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Haptophytes are one of the most ecologically successful phytoplankton groups in the modern ocean and tend to maintain balanced and stable communities across various environments. However, little is known about the mechanisms that enable community stability and ecological success. To reveal the community characteristics and interactions among haptophytes, we conducted comprehensive observations from the upstream to downstream regions of the Kuroshio Current. Haptophyte abundance and taxonomy were assessed using quantitative polymerase chain reaction and metabarcoding of 18S rRNA sequences, respectively. The haptophyte community structure changed abruptly at sites on the shelf-slope of the East China Sea, indicating the strong influence of shelf waters with high phytoplankton biomass on downstream communities. Correlation network analysis combined with the phylogeny suggested that haptophytes can coexist with their close relatives, possibly owing to their nutritional flexibility, thereby escaping from resource competition. Consistently, some noncalcifying haptophyte genera with high mixotrophic capacities such as Chrysochromulina constituted a major component of the co-occurrence network, whereas coccolithophores such as Emiliania/Gephyrocapsa were rarely observed. Our study findings suggest that noncalcifying haptophytes play crucial roles in community diversity and stability, and in sustaining the food web structure in the Kuroshio ecosystems.

In the seasonally hypoxic semi-enclosed Omura Bay, Japan, regression equations estimating year-round nutrient [NO3 + NO2, NH4, PO4, Si(OH)4] concentrations in pore water were successfully constructed based on water temperature and the concentration of biogenic elements in surface sediments, except for NH4 and PO4 concentrations in summer when the hypoxic water mass develops. Core incubation studies revealed that pore water NH4 and PO4 concentrations under anaerobic conditions increased 1.4–2.2 and 1.8–2.1 times, respectively, above levels estimated with the regression equations. These were in good agreement with the values observed in the field during summer. The gradient in nutrient concentrations between the measured values in the overlying water column and the estimated values in the pore water revealed that spatial fluctuations in nutrient fluxes at the sediment–water interface across the Omura Bay ranged from 1.5 to 2 times, reaching up to 3–4 times in the summer. Annual nutrient fluxes from the sediment (690–780 ton-N year−1 for DIN, 77–85 ton-P year−1 for DIP, and c.a. 4700 ton-Si year−1 for DSi) were comparable to those from terrestrial areas. Accurate estimation of internal loadings is essential to properly evaluate residual nutrients and organic matter in the water column and their effect on aquatic ecosystems.

Hypoxic water masses are usually formed in the bottom layer of stratified water columns. However, we found that hypoxia was formed throughout the water column in a well-mixed macrotidal estuary, the Rokkaku River Estuary, Japan. There was a spring-neap variation in the dissolved oxygen (DO) concentration where hypoxia developed during spring tides. Minimum DO concentration was observed 1 or a few days after the spring tide. When hypoxia developed, all the water column from bottom to surface became hypoxic ( 2 mg l−1 day−1). Potential oxygen consumption by microorganisms was positively correlated with turbidity. Concentration of suspended solid (SS) at the ETM exceeded 1000 mg l−1. Furthermore, particulate organic carbon (POC) concentration in the estuary increased in spring tides. C/N ratio and δ13C of organic matter suggested that POC was transported from the sea into the estuary. These results suggest that the active resuspension of organic matter caused by the strong tidal current during spring tides enhanced the respiratory activity of microorganisms and the main source of the organic matter was detrital material from the sea. Such a large amount of organic matter should fuel microbial respiration effectively. Since microbial oxygen consumption was larger than the oxygen supply from the water surface even in the well mixed condition, a hypoxic water mass was formed in this estuary.

East Asian oceans are possibly affected by a high nitrogen (N) burden because of the intense anthropogenic emissions in this region. Based on high-resolution regional chemical transport modeling with horizontal grid scales of 36 and 12 km, we investigated the N burden into East Asian oceans via atmospheric deposition in 2010. We found a high N burden of 2–9 kg N ha-1 yr-1 over the Yellow Sea, East China Sea (ECS), and Sea of Japan. Emissions over East Asia were dominated by ammonia (NH3) over land and nitrogen oxides (NOx) over oceans, and N deposition was dominated by reduced N over most land and open ocean, whereas it was dominated by oxidized N over marginal seas and desert areas. The verified numerical modeling identified that the following processes were quantitatively important over East Asian oceans: the dry deposition of nitric acid (HNO3), NH3, and coarse-mode (aerodynamic diameter greater than 2.5 μm) NO3-, and wet deposition of fine-mode (aerodynamic diameter less than 2.5 μm) NO3- and NH4+. The relative importance of the dry deposition of coarse-mode NO3- was higher over open ocean. The estimated N deposition to the whole ECS was 390 Gg N yr-1; this is comparable to the discharge from the Yangtze River to the ECS, indicating the significant contribution of atmospheric deposition. Based on the high-resolution modeling over the ECS, a tendency of high deposition in the western ECS and low deposition in the eastern ECS was found, and a variety of deposition processes were estimated. The dry deposition of coarse-mode NO3- and wet deposition of fine-mode NH4+ were the main factors, and the wet deposition of fine-mode NO3- over the northeastern ECS and wet deposition of coarse-mode NO3- over the southeastern ECS were also found to be significant processes determining N deposition over the ECS.

