Food Web Structures and Mercury Exposure Pathway to Fish in Minamata Bay.

We analyzed total mercury content (THg) and carbon (δ13C) and nitrogen (δ15N) stable isotope ratios in fish, subtidal macrobenthos, and particulate organic matter (POM) as a proxy for pelagic phytoplankton and attached microalgae as a proxy for microphytobenthos to investigate the mercury exposure pathway in fish. For four seasons, samples of the above-mentioned organisms were collected on five occasions (July and October 2018 and January, April, and July 2019) in Minamata Bay. Isotope analysis showed that Minamata Bay food web structures were almost entirely fueled by microphytobenthos. The THg values of the fish and macrobenthos species were positively correlated with their δ13C. This indicates that their diets, which were highly fueled by microphytobenthos, led to high THg bioaccumulation in both macrobenthos and fish. The feeding habits of fishes differ depending on the species, and they prey on organisms of many taxa, including fish (mainly Japanese anchovy), crabs, shrimp, copepods, annelids, and algae. Fish species that preyed on benthic crustaceans had high THg. These results suggest that the main pathway of Hg bioaccumulation in fish from Minamata Bay is the benthic food chain, which is primarily linked to benthic crustaceans fueled by microphytobenthos.

The mitochondrial genome of the threatened tideland snail Pirenella pupiformis (Mollusca: Caenogastropoda: Potamididae) determined by shotgun sequencing.

The nearly complete mitochondrial genome of the threatened tideland snail Pirenella pupiformis (Mollusca: Cerithioidea: Potamididae) was determined by shotgun next-generation sequencing. The mitogenome is comprised of 13 protein-coding genes (PCGs), two ribosomal RNA (12S and 16S) genes, and 22 transfer RNA genes (tRNAs). This gene order is consistent with the previously published mitochondrial genomes of other species belonging to the family Potamididae. The family Potamididae including P. pupiformis was recovered as a monophyletic group in the superfamily Cerithioidea.

Differences in the isotopic signature of activated sludge in four types of advanced treatment processes at a municipal wastewater treatment plant.

The natural abundance of stable isotopes is a powerful tool for evaluating biological reactions and process conditions. However, there are few stable isotope studies on the wastewater treatment process. This study carried out the first investigation on variations in natural abundance of carbon and nitrogen stable isotope ratios (δ13C and δ15N) of activated sludge in four types of advanced treatment process (extended aeration activated sludge (EAAS), aerobic-anoxic-aerobic (A2O), recycled nitrification-denitrification (RND), and modified Bardenpho (MB)) at a municipal wastewater treatment plant. The δ13C and δ15N values of influent suspended solids settled in the primary sedimentation tank (i.e., primary sludge) ranged from -25.4‰ to -24.6‰ and 0.5‰-2.9‰, respectively, during monitoring periods. The δ13C values of the activated sludge were -24.6‰ to -23.6‰ (EAAS), -25.4‰ to -24.3‰ (A2O), -25.7‰ to -24.9‰ (RND), and -25.7‰ to -24.3‰ (MB). The δ13C values of the activated sludge were similar to those of influent suspended solids. However, the δ13C values of activated sludge in EAAS was significantly higher than in A2O, RND, and MB. Meanwhile, the δ15N values of activated sludge were obviously higher than influent suspended solids; 5.8‰-7.5‰ (EAAS), 6.6‰-8.1‰ (A2O), 5.5‰-7.5‰ (RND), and 5.3‰-7.6‰ (MB). Changes in δ13C and δ15N values of the activated sludge within the treatment system were also found. These findings indicate that changes in δ13C and δ15N values of the activated sludge rely on important function for biological wastewater treatment such as nitrification, denitrification, and methane oxidation through wastewater treatment over time.

Evaluation of stable isotope ratios (δ15N and δ18O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types.

Stable isotope ratios of nitrate are a powerful tool to evaluate aquatic environment stress from treated and untreated sewage. However, there is generally a lack of knowledge on the change in stable isotope ratios within wastewater treatment plants. We investigated nitrogen and oxygen stable isotope ratios (δ15N and δ18O) of nitrate in four types of advanced treatment processes operated in parallel; (A) extended aeration activated sludge, (B) anaerobic-anoxic-aerobic (A2O), (C) recycled nitrification-denitrification, and (D) modified Bardenpho. The results exhibited spatial variation of δ15N and δ18O for nitrate within the treatment steps. The changes in δ15N and δ18O may result from the reactor conditions (aerobic, anoxic, and anaerobic) and the order of these processes. As decreasing nitrate concentration in the anoxic stages, the δ15N/δ18O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ15N and δ18O signatures were attributed to process performance in regard to nitrogen removal. In particular, the modified Bardenpho process has higher nitrogen removal efficiency over other processes, producing effluent with lower nitrate concentration and higher stable isotopes (δ15N: 23.6 to 25.5‰, δ18O: 2.8 to 4.5‰). We concluded that the stable isotope signatures mirrored the treatment efficiency and effluent characteristics.

