, starvation and metamorphosis. Isotope changes in chironomid larvae occurred mainly during growth and rarely afterward. This finding indicates that chironomid isotope turnover mainly occurs in conjunction with growth and suggests that chironomid larvae only break down newly assimilated food for energy during periods of no growth. Chironomid delta(13)C values significantly increased throughout the starvation experiment, indicating that chironomids preferentially break down components with lower delta(13)C content during starvation. We found significant changes in chironomid isotope ratios ((15)N enrichment) during pupation. This evidence suggests that the physiological condition of animals (such as during an active growth phase or pre- or post-molting) is important to their stable isotope ratios.

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Food sources for the deposit-feeding gastropods Assiminea japonica and Angustassiminea castanea (Gastropoda: Assimineidae) were estimated using stable carbon and nitrogen isotope ratios. We collected animals and potential food materials in reed marshes of the Nanakita River estuary, the eastern part of Miyagi Prefecture, Japan. Feeding experiments were also conducted to confirm whether snails assimilate 4 types of diets. The stable carbon isotope ratios of both assimineid species (mean ± 1 SD: A. japonica, -20.7 ± 0.3 ‰; A. castanea, -19.8 ± 0.5 ‰) were closer to that of deposited organic matter from lagoon water (-20.7 ± 0.3 ‰) than to those of reed litter (-25.4 ± 0.1 ‰) and the surface soil of the reed marsh (-26.3 ± 0.1 ‰). The snails that were fed deposited organic matter showed δ 13 C values similar to the control animals before feeding experiments for both species. The δ 13 C values of the snails fed litter or soil diet, however, revealed that these snails were able to assimilate organic matter from reed detritus under laboratory conditions. These findings suggest that the salt marsh snails utilized mainly deposited organic matter from lagoon water in the field. Microalgae such as phytoplankton and benthic diatoms in deposited matter are considered to be important food sources for 2 species of assimineids inhabiting salt marshes of the Nanakita River estuary.

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A huge tsunami is one of the greatest disturbance events in coastal benthic communities, although the ecological consequences are not fully understood. Here we examined the tsunami-induced changes in the sediment environment and macrozoobenthic assemblage in a eutrophic brackish lagoon in eastern Japan. The 7.2-m-high tsunami completely replaced muddy sediment with drifting sea sand throughout the lagoon, leading to the drastic changes in quantity and quality of sedimental organic matters, sulfide contents, and sediment redox condition. Intensive physical stress devastated the benthic community, but the disappearance of sulfidic muddy bottoms significantly improved the habitat quality for macrozoobenthos. The re-established macrozoobenthic community after 5 months was characterized by (1) a 2-fold higher total density, but sharp declines in species richness, diversity, and evenness; (2) an increased density of opportunistic taxa (e.g., polychaete Pseudopolydora spp. and amphipod Monocorophium uenoi) in newly created sandy bottoms; and (3) disappearance of several dominant taxa including bivalves and chironomid larvae. These findings indicate that the sensitivity and recovery potential of macrozoobenthos were highly taxa-specific, which was closely related to the taxa’s ecological characteristics, including tolerance to physical disturbance, life-history traits, and life form. Our data revealed the rapid recolonization of opportunistic macrozoobenthos after a huge tsunami, which would contribute to the functional recovery of estuarine soft-bottom habitats shortly after a disturbance event.