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Dissolved organic carbon (DOC) comprises nearly half of the riverine organic carbon flux into oceans. Although riverine DOC is involved in numerous important ecosystem functions, excessive labile DOC inputs from rivers may contribute to hypoxia in coastal systems. Furthermore, many aspects of the contribution of riverine DOC to hypoxia are unknown. The natural carbon stable isotope ratio (δ13C) in organic matter can be used to identify sources of organic carbon in coastal oceans. In this study, we analyzed the concentrations and δ13C values of DOC in the bottom layer of the northern Ariake Sea, Japan, in which hypoxic water develops during summer. Additionally, we compared the δ13C values of DOC at the beginning and end of an incubation experiment to determine the contribution of riverine DOC decomposition to hypoxia. The results of this study indicate that the bottom DOC concentration is influenced by both phytoplankton and river water, with the former likely having a stronger impact in the northwestern Ariake Sea in summer. Nevertheless, we also found major contributions (~ 49.7%) of DOC decomposition to total organic carbon decomposition at some stations. The initial δ13C value of DOC ranged from − 24.0 to − 22.2‰, and the DOC δ13C increased (from + 0.3 to + 0.8‰) in three of the five incubation bottles after incubation. This might be caused by selective decomposition of 13C-depleted organic matter. The decomposed DOC in those bottles must be mainly derived from terrestrial sources. This result implies that riverine DOC decomposition contributes to hypoxia formation in the bottom layer of the northern Ariake Sea., Aquatic Sciences, 85(1), art. no. 17; 2022

The Osumi Strait and its neighboring waters are major spawning grounds for commercially important small pelagic and coastal fish in the Pacific coastal area of Japan. Mesopelagic fish larvae are also abundant here, co-occurring with the larvae of commercial species. However, it is unknown whether these fish groups compete for prey. We examined the diet of four commercial fish groups (Engraulis japonicus, Trachurus japonicus, Triglidae spp., and Sebastiscus spp.) and two mesopelagic species (Sigmops gracilis, Myctophum asperum) during their larval stages to determine whether they had the same prey source. Morphological analysis and DNA metabarcoding of gut contents showed that the main prey species of all six fish groups were calanoid copepods. However, results from metabarcoding differed greatly from morphological analysis, showing that appendicularians were abundant in the guts of S. gracilis, M. asperum, and Sebastiscus spp. This may be because the methods differ in how they determine prey composition, with morphological analysis relying on counts of identifiable prey parts and metabarcoding being only a semi-quantitative method. In addition, appendicularians might be underestimated by metabarcoding analysis due to variation in genome size, copy number of target genes, and primer mismatches. Stable isotope analysis supported the importance of appendicularians as prey for S. gracilis, M. asperum, and Sebastiscus spp. Our results indicate that the two mesopelagic species and Sebastiscus spp. may compete for prey, but E. japonicus, T. japonicus, and Triglidae spp. occupy niches different from those of the two mesopelagic species. Our results imply that the trophic pathway via appendicularians may support the feeding of dominant mesopelagic species and enable coexistence with commercial species in the study area.

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.

The objective of this study was to determine the effects of transported hypoxic water on macrobenthic species composition in Isahaya Bay, Kyushu, Japan. To this end, we conducted a field survey of water quality, sediment quality, and the macrobenthic community across seasons. Hypoxic intensity, defined as the percentage of the time that a region exhibits hypoxic conditions [dissolved oxygen (DO)

To evaluate the bioaccumulation potential of perfluoroalkyl acids (PFAAs) in an aquatic food web, we measured the concentrations of nine PFAAs in the water and aquatic organisms from an estuary of the Omuta River, Japan. Average log bioaccumulation factors for all PFAAs ranged from 2.0 to 2.8. There was no positive correlation observed between PFAA carbon chain length and there was no evidence of trophic magnification demonstrated among the sample types collected. These results differed from the findings of previous studies in enclosed bodies of water, perhaps because river mouth–estuarine ecotones are more variable spatially and temporally and include some fish that are highly migratory. Further investigations of bioaccumulation factors will be needed to elucidate the tendency of amphiphilic chemicals to bioaccumulate in these river mouth–estuarine ecotones.

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.

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.

