Seiichiro Shibanuma

Food Sources for Ruditapes philippinarum in a Coastal Lagoon Determined by Mass Balance and Stable Isotope Approaches

The relationship between the food demand of a clam population (Ruditapes philippinarum (Adams & Reeve 1850)) and the isotopic contributions of potential food sources (phytoplankton, benthic diatoms, and organic matter derived from the sediment surface, seagrass, and seaweeds) to the clam diet were investigated. In particular, we investigated the manner in which dense patches of clams with high secondary productivity are sustained in a coastal lagoon ecosystem (Hichirippu Lagoon) in Hokkaido, Japan. Clam feeding behavior should affect material circulation in this lagoon owing to their high secondary productivity (ca. 130 g C m−2 yr−1). Phytoplankton were initially found to constitute 14–77% of the clam diet, although phytoplankton nitrogen content (1.79–4.48 kmol N) and the food demand of the clam (16.2 kmol N d–1) suggest that phytoplankton can constitute only up to 28% of clam dietary demands. However, use of isotopic signatures alone may be misleading. For example, the contribution of microphytobenthos (MPB) were estimated to be 0–68% on the basis of isotopic signatures but was subsequently shown to be 35±13% (mean ± S.D.) and 64±4% (mean ± S.D.) on the basis of phytoplankton biomass and clam food demand respectively, suggesting that MPB are the primary food source for clams. Thus, in the present study, the abundant MPB in the subtidal area appear to be a key food source for clams, suggesting that these MPB may sustain the high secondary production of the clam.

Is there any seasonal variation in marine nematodes within the sediments of the intertidal zone

The sediment parameters and nematode assemblages in the intertidal zone of the Hichirippu shallow lagoon, Hokkaido, Japan, were investigated. The objectives of this study were to observe the seasonal variation in the nematodes in the sediment, and to investigate the relationships between the nematodes and environmental factors. Samples were collected bi-monthly from five stations on the tidal flat from April 2003 to February 2004. It was found that the sediment parameters (Chl a concentration, AVS, TOC and TN contents) varied throughout the 10-month study. Fifty-four species of nematodes were found in the study area. The density and biomass of the nematodes varied in accordance with the sediment temperature during the sampling period. In this study, there was a seasonal variation in the nematode assemblage found in the intertidal zone of this shallow lagoon. The important factors affecting this variation were sediment temperature, and food competition among the nematodes themselves. The seasonal variation of the nematode also showed a relationship with the Chl a concentration in the sediment during the sampling period.