Abstract: Midori River estuary faces Ariake Bay, and has the largest sandy tidal flat(approximately 2, 100 ha)in Japan. We investigated the spatial distribution of particle size composition of the sediment, microphytobenthos on the surface sediment and macrobenthic animals at the stations set along a cross transect line in Midori River estuary. The Chl-a concentrations of the surface sediment that reflected the biomass of the microphytobenthos ranged between 19.3 and 113 mg/m2 at the stations on the tidal flat, and tended to decrease from the upper part to the lower part of the tidal flat and the offshore area. However, 92.0%of the total biomass of the macrobenthic animals collected at the sampling stations on the tidal flat was concentrated at the stations on the middle to lower part of the tidal flat. The highest biomass reached 3, 760g WW/m2, mainly due to the occurrence of dense patches of two suspension feeding bivalves, Ruditapes philippinarum and Mactra veneriformis. These two bivalves alone occupied 93.2%of the biomass of all the benthic animals collected at all sampling stations. The consumption of the primarily produced organic matter estimated from the secondary production of only a single species of Ruditapes apparently exceeded the potential primary production at the same place. The results of this study suggest that the re-suspension of primarily produced organic particles by the microphytobenthos on“ the upper part” of the tidal flat and its lateral transportation to“ the middle to lower part” of the tidal flat is essential to sustain the high secondary production of the suspension feeding benthic bivalves on the middle to lower part of this tidal flat.
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.
Hypoxia-tolerant bivalve populations could be a key component of ecosystem function because of their predominance in hypoxic environments. In this study, we conducted field surveys at two sites to elucidate the biological traits of the hypoxia-tolerant bivalve Veremolpa micra. We detected new settlements at both sites during summer. More highly reduced conditions in the surface sediment were recorded at Site-A than at Site-B in August 2013. Following this hypoxic event, shell growth of V. micra at Site-A decelerated, whereas that at Site-B increased. The population density after recovery from the hypoxic event decreased more at Site-B than at Site-A. Body size exhibited very little growth at both sites during the fall and winter, but there was rapid growth during the subsequent spring and summer, with sexual maturity being reached in early summer. Our findings demonstrated that the growth rates and survival patterns of the hypoxia-tolerant bivalve populations differed between sites with different degrees of sediment reduction. To understand the biological traits of hypoxia-tolerant bivalve species, it is necessary to consider and evaluate spatial differences in hypoxic stress at the local site level.
The dominant food sources of benthic suspension feeding bivalves living in shallow coastal areas have been known to be phytoplankton and microphytobenthos. However, there has been considerable debate over which is the primary food source. In this paper, we reviewed previous studies that deal with microalgae (i.e. phytoplankton and microphytobenthos) as a food source for benthic suspension feeding bivalves living in shallow coastal areas. Results of previous studies examining the stable carbon isotope composition of the cockle, Cerastoderme edule, in various coastal areas showed that the main food source for C. edule varied spatially. We estimated the main food source for the clam, Ruditapes philippinarum, on a tidal flat at the mouth of the Midori River. We concluded that R. philippinarum was predominantly feeding on the microphytobenthos. In our sampling station, the annual mean micropytobenthos biomass of the sediment was 30.2±10.2 mgChl-a m-2; this was approximately three times higher than that of phytoplankton in the overlying water column. The high biomass of the microphytobenthos coupled with high tidal and turbulent flow at the mouth of the Midori River results in resuspension and high relative abundance of the microphytobenthos to the overlying phytoplankton. This resuspension provides access to the microphytobenthos as the dominant food for R. philippinarum. To clarify whether the relative importance of phytoplankton and microphytobenthos as food for benthic suspension feeding bivalves living in shallow coastal areas, we need to understand the environmental factors influencing their main food source; additional case studies in this regard is necessary.