We herein report on the dynamics of a sulfate-reducing bacteria (SRB) community structure in the surface sediment of a seasonally hypoxic enclosed bay for two consecutive years (2012 and 2013). The uppermost (0-5 mm) and subsurface (5-10 mm) layers of sediment were examined with a terminal-restriction fragment length polymorphism (T-RFLP) analysis based on the dissimilatory sulfite reductase (dsrA) gene. The SRB community significantly differed between the two sediment layers over the sampling period. This difference was mainly attributed to operational taxonomic units (OTUs) that were unique to either of the sediment layers. However, nearly 70% of total OTUs were shared between the two layers, with a few predominating. Therefore, no significant shift was observed in the SRB community structure under varying dissolved oxygen (DO) conditions in bottom water overlying the sediment surface. An additional analysis of 16S rRNA gene amplicon sequences, conducted for three uppermost sediment samples (July, August, and September in 2012), revealed that Desulfococcus, a member of SRB with high tolerance to oxygen, was the predominant Deltaproteobacteria across the uppermost sediment samples. Based on the predominance of shared OTUs across the SRB community in the sediment (0-10 mm) regardless of bottom-water DO, some SRB that are physiologically tolerant of a wide range of DO conditions may have dominated and masked changes in responsive SRB to DO concentrations. These results suggest that the SRB community structure in the enclosed bay became stable under repeated cycles of seasonal hypoxia, but may be compromised if the severity of hypoxia increases in the future.

To clarify the factors that affect spatiotemporal isotopic variations in fishes, we examined stable carbon and nitrogen isotope ratios (δ13C and δ15N) of Japanese jack mackerel Trachurus japonicus larvae and juveniles sampled in the shelf break region of the East China Sea (ECS) in April during six consecutive surveys (2005–2010). The δ13C values in fish tissues were similar between northern and southern ECS, while δ15N values were higher in the southern ECS, probably because of lower nitrogen fixation and the presence of fish with larger body size occupying a higher trophic level. Interannual variations in δ15N of T. japonicus juveniles might be mainly contributed by the variation in the abundance of the main diet such as Oncaeidae copepodites and Corycaeus. The isotopic overlaps between epipelagic T. japonicus larvae and other dominant mesopelagic fish larvae were negligible, suggesting lesser competition for food resources. Knowledge regarding spatiotemporal variation in stable isotope ratios of T. japonicus can be applied to understand the feeding habits and migrations of fishes in the ECS.

The field diet of meroplanktonic decapod crustacean larvae is poorly known, despite standard use of microzooplankton as food in laboratory culture. Using callianassid shrimp Nihonotrypaea harmandi larvae collected from a 65 m deep inner-shelf location off mid-western Kyushu, Japan, between June and August 2012 and 2013 and mass-reared in the laboratory, a phytoplankton-based diet through larval development (zoeae I?VI to decapodid) was demonstrated. When the pure-cultured diatom Chaetoceros gracilis was fed to zoeae, survival rate to decapodids was 3.4 to 3.9% in 26 to 40 d at 22°C, which was comparable to previous rearing results for zoeae fed microzooplankton. Trophic enrichment factors (TEFs) from stable isotope (SI) analysis of zoeal whole-body tissue in the laboratory were 2.0‰ for δ13C and 1.9‰ for δ15N. In the field water column, diatoms dominated the nano- to micro-sized plankton, accounting for 38 to 81% of the biovolume, followed by heterotrophic protists. The trophic position (TP) estimated from amino acid-specific δ15N values for the field-collected zoeae VI was 2.1 (TPGlu/Phe) or 2.7 (TPAla/Phe), suggesting that those zoeae fed on mixtures of phytoplankton and heterotrophs including protists. Bulk SI analyses were performed for particulate organic matter (POM; proxy for phytoplankton), microzooplankton (mainly calanoid copepods), and shrimp zoeae to elucidate the diet of larvae in the water column. A shift in SI from fresh to degraded POM was determined through the incubation of field-collected POM. Based on this shift during degradation and larval TEFs, phytoplankton and their sinking detritus with heterotrophic protists were estimated to be the principal diet for those larvae residing mostly below the chlorophyll maximum layer., Marine Ecology Progress Series, 592, pp.141-158; 2018

Bulk carbon and nitrogen stable isotope (SI) ratios (δ13C and δ15N) were analysed to investigate the feeding habits of six taxa of mesopelagic fish larvae inhabiting the Kuroshio waters of the northeastern East China Sea. Large variation in tissue SI during early larval periods suggested maternal effects from parent fishes, and non-selective feeding on a variety of plankton species due to poor swimming ability. The similarity between SI ratios measured in larval tissues and those estimated for eggs of an “income breeder” in the spawning area support an “income breeder” strategy in Diaphus slender type and Vinciguerria nimbaria, while Lipolagus ochotensis seemed to show “capital breeder”-like characteristics. SI ratios of the fish larvae studied became relatively constant at species-specific body dry-weights (0.5–1.0 mg), probably due to the commencement of selective feeding, meaning SI ratios during late larval periods could be used for trophic position analysis. There was great overlap (44.6–76.5%) in trophic niche among the larval fishes within the same taxonomic family of Myctophidae. Even if principal diet components cannot be identified with gut contents analyses, diet information from other fish species occupying a similar isotopic niche can thus improve our understanding of the diets of larval fishes.

