Isahaya Bay Research Articles

Lability of sedimentary organic nitrogen estimated from amino acids in a reservoir on land reclaimed from Isahaya Bay, Kyushu, Japan

In 1997, the inner part of Isahaya Bay on the west coast of the Ariake Sea was separated from the sea by dikes, leading to the formation of a freshwater reservoir. Since the Isahaya Reclamation Project began in the 1990s, red tides have become more common in the bay. Studies suggested that a non-negligible amount of NH4+ was regenerated from organic matter decomposition in the turbid water drained from the reservoir into the bay, and this drainage water might be an important source of NH4+ for red tides in the bay. Although labile organic matter in the turbid water might also originate from resuspended sediments in the reservoir induced by the drainage process, the potential for the resuspended sedimentary organic matter to serve as an NH4+ source has never been evaluated. In this study, we found that sediments near the drainage gates on the dike in the reservoir contained high levels of amino acids and NH4+. In addition, the NH4+ concentrations were positively correlated with amino acids concentrations. This result can be explained by increasing NH4+ production according to labile organic nitrogen accumulation in the sediments near the dike. The result of our study suggests that the sediment resuspension near the dike induced by drainage might be a source of bioavailable nitrogen for Isahaya Bay.

Influence of terrestrial organic matter loading on the occurrence of red tides and hypoxia in Isahaya Bay, Japan

Influence of terrestrial organic matter loading on the occurrence of red tides and hypoxia in Isahaya Bay, Japan

Effects of microtopography on soil development of the conserved area in the Isahaya Bay polder

ABSTRACT After reclamation of the Isahaya Bay polder, a conserved area is covered with naturally developed vegetation under high groundwater level conditions. This study investigated how alluvial deposits transform into soil without utilizing artificial water drainage systems for agricultural land uses. Soils were surveyed under four vegetation compositions, including those dominated by goldenrods (GR), occupied by high-grown reeds (HRD), covered with low-grown reeds (LRD), or a mixture of goldenrods and reeds (MIX). The microtopography of the polder differentiated the research site with a higher elevation at the GR location, followed by a MIX, HRD, and a lower elevation at the LRD location. The soils were characterized by their physicochemical properties and ionic composition of the soil water. Electrical conductivity (EC), which can be an indicator of residual seawater, was lowest in the GR, followed by the MIX, HRD, and LRD. The soil moisture regimes affected by microtopography led to different frequencies of wet-dry cycles in the soil, resulting in a developing sequence of soil structure from LRD to GR with decreasing EC. The ionic composition of soil water varied with elevation and soil depth. The residual seawater still influenced the ionic composition of deeper soil horizons at lower elevations, while specifically high rates of Ca2+ and SO4 2− were observed in GR. The oxidation of pyrite is stimulated by soil aeration with lowering water level, resulting in soil acidification and the simultaneous dissolution of calcium carbonate from seashells. The sequential changes in soil properties with elevation indicated the effects of microtopography on soil moisture dynamics, which led to variations in soil formation and vegetation in the conserved area.

High contribution of picophytoplankton to phytoplankton biomass in a shallow, eutrophic coastal sea

We explored picophytoplankton in the surface (0 m) and bottom (2.3–8.7 m) layers of a shallow (<10 m) eutrophic coastal system (Isahaya Bay, Japan). We found that picophytoplankton (principally Synechococcus) constituted the major phytoplankton in spring and summer. The chlorophyll a (chl.a) concentration in the 0.7–2.0-μm picophytoplankton fraction (hereinafter ‘pico-sized chl. a’) and picophytoplankton abundance in Isahaya Bay were higher than those in other eutrophic coastal waters. The pico-sized chl. a concentration and the picophytoplankton abundance counted with an epifluorescence microscope was up to 49.31 μg L−1 and 1.9 × 106 cells mL−1, respectively. Higher contributions of pico-sized chl. a to the total chl. a were evident in summer (up to 63.5%), relative to spring (up to 32.1%), at both depths. Picophytoplankton abundance and the pico-sized chl. a concentration was positively correlated with water temperature and dissolved inorganic phosphorus (DIP) concentrations. Thus, both temperature and DIP may be major controllers of picophytoplankton in Isahaya Bay. The pico-sized chl. a concentration and picophytoplankton cell number at the bottom layer were positively correlated with those in the surface layer, suggesting that picophytoplankton in bottom layers may have sunk from the surface layers. The results imply that the picophytoplankton affects the biogeochemical processes in the bottom of Isahaya Bay more than previously thought. This may be true not only for this estuary but also for other eutrophic coastal seas.

