We investigated the toxicity and lethal cell density of Chattonella ovata in relation to the mass mortality of yellowtail Seriola quinqueradiata. Two bioassays for juvenile yellowtail were performed using three C. ovata and one C. marina culture strain. In addition, plankton survey data collected from aquaculture farms in the Seto Inland Sea, Japan were analyzed. The lethal times of juvenile yellowtails for all C. ovata culture strains were significantly longer than those of C. marina. Juvenile yellowtail died faster when the cell density of C. ovata cultures increased. In aquaculture farms, the cell density of C. ovata was 106–261 cells mL−1 when the mass mortality of yellowtail was observed, whereas no mass mortality was observed in C. ovata of at least several tens of cells mL−1. It was concluded that the toxicity of C. ovata was weaker than that of C. marina used in this study’s culture strains. The cell density of C. ovata, which potentially causes mass mortality of yellowtail in aquaculture farms, is several hundred cells mL−1. Considering the measures that should be implemented with a precautionary approach, we propose that stop-feeding criteria for C. ovata to yellowtail should be established at 10 cells mL−1.

This study statistically evaluated the associations among seafloor plowing, sediment properties, and the density of the opportunistic bivalve Arcuatula senhousia in a semi-enclosed coastal embayment of the Seto Inland Sea, Japan. Monitoring data from FY2012–FY2014 at five stations across three treatment areas (plowing, control, and fishing) were analyzed using generalized linear mixed models (GLMMs) with a negative binomial error distribution, appropriate for the highly skewed, overdispersed count data with a high proportion of zeros characteristic of this species. Density was significantly higher in the plowing area than in the unplowed control (incidence rate ratio IRR = 375, p < 0.001), and elevated density was also detected in the fishing area (IRR = 258, p = 0.023), though both estimates carried wide confidence intervals reflecting limited replication of treatment areas. No significant difference in density was found between the plowing and fishing areas (p = 0.905). Standardized ORP showed a negative relationship with density, but this was not statistically significant (p = 0.441), and an independent effect of ORP could not be established. Seafloor plowing was associated with sediment coarsening and may have functioned as a “habitat reset” that facilitated mass recruitment of this opportunistic bivalve; biologically driven sediment reduction from dense byssal mat formation warrants further investigation. These findings provide empirical evidence for the biological consequences of active sediment management in a eutrophicated semi-enclosed coastal system, with implications for benthic ecosystem management in similarly degraded bays worldwide.

Although primary production (PP) in the Seto Inland Sea, Japan, has decreased owing to declining nutrient concentrations, concrete data remain limited. We measured phytoplankton PP monthly over four years (February 2019–July 2022) using an in situ 13C method. Annual PP in the Harima-Nada was estimated at 291 ± 12 gC m−2 yr−1, categorizing the area as transitional between mesotrophic and eutrophic conditions. PP variability was controlled by light, water temperature, and nutrients, enabling the development of an empirical estimation equation. Compared with the 1990s, dissolved inorganic nitrogen concentrations decreased by half, reducing surface PP. However, depth-integrated PP remained comparable with that of the 1990s due to improved water transparency, enabling PP throughout the water column. Chlorophyll-a-specific PP increased under high PP and low-nutrient conditions, suggesting phytoplankton adaptation through community size reduction. These findings demonstrate how coastal ecosystems maintain productivity under nutrient-reducing conditions through vertical redistribution and biological adaptation.

This study investigated the inter-specific competition between the diatom Chaetoceros tenuissimus and the harmful algae Karenia mikimotoi and Chattonella marina under varying initial cell densities. Experiments were conducted using both monoculture and co-culture methods in a controlled laboratory environment. The results indicated that the growth of C. tenuissimus, K. mikimotoi, and C. marina in monoculture was not influenced by the initial cell densities (1000 or 10 000 cells ml(-1)). However, an increase in initial inoculation density accelerated the time to reach maximum cell density and the onset of the stationary growth phase. The initial cell density significantly impacted the inter-specific competition between C. tenuissimus and K. mikimotoi, with the species at higher concentrations prevailing in the competition. In the competition experiments with four different initial concentration combinations of C. tenuissimus and C. marina, neither species were eliminated by the other. Simulation of the co-culture experiments using the Lotka-Volterra inter-specific competition model revealed that C. marina could only outcompete C. tenuissimus when its initial cell concentration was significantly higher. In the experiment in which each filtrate was exposed to the other species, no clear inhibitory effect was observed in any of the combinations. This study contributed to a better understanding of the inter-specific relationships between harmful algae and diatoms.