Long term monitoring of atmospheric wet and dry depositions and associated nutrients fluxes was conducted on the coast of Japan facing the East China Sea continuously for 1 year and 2 months, with the origin of air mass investigated based on isotope analyses (Sr, Nd, and NO3). During the same period, intensive observations of ocean conditions and the chemical composition of sinking particles collected using sediment traps were conducted to investigate the effects of atmospheric deposition-derived nutrients on phytoplankton blooms. Dry-deposition-derived nutrient inputs to the surface ocean were larger during autumn to spring than in summer due to the effect of continental air mass occasionally carrying Asian dust (yellow sand). However, these nutrients fluxes were limited (1.1–1.5 mg-N m−2 day−1 on average) and didn't appear to cause phytoplankton blooms through the year. Although average dissolved inorganic nitrogen (DIN) concentrations in rainwater were lower in oceanic air masses compared to continental air masses, wet-deposition-derived nutrient inputs to the surface ocean on rainy days during the summer (26.0 mg-N m−2 day−1 on average) were large due to higher precipitation from oceanic air masses. Wet-deposition-derived nutrients significantly increased nutrient concentrations in the surface ocean and seemed to cause phytoplankton blooms in the warm rainy season when nutrients in the surface were depleted due to increased stratification. The increase in phytoplankton biomass was reflected in increased particle sinking into the bottom layer, as well as changing chemical characteristics. The supply of flesh phytoplankton-derived labile organic matter into the bottom layer could be expected to promote rapid bacterial decomposition and contribute to the formation of hypoxic water masses in early summer when the ocean was strongly stratified. Atmospheric deposition-derived nutrients in East Asia will have important impacts on not only the oligotrophic outer ocean but also surrounding coastal areas in the warm rainy season.

Acute and severe hypoxia associated with harmful algal bloom has become one of the major causes for the environmental deterioration of coastal areas. Although it is generally thought that a large part of the dissolved oxygen consumption at a bloom site is initiated by heterotrophic bacteria that attack organic matter derived from dead or dying algal cells, precise microbial processes leading to the hypoxia are yet to be examined. Here we show temporal dynamics of extracellular dissolved organic carbon (DOC) of the red tide forming raphidophyte Chattonella marina and bacterial populations associating with the algae under laboratory conditions. During the growth of non-axenic strains of C. marina, we monitored abundance of algae, associated bacteria, and DOC in the culture media. Bacterial cell abundance increased in response to the increase in DOC both at the beginning and the late log phase of the algal growth. Flow cytometric analysis revealed that transient increase in the percentage of respiratory-active bacterial cells also coincided with the timing of the increase in bacterial abundance and DOC. These results strongly suggest that DOC released from growing C. marina fuels respiration and growth of planktonic bacteria surrounding the algae. This has implications for the role of DOC released from C. marina bloom before the collapse in mediating interactions between neighboring algae and bacterial assemblage which may eventually lead to algal bloom-associated hypoxia.

Macroalgal beds have drawn attention as one of the vegetated coastal ecosystems that act as atmospheric CO2 sinks. Although macroalgal metabolism as well as inorganic and organic carbon flows are important pathways for CO2 sequestration by macroalgal beds, the relationships between macroalgal metabolism and associated carbon flows are still poorly understood. In the present study, we investigated carbon flows, including air–water CO2 exchange and budgets of dissolved inorganic carbon, total alkalinity, and dissolved organic carbon (DOC) in a temperate macroalgal bed during productive months of the year. To assess the key mechanisms of CO2 sequestration by the macroalgal bed, we estimated macroalgal metabolism and lateral carbon flows using a field-bag method, a degradation experiment, and mass balance modelling over a diurnal cycle. Our results showed that macroalgal metabolism and lateral carbon flows driven by water exchange affected air–water CO2 exchange in the macroalgal bed and the surrounding waters. Macroalgal metabolism caused overlying waters to contain low concentrations of CO2 and high concentrations of DOC that were efficiently exported offshore from the macroalgal bed. The exported water lowered CO2 concentrations in the offsite surface water and enhanced atmospheric CO2 uptake. Our findings suggest that macroalgal beds in habitats associated with high water exchange rates can create extensive CO2-sinks around them.

The bacterial community strongly drives carbon and other biogeochemical cycles in marine sediment. However, little is known about the impact of dissolved oxygen (DO) availability on bacterial community composition. To fill this gap, we examined diversity, richness and structure of the bacterial population for three consecutive years (2011-2013) in the uppermost (0-5 and 0-7 mm depth) and the subsurface layers (5-10 and 7-14 mm depth) of Omura Bay, Kyushu, Japan, a seasonally hypoxic enclosed bay. Automated ribosomal intergenic spacer analysis revealed a unimodal pattern of diversity indices with DO, peaking at the suboxic (11 μM O2) condition. Shifts in the bacterial communities were also evident in response to the availability of DO. Changes in the operational taxonomic units (OTUs) that were less abundant accounted for a large part of the community dissimilarity. It was further demonstrated that the relative abundance of OTUs affiliated with Gammaproteobacteria was correlated positively with DO, while that with Deltaproteobacteria was inversely correlated with DO. These results strongly suggest that DO availability of bottom water plays a fundamental role in shaping the bacterial community in sediment surfaces of shallow coastal areas.

Phosphorus (P) supplies from the sediment are important when evaluating the impacts of nutrient dynamics on the ecosystem. Many studies have reported P dynamics based on gradients of pore water (PW)-phosphate (Formula presented) concentrations at the sediment?water interface and iron-related abiotic reactions in response to redox conditions. However, the aim of the present study is to evaluate the contributions of intracellular (IC)-P and marine organism-mediated reactions to P-dynamics under variable environmental conditions. The focus of this study is the intertidal mudflat habitat of the innermost part of Ariake Bay, Kyushu, Japan, during winter. The vertical distribution of several forms of P fraction [i.e. PW- and IC-dissolved inorganic/organic-P (DIP/DOP), and particulate inorganic/organic-P (PIP/POP)], the number of microalgae and bacteria, and chlorophyll a concentrations in the sediment were measured. The results obtained revealed that the IC-P pool, especially IC-DOP, accounted for 76% of the total dissolved P in the surface sediment layer, mainly associated with the microphytobenthos. Additionally, phytoplankton, which comprised more than 60% of the surface microalgae in tidal-flat sediments, are considered to carry river-derived-P to the intertidal flats via inner bay-specific physical movements and disturbances. Incubation experiments on mud sediment under oxic/anoxic conditions suggest that dissolved P fluxes between the IC pool and PW occur actively in response to redox conditions, according to the balance between microalgal uptake and bacterial degradation. These results indicate that evaluation of the IC-P pool is indispensable to understanding P cycling in intertidal mudflats, and that microalgae could play important roles not only as food sources, but also as P reservoirs and sources of (Formula presented)., Plankton and Benthos Research, 10(1), pp.1-10; 2015

