Yuichi Hayami

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.

Fixation of Chattonella antiqua and C. marina (Raphidophyceae) using Hepes-buffered paraformaldehyde and glutaraldehyde for flow cytometry and light microscopy

Katano T., Yoshida M., Lee J., Han M.-S. and Hayami Y. 2009. Fixation of Chattonella antiqua and C. marina (Raphidophyceae) using Hepes-buffered paraformaldehyde and glutaraldehyde for flow cytometry and light microscopy. Phycologia 48: 473–479. DOI: 10.2216/08-102.1. Chattonella antiqua and C. marina are harmful algal species that cause massive fish kills in coastal environments. Generally, Chattonella cells cannot be preserved well by fixation because of their fragile nature. In the present study, we developed a new fixative for Chattonella. Phosphate-buffered saline, which is generally used as a buffer for fixative, caused precipitation after fixation. In contrast, Hepes or sodium cacodylate prevented the precipitation. Moreover, these buffers worked well in the fixation to preserve cell morphology. Since Hepes is not as noxious as cacodylate, we selected Hepes as the buffer for the fixative. Cell counting revealed that the decrease in cell numbers by fixation was negligible and did not start until at least 8 days after fixation. We successfully analysed the DNA amount in fixed cells of Chattonella by flow cytometry. The present study demonstrated that Hepes-buffered paraformaldehyde and glutaraldehyde are superior to other fixatives for flow cytometry and light microscopy.

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.

Using foraminiferal Mg/Ca ratios to detect an ocean-warming trend in the twentieth century from coastal shelf sediments in the Bungo Channel, southwest Japan

To assess abnormal warming of temperature in southwest Japans coastal seas during the twentieth century, we developed a 200 yr interannually resolved record of planktonic and benthic foraminiferal Mg/Ca-based temperature using neritic coastal sediment cores. The winter to late spring bottom temperature (50 m) record, based on benthic foraminiferal (Nonion japonicum) Mg/Ca ratios in the Bungo Channel (BC), showed consistent variation with observed temperatures on a five-year average basis. The BC bottom temperature record showed a significant increasing trend of 1.5°C /100 yr during the twentieth century, which was never apparent in nineteenth century. That result suggests that our Mg/Ca-based thermometry approach using coastal benthic foraminifera can detect abnormally rising temperatures in neritic coastal seas in southwest Japan. The abnormal warming of winter to late spring bottom temperature in the BC contrasts with the lack of an increasing trend in the Globigerinoides ruber-based summer to autumn temperature for the upper 20 m in the slope region of southwest Japan and the lack of an increasing trend in the steric sea level in the region. These results indicate a warming trend of the neritic coastal ocean in southwest Japan, especially in winter to late spring. The timing of the onset of the prolonged abnormal high-temperature stage, which started in the early twentieth century, suggests a link of the neritic coastal ocean in Japan with human-induced global warming.

Decadal-scale variation in COD and DIN dynamics during the summer in the inner area of the Ariake Sea, Japan

More than 30 years of chemical oxygen demand (COD) and dissolved inorganic nitrogen (DIN) data for the inner area of the Ariake Sea were analyzed with a box model to show the changes in the average seasonal budget and the decadal-scale variation during the summer. The COD peaked in August and March on average. This summertime peak can be explained by an enhanced riverine load and increased primary production. The peak in March suggested additional organic matter production. There were also two peaks in DIN concentration on average: a summertime peak that could be explained by an enhanced riverine load, and a peak in December that was more complicated to explain. From the 1970s to the early 1990s, the bottom water in this area became increasingly hypoxic due to increased COD during the summer, even though there were minimal increases in terrestrial COD and nutrient loads and there were tidal flats covering a widespread area during this period. The increase in COD was caused by increased net ecosystem production, which was due to enhanced primary production induced by an increased freshwater residence time and decreased bivalve grazing. There was a negative feedback control in which hypoxia progressively increased, leading to declines in bivalve biomass, which in turn decreased the grazing pressure limiting primary production, meaning that primary production increased and the area became even more hypoxic. The net DIN production decreased during the 1980s and the 1990s. This was consistent with the change in net ecosystem production according to the COD.