Noctiluca scintillans, which is common in coastal waters, significantly affects coastal biomass through bloom formation. This study has measured the cellular carbon, nitrogen, and chlorophyll a (Chl-a) content of natural green Noctiluca from the western Upper Gulf of Thailand. The carbon content ranged from 195-556 ng-C cell-1 (mean: 241 +/- 132 ng-C cell-1), while nitrogen content varied between 17-55 ng-N cell-1 (mean: 36 +/- 6 ng-N cell-1). Chl-a content averaged 9.80 +/- 0.78 ng cell-1. Notably, green Noctiluca exhibited higher carbon content than red Noctiluca of identical cell size, potentially because of their endosymbionts.

The eastern part of the Seto Inland Sea is significantly influenced by anthropogenic and territorial nutrient sources, exacerbated by high population and industrial activities. Several rivers, with the Yodo River in Osaka Bay as the largest contributor, play a vital role by providing freshwater, substantial nutrients, and pollution loads to this coastal region. We aimed to understand the responses of ecosystem and coastal dynamics to the changes in nutrient supply from the rivers. Accordingly, a comprehensive three-dimensional physical model coupled with a complicated biogeochemical model was developed to replicate processes in this area. The findings reveal significant differences in nutrient dynamics, and phytoplankton community structures and biomass between Osaka Bay and Harima-Nada. Osaka bay exhibits higher nutrient concentrations, leading to elevated phytoplankton biomass, primarily dominated by a large micro-phytoplankton. Contrastingly, Harima-Nada exhibits lower concentrations, resulting in diminished phytoplankton biomass, with nano-phytoplankton prevailing. The experiment with heightened nutrient inputs from rivers significantly elevated nutrient concentrations for the entire areas in both Osaka Bay and Harima-Nada. Contrastingly, phytoplankton biomass remarkably increased only in the nearshore areas. Furthermore, the response of phytoplankton community structures differs between the two regions. Specifically, in Harima-Nada, nano-phytoplankton shifted primarily to large micro-phytoplankton, while large micro-phytoplankton remains predominant in Osaka Bay. This emphasizes the vital role of nutrient availability in influencing the structure of phytoplankton communities in these regions. The interplay between nutrient availability and phytoplankton dynamics stands as a key factor in comprehending and effectively managing these coastal ecosystems.

The nutrients essential for primary production are inorganic salts and it is important to fully understand nutrient dynamics in coastal oceans. There are three major sources of nutrients in the Seto Inland Sea: loadings from terrestrial run-off, advection from the open sea, and release from bottom sediments. Among these sources, information concerning release from bottom sediments has been limited due to infrequent observations. In this study, we took monthly samples over a two-year period, measuring nutrient concentrations in pore water from 0 to 12 cm at a sampling station in Harima Nada (depth 43 m), with the aim of clarifying the variation of nutrient concentrations in, and fluxes from, the sediment. Nutrient concentrations in the pore water were characterized by high concentrations of dissolved ammonium, dissolved inorganic phosphorus (DIP) and dissolved silicate (DSi), with minor amounts of nitrate and nitrite. The daily nutrient supply to the water column was 8.7 ± 4.6 mg-N m−2 day−1 for dissolved inorganic nitrogen (DIN), 2.5 ± 3.0 mg-P m−2 day−1 for DIP and 65.1 ± 36.7 mg-Si m−2 day−1 for DSi. DIN and DIP fluxes from sediments were compared to those of terrestrial loadings into Harima Nada, which were estimated to be comparable to or about two times higher than river loadings for DIN and more than ten times higher than terrestrial loading for DIP. Overall, the results of this study provide important basic information on nutrient dynamics from sediments and their management in Harima Nada.