To investigate the spatial variation in bivalve food sources along a pollution gradient and assess bivalve contribution to biogeochemical cycles in tropical coastal ecosystems, the δ13C and δ15N values of bivalves and their potential food sources were studied in northwestern Philippines. In a semi-enclosed embayment affected by river discharge and mariculture activities, bivalves depended primarily on 13C-depleted suspended particulate organic matter such as phytoplankton and/or fish feeds. However, toward the relatively oligotrophic seagrass beds, the bivalve food source gradually shifted to more 13C-enriched resuspended and/or settled particles. Furthermore, in the outer seagrass beds exposed to the open ocean, bivalves mainly relied on similar food sources, such as detritus of microalgae, regardless of the distance from the embayment. These trends appear to reflect the ready availability of the food sources. Especially in the outer seagrass beds, a semi-closed material cycle within the vicinity of the sea bottom likely emerged between bivalves and algae, but not between the phytoplankton in the overlying water column. This resulted in a relatively weak benthic-pelagic coupling for bivalves. These cycles would need to be taken into account when estimating the biogeochemical cycles in eutrophicated coastal areas.

Low-lying, small reef islands on Pacific atolls are vulnerable to coastal erosion and flooding, mainly due to rises in sea level, as well as local stresses associated with overpopulation. Because reef island sediments on Pacific atolls are composed mostly of shells of large benthic foraminifers (LBFs), their future stability depends on understanding foraminiferal shell sources and their production rates around the islands. Here, we report on the distribution and population density of LBFs around Fongafale Island, the main populated island of Tuvalu in the South Pacific, where coastal erosion and inundation have occurred. We discuss their controlling factors, in particular, anthropogenic influences on foraminiferal distribution. The results based on multivariate analyses (canonical correlation analysis and multiple regression analysis) demonstrated that Baculogypsina sphaerulata and Amphistegina lobifera were more common on the high-energy, intertidal ocean reef flats (≤ 10 6 and ≤ 10 4 individuals m − 2 , respectively), and B. sphaerulata was less common around the populated area of the island. On the relatively low-energy subtidal lagoon reef flats, Amphistegina lessonii was the most common species, particularly in the dry (leeward) season, with an increasing density in deeper offshore zones (≤ 10 4 individuals m − 2 ). Sorites orbiculus was less common than the other species, but was found on both sides of the reef flats (≤ 10 3 individuals m − 2 ). Macroalgal δ 15 N values indicated that upwelling nutrient-rich deep water could be the source of nitrogen for oceanic waters, whereas nutrients in lagoon seawater could be partly derived from groundwater mixing with domestic wastewater, especially near the populated area. These results suggest that the distribution and population density of LBFs in the study area are influenced by spatial differences in water energy and water quality, and that B. sphaerulata is less tolerant of nutrient loading than other species. Comparison of distributions between living individuals and empty shells indicated that ocean reef flats are the primary source of foraminiferal shells for reef islands and lagoon beaches.

The northern East China Sea (ECS) serves as a spawning and nursery ground for many species of fish and squid. To clarify the basis of the food web in the northern ECS, we examined the nitrate (NO3) dynamics along four latitudinal transects based on stable nitrogen and oxygen isotopes of NO 3 (δ15NNO3 and δ18ONO3) and temperature-salinity dynamics in both winter (February 2009) and summer (July 2009 and July 2011). The δ15NNO3 and δ18ONO3, which were distinctly different among the potential NO3 sources, were useful for clarifying NO3 sources and its actual usage by phytoplankton. In winter, Kuroshio Subsurface Water (KSSW) and the Yellow Sea Mixed Water (YSMW) predominantly contributed to NO3 distributed in the shelf water. In the surface water of the Okinawa Trough, NO3 from the KSSW, along with a temperature increase caused by an intrusion of Kuroshio Surface Water (KSW), seemed to stimulate phytoplankton growth. In summer, Changjiang Diluted Water (CDW), Yellow Sea Cold Water Mass (YSCWM), and KSSW affected the distribution and abundance of NO 3 in the northern ECS, depending on precipitation in the Changjiang drainage basin and the development of the YSCWM in the shelf bottom water. Although isotopic fractionation during NO3 uptake by phytoplankton seemed to drastically increase δ15NNO3 and δ18ONO3 in summer, relatively light nitrate with δ15NNO3 lower than expected from this fractionation effect might be explained by contribution of atmospheric nitrogen and/or nitrification to NO3 dynamics in the surface and subsurface layers. If the latter were a dominant process, this would imply a tightly coupled nitrogen cycle in the shelf water of the northern ECS., Biogeosciences, 11(4), pp.1297-1317; 2014

The source of anthropogenic nutrient and its spatial extent in three fringing reefs with differing human population gradients in Kenya were investigated using stable isotope approaches. Nutrient concentrations and nitrate-δ15N in seepage water indicated that population density and tourism contributed greatly to the extent of nutrient loading to adjacent reefs. Although water-column nutrient analyses did not show any significant difference among the reefs, higher δ15N and N contents in macrophytes showed terrestrial nutrients affected primary producers in onshore areas in Nyali and Bamburi reefs, but were mitigated by offshore water intrusion especially at Nyali. On the offshore reef flat, where the same species of macroalgae were not available, complementary use of δ15N in sedimentary organic matter suggested inputs of nutrients originated from the urban city of Mombasa. If population increases in the future, nutrient conditions in the shallower reef, Vipingo, may be dramatically degraded due to lower water exchange ratio.