Food sources are more important than biomagnification on mercury bioaccumulation in marine fishes.

Marine animals often accumulate various harmful substances through the foods they ingest. The bioaccumulation levels of these harmful substances are affected by the degrees of pollution in the food and of biomagnification; however, which of these sources is more important is not well-investigated for mercury (Hg) bioaccumulation. Here we addressed this issue in fishes that inhabit the waters around Minamata Bay, located off the west coast of Kyushu Island in Kumamoto Prefecture, Japan. The total Hg concentration (hereafter [THg]) and carbon and nitrogen stable isotope ratios (δ13C and δ15N) were analyzed in the muscle tissue of 10 fish species, of which more than five individuals were caught by gillnet. Except one species, each was separated into two trophic groups with respective lower and higher δ13C values ranging from -17‰ to -16‰ and -15‰ to -14‰, which suggested that the fishes depended more on either phytoplankton- and microphytobenthos-derived foods (i.e., pelagic and benthic trophic pathways), respectively. Linear mixed effects models showed that the Hg levels were significantly associated with both δ15N and the differences in the trophic groups. [THg] increased with δ15N (i.e., indicative of higher trophic levels), but the slopes did not differ between the two trophic groups. [THg] was significantly higher in the group with higher δ13C values than in those with lower δ13C values. The effect size from marginal R squared (R2) values showed that the variation in [THg] was strongly ascribed to the trophic group difference rather than δ15N. These results suggest that the substantial Hg bioaccumulation in the fishes of Minamata Bay is mainly an effect of ingesting the microphytobenthos-derived foods that contain Hg, and that the subsequent biomagnification is secondary.

Distribution in natural abundances of stable isotopes of nitrate and retained sludge in a nitrifying bioreactor: Drastic changes in isotopic signatures.

This study determined the spatial and temporal changes in natural abundance of stable isotopes (δ13C, δ15N, and δ18O) with regard to nitrate (NO3-) and retained sludge in a nitrifying bioreactor. The bioreactor was continuously fed with synthetic wastewater including ammonium for 61 days at 20 °C. After the start-up period of the bioreactor, the NO3- concentration in the effluent gradually increased. The stable isotopes (δ15N and δ18O) of NO3- in the effluent also increased in a phase of incomplete nitrification. The profile experiments showed that the concentration and stable isotopes of NO3- changed simultaneously along the wastewater flow in the bioreactor. The stable isotope analysis revealed that nitrification efficiency seems to be strongly related to the δ15N of NO3-. Moreover, the δ13C and δ15N of the retained sludge drastically changed along the reactor length, from -26‰ to -18‰ and from 5‰ to 30‰, respectively, after 61 days of operation. The isotopic composition of the retained sludge might be affected by the isotope ratios (δ15N and δ18O) of NO3- in the bioreactor. Therefore, the isotope signatures of the retained sludge seem to closely reflect process performance such as nitrification efficiency throughout the operational period. Our findings suggest that the spatial distribution of the isotopic composition of the retained sludge can be used to detect process occurrence within the bioreactor over time.

Prevalence and species richness of trematode parasites only partially recovers after the 2011 Tohoku, Japan, earthquake tsunami.

Trematode parasites have complex life cycles and use a variety of host species across different trophic levels. Thus, they can be used as indicators of disturbance and recovery of coastal ecosystems. Estuaries on the Pacific coast of northeastern Japan were heavily affected by the 2011 Tohoku earthquake tsunami. To evaluate the effect of the tsunami on the trematode community, we examined trematodes in the mud snail, Batillaria attramentaria, at five study sites (three sites severely exposed to the tsunami and two sites sheltered from the tsunami) in Sendai Bay for 2 years prior to and 8 years after the tsunami. While the trematode prevalence decreased at all study sites, the species richness decreased only at the sites exposed to the tsunami. Although parasitism increased over the study period post-tsunami, the community had not fully recovered 8 years after the event. Trematode community structure has changed every year since the tsunami and has not stabilised. This could be explained by the alteration of first and second intermediate host communities. Our study suggests that it will take more time for the recovery of the trematode community and the associated coastal ecosystem in the Tohoku region.

