Kagoshima Bay Research Articles

Ovarian maturation, size at sexual maturity, and spawning season of jack-knife shrimp Haliporoides sibogae (Decapoda: Solenoceridae) in Kagoshima Bay, southern Japan

Ovarian maturation, size at sexual maturity, and spawning season of jack-knife shrimp Haliporoides sibogae (Decapoda: Solenoceridae) in Kagoshima Bay, southern Japan

Temporal changes in the microplankton community due to Kuroshio branch current inflow

Inflow from the Kuroshio Current into coastal areas stimulates biological production. However, to date there are no detailed reports on its temporal changes. In the target area of this study, Kagoshima Bay, Japan, the time of inflow of the Kuroshio branch current can be identified a posteriori based on water temperature data. This study aimed to determine the relationship between the Kuroshio branch current inflow from the surface layer and phytoplankton bloom at the mouth of the bay during the mixing season. Therefore, we compiled data from ship observations in the Kagoshima Bay from 2016 to 2019 to examine changes in water quality (temperature, salinity, and nutrients), microphytoplankton, and microzooplankton versus time since current inflow. In this study, significant relationships were obtained for microplankton in terms of both biomass and cell abundance, and specific growth rates (0.060–0.079 d–1) comparable to that of chlorophyll a. In addition, the current inflow provided nutrients to the surface in the absence of strong wind and waves. Over time, phytoplankton, mostly athecate dinoflagellates, as well as microzooplankton, mainly naked ciliates, proliferated and were quickly preyed upon by mesozooplankton. Over the study period, such events could be observed once every two weeks in most years, suggesting that these frequent phytoplankton blooms support the high biomass of mesozooplankton and larval fish found in this area.

Mineralogical and geochemical facets of the massive deposition of stibnite-metastibnite at a seafloor hydrothermal field (Wakamiko Crater, Kagoshima Bay, Ryukyu Volcanic Arc)

Abstract Stibnite precipitates in the form of massive boulders at two active hydrothermal mounds in the submarine Wakamiko Crater (Ryukyu Volcanic Arc) as opposed to commonly observed accessory stibnite in the seafloor hydrothermal deposits. The stibnite dimorph, metastibnite, found here for the first time on the seafloor, appears to always form whenever stibnite is precipitated under submarine hydrothermal conditions. Our study shows that hydrothermal conditions of low temperatures (<50 °C) and low values of pH (<6) are favorable for the precipitation of stibnite on the seafloor. The stibnite probably does not precipitate at the measured vent fluid temperatures (i.e., 177.6 – 187.0 °C) along the chimney conduits, but rather at temperatures <50 °C and at slightly reduced to slightly oxic conditions (Eh = -0.5 to +0.5 V) within the chimney walls and hydrothermal mounds. Metastibnite deposition appears to be the result of rapid quenching of hot hydrothermal fluid when mixed with cold seawater and rapid precipitation at the interface between stibnite and vent fluid. The low concentrations (usually below detection limits) of the trace elements (Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sr, V, Zn) in the stibnite deposits from Wakamiko Crater are likely a result of decreased metal-transporting capacity of the precipitating vent fluid due to its low chlorinity. Low-chlorinity venting implies sub-seafloor boiling and phase separation of the hydrothermal fluid. Sluggish hydrothermal fluid/seawater mixing within the walls of the chimneys and mounds favors the reduction of sulfate dissolved in the hydrothermal fluids and results in a heavy S isotope composition of the sulfate in the vent fluids. Sulfate reduction and disproportionation of magmatic SO2, both leading to heavy S isotope composition of sulfate in the vent fluids, seem to be common processes in volcanic arc/back-arc submarine hydrothermal settings.

Distribution of selenium and mercury concentrations in sediments of Kagoshima Bay: can submarine volcanic activity affect the distribution of selenium in sediments?

Distribution of selenium and mercury concentrations in sediments of Kagoshima Bay: can submarine volcanic activity affect the distribution of selenium in sediments?