In this study, we used 454-pyrosequencing to report the highly diverse diazotroph communities in the Kuroshio and its adjacent waters along a transect across the Tokara Strait, Japan. Terrestrial input from the islands resulted in a highly heterogeneous diazotroph community within a relatively small geographic region, which was presumably caused by the remarkably different responses of UCYN-A2, UCYN-C and Trichodesmium to the steep environmental gradient. On the other hand, most major cyanobacterial OTUs found in this study were also detected in an unpublished dataset from the upstream Kuroshio, which suggests transportation of diazotrophs by the Kuroshio in large geographic scale. A significant amount of UCYN-C was found in the Kuroshio and offshore stations, suggesting the importance of this potentially overlooked group in the western North Pacific Ocean (WNPO). Moreover, a novel sublineage of UCYN-B was defined, which was predominant in an oligotrophic water sample; and it was also found to be widely distributed in oceanic waters. In addition, the apparent increase in relative abundance of UCYN-A2 from offshore to near-shore water provides evidence for the earlier and under-debating view that UCYN-A2 prefers coastal conditions. Our report provides new knowledge for understanding the phylogeny and ecology of unicellular cyanobacterial diazotrophs in WNPO.

Monitoring sediment microbial community metabolism and structure is instrumental to understanding biogeochemical processes in and ecological impacts on bottom environments. The aim of this study was to determine potential community respiration and to reveal community dynamics of the microorganisms in the dead zone sediments of Omura Bay, Japan. The bay is highly enclosed and develops severe hypoxia in the central regions every summer. We collected sediment core samples from the center of the bay during hypoxia, estimated sediment oxygen consumption by using an adapted in vivo electron transport system activity (in vivo ETSA) assay, enumerated abundance of bacteria, and analyzed bacterial community structure by automated ribosomal intergenic spacer analysis. Higher ETSA and bacterial diversity were found in upper sediments (within 3 cm depth) from the center than the fringe of the bay. Sediment bacterial community structure of the bay center was distinct from that of the fringe. From these results, upper sediment in the dead zone of Omura Bay was characterized by (1) greater community respiration and (2) greater diversity of bacterial components compared with the non-hypoxic sediment of the bay fringe. These characteristics have important implications for understanding the interaction between microbial communities and the development of hypoxia in Omura Bay.

The territorial damselfish, Stegastes nigricans, maintains algal farms by excluding invading herbivores and weeding unpalatable algae from its territories. In Okinawa, Japan, S. nigricans farms are exclusively dominated by Polysiphonia sp., a highly digestible filamentous rhodophyte. This study was aimed at determining the diet of S. nigricans in Okinawa and its dependency on these almost-monoculture algal farms based on stomach content and chemical analyses. Stomach content analyses revealed that all available food items in the algal farms (i. e., algae, benthic animal inhabitants, trapped detritus) were contained in fish stomachs, but amorphous organic matter accounted for 68% of the contents. Therefore, carbon and nitrogen stable isotope ratios and fatty acid (FA) compositions were analyzed to trace items actually assimilated in their bodies. Stable isotope analyses showed that benthic animals were an important food source even for this farmer fish. Two essential fatty acids (EFAs), 20:4n6 and 20:5n3, which are produced only by rhodophytes among available food items, were rich in the muscle tissue of S. nigricans as well as in algal mats and detritus, suggesting that algal mats contribute EFAs to S. nigricans directly and indirectly through the food web. In conclusion, S. nigricans ingested algal mats, detritus, and benthic animals maintained within its farm. Algae and detritus were original sources of EFAs, and benthic animals, which were much more abundant in the farms than in outside territories, provided a nitrogen-rich dietary source for the fish., Ecological Research, 26(4), pp.809-818; 2011

The 210 Pb geochronology, heavy metal concentrations (Zn, Cu, and Pb), and stable Pb isotope ratios ( 206 Pb/ 207 Pb) of three sediment cores collected from Jakarta Bay were analyzed to decipher the history of heavy metal contamination in the period 1900–2006. The chemical and isotopic analyses clearly suggest that anthropogenic metal accumulation in the sediments began in the 1920s and increased greatly from the 1970s until the end of the 1990s. From the end of the 1990s to 2006, accumulation rates were constant or decreased for Zn and Pb near the coastal industrialized area. Comparison of economic data and sociological information suggests that the decline in the concentrations of heavy metals could be attributed to the stricter environmental regulations which were enforced at the end of 1990s. However, metal contamination is currently still an important cause of concern in dealing with environmental preservation and protection in Jakarta Bay.