Freshwater cyanobacteria produce toxic microcystins (MCs), which travel from freshwater areas into the sea. The MCs produced by cyanobacteria in a freshwater reservoir were discharged frequently into the adjacent Isahaya Bay, remained in the surface sediments, and then accumulated in various macrobenthic animals on the seafloor. The MCs were transported further outside of Isahaya Bay (Ariake Bay), and the median values of the MC contents in the sediments were in the same levels in both bays, while their temporal variations were also similar during the study period. Therefore, the fluctuations of the MC contents in the surface sediments were physically controlled by the timing of the discharge from the reservoir. The MC contents in polychaetes and oysters collected in Isahaya Bay increased markedly during winter. The median values of the carbon-based MC contents in the sediments, primary consumers, and secondary consumers in the bay were 87, 160, and 250 ngMC gC-1, respectively. These results demonstrated bio-accumulation at lower trophic levels in benthic marine ecosystems. An understanding of the processes occurring between sediments and macrobenthic animals is important for clarifying MC dynamics in ecosystems.

Monthly field surveys conducted from April to November 2009 in a tidal front in the Seto Inland Sea, Japan provided a spatial and temporal dataset for investigating seasonal variations in nutrient supply and the formation of a chlorophyll a (Chl-a) maximum. The upward diffusive flux of nutrients is estimated from observational data but it accounted for less than 5% of the nutrients needed to support the primary production of phytoplankton in the front area of the stratification region when the density difference between the surface layer and bottom layer is greater than 0.5 kg m−3. Instead of vertical diffusion, the lateral intrusion of water with high nutrient concentration from the mixed area represented the major nutrient supply in the front area. The depth of the lateral intrusion changed with the month: the surface layer in July became the middle layer in August. According to the calculation of numerical model, an anti-clockwise circulation is intensified by removing river runoff (i.e., low precipitation) in this study area, and the change of lateral intrusion is likely caused by the change of anti-clockwise circulation along with the temporal variation in river runoff. Consequently, the Chl-a peak appeared in the vicinity of the surface front (up to 3 µg L−1) in July, but was in the subsurface (up to 9 µg L−1) in August. Diatom species were a relatively minor taxa of the phytoplankton community up to July, although a relatively high Si(OH)4-Si concentration (up to 20 µmol L−1) was confirmed. In contrast, the subsurface Chl-a maximum (SCM) in August was mainly comprised of diatoms as evidenced by the reduction of both Si(OH)4-Si and Si/N from the surface to subsurface layer (0–20 m depth). Therefore, the supply of both nutrients and the seed population necessary for the formation of the SCM results from the tidal frontal system and phytoplankton assemblages within the tidal front system should be varied on a monthly basis.

To test a hypothesis that the development of the stratification of water enhances the phytoplankton blooming and hypoxia without any increase in the nutrient loading in an enclosed bay, the present study examines the spatial relationship among the hydrographic structures, physico-chemical conditions of the sediments, and macrobenthic communities at 21 stations in the inner part of Ariake Bay, Kyushu, Japan in August 2010, and compares the distribution of the mud content of the sediment in 2014 with those of last few decades focusing on the succession of sedimentary environment. The stratification of water developed in the innermost part of Ariake Bay, and hypoxia occurred below the well-stratified water. The distribution of the muddy bottom with high content of organic matter derived from phytoplankton almost overlapped with the stratified water. Cluster analysis concerning the grain size composition of the sediment corresponded greatly to that of the macrobenthic communities. These results indicate that the development of the stratification of water strongly influences the sedimentary environments and the community structure of macrobenthic animals in the inner part of Ariake Bay. Comparison with the past distribution of mud content revealed eastward expansion of the muddy sediment, and it suggests the distribution of the well-stratified water has expanded recently. The results of the present study show the possibility that the development of the stratified water causes a series of phenomena, which closely resemble ones following the development of the eutrophication of the water, without any increase in the nutrient loading in an enclosed bay or estuary.

Freshwater cyanobacteria produce highly toxic substances such as microcystins (MCs), and water containing MCs is often discharged to downstream and coastal areas. We conducted field monitoring in Isahaya Bay to clarify the short-term dynamics of MCs discharged from a reservoir following a cyanobacteria bloom in the warm season. MCs were detected in the seawater of the bay (max. 0.10 μg L−1), and were deposited on the sea floor, with the MC content of the surface sediment increasing by approximately five times (0.11 ± 0.077–0.53 ± 0.15 μg kgww−1, mean ± SD) at the four stations near the reservoir drainage gate before and after the discharge. The MCs was then transported from the mouth of the bay by tidal currents during the period of the study. Therefore, the MCs were moved away from the closed water area where the cyanobacteria blooms, and spread throughout the coastal area.