This study examines microplastic (MP, 1-5 mm) densities in convergence zones in a coastal sea, the Seto Inland Sea, comparing them to those of non-convergence zones and other areas. Notably, Seto convergence zones exhibit MP densities 40 to 300 times higher than non-convergence zones, with an extraordinary density of 3.7 ± 6.3 pieces m-3, similar to densities found in Tokyo Bay as known a MP hotspot. The predominant polymer found was expanded polystyrene, varying seasonally and peaking in summer. Juvenile fish associated with driftweed in these convergence zones face a risk of long-term MP exposure, potentially up to four months. This large number of MPs found in coastal convergence zones is similar to accumulation zones formed in the gyres of open oceans, with strong implications for detrimental effects on coastal marine life. However, these MPs are autochthonous, and may be manageable through local marine plastic waste management.

Feeds based on small (trash) fish contain much persistent phosphorus (P), such as Ca bound (Ca-P) and refractory P (Ref-P), which accumulates in high concentrations in the bottom sediments immediately below fish farms using this type of feed. This study designated three sampling locations based on their history concerning yellowtail fish farming (current, formerly or never) to investigate the sediment vertical distribution of phosphorus fractions and to test for detection of P impact on the bottom sediment. The aquaculture-free site was relatively low in Ca-P and Ref-P compared with continuous and previous aquaculture sites. The latter had high Ca-P and Ref-P content only in the depth range 17–29 cm, coinciding with the period during which aquaculture was conducted (1960–1998). The techniques used are expected to be useful to indicate bottom condition improvements and as a tracer of the effects of previous aquaculture farming.

The horizontal distributions of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) in the surface sediments where complex aquaculture is conducted in Shido Bay, Seto Inland Sea, Japan, were determined to investigate the impact of fish and oyster farming. We also determined five fractions of phosphorus in the surface sediments. Relatively high phosphorus contents (P/C ratios) were observed in fish farming areas, likely due to the waste feed and fecal matter. Moreover, relatively high abundances of the Ca-bound P (Ca-P) and detrital P (Ref-P) fractions were observed in fish farming areas, likely due to bones of small fish used as feed. Our results strongly suggest that the P/C ratio and the Ca-P and Ref-P fractions in the surface sediments are good indicators of the effects of fish farming and allow its characterization, and are particularly valuable in the area of complex aquaculture.

The coastal ocean is an important component of the global carbon budget and contains both terrestrial and internally produced organic matter. The use of specific organic compounds to estimate the origin of the carbon can improve understanding of organic material cycling in coastal ecosystems and the origins of food resources consumed by suspension feeders. In this study, samples of suspended particulate matter (SPM) in surface waters were collected in summer from the Harima-Nada, Seto Inland Sea, and spatial variations in the distributions of organic compounds were investigated and compared with that of chlorophyll as an indicator of primary production. High chlorophyll a (Chl a) concentrations were associated with riverine nutrient inputs in the northern Harima-Nada and with a tidal front in the south. Sterols, fatty acids, phytol, and lignin phenols were identified in the SPM samples. Short-chain fatty acids were dominant, with minor amounts of lignin phenols; this result indicates that the major carbon source of the SPM was aquatic organisms, especially microalgae. This interpretation is consistent with the bulk geochemical tracer results. Phytol showed a strong positive linear correlation with Chl a. The major sterols in the SPM samples were 4-methylsterols and 24-methylenecholesterol, which are derived mainly from dinoflagellates and diatoms, respectively. The strong correlation between phytosterol contents and Chl a indicates that total phytosterols reflect the total microalgal community abundance. The relative proportions of 4-methylsterols and other sterols were used to estimate the contributions of dinoflagellates and diatoms to Chl a: 34% of the total mean Chl a concentration was estimated to be derived from dinoflagellates and 66% from diatoms and other phytoplankton. This result is consistent with the reported occurrence of a red tide due to dinoflagellates on the day of observation.