Summary Deterioration of the quality of groundwater under urban areas has become a major environmental concern worldwide. This paper presents a pilot case study (in the Taipei urban area) using multiple stable isotopes ( δ D H 2 O and δ 18 O H 2 O , δ 15 N NO 3 and δ 18 O NO 3 , δ 34 S SO 4 and δ 18 O SO 4 , and 87Sr/86Sr) with the aim of understanding the subsurface nature and environmental status of such areas. A previous study has already characterized Taipei shallow groundwaters as sulfate-rich in comparison to other Southeast Asian metropolitan cities. Our chemical measurements clarified that the arsenic is the important environmental concerns in the aquifers. Dual isotope approaches for nitrate and sulfate suggested these ions were sourced mainly from municipal sewage leaking from sewer pipes in urbanized area, while chemical fertilizers applied in the local agricultural fields partly contribute in the suburb area. Significant decreases in nitrate and sulfate concentrations due to denitrification and sulfate reduction were observed in some groundwaters, which induced increases in isotope ratios up to 24.9‰ for δ 15 N NO 3 and 27.2‰ for δ 34 S SO 4 . Nitrate groundwater pollution in the Taipei Basin is not a subject of serious concern ( NO 3 - for all analyzed samples) at the present time due to attenuation by denitrification. However, arsenic is preferentially released in groundwater (>100 μg/L) in anoxic environment under which sulfate reduction occurs. The Sr isotope data suggested that the arsenic is derived from alluvial deposits of Oligocene–Miocene sedimentary rock origin. This study disclosed the complex causes of environmental changes in Taipei urban shallow groundwater. It is also important as an excellent case study demonstrating the efficient use of multiple isotope diagnosis in the urban groundwater environmental field.

To characterize water quality in terms of dissolved elements and to investigate both the origin of the water and the source and behavior of groundwater contaminants in Metro Manila, 33 water (groundwater and surface water) samples were analyzed for ion and element concentrations, H and O isotope ratios (δD-H2O and δ18O-H2O), SO 4 2 - isotope ratios (δ34S- SO 4 2 - and δ18O- SO 4 2 - ), and Sr isotope ratio (87Sr/86Sr). The chemical measurements showed that the primary important environmental concerns within Metro Manila are groundwater salinization for both shallow and deep aquifers, and As concentrations (up to 22.5 μg/L) in shallow groundwaters. Comparison of SO 4 2 - and Sr isotope values with possible source materials revealed that contamination by man-made materials such as fertilizers and detergents are present in some shallow groundwaters. Shallow groundwater having higher δD-H2O and δ18O-H2O values (av. −44‰ ± 5.6‰ and −6.8‰ ± 0.6‰, respectively, n = 15) than deep groundwater (av. −48‰ ± 4.4‰ and −7.4‰ ± 0.7‰, respectively, n = 7) suggests that the origins of H2O in both groundwaters are different from each other. Since the mixing of shallow and deep groundwater does not commonly occur under Metro Manila, the contaminants in the shallow aquifers are unlikely to be transported into the deep aquifers. Sulfate reduction by bacterial activity was observed for some groundwaters, resulting in a maximum elevation in δ34S- SO 4 2 - values of around 10‰. By using SO 4 2 - isotope ratios as an indicator of changes in redox conditions and Sr isotope ratio as a source indicator, it was shown that As was dissolved from unconsolidated sedimentary deposits of volcanic origin which enclose shallow unconfined aquifers, but was not the result of changes in redox conditions. The study demonstrated that multi-isotope ratios are useful for evaluating water quality problems in urban groundwaters.

The net production of dissolved organic matter (DOM) and dissolved combined and free amino acids (DCAA and DFAA, respectively) by the hermatypic coral Acropora pulchra was measured in the submerged condition, and the production rates were normalized to the coral surface area, tissue biomass, and net photosynthetic rates by zooxanthellae. When normalized to the unit surface area, the production rates of dissolved organic carbon and nitrogen (DOC and DON, respectively) were 37 and 4.4 nmol cm − 2 h − 1 , respectively. Comparing with the photosynthetic rate by zooxanthellae, which was measured by 13 C-tracer accumulation in the soft tissue of the coral colony, the release rate of DOC corresponded to 5.4% of the daily net photosynthetic production. The tissue biomass of the coral colony was 178 µmol C cm − 2 and 23 µmol N cm − 2 , indicating that the release of DOC and DON accounted for 0.021% h − 1 and 0.019% h − 1 of the tissue C and N, respectively. The C:N ratios of the released DOM (average 8.4) were not significantly different from those of the soft tissue of the coral colonies (average 7.7). While DFAA did almost not accumulate in the incubated seawater, DCAA was considerably released by the coral colonies at the rate of 2.1 nmol cm − 2 h − 1 on average. Calculating C and N contents of the hydrolyzable DCAA, it was revealed that about 20% and 50%–60% of the released bulk DOC and DON, respectively, were composed of DCAA.

Asian megacities have severe pollution problems in both coastal and urban areas. In addition, the groundwater potential has decreased and land subsidence has occurred because of intensive groundwater pumping in urban areas. To prevent the adverse effects of urbanization on groundwater quality, it is necessary to confirm the changes in groundwater flow and contaminant transport caused by urbanization. We examined the effects of urbanization on contaminant transport in groundwater. The research areas were located around Bangkok, Thailand, and Jakarta, Indonesia, cities with populations of approximately 8 and 12 million, respectively. Each metropolitan city is located on a river delta and is adjacent to a bay. We measured the water level and collected water samples at boreholes at multiple depths (100 to 200 m) in 2004 and 2006 in Bangkok and Jakarta, respectively. The current hydraulic potential is below sea level in both cities because of prior excess abstraction of groundwater. As a result, the direction of groundwater flow is now downward in the coastal area. The Cl- concentration and delta18O distributions in groundwater suggest that the decline in hydraulic potential has caused the intrusion of seawater and shallow groundwater into deep groundwater. Concentrations of Mn and NO3--N in groundwater suggest the intrusion of these contaminants from shallow to deep aquifers with downward groundwater flow and implies an accumulation of contaminants in deep aquifers. Therefore, it is important to recognize the possibility of future contaminant transport with the discharge of deep groundwater into the sea after the recovery of groundwater potential in the coastal areas.