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.

Effect of temperature and light on growth of planktic green algae isolated from Lake Hövsgöl, Mongolia

Four species of planktic green algae, Oocystella mongolica, Monoraphidium contortum, Pseudodidymocystis planctonica and Closterium parvulum were isolated from the ancient ultraoligotrophic Lake Hovsgol, Mongolia. While O. mongolica is considered to be an endemic species of the lake, the others are cosmopolitan species. Lake Hovsgol freezes between November and May, and the maximum water temperature in the pelagic area is ca. 14°C in summer. In order to clarify the growth characteristics on temperature and light of the strains, culture experiments were conducted. Although these algae live in cold habitats, the optimum temperatures for growth of the strains were 20-25 °C under high light conditions. All strains could grow at 5 °C, which was the lowest temperature examined, and showed their best growth rate at 10°C under the lowest light conditions (10 μmol photons m -2 . s -1 The results of culture experiments were discussed with regard to their natural habitats.

Special section on the Cooperative Monitoring Program of Ariake Sea (COMPAS)

The Cooperative Monitoring Program of Ariake Sea (COMPAS) is a research project of the Ariake Sea started in 2013. It is a 9-year project organized by Saga University consisting of researchers from four prefectures surrounding the Ariake Sea; Saga University, Kyushu University, Nagasaki University and the Prefectural University of Kumamoto. There are many environmental and fisheries problems occurring in the Ariake Sea now; e.g., decrease of bivalve catch, decrease of demersal fish catch, occurrence of hypoxic water mass and increase of red tide occurrence. Many fishermen insist that one of the reasons for the problems must be the construction of the Isahaya Sea dike, and have filed lawsuits in response. A final judgement ordered the Japanese government to open the gates of the dike for 5 years to clarify the environmental influence of the dike construction. In these situations, COMPAS has two aims: One is to assess the environmental influence of the Isahaya gate opening as a third party. The other is to investigate the mechanisms of the long-term environmental change in the Ariake Sea. Despite the final judgement of the court, the Isahaya gate opening has not started since there are protests against the gate opening by the farmers in Isahaya reclamation land. Therefore, COMPAS focuses on investigating the mechanisms of the long-term environmental change in the Ariake Sea. The causes for the longterm change include not only anthropogenic impacts but also natural phenomena. This special section of the Journal of Oceanography consists of the papers that show the major results of the early half of COMPAS. The topics of the papers in this section include the evaluation of the influences of the Isahaya reclamation land on the formation of hypoxic water mass, dynamics of the hypoxic water mass at the mouth of Isahaya Bay, particulate organic matter transport from Isahaya Bay, longterm change in the material budget, influence of hypoxic water mass on benthic communities and the dynamics of dissolved organic carbon produced by red tide. As guest editors, we appreciate the Oceanographic Society of Japan for giving the opportunity to publish our activity. We also thank Professor Joji Ishizaka of Nagoya University, the editor-in-chief of Journal of Oceanography and anonymous reviewers of this special section. We hope that this special section would contribute to solving the problems in the Ariake Sea. Cooperative Monitoring Program of Ariake Sea (COMPAS)

The mechanism of bottom water DO variation in summer at the northern mouth of Isahaya Bay, Japan

Recently, bivalves have been massively killed by anoxia or hypoxia in summer at the northern part of Isahaya Bay, Japan, which constituted a major problem for fisheries. However, the mechanism behind the occurrence of hypoxic water masses is unclear. It is known that the bottom water dissolved oxygen (DO) in this area is affected by the inflow of seawater into the northern mouth of Isahaya Bay. To understand the mechanism of hypoxia, it is necessary to determine the physical processes that cause changes in the bottom DO concentrations in this area. This study shows that there is a neap-spring tidal variation in bottom DO due to a change in vertical tidal mixing, and it also suggests that the decrease in bottom DO was generated by a baroclinic flow, which is due to the internal tide, and a shear flow, which is induced by the external tide in the bottom boundary layer. In addition, our study suggests that the source of cold and hypoxic water that appears in the bottom layer at low tide is the inner area of the Ariake Sea.