Urea is a potentially important nitrogen source for phytoplankton during red tide formation in Isahaya Bay, Japan

Urea is a potentially important nitrogen source for phytoplankton during red tide formation in Isahaya Bay, Japan

Stratification potential-energy anomaly index standardized by external tide level

Simpson's stratification index φ is widely used in oceanography as an index of vertical stratification. However, the use of φ can be problematic when the tidal amplitude is significant compared to the water depth, since φ fluctuates periodically with the external tide even if the vertical density structure does not change. The stratification potential-energy anomaly V is proportional to the square of the water column thickness D; therefore, V∗=(H/D)2V (where H is the mean water depth) is an appropriate standardized stratification index for estuaries and regions of freshwater influence. The usefulness of this index is demonstrated using time series fk=H/DkVk=0,1,2,3(f0=V,f1=Hφ,f2=V*) and their power spectra as obtained from tower B3 in Isahaya Bay in Japan. Tidal cycle fluctuation is removed most effectively for f2(=V∗).

Temporal changes in benthic amphipod assemblages in the central part of the Ariake Sea during the five years following the dike closure in Isahaya Bay, Japan

Temporal changes in benthic amphipod assemblages in the central part of the Ariake Sea during the five years following the dike closure in Isahaya Bay, Japan

Twenty years after the shut-off of Isahaya Bay, the present status and future of Ariake Sea (2)

Twenty years after the shut-off of Isahaya Bay, the present status and future of Ariake Sea (2)

Interannual variation in water quality, bottom sediments, and community structures of macrobenthic fauna in the Isahaya Bay reservoir, western Kyushu, Japan

Interannual variation in water quality, bottom sediments, and community structures of macrobenthic fauna in the Isahaya Bay reservoir, western Kyushu, Japan

Occurrence of Chironomid larvae (Insecta: Chironomidae) in sediments of the Isahaya Bay reservoir from 1998 to 2018

Occurrence of Chironomid larvae (Insecta: Chironomidae) in sediments of the Isahaya Bay reservoir from 1998 to 2018

Effects of change of density stratification due to Isahaya Sea-Dyke on the fate of anoxic water in the Ariake Sea

In this study, we aim to investigate the possibility that the construction of the Isahaya Sea-Dyke significantly causes anoxic water development in the bottom layer in the Ariake Sea, Japan. We numerically compared the vertical distribution of dissolved oxygen (DO) in the water column between two cases with the dyke and without it. As a result of this research, the followings are clarified: 1) DO totally decreased in Isahaya Bay due to construction of Isahaya Sea-Dyke; 2) The effect is relatively small during neap tide, but is larger in spring tide; and 3) The effect is not shown in the main body of the Ariake Sea.

Interannual changes in the oligochaete communities in the reservoir pond of Isahaya Bay in the Ariake Sea, Kyushu, since shut of the floodgate

Interannual changes in the oligochaete communities in the reservoir pond of Isahaya Bay in the Ariake Sea, Kyushu, since shut of the floodgate

Twenty years after the shut-off of Isahaya Bay, the present status and future of Ariake Sea

Twenty years after the shut-off of Isahaya Bay, the present status and future of Ariake Sea

Changes in distributional patterns in water quality, bottom sediments, and community structures of bivalves, gammaridean amphipods, and polychaetes in the Ariake Sea after dike construction for the reclamation of Isahaya Bay, western Kyushu, Japan

Changes in distributional patterns in water quality, bottom sediments, and community structures of bivalves, gammaridean amphipods, and polychaetes in the Ariake Sea after dike construction for the reclamation of Isahaya Bay, western Kyushu, Japan