To evaluate trophic biomagnification of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in an estuary of the Ariake Sea, Japan, we measured concentrations of 209 PCB congeners and 28 PBDE congeners, and nitrogen stable isotope (δ15N) levels in living aquatic organisms. The trophic magnification factor (TMF) for ΣPCBs (all 209 congeners) was 1.52, and TMFs for 58 PCB congeners ranged from 0.90 to 3.28. In contrast, TMF for ΣPBDEs was 1.17, and TMFs for 7 PBDE congeners ranged from 0.46 to 1.66. TMFs of PCB and PBDE congeners in this study were lower than those in marine food webs, and were similar to those in a lake food web. However, although negative relationships were observed between TMF and log octanol–water partition coefficient (KOW) values among PCB congeners in this study (log KOW up to 7), positive relationships have been reported in several other studies. In the present estuary, PCB concentrations in sea bass may not reach a steady state because sea bass are migratory species. Therefore, TMFs of highly chlorinated congeners with high log KOW values take longer to reach the steady state and may not increase with increasing log KOW.

In the inner part of Ariake Bay, located on the west coast of Kyushu, in western Japan, red tides have occurred with more frequency since the second half of the 1990s. Hypoxic waters have occurred during the summer months since the 2000s, despite the fact that nutrient loading from the land to the bay has not increased over the last five decades. We monitored water conditions at nine stations in the inner part of the bay, conducted benthic environmental surveys, and quantitative samplings of macro-benthic communities at the innermost four stations between 2002 and 2008. Each summer, the water was well-stratified due to the development of a halocline and a thermocline. The DO of the water below the pycnocline fell to hypoxic conditions. At the innermost three stations in the bay, the mud content and organic matter content of the sediment increased significantly, and the carbon stable isotope ratios of the organic matter contained in the sediment ranged between −21.3±0.5‰ and −20.7±0.5‰ of δ13C. These facts indicated that the organic matter was derived photosynthetically from marine phytoplankton. The increase in the mud content of the sediment indicates a deceleration in the tidal current. This may be a key event that induces a series of environmental changes and disturbances, including the stratification of the water, the more frequent occurrence of red tides, the progress of the organic enrichment of the sediment, and the occurrence of hypoxic water during the summer.

We conducted field surveys to determine spatio-temporal variations in the water quality (including inorganic nutrient concentration) of a coastal lagoon, Hichirippu Lagoon (3.56 km2), in Hokkaido, Japan. During a typical supply period (winter), a 24 h continuous survey was conducted to evaluate nutrient supply from the open ocean and examine nutrient utilization by microphytobenthos (MPB) based on the mass balance of a biophilic element. During February 2008, the majority of NO3+NO2-N was supplied from the open ocean, and the dissolved inorganic nitrogen (DIN) budget was 3.25 kmol N d−1. In March 2008, the total amount of organic nitrogen was 39.0 to 588 kmol N for MPB in the surface sediment (depth, 0.5 cm). Assuming that MPB in the surface sediment used all NO3+NO2-N input (3.93 kmol N d−1), their temporal division rate (NO3+NO2-N uptake/total amount of MPB) would be 0.01 to 0.10 d−1. Here the annual DIN budget was estimated to be 258 kmol N yr−1, which mainly depended on MPB-fixed oceanic DIN during winter.

The small gastropod, Lacuna decorata Adams, living on macrophytobenthos or surface sediment, is one of the most dominant species of macrozoobenthos in Hichirippu lagoon covered with seagrass and macroalgae, eastern Hokkaido, Japan. We measured the standing stocks of primary producers and macrozoobenthos, and determined the stable carbon and nitrogen isotope ratios of the primary producers and L. decorata. With these results, we identify the main food items for L. decorata and discuss the feeding strategy of the small gastropod. This gastropod occupied about 64% in density and about 25% in biomass of the macrozoobenthos at all six sampling stations in the lagoon. It occurred densely on the surface of the sediment with dense patches of benthic microalgae (BMA), which contained extremely high levels of Chl.-a between 84 to 226 mg m �2 throughout the period of this study. Nevertheless, the stable isotope sig- natures of carbon and nitrogen of this gastropod clearly show the direct utilization of organic matter derived from sea- grass, Zostera japonica, in the areas where the seagrass luxuriated. However, it shows also a flexible feeding strategy in food preference. It fed green algae such as Ulva pertusa and Urospora wormskioldii in the areas where the seagrass grew scarcely.