The stable nitrogen isotope ratio (δ 15N) in macroalgae is effectively used as a time-integrated bioindicator to record nitrogen sources for primary producers during their growing periods in aquatic ecosystems. However, the utility of this tool is limited because the occurrence of these organisms is often restricted in space and time. To investigate the potential of chemical composition in sedimentary organic matter (SOM) as a proxy for time-integrated environmental conditions, nitrogen (N) and carbon (C) contents and their stable isotope ratios (δ 15N and δ 13C) were determined, and systematically cross-checked against corresponding values in macroalgae at the Shiraho fringing reef in Okinawa, Japan. Preliminary trials showed that δ 15N in SOM processed by the “wash-out method” for δ 13C analysis yielded similar δ 15N values to the bulk sediment, despite the loss of some SOM during the process. The amounts of organic matter and the ratio of the HCl-insoluble portion were variable within the reef, probably reflecting local vegetation and subsequent decomposition. The distribution of δ 15N and δ 13C in SOM showed similar trends to those of macroalgae, with mostly constant differences of 1.4‰ and −6.7‰, respectively. These differences throughout the reef appeared to be explained in terms of mixed contributions from macrophyte and epibenthic microalgae growing in different seasons and years, with their debris undergoing diagenetic alteration. Therefore, macroalgae and SOM δ-values can be used in a complementary manner, over various time scales, as indicators of the integrated effect of dissolved inorganic nitrogen (DIN) sources on coral reef ecosystems.

Asian megacities have severe pollution problems in both coastal and urban areas. In addition, the groundwater potential has decreased and land subsidence has occurred because of intensive groundwater pumping in urban areas. To prevent the adverse effects of urbanization on groundwater quality, it is necessary to confirm the changes in groundwater flow and contaminant transport caused by urbanization. We examined the effects of urbanization on contaminant transport in groundwater. The research areas were located around Bangkok, Thailand, and Jakarta, Indonesia, cities with populations of approximately 8 and 12 million, respectively. Each metropolitan city is located on a river delta and is adjacent to a bay. We measured the water level and collected water samples at boreholes at multiple depths (100 to 200 m) in 2004 and 2006 in Bangkok and Jakarta, respectively. The current hydraulic potential is below sea level in both cities because of prior excess abstraction of groundwater. As a result, the direction of groundwater flow is now downward in the coastal area. The Cl(-) concentration and delta(18)O distributions in groundwater suggest that the decline in hydraulic potential has caused the intrusion of seawater and shallow groundwater into deep groundwater. Concentrations of Mn and NO3(-)-N in groundwater suggest the intrusion of these contaminants from shallow to deep aquifers with downward groundwater flow and implies an accumulation of contaminants in deep aquifers. Therefore, it is important to recognize the possibility of future contaminant transport with the discharge of deep groundwater into the sea after the recovery of groundwater potential in the coastal areas.

The seasonal variability of specific growth rate and the carbon stable isotope ratio ( δ 13 C) of leaf blades ( δ 13 C leaf ) of a temperate seagrass, Zostera marina (within 10 days old) were measured simultaneously, together with the δ 13 C of dissolved inorganic carbon ( δ 13 C DIC ) at three sites in the semi-closed Akkeshi estuary system, northeastern Japan, in June, September, and November 2004. The δ 13 C leaf ranged from −16.2 to −6.3‰ and decreased from summer to winter. The simultaneous measurement of the δ 13 C leaf , growth rate, and morphological parameters (mean leaf length and width, mean number of leaves per shoot, and sheath length) of the seagrass and δ 13 C DIC in the surrounding water allowed us to compare directly the δ 13 C leaf and specific growth rate of seagrass. The difference in the δ 13 C of seagrass leaves relative to the source DIC (Δ δ 13 C leaf − DIC ) was the least negative (−11 to −7‰) in June at all three sites and became more negative (−17 to −8‰) as the specific growth rate decreased. This positive correlation between Δ δ 13 C leaf − DIC and specific growth rate can be used to diagnose the growth of seagrasses. Δδ 13 C leaf − DIC changed by −1.7 ± 0.2‰ when the leaf specific growth rate decreased by 1% d −1 .

Concentration of dissolved and particulate carbon (C), nitrogen (N), and phosphorus (P) were measured daily and seasonally in the water column of a coral reef lagoon at Ishigaki Island. Concentrations of dissolved inorganic N and P (DIN, DIP), dissolved organic C and N (DOC, DON), particulate organic C, and particulate N and P (POC, PN, PP) were usually higher in the lagoon than at offshore stations, while dissolved organic P (DOP) was higher at the offshore stations than in the lagoon. Concentration ratios of DIN:DIP and PN:PP were near 16:1 in winter and >20:1 in summer. DON:DOP was always ≥30. Total P (DIP + DOP + PP) was fairly constant around 0.177 ± 0.025 μmol l–1 (mean ± SD), suggesting conservative behavior of water-column P around this reef. DIP and PP seemed to be produced at the expense of DOP, and PP rather than DIP increased when gross primary production (GPP) was high. In contrast, total N (DIN + DON + PN) showed large spatial and seasonal variation from 4.3 to 12.4 μmol l–1. DON and PN increased when GPP and DIN:DIP were high, which suggests that export production of organic N by the lagoon biota was enhanced by primary production and the availability of N relative to P. Estimation of C, N, and P exchange fluxes between the lagoon and the outer ocean using a simple hydrodynamic model showed that the lagoon ecosystem released N (especially DIN) but not P to the surrounding ocean. Oceanic DOP could be the major source for the P budget of the lagoon. While the lagoon was a net source of organic C to the outer ocean, the net export was only a minor fraction (