Estimation of production and sedimentation of cyanobacterial toxins (microcystin) based on nutrient budgets in the reservoir of Isahaya Bay, Japan

The increasing eutrophication of freshwater and brackish habitats globally has led to a corresponding increase in the occurrence of harmful cyanobacterial blooms. Cyanobacteria can produce highly toxic substances such as microcystins (MCs) that affect the health of livestock, wildlife, and humans. The present study broaden the understanding of cyanobacteria ecology and MC dynamics in the field, focusing on the estimation of the production and sedimentation rates of MCs in a natural habitat. The nutrient concentrations of the reservoir water and sediment pore water were monitored at 3-h intervals for 24 h during the summer cyanobacterial bloom. The DIN uptake rate of Microcystis in the Isahaya reservoir was estimated and the large-scale blooms in the reservoir were largely controlled by the interactions between rainfall and nutrient levels in the warm season. By using calculations based on the nitrogen budgets and tracking changes of the MC concentrations in the water column, the total MC production and sedimentation rates were estimated to be 52.2 kg MCs d−1 and 21.5 kg MCs d−1, respectively. Although MCs could be degraded in the environment, the MC sedimentation still comprised 41% of the in-water production.

A relationship between the mechanisms of the environmental crisis in the inner part of Ariake Bay and the reclamation project of the tidal flats conducted in Isahaya Bay

A relationship between the mechanisms of the environmental crisis in the inner part of Ariake Bay and the reclamation project of the tidal flats conducted in Isahaya Bay

The drainage from Isahaya Bay reservoir has further adverse effects on the damaged ecosystem of Ariake Sound

The drainage from Isahaya Bay reservoir has further adverse effects on the damaged ecosystem of Ariake Sound

An opinion survey of members of a mailing list ’benthos’ about the present state of Isahaya Bay with a questionnaire method: The background and an outline

An opinion survey of members of a mailing list ’benthos’ about the present state of Isahaya Bay with a questionnaire method: The background and an outline

Influence of water discharged from a reservoir on reclaimed land into Isahaya Bay (Kyushu, Japan) on the regeneration of NH4+ in the water column

In 1997, the inner part of Isahaya Bay on the west coast of Ariake Bay, western Japan, was separated from the sea by dikes, leading to the formation of a freshwater reservoir. Since the Isahaya Reclamation Project began in the 1990s, red tides and hypoxia have become more common in the bay. Eutrophic high-turbidity water from the reservoir is frequently discharged into the bay, so NH4+ regenerated from organic matter in the discharge may be a source of nutrients for the red tides. In this work, we mixed reservoir water and seawater from the bay in experiments aimed at quantifying the NH4+ regeneration caused by the direct decomposition of labile organic matter in the high-turbidity drainage water. By comparing the results of field monitoring and mixing experiments, we estimated that the NH4+ regeneration rate under the influence of the drainage water was 192–501 μmol m−2 h−1, which was 31.5–46.8% higher than the natural NH4+ regeneration rate. This suggests that the additional NH4+ regeneration induced by the drainage water is non-negligible and that the organic matter in the drainage water may be an important source of NH4+ for red tides in the bay. This is the first study to provide data on the non-negligible amount of NH4+ regenerated upon the direct decomposition of labile organic matter in high-turbidity water drained from a reservoir on reclaimed land into the bay.

Characteristics of Organic Matter in and around Isahaya Bay Regulating Reservoir

諫早湾干拓調整池及びその流入河川等における有機物特性の把握を目的として, 有機物の分光学的特性 (紫外部吸光度, 三次元励起・蛍光スペクトル法) や化学的特性 (脂肪酸分析) の違いを利用した難分解性有機物等の特性把握及び起源推定に関する研究を実施した。その結果, 調整池やその流入河川等における溶存有機物は土壌由来のものが優位であることが示唆された。なお, 溶存態の難分解性有機物にはフミン物質, 易分解性有機物にはタンパク質が含まれていることが推察された。また, 懸濁有機物に関して, 調整池やその流入河川等において植物プランクトン由来の有機物は比較的易分解性であり, 陸上植物に由来する懸濁有機物は植物プランクトン由来の懸濁有機物より微生物分解されにくいことが示唆された。