Impact of mercury discharged from submarine volcano on inner bay ecosystems

Organic mercury, inorganic mercury and total mercury concentrations in phytoplankton (<0.1 mm) and zooplankton (>0.1 mm) collected in Kagoshima Bay, Japan were measured from 2017 to 2019 to estimate the impact of mercury discharged from submarine volcanoes on ecosystems; submarine volcanic activity continues at a depth of 200 m in the inner part of Kagoshima Bay. The total mercury concentrations in phyto- and zooplankton collected by vertical hauling at 0–200 m at just above the submarine volcano were in the range of 0.11–2.0 mg kg−1 (avg. 0.67 mg kg−1) and 0.090–0.56 mg kg−1 (avg. 0.21 mg kg−1), respectively. These values were one order of magnitude higher than the values in plankton collected in the central part of Kagoshima Bay. Organic mercury concentrations in phyto- and zooplankton were <0.010–0.071 mg kg−1 (avg. 0.028 mg kg−1) and 0.012–0.25 mg kg−1 (avg. 0.10 mg kg−1), respectively, for the inner part, and <0.010–0.040 mg kg−1 (avg. 0.010 mg kg−1) and <0.010–0.025 mg kg−1 (avg. 0.012 mg kg−1), respectively, for the central part. The values obtained in the inner part of the bay increased in summer and decreased in winter, which was consistent with changes in seawater mercury concentrations affected by volcanic activity. The organic mercury concentration in zooplankton collected just above the submarine volcano showed a size dependency, and a higher value was observed in the larger size, which suggested that the discharged mercury from the volcano was absorbed and concentrated through the ecosystem.

Recruitment and growth patterns of the portunid crab Charybdis bimaculata in Kagoshima Bay, southern Japan

Recruitment and growth patterns of the portunid crab Charybdis bimaculata in Kagoshima Bay, southern Japan

A new species of the genus Nebalia (Crustacea: Phyllocarida: Leptostraca) from hydrothermal vent fields of the submarine Piip Volcano (the southwestern Bering Sea) described through molecular and morphological analysis

A new leptostracan species, Nebalia piipensis sp. nov., collected during the 82nd cruise of the RV Akademik M.A. Lavrentyev on the submarine Piip Volcano (the southwestern Bering Sea) at depths of 400–472 m, has been described. These leptostracans were abundant at bacterial mats and in assemblages of chemosymbiotrophic bivalves Calyptogena pacifica (Pliocardiinae). Nebalia piipensis sp. nov. differs from the known Nebalia species by a set of features including a distally tapering eye-stalk mostly covered by an ommatidial part; a rostrum with the length 2.5 times the width; an article 2 of mandibular palps bearing three thin plumose and one thin simple seta; an article 1 of the second maxilla endopod 1.8 times longer than an article 2; furcal rami longer than the combined length of the pleonite 7 and telson; rounded denticles of pleonites 6 and 7; an indistinct anal-plates ‘shoulder’. Nebalia piipensis sp. nov. is the second species of the genus Nebalia reported from hydrothermal vent fields after N. tagiri, described from hydrothermal vents in Kagoshima Bay, at 200 m. The sequences of COI barcoding region, 16S and Histon H3 loci for the new species were obtained. The phylogenetic analysis based on COI sequence showed that all the northwestern Pacific species, including N. tagiri and N. piipensis sp. nov. are clustering together. However, N. piipensis sp. nov. is genetically closer to N. dolsandoensis from regular habitats than to the vent species N. tagiri. A preliminary examination of the body surface of the new leptostracan species using scanning electronic microscopy revealed a variety of epibiotic bacteria.

Seasonal influence of intrusion from the Kuroshio Current on microplankton biomass and community structure in the northern Satsunan area, western Japan

Seasonal variations in hydrographic conditions, nutrients, and microplankton abundance and biomass were investigated inside and outside Kagoshima Bay in the northern Satsunan area. The area is a nursery ground for the migrating fish. During the mixing season from November to April, highly saline water (>34.5) originating from a branch of the Kuroshio Current, the western boundary current of the North Pacific Subtropical Gyre, intruded from the surface layer into the bay causing extrusion of coastal bottom water. This intrusion generated vertical mixing between surface water and nutrient-rich bottom water. Consequently, chlorophyll a concentrations increased (>1 μg l−1) around the bay mouth. During the stratified season from May to August, the coastal surface water was extruded due to intrusion of saline water from the bottom layer. High chlorophyll a concentrations (>1 μg l−1) were also observed in the surface layer in July during the rainy season. Such differences in hydrographic conditions between mixing and stratified seasons were due to vertical distribution of highly saline water from outside the bay, which seasonally migrated between surface and subsurface layers. Relatively distinct redundancy analysis plots and boxplots generated from a generalized linear model showed that microplankton community structure differed between mixing and stratified seasons. The mixing and stratified seasons were characterized by high diatom abundance and low microplankton abundance, respectively, suggesting that vertical mixing increases nutrient supply and consequently increases diatom abundance. Thus, microzooplankton (mainly naked ciliates and athecate dinoflagellates) abundance was related to the abundance of prey organisms, such as diatoms, thecate dinoflagellates and cryptophyte-like flagellates, rather than hydrographic conditions. Thus, saline water intrusion from a branch of the Kuroshio Current likely governs hydrographic conditions in Kagoshima Bay and consequently affects temporal variations in abundance and taxonomic composition of phytoplankton and microzooplankton communities.