To clarify the major factors that control stable carbon and nitrogen isotopic compositions (δ13C and δ15N) of suspended particulate organic matter in a hypertrophic estuary, seasonal variation in concentrations and stable isotopic compositions of particulate organic carbon and nitrogen (POC, PN), chlorophyll a content (Chl) and dry weight of suspended solid (SS) was studied in the Sumida River estuary of Tokyo Bay. Particulate material was fractioned into four size-fractions (3–20, 20–53, 53–250, 250–1000 μm). At the lower-estuarine (LE) site, the finer size fraction ( 80% Chl) during the autumnal bloom (Aug–Sep 2003). The middle size fractions (20–53 and 53–250 μm) however increased during the spring bloom (50–80% Chl; Feb–Apr 2004), reflecting difference in taxonomic composition of phytoplankton. At the upper-estuarine and the riverine sites, the

We analyzed the content of tissue nutrients (carbon, nitrogen, and phosphorus), the C:N:P ratio, the carbon stable isotope ratio ( δ 13 C ) and the nitrogen stable isotope ratio ( δ 15 N ) of leaves of Enhalus acoroides (L. f.) Royle growing in two submerged beds in southwest Thailand. The N and P concentrations and δ 15 N decreased with increasing distance along the leaf. The maximum ranges in one leaf were 1.80–4.05% for N, 2.85–6.64 mg g−1 for P, and 1.28–2.93‰ for δ 15 N . The C:N, C:P, and N:P ratios increased as the distance along the leaf increased. The maximum ranges in one leaf were 12.5–24.3 for C:N, 162–622 for C:P, and 14.0–28.9 for N:P. In contrast, C concentration and δ 13 C fluctuated with distance along the leaf, and the longest leaves from each beds showed similar patterns of fluctuation. The variation of the chemical signature within leaves suggests that special attention is needed to apply these data to the evaluation of nutrient conditions during growth in food-web analyses involving seagrass beds.

Groundwater discharge from adjacent terrestrial areas can be a potentially important nutrient source to coastal coral reefs, since adjacent lands are often overlaid with permeable bedrock such as limestone. The quantity of groundwater nitrogen discharged into the Shiraho and Kabira coral reefs from their namesake watersheds on Ishigaki Island southwest of the Ryukyu Islands, Japan (24°19′–37′N, 124°4′–21′E) was monitored. These watersheds were subject to different types of nitrogen loading. The groundwater nitrogen discharge was compared by two independent methods, one based on measuring dissolved inorganic nitrogen (DIN) concentrations in the groundwater near the coastline, the other by estimating nitrogen loading from various land uses within the watershed. For a common watershed, the two methods agreed within a factor of two. The Shiraho reef received 4- or 5.5-fold more nitrogen than the Kabira reef. Groundwater discharge contributes significantly to the reef nitrogen budget, and is potentially a key factor controlling the biomass and succession of aquatic vegetation of the reefs.

We measured the C/N ratio and d 15 N values of two brown macroalgae— Padina spp. and Dictyota sp., which are distributed over all the subtropical fringing reefs of the Ryukyu Islands, Japan—to evaluate the feasibility of these algae as indicators of the terrestrial nitrogen load to the reef. The correlations between the distance from the shoreline and algal C/N ratio and surrounding NO concentrations were not clear, although their average values among the 2 3 reefs seemed to indicate differences in nitrogen loadings from the land. The d 15 N values of these algae, on the other hand, linearly or curvilinearly decreased from 18‰ to 12‰ with increasing distance from the shoreline, indicating the difference in nitrogen sources available to macroalgae. The slope of the decline among eight study areas had different characters, which seemed to depend on the residence time of reef seawater and the fluxes of terrestrial nitrogen. Using d 15 N values of brown algae as an indicator, we confirmed that primary producers, such as macroalgae on the reefs, assimilated land-derived nitrogen and successfully evaluated time-integrated effects of terrestrial nitrogen on coral reef algae, which had been missed by conventional monitoring of the water column nutrients.

The phosphorus (P) cycle that maintains the production of microphytobenthic communities in carbonate sediments was investigated in the fringing Shiraho coral reef system located off the east coast of Ishigaki Island, Japan. Production rates of the diatom-dominated microphytobenthic communities and the P flux at the sediment‐water interface were determined by in situ benthic enclosure experiments. The daily net production (6.9‐24 mmol C m 22

Halophila ovalis on the intertidal flat of SW Thailand maintains its presence under heavy grazing by dugongs; nevertheless, the site provides lower availability of nutrients in the water column to the seagrasses due to emersion during ebb tide. To examine the seasonality of the nutrient status of fast-growing H. ovalis in the intertidal flat, and to assess whether emersion at low tide affects the availability of nutrients and inorganic carbon, monthly changes in carbon (C), nitrogen (N), and phosphorus (P) concentrations of H. ovalis were determined. Carbon and nitrogen stable isotope ratios were also determined to examine the carbon and nitrogen sources. CNP concentrations of the leaves and petioles of H. ovalis at the intertidal flat were considerably lower than those of the same species previously reported from a subtidal environment throughout the observation period of 8 months. The δ13C of the above-ground parts of H. ovalis was lower than in previous reports, suggesting that CO2 was sufficient for its growth. The CNP concentrations of H. ovalis on the intertidal flat increased after a decrease in grazing pressure by dugongs. It is suggested that H. ovalis sustained rapid growth on the intertidal flat by lowering its nutrient concentrations under heavy grazing by dugongs to maintain its presence.

Nitrogen fixation (NF), denitrification (DN), microalgal uptake (MU) of dissolved inor- ganic nitrogen (DIN) and the net diffusive DIN flux (FL) between the sediment and the overlying water were measured in bare carbonate (sand) sediment and seagrass-bed sediment of the shallow (