Previous estimations of nutrient mineralization in the water column by infaunal bivalves might have been overestimated because of underestimation of the uptake process by microphytobenthos in the field. We conducted field surveys of environmental conditions and quantitative sampling of Ruditapes philippinarum in a shallow lagoon system (Hichirippu Lagoon, eastern Hokkaido, Japan) in August 2006. We recorded the spatial distribution pattern and the molar ratio of dissolved inorganic nutrients to determine the limiting nutrients for microphytobenthos, to evaluate the input and output of nutrients at the entrance of the lagoon station, and to estimate potential nutrient mineralization by R. philippinarum . Our aim was to reevaluate the nutrient mineralization process by infaunal bivalve species. In this study, the mean standing stock of microphytobenthos inhabiting surface sediment (5 mm thick) on the tidal flats was 100 times higher than that of phytoplankton (1 m depth). Low N/P and high Si/N ratios (mean = 2.6 and 17.6, respectively) near the entrance of the lagoon compared to those of microphytobenthos (N:P:Si = 10.1:1:18) clearly suggest N deficiency. The flux of NH 4 –N coming into the lagoon was 3.4 kmolN d − 1 , and the flux out was − 3.7 kmolN d − 1 . Thus, assuming that there would have been no phytoplankton and microphytobenthos uptake during the day, 0.3 kmolN d − 1 of NH 4 –N was produced within the lagoon. However, the NH 4 –N mineralization rate of the clams has been estimated to be approximately 7.7 ± 6.8 kmolN d − 1 . Thus, 96% (7.4 kmolN d − 1 , i.e., 7.7 kmolN d − 1 minus 0.3 kmolN d − 1 ) of the NH 4 –N mineralized by the clam was consumed by microphytobenthos. In contrast, if all the NH 4 –N inflow (3.1 kmolN d − 1 ) was consumed by the microalgae before outflow, 52% (4.0 kmolN d − 1 , i.e., 7.7 kmolN d − 1 minus 3.7 kmolN d − 1 ) of the NH 4 –N mineralized by the clams should have been consumed by microphytobenthos. Microphytobenthos on the tidal flats (11.3 ± 11.8 kmolN) used all of the surplus nutrients (between 4.0 and 7.4 kmolN d − 1 ), and the temporal division rate [=(NH 4 –N uptake)/(standing stock of microphytobenthos)] of microphytobenthos would have to be between 0.35 and 0.65 d − 1 . Residual NH 4 –N (0.3 – 3.7 kmolN d − 1 ) was the water-column source and accounted for 12–148% of NH 4 –N in the water column near the entrance of the lagoon (2.5 ± 1.4 kmolN) per day. This is the first field-based observation with a quantitative evaluation of nutrient mineralization by infaunal bivalves and nutrient uptake by microphytobenthos.

s: We conducted field surveys to monitor the population dynamics of short-neck clam, Ruditapes philip- pinarum, in Hichirippu Lagoon, in eastern Hokkaido, Japan, focussing on the negative impact of extremely low temper- atures during the winter on juveniles in the post-settlement period. The clam population had a short annual breeding season between September and November. The newly settled juveniles experienced the severe winter as the daily mean temperature of the surface sediment descended below 0°C soon after settlement. The mortality of small juveniles (less than 3 mm in mean shell length) in the post-settlement period between October 24, 2006, and August 1, 2007, was esti- mated at 1.34% d � 1 , which was approximately 5.6 times higher than after the post-settlement periods (0.24% d � 1 and 0.17% d � 1 ). We compared the mortality of the juvenile in and after the post-settlement period with the results of the population study conducted on Kikuchi River Tidal Flat, Kumamoto, Kyushu (Tsukuda 2008), where the daily mean temperature was approximately 6°C higher in the post-settlement period of the juveniles. On Kikuchi River Tidal Flat, the mortality in and after the post-settlement period of the juvenile was estimated at 1.62% d � 1 , and 0.49% d � 1 and 0.44% d � 1 , respectively. These facts indicate that the extremely low winter temperatures in this study area (eastern Hokkaido) does not bring about large-scale mortality, and this is true not only for juveniles after the post-settlement pe- riod but also small juveniles in the post-settlement period.