Complete gammaproteobacterial endosymbiont genome assembly from a seep tubeworm Lamellibrachia satsuma.

Siboglinid tubeworms thrive in hydrothermal vent and seep habitats via a symbiotic relationship with chemosynthetic bacteria. Difficulties in culturing tubeworms and their symbionts in a laboratory setting have hindered the study of host-microbe interactions. Therefore, released symbiont genomes are fragmented, thereby limiting the data available on the genome that affect subsequent analyses. Here, we present a complete genome of gammaproteobacterial endosymbiont from the tubeworm Lamellibrachia satsuma collected from a seep in Kagoshima Bay, assembled using a hybrid approach that combines sequences generated from the Illumina and Oxford Nano-pore platforms. The genome consists of a single circular chromosome with an assembly size of 4,323,754 bp and a GC content of 53.9% with 3,624 protein-coding genes. The genome is of high quality and contains no assembly gaps, while the completeness and contamination are 99.33% and 2.73%, respectively. Comparative genome analysis revealed a total of 1,724 gene clusters shared in the vent and seep tubeworm symbionts, while 294 genes were found exclusively in L. satsuma symbionts such as transposons, genes for defense mechanisms, and inorganic ion transportations. The addition of this complete endosymbiont genome assembly would be valuable for comparative studies particularly with tubeworm symbiont genomes as well as with other chemosynthetic microbial communities.

Seasonal variation in mercury species in seawater of Kagoshima Bay, southern Kyushu, Japan: The impact of active submarine volcanos on the inner bay

Wakamiko Proto-Caldera, a submarine volcano, is located at the bottom of the sea in the bay head area of Kagoshima Bay. In Kagoshima Bay, several species of fish with total mercury exceeding the provisional regulation value of 0.40 mg kg−1 were found in the 1970s. The effects of mercury released from submarine volcanoes are suspected. However, the path from volcanic mercury to the broader ecosystem has not yet been clarified. In this study, as the first step to elucidate the impact of volcanic activity on the ecosystems of Kagoshima Bay, seasonal changes in the concentrations of dissolved gaseous mercury (DGM), Hg(II), filtered total mercury (FT-Hg), particulate mercury (P-Hg), and mercury in suspended particulate matter (Hg in SPM) in seawater of Kagoshima Bay were tracked every two months from Feb. 2014 to Dec. 2017. The dissolved total mercury (DT-Hg) concentration was calculated by summing the DGM and FT-Hg concentrations. The DT-Hg at the location in the caldera ranged from 0.09 to 6.83 ng L−1 showing higher values in summer compared to winter, that is, in the stratified period. The P-Hg showed similar periodic variation as DT-Hg, and the highest Hg in SPM was 2.15 mg kg−1, which is one order of magnitude higher than that observed at the reference site. The high mercury concentration in SPM suggests the accumulation of dissolved mercury discharged from volcanic activity on SPM. Since mercury loaded on SPM can be taken up by suspension feeders and enter the food chain, the Hg in SPM may be a key factor affecting Hg in fishes in Kagoshima Bay.

Post-caldera volcanism and environmental changes in Aira Caldera, inner Kagoshima Bay, SW Japan

Post-caldera volcanism and environmental changes in Aira Caldera, inner Kagoshima Bay, SW Japan

First records of Plectranthias azumanus (Jordan and Richardson, 1910) (Serranidae: Anthiadinae) from Tobi-shima island (Yamagata Prefecture), and Kagoshima Bay and the Amami Islands (Kagoshima Prefecture), Japan, with notes on relationships of localities, water depths, and water temperatures

First records of <i>Plectranthias azumanus</i> (Jordan and Richardson, 1910) (Serranidae: Anthiadinae) from Tobi-shima island (Yamagata Prefecture), and Kagoshima Bay and the Amami Islands (Kagoshima Prefecture), Japan, with notes on relationships of localities, water depths, and water temperatures

