Nitrogen fluxes across the sediment-water interface and nitrogen removal from sediments are essential components of nitrogen cycle in semi-enclosed inland seas. However, the difficulty in observational sampling hinders continuous data availability that is necessary to understand their seasonal variations and underlying mechanisms. To address this issue, we developed a one-dimensional sediment nitrogen-cycle model and used sensitivity experiments to quantify the relative roles of environmental and biogeochemical drivers. Model results indicate that 48 % of particulate organic nitrogen (PON) settling into sediments is returned to the bottom water as dissolved inorganic nitrogen (DIN), while 6 % is removed via N-loss flux (dinitrogen gas and nitrous oxide). The seasonal variations of PON and DIN fluxes are controlled by fundamentally different mechanisms, with PON flux primarily regulated by bottom-water PON concentration and bottom stress, while DIN flux is mainly governed by temperature-dependent biogeochemical transformations and nitrogen availability. These contrasting responses reveal a decoupling between particulate and dissolved nitrogen fluxes, reflecting the buffering capacity of sediments. Denitrification controls nitrogen removal but its amount is not large because the oligotrophic conditions of the study site limits the nitrate availability.

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.

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.

In recent years, algae cultivation in the Seto Inland Sea has suffered damage due to low nutrient concentrations, and so there is a need to develop a technology for applying fertilizer with less environmental impact. In this study, a technique was developed to efficiently supply nutrients by filling cylinders with small holes (Kagawa-style fertilizer cylinder) with fertilizer and gradually releasing the dissolved fertilizer solution inside the Pyropia thalli growing-nets, where the flow velocity is slower than that outside. The vigor and color of Pyropia thalli were improved by this new method and the quality of the product was well recognized by the market.

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.

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.

We investigated the occurrences of Noctiluca scintillans in the coastal waters of Harima-Nada, the Seto Inland Sea, using the red tide data set. These data concerned red tide occurrences that were reported in an annual report entitled “ Red tides in the Seto Inland Sea ” which was published by the Seto Inland Sea Fisheries Coordination Office of the Fisheries Agency, Ministry of Agriculture, Forestry and Fisheries of Japan. These data were imported into a geographic information system, and red tide areas were calculated. During the 26 years from 1979 to 2004, in the Seto Inland Sea, eutrophication improved, nutrient concentrations in seawater decreased dramatically, and the number of red tide occurrences excluding those concerning Noctiluca scintillans , decreased. However, the number of occurrences, area, and durations of Noctiluca scintillans red tides did not decrease. The ratio of Noctiluca scintillans red tides to total red tides for an average red tide index, which permits the assessment of spatial and temporal scales, was 54%. This was higher than that of the number of occurrences ( 36% ) . This indicates the importance of N. scintillans as a red tide organism.

The discoloration of Pyropia thalli resulting from a shortage of nutrients is a serious problem for the Pyropia industry and has often occurred during cultivation in the Seto Inland Sea, Japan. Therefore, a new method was developed to fertilize areas of Pyropia cultivation. Pyropia thalli growing-nets placed in a calm area were enclosed by a fence, using a Kagawa-style "nori skirt" made from polypropylene sheets and cultivation nets. Nutrients were supplied to the area enclosed by the fence. The concentrations of nutrients in the enclosed area were kept higher than those in the open area (which acted as a control). The color of Pyropia thalli was improved by this new method and the quality of the product was well recognized at market. The salinity in the area enclosed by the fence was lower following rainfall, suggesting that nutrients were effectively supplied to the cultivated thalli not only with fertilizer but also rainwater, because rainwater contains a high concentration of dissolved inorganic nitrogen.

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.

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.

Noctiluca scintillans is an important member of the mesozooplankton in terms of biomass and production in the Seto Inland Sea, Japan. The densities and adenosine triphosphate (ATP) contents of N. scintillans cells were measured. Vertical profiles of N. scintillans cellular activity in the coastal water were determined and the ATP contents were high at middle layers, with a maximum depth of 10 m. ATP contents were low in the surface and lower layers. These results suggest that active N. scintillans cells in subsurface layers with low density play an important role in the coastal ecosystem, and high-density cells in the surface water are not active. (C) 2020 Institute of Oceanology of the Polish Academy of Sciences. Production and hosting by Elsevier B.V.