The use of stable isotopes of carbon (δ13C) and nitrogen (δ15N) from feces and breath offers potential as non-destructive tools to assess diets and nutrition. How stable isotope values derived from breath and feces compare with those from commonly used tissues, such as blood fractions and liver, remains uncertain, including understanding the metabolic routing of dietary nutrients. Here, we measured δ13C and δ15N from feces and δ13C of breath from captive Red-necked Stints (Calidris ruficollis) and 26 species of wild-caught migratory shorebirds (n = 259 individuals) and compared them against isotopic values from blood and feathers. For captive birds fed either cereal- or fish-based diets, differences in δ13C between feces and lipid-free diet were small, − 0.2 ± 0.5‰ and 0.1 ± 0.3‰, respectively, and differences in δ15N, − 0.7 ± 0.5‰ and − 0.5 ± 0.5‰, respectively. Hence, δ13C and δ15N values from feces can serve as proxies for ingested proteinaceous tissues and non-soluble carbohydrates because isotopic discrimination can be considered negligible. Stable isotope values in plasma and feces were strongly correlated in wild-caught shorebirds, indicating feces can be used to infer assimilated macronutrients. Breath δ13C was 1.6 ± 0.8‰ to 5.6 ± 1.2‰ lower than bulk food sources, and breath C derived from lipids was estimated at 47.5% (cereal) to 96.1% (fish), likely underlining the importance of dietary lipids for metabolism. The findings validate the use of stable isotope values of feces and breath in isotopic assays to better understand the dietary needs of shorebirds.

This study presents vertical profiles of biophilic elements in core sediments from the Seto Inland Sea, Japan. We determined total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), biogenic silica (BSi), and phaeopigments in sediments from 80-cm-long cores collected from Hiuchi-Nada (Site A) and Harima-Nada (Site B). The vertical profiles are generally similar: all concentrations decrease downward from the top layer and show maximum values in the top layers for each parameter at both study areas. Among these biophilic elements, the correlation between BSi and TOC is particularly high, suggesting that diatoms are the main source of TOC in the sediments. The degradation rates at Site A in increasing order, as estimated by a steady-state diagenetic model (one-G model), are TOC ≈ BSi < TP < TN ≈ phaeopigments. However, these rates differ substantially from those at Site B, suggesting that the degradation rates might differ among sedimentation processes and environments. Using the one-G model and molecular composition (C:N) ratios, we estimate the origin (terrestrial and marine) for the amount of carbon before decomposition and followed its variation over the past approximately 200 years. We found that marine carbon is a major component and tends to be higher during the eutrophication period (1960–1990) at Site B, although not so much at Site A. At Site B the marine carbon content increases by about 25% during the eutrophication period, but at Site A no remarkable increase can be confirmed. These results are consistent with previous studies showing that the trend in primary production in the Seto Inland Sea since the eutrophication period has not been regionally homogeneous.

Fish farming in coastal waters generates large amounts of particulate organic wastes, such as waste feed (unconsumed feed) and fecal matter. We collected sediment core samples in a small bay where fish farming has been actively conducted and determined the vertical profiles of total organic carbon (TOC), total nitrogen (TN) and phosphorous (TP). Fish farm TOC and TN sediment contents showed similar profiles with those of a non-<^>sh farming area (reference area), but TP content was higher. It is thought that the TP content in fish feed is high due to the bones of small fish; is hard to be assimilated by cultured fish; and is hard to be decomposed in the sediment. Our results suggest that the environmental impacts of fish farming can be evaluated from the TP content in core samples, as an indicator of organic waste produced by fish farming.

AbstractThe dynamics of phytoplankton biomass in the vertically mixed south-eastern part of the Bisan Strait, in the Seto Inland Sea of Japan, may be considered to be strongly dependent not only on nutrients but also on light status. This was investigated by examining variations in chlorophyll-a (Chl-a), nutrients, and Secchi-disc depth through high-frequency sampling (a mean of once every 1.7 days) at the same station from April to October 2019. Precipitation during the Japanese rainy season (East Asian monsoon rains) was associated with a decrease in salinity from 32 to 31 in late July. The highest concentration of dissolved inorganic nitrogen (DIN), the most deficient nutrient, also was recorded in late July in association with seasonal precipitation. However, the measured Chl-a peak (max. 4 μg l−1) in early August was not as high as expected, possibly due to low water clarity. A relatively small but substantial peak of DIN (max. 4 μM) was recorded in mid-August, which coincided with the passing of a typhoon. The small peak of DIN coupled with higher water clarity thereafter was followed by a phytoplankton bloom from mid-August to early September, at which the highest Chl-a (7 μg l−1) occurred. It is suggested that increased light penetration enhanced the efficiency of nutrient assimilation and thereby triggered the late-summer phytoplankton bloom. In contrast to the adjacent stratified areas, light rather than nutrient status appears to be the key determinant for the onset of phytoplankton blooms in the Bisan Strait.