Analysis of workload required for removal of drifting pumice after a volcanic disaster as an aspect of a port business continuity plan: A case study of Kagoshima Port, Japan

For most major ports in Japan, a business continuity plan (Port-BCP) has been developed to cope with potential functional disorder in port operations caused by natural disasters. The objectives of a Port-BCP are to maintain the minimum vital activities, such as transportation of emergency supplies, under critical conditions, and to enable resumption of normal operations within a reasonable period. However, Japan's existing Port-BCP plans focus only on earthquake and/or tsunami disasters and do not consider volcanic events. This study assessed the impact of pumice fallout in the Kagoshima Port area following a supposed large-scale eruption of the adjacent volcano, Sakurajima. To estimate the potential volume and mass of pumice deposited in Kagoshima Bay, numerical simulations were conducted using a tephra transport model with varying wind parameters. Based on the results, the duration of the work required to remove the deposited pumice from the port area and navigation channels was estimated. Then, analysis of tidal circulations under surface wind stress was conducted using a 3D ocean model to clarify the characteristics of the currents in Kagoshima Bay. On the basis of the calculations of both pumice fallout and tidal currents, the surface drift of the floating pumice was analyzed. The total mass of floating pumice, as the object for removal, is presumed to decrease with time. Therefore, the temporal variation of the total mass of floating pumice was investigated by considering several physical mechanisms.

Spring phytoplankton blooms in the Northern Satsunan region, Japan, stimulated by the intrusion of Kuroshio Branch water

The Kuroshio current greatly affects biological processes in coastal areas. The Kagoshima Bay has good spawning ground for major migratory fish, serves as an important feeding area for fish larvae, and is affected by the Kuroshio current; however, there is little information concerning the biological response to the Kuroshio intrusion in this location. During the mixing period, the Kuroshio current approaches the Kagoshima Bay and a density flow, consisting of surface inflow and bottom outflow, is generated. We hypothesize that the bottom water in the bay, having a high nutrient concentration, mixes with the surface layer when the water flows out of the bay and that this contributes to the high biological production at the bay mouth. In this study, we conducted a field survey of the water quality in March 2016 and, using a three-end member mixing model, examined whether the bottom water of the bay could be a nutrient source supporting the phytoplankton bloom in the bay mouth. We discuss the fate of the organic matter derived from the primary production promoted by a nutrient supply event. The results indicate that the N-based phytoplankton biomass represented less than 80% of the N uptake by the phytoplankton, as estimated by subtracting the measured nitrate concentration from the supplied nitrate concentration based on the mixing model, indicating that the phytoplankton bloom at the bay mouth was supported by the supply of bottom water from the bay. Based on the C/N molar ratio (6.4) calculated by C/chlorophyll a (Chl-a) and N/Chl-a in this study, the highest value of nitrogen uptake was 12 mg C m−3 d−1 in terms of the carbon assimilation. The feeding rate of the mesozooplankton was estimated to be 13.6 mg C m−3 d−1 using a physiological model. An analysis of the food web structure is required to assess the effects of feeding by mesozooplankton; however, most of the phytoplankton bloom caused by mixing could be efficiently transferred to mesozooplankton in this study area.

Latest Pleistocene to Holocene calcareous microfossil assemblages from the Shinjima (Moeshima) Island and reconstruction of the paleoenvironments in the inner part of Kagoshima Bay, SW Japan

Latest Pleistocene to Holocene calcareous microfossil assemblages from the Shinjima (Moeshima) Island and reconstruction of the paleoenvironments in the inner part of Kagoshima Bay, SW Japan

Fault‐Valve Behavior Estimated From Intensive Foreshocks and Aftershocks of the 2017 M 5.3 Kagoshima Bay Earthquake Sequence, Kyushu, Southern Japan

AbstractDetermining fluid migration and pore pressure change within the Earth is key to understand earthquake occurrences. We investigated the spatiotemporal characteristics of the intense foreshocks and aftershocks of the 2017 ML 5.3 earthquake in Kagoshima Bay, Kyushu, southern Japan, to examine the physical processes governing this earthquake sequence. Our relocated hypocenters show the foreshocks moved on a sharply defined plane with a steep dip. The mainshock rupture initiated at the edge of the foreshock seismic gap. The size of the foreshock seismic gap is comparable to that of the mainshock estimated from the source corner frequency, suggesting this seismic gap corresponds to the large slip region of the mainshock. The aftershocks migrated upward along several steeply dipped planes with a seismicity pattern that deviated from the typical mainshock–aftershock type. This deviation of seismicity pattern, together with the hypocenter migrations, suggests aseismic processes, such as pore pressure migration and aseismic slip, affected this earthquake sequence. We established the following hypothesis. First, fluids originating from the subducting slab migrated upward and intruded into the fault plane, reducing the fault strength and causing the foreshock sequence and potentially aseismic slip. Second, the mainshock rupture occurred due to the decreased fault strength and the increased shear stress in an area with relatively high strength. Third, pore pressure increase associated with post‐failure fluid discharge caused the upward aftershock migration. These observations are consistent with the fault‐valve model and show the importance of fluid movement at depth not only in earthquake swarms but also in foreshock–mainshock–aftershock sequences.

