Takaharu Hamada

Effects of hypoxia and organic enrichment on estuarine macrofauna in the inner part of Ariake Bay

In the inner part of Ariake Bay, Japan, hypoxia frequently occurs in summer at the organically enriched bottom with salinity stratification caused by flooding after the rainy season. Sediment organic enrichment can work as a stressor for macrobenthos. To investigate the effects of both hypoxia and sediment organic enrichment on macrobenthos, samples were collected at 20 stations by grab sampling in May and August, representing the situation before and after hypoxia, respectively. Although sediment grain size did not change, sediment TOC increased significantly in August. Multivariate analyses showed that the community structure changed significantly in August. The variation in the community structure among stations also increased, which indicated disturbance by stressors during the study period. Similarly, the species richness and total abundance of macrobenthos decreased significantly after hypoxia even after the TOC effect was removed. In addition, the amount of TOC change and the abundance of the main species did not correlate in any case. These results suggest that the community changes during the study period are not caused by stress from the increased sediment TOC but are mainly from the subsequent hypoxic stress.

Effect of nutrient concentration and salinity on diel vertical migration of Chattonella marina (Raphidophyceae)

Abstract Vertical migration plays an important role in the development of Chattonella populations in the natural environment; however, this behaviour has remained largely undescribed. In the present study, the diel vertical migration of Chattonella marina in the Ariake Sea was examined along with nutrient concentrations and salinity at four sampling sites during the bloom in the summers of 2008, 2009 and 2010. Cells of Chattonella were abundant at the surface (0 m depth) in the daytime in most cases. However, when surface salinity was 9, which is the lower limit for growth, the population accumulated at a depth of 2 m, where salinity was 15, suggesting that Chattonella moves to avoid water with low salinity. Chattonella cells actively migrated, even under nutrient-replete conditions (mean DIN concentration, 12.73 µM). The present study demonstrates that Chattonella cell migration is independent of nutrient availability. In addition, Chattonella cells stop upward migration when the surface salinity is low (< 15). These insights are important for making predictions of Chattonella population dynamics after heavy rains.

Morphological variation of pallial organs at sites of differing turbidity: a case study of an arcid bivalve

We investigated morphological differences in specimens of the arcid bivalve Scapharca kagoshimensis collected from two sites differing in turbidity in the inner part of Ariake Bay. First, we confirmed if the specimens collected from the two sites were the same species by comparing the sequences of their nuclear 18S ribosomal DNA (18S rDNA) and cytchrome c oxidase subunit I gene (COI) of mitochondrial DNA (mtDNA), since the closely related species S. inequivalvis could be distributed in one site. The results of DNA analyses showed that specimens from both sites belonged to the same species. Shell morphology, gill and posterior adductor muscle size did not differ between the two populations. However, the size of the labial palp was significantly larger in bivalves living in an area of high turbidity compared with those living in an area with low turbidity. This difference could not simply be attributed to differences in meat content because the total weight of the soft body parts did not differ between the two populations. The labial palp is an organ functioning in preingestive particle selection. Hence, the large palps would presumably be a response to high turbidity conditions in which the need for particle processes increases. To the best of our knowledge, this is the first study demonstrating palp size flexibility in arcid bivalves, and such flexibility could be a factor enabling S. kagoshimensis to successfully exploit a wide area of shallow water in the inner part of Ariake Bay.

Density and undrained shear strength of bed sediment from ND-CPT

The density and the undrained shear strength (su) of bed sediments at either side of the Isahaya Bay dike in the Ariake Sea, Japan, were investigated using nuclear density cone penetration tests (ND-CPTs). The nuclear density cone penetrometer (ND-CP) was operated from a boat, conducting 71 ND-CPTs. Furthermore, 26 undisturbed samples were obtained for soil density and su measurements to calibrate the ND-CPT data. The results show that the density and su obtained from in situ with the ND-CPTs agree well with those from the laboratory tests on undisturbed samples, and the obtained density profiles show good repeatability. The vertical variation in density and su of the bed sediment can be obtained from the ND-CPT results. The relations between the density and su show that su increases with an increase in density, but that this relation is site specific. The values of su show an exponentially growing trend and the values of log su show a linearly increasing trend with density. The vertical distribution of the bed sediments can be described using the density values obtained with the ND-CPT.

Isahaya Bay, freshwater lake to an estuary again

The inner 1/3 of Isahaya Bay which is a tributary of Ariake Sea in Japan was shut off from the sea by a dike for the reclamation and disaster prevention in 1997. On the other hand, several environmental and fisheries problems occurred in Ariake Sea after 1990s. Some fishermen insisted that the major reason for the decrease of fishing must be the influence of the dike construction and filed lawsuits. Now the court decision is fixed and Japanese government must open the gates to reintroduce sea water into the reservoir. We made numerical simulations of currents, hydrography and sediment transport to assess the influences of the gate opening. To choose the environmentally wise procedure of gate opening, it is needed to reduce the erosion and deposition of bottom sediments caused by the enhanced tidal current and to minimize the occurrence of hypoxia in the reservoir.

Life history of Sternaspis costata (Sternaspidae: Polychaeta) in Ariake Bay, Japan

The life history of the Japanese sternaspid polychaete Sternaspis costata von Marenzeller, 1879 was investigated in the inner part of Ariake Bay from May 2010 to May 2011, with additional sampling in July and September 2011. All the worms were measured by the width of the ventral shield (WS) as an indicator of body size, and their coelomic contents were also examined. Ovigerous females occurred throughout the year, except during October and March, with the highest ovigerous rate (number of ovigerous females/number of adults with a WS larger than that of the smallest ovigerous female, i.e. 1.8 mm) noted in September (43.8% in 2010, 34.7% in 2011). The ovigerous rate was less than 10% in the other months, except in April 2011 (30%). Although the coelomic oocytes ranged from 90 to 160 μm in diameter, the diameters of most of the oocytes were 140–160 μm in September. The number of adults drastically decreased from September to October, suggesting that most of the adults died after spawning in the major reproductive period, although a small fraction of adults seemed to reproduce earlier or later. A cohort of recruits with an approximately 1.2 mm WS was identified in March and April. Most of the recruits grew to adult size in July–September, when many ovigerous females occurred. These results indicated that this species is fundamentally semelparous, with longevity around 1 year, and they mostly reproduce in September, with a few adults reproducing almost throughout the year.

RESUSPENTION AND DEPOSITION OF MUD IN THE DAIJYUGARAMI TIDELAND, THE ARIAKE SEA, JAPAN

To clarify the sediment transport on the tidal flat of the Ariake Sea, we studied SS concentration and nutrient concentration by the simultaneous observation by mooring and shuttle ship over 10 hours. The contribution of the erosion, deposition, storage by water depth change and advection were calculated by using empirical formulae of the erosion and deposition. It was clarified that high concentration of SS was observed around the mud shore line and its advection affected the SS concentration on the tidal flat as well as re-suspension caused by bottom shear stress. The water mass of high nutrient, which came from the river was transported by tide.