2. The outbreak of phytoplankton causing shellˆsh poisoning induced by the Tsunami YUTAKA OKUMURA,1a SETSUKO SAKAMOTO,2 YOSHIO MASUDA,3 TORU TANABE,3 SHINNOSUKE KAGA,4 SHIHO WATANABE,4 RINAKO NAKANO,5 TORU HIRAWAKE,5b KAZUHIKO ICHIMI,6 KENJI KANEKO7 AND MOTOYUKI HARA8 1Fisheries Technology Institute, National Research and Development Agency, Japan Fisheries Research and Education Agency, Shiogama, Miyagi 9850001, 2Fisheries Technology Institute, National Research and Development Agency, Japan Fisheries Research and Education Agency, Hatsukaichi, Hiroshima 7390452, 3Miyagi Prefecture Fisheries Technology Institute, Ishinomaki, Miyagi 9862135, 4Iwate Fisheries Technology Center, Kamaishi, Iwate 026 0001, 5Graduate School/Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 0418611, 6Seto Inland Sea Regional Research Center, Kagawa University, Kida, Kagawa, 7610795, 7Faculty of Life Science and Biotechnology, Fukuyama University, Onomichi, Hiroshima, 7222101, 8Graduate School of Agricultural Science, Tohoku University, Sendai, Miyagi 9800845, Japan

Drifting seaweeds are utilized by some species of juvenile fishes. We collected drifting seaweeds and associated fishes off Kagawa Prefecture, Seto Inland Sea, monthly from June 2017 to June 2019. A total of 206 clumps of drifting seaweeds, comprised of 15 species, were collected. Average clump weight in each month ranged from 0.05 to 5.97 kg/clump, increasing from winter, reaching a peak in spring, and decreasing in summer and autumn. Sargassum horneri, S. muticum and S. fulvellum dominated in winter and spring, and Zostera marina did in summer and autumn. A total of 6,053 individual fish, belonging to 24 species, were collected. The density (number of individuals per kg of drifting seaweed) ranged from 0.1 to 209.0 and generally was low in winter and high in autumn. The numerically dominant species were Rudarius ercodes (54.7%), Sebastes cheni (21.7%), S. ventricosus (11.0%), Petroscirtes breviceps (4.4%) and S. schlegelii (2.8%). Rudarius ercodes was predominant between summer and autumn. Sebastes cheni and S. ventricosus were predominant species in spring, but the peak of their appearance in number was two months earlier in S. cheni than in S. ventricosus.

We examined phytoplankton productivity in Shido Bay, an active oyster farming area in the eastern Seto Inland Sea, Japan, by an in situ 13C method from March 2016 to January 2017. The depth-integrated daily phytoplankton production ranged from 0.13 to 1.61 g C m−2 day−1, and the estimated annual production was 218 g C m−2 year−1. The daily production peaked in the rainy season and autumn, when phytoplankton blooms were observed, and production during the bloom events largely contributed to the annual production. There was a strong correlation between daily production and phytoplankton biomass (chlorophyll a) throughout the study period. In the study area, due to the shallow depth (6 m) and moderate light attenuation coefficient of the water column (0.38 ± 0.07 m−1), the mean light intensity in the water column was maintained at high levels (36 ± 5% of incident photosynthetically active radiation) throughout the study period. In contrast, the dissolved inorganic nitrogen concentration of the water was persistently low (< 1.5 µM) from late winter to summer. As a consequence, an  increase in nitrogen supply seemed to be the key factor causing the increases in daily production and directly triggering the phytoplankton blooms in the bay.