In a recent review of the seagrass genus Halophila in Japan, eight taxa including four new species were listed: (i) H. decipiens, (ii) H. minor, (iii) H. major, (iv) H. ovalis, (v) H. mikii, (vi) H. nipponica, (vii) H. okinawensis, and (viii) H. gaudichaudii. In this study, the specific concept of Halophila in Japan as well as some other geographic areas was re-assessed by a combination of nuclear ribosomal ITS1, 5.8S rDNA, and ITS2 sequence analyses and morphological examinations of both recently collected materials and available type collections. The results confirm the distinctness of the Australian endemic species H. australis and the putative pan-tropical taxon H. decipiens; but also demonstrate conspecificity of: (i) H. minor and H. ovalis; (ii) H. major and H. mikii; and (iii) H. nipponica, H. okinawensis and H. gaudichaudii. As an outcome of this study, we recommend restricting the list of Halophila representatives in Japan to the following four species: (i) H. decipiens, (ii) H. major, (iii) H. ovalis, and (iv) H. nipponica. In addition, we predict the possibility of H. major occurring further south than currently recorded.

Effect of water disturbance on grazing by the sea urchin with the refuge habitat was estimated. We tested the hypothesis that sea urchins ceased feeding and remained in refuge habitat for protection against disturbance even though water flows were not high to inhibit their grazing. Fecal density from sea urchins was positively related to their residence time under the undisturbed condition. This result suggested that the fecal density is a useful criterion for understanding the residence time of sea urchins in the calm condition. Five types of structure (L-shaped, cave shaped, crack-shaped, smaller and larger rectangular structure) as refuge habitats were deployed in the undisturbed tank with sea urchins and the fecal densities at each structure were measured to examine the habitat forms which sea urchins prefer. This experiment indicated that the crack-shaped form was used for sea urchins as the refuge habitat. Feeding rate of sea urchins inversely corresponded to water flow. Feeding rates with the refuge habitat were lower than in the absence of habitat, although water disturbances were almost the same. The presence of refuge habitat seemed to be a factor responsible for the reduction of their grazing, because sea urchins sheltered to avoid water disturbances.

We tested the hypothesis that mineralization of molybdenum unreactive phosphorus (MUP) in pore water is the major pathway for the changes in the concentration of molybdenum‐reactive P (MRP) in pore water and inorganic P in sediment particles. The concentration of inorganic P in the sediment particles increased from December to April in Lake Kasumigaura, whereas concentrations of organic P in the sediment particles and MUP in pore water decreased. These results suggest that MUP mineralization plays a key role as the source of MRP, whereas desorption of inorganic P from the sediment particles into the pore water is a minor process. One‐dimensional numerical simulation of sediment particles and the pore water supported the hypothesis. Diffusive flux of MUP was small in pore water, even in near‐surface layers, so mineralization was the dominant process for changing the MUP concentration in the pore water. For MRP, diffusion was the dominant process in the surface layer, whereas adsorption onto the sediment was the dominant process in deeper layers. Researchers usually ignore organic P in the sediment, but organic P in sediment particles and the pore water is a key source of inorganic P in the sediment particles and pore water; our results suggest that in Lake Kasumigaura, organic P in the sediment is an important source, even at depths more than 1 cm below the sediment surface. In contrast, the large molecular size of MUP in pore water hampers diffusion of MUP from the sediment into the overlying water.

A numerical model was developed to calculate the rate of dissolved-oxygen (DO) diffusion across a sediment surface taking into account the surface roughness and biochemical reactions of the sediment. Estimates of DO transfer rate from the model were compared with results from laboratory experiments conducted in a rectangular flume using roughness elements. In experiments, there was maximum value for the nondimensionalized DO transfer rate (Stanton number, St ) in the transitional region of surface roughness, in which the mass flux was two to five times larger than that of the smooth surface. The reproducibility of the experimental results by numerical analysis was significantly improved by including terms for flushing frequency of water in cavities between the roughness elements and for nonsteady variations in the diffusion rate due to step changes in DO concentration in the flushed region. A simple method to estimate enhancement effect for St caused by nonsteady variations was also presented.