The influence of submarine volcano on seasonal changes in arsenic in the waters of Kagoshima Bay, southwestern Japan

Volcanic activity is one of the primary sources of arsenic in nature, and chemical forms of arsenic discharged by volcanic activity are inorganic. Kagoshima Bay has active submarine volcano located in the 200 m deep basin of its northern part. Since dissolved inorganic arsenic is more toxic than organic arsenic it is important to understand the dispersion of discharged arsenic in seawater. To estimate the influence of arsenic emitted from the bottom vents seasonal changes in As(III) and As(V) concentrations in seawater were studied. There are few studies on the impact of arsenic discharged by submarine volcanos in the inner bay. The total concentrations of As(III) and As(V) (T-As) in seawater at the background site in Kagoshima Bay was 1.71 ± 0.35 μg L−1 (av. ± s.d.), similar to the world average value of 1.7 μg L−1. The highest T-As value at the nearest point to the submarine volcano, 3.48 μg L−1, was observed in summer. Since the vertical mixing of seawater cannot occur in summer due to the formation of the thermocline, the deep water is strongly affected by volcanic activity. The increase in T-As was caused by the increase in As(III) concentration suggesting the discharge of As(III) from the vents.

Ammonium Behavior and Nitrogen Isotope Characteristics of 2:1 Clay Minerals from Submarine Hydrothermal System in the Wakamiko Crater of Kagoshima Bay, Southwestern Japan

Ammonium Behavior and Nitrogen Isotope Characteristics of 2:1 Clay Minerals from Submarine Hydrothermal System in the Wakamiko Crater of Kagoshima Bay, Southwestern Japan

Stratigraphy and origin of the uppermost Pleistocene to Holocene succession exposed on the Shinjima (Moeshima) Island, inner Kagoshima Bay, SW Japan

新島に露出する姶良カルデラ底下の最上部更新統〜完新統堆積物について，これまで議論のあった岩相層序を見直し，下位から順に下部新島シルト層，新島軽石，新島南部軽石，上部新島シルト層，燃島貝層に区分した．堆積物の産状や貝類，珪藻，有孔虫，貝形虫などの化石に基づけば，上部・下部新島シルト層は水深100-200mの内湾静水域の堆積物で，桜島などに由来するテフラを挟む．燃島貝層は桜島起源のラハール堆積物で，新島軽石と新島南部軽石は13-12kaの短期間に噴出した若尊カルデラ起源の水底密度流堆積物と考えられる．既知のテフラとの対比と14C年代などによれば，これらが堆積した時期は13 cal ka BPから2cal ka BPまでといえる．

Cryptic fungal diversity revealed in deep-sea sediments associated with whale-fall chemosynthetic ecosystems

ABSTRACT In this study, sediments from whale-fall chemosynthetic ecosystems (two different sites, one naturally occurring at 4200 m water depth in South Atlantic Ocean and one artificially immersed at 100 m water depth in Kagoshima Bay, Japan) were investigated by Ion Torrent PGM sequencing of the ITS region of ribosomal RNA to reveal fungal communities in these unique marine environments. As a result, a total of 107 (897 including singletons) Operational Taxonomic Units (OTUs) were obtained from the samples explored. Composition of the 107 OTUs at the phylum level among the five samples from two different whale-fall sites was assigned to Ascomycota (46%), Basidiomycota (7%), unidentified fungi (21%), non-fungi (10%), and sequences with no affiliation to any organisms in the public database (No-match) (16%). The high detection of the unidentified fungi and unassigned fungi was revealed in the whale-fall environments in this study. Some of these unidentified fungi are allied to early diverging fungi and they were more abundant in the sediments not directly in contact with whalebone. This study suggests that a cryptic fungal community exists in unique whale-fall ecosystems.