Evaluation of trophic transfer in the microbial food web during sludge degradation based on 13C and 15N natural abundance.

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) were determined in activated sludge, which was exposed to endogenous conditions for 36 days and contained a wide diversity of organisms across several trophic levels. The aim of this study was to elucidate the fluctuation of δ13C and δ15N through trophic transfer in the microbial consortia. The sludge was evaluated in view of sludge mass, bacterial community, higher trophic organisms, sludge δ13C and δ15N, and δ15N and δ18O of nitrate. The results show that the activated sludge became more enriched with 15N as degradation proceeded. Eventually, the mixed liquor volatile suspended solid concentrations in the activated sludge decreased from 1610 to 710 mg/L and the δ15N of the sludge increased from 8.3‰ to 10.8‰. In contrast, the δ13C values of the sludge were stable. Microscope observations confirmed that consumers such as Rotifera, Tardigrada and Annelida (Aelosoma sp.) were present in the activated sludge for the entire operational period. The abundance of those organisms drastically changed during the operational periods, and the diversity in bacterial community also changed, resulting in community succession. Changes in biotic community, reduction in sludge mass, and increase in δ15N of the sludge occurred during the sludge degradation processes. This implies that the sludge degradation was partly caused by the trophic conversion of the sludge-derived nitrogen in the food web. The δ15N of the sludge can be used as an indicator of the sludge degradation through trophic transfer in wastewater treatment reactors. These findings provide new insights into understanding trophic transfer during microbial community succession and the effects of the feeding process on sludge degradation.

Ecological thresholds of hypoxia and sedimentary H2S in coastal soft-bottom habitats: A macroinvertebrate-based assessment.

Ecological thresholds of dissolved oxygen (DO) and sedimentary hydrogen sulfide (H2S) for macrozoobenthos were examined during a 30-month monitoring of two stations in a highly eutrophic canal in inner Tokyo Bay, Japan. Bottom DO and H2S concentrations fluctuated seasonally, and were significantly correlated with water and sediment temperatures. Red tide-derived phytodetritus was a major source of sediment organic matters in the canal bottom, and the sediment became highly reduced and sulfidic condition in warmer months (sedimentary H2S; up to 8.5 mM). Dominant opportunistic taxa, including polychaetes and amphipods, were eliminated under low DO and high H2S conditions (i.e., population thresholds), and devastation of community structure occurred at 2.4-3.3 mg l-1 DO and 1.8-2.7 mM H2S (i.e., community thresholds). To maintain ecosystem function in anthropogenically degraded habitats and ensure colonization by macrozoobenthos throughout the year, DO and H2S levels should be maintained below these thresholds.

Ecological and genetic impact of the 2011 Tohoku Earthquake Tsunami on intertidal mud snails.

Natural disturbances often destroy local populations and can considerably affect the genetic properties of these populations. The 2011 Tohoku Earthquake Tsunami greatly damaged local populations of various coastal organisms, including the mud snail Batillaria attramentaria, which was an abundant macroinvertebrate on the tidal flats in the Tohoku region. To evaluate the impact of the tsunami on the ecology and population genetic properties of these snails, we monitored the density, shell size, and microsatellite DNA variation of B. attramentaria for more than ten years (2005-2015) throughout the disturbance event. We found that the density of snails declined immediately after the tsunami. Bayesian inference of the genetically effective population size (Ne) demonstrated that the Ne declined by 60-99% at the study sites exposed to the tsunami. However, we found that their genetic diversity was not significantly reduced after the tsunami. The maintenance of genetic diversity is essential for long-term survival of local populations, and thus, the observed genetic robustness could play a key role in the persistence of snail populations in this region which has been devastated by similar tsunamis every 500-800 years. Our findings have significant implications for understanding the sustainability of populations damaged by natural disturbances.

High spatial resolution analysis of the distribution of sulfate reduction and sulfide oxidation in hypoxic sediment in a eutrophic estuary.

Bottom hypoxia and consequential hydrogen sulfide (H2S) release from sediment in eutrophic estuaries is a major global environmental issue. We investigated dissolved oxygen, pH and H2S concentration profiles with microsensors and by sectioning sediment cores followed by colorimetric analysis. The results of these analyses were then compared with the physicochemical properties of the bottom water and sediment samples to determine their relationships with H2S production in sediment. High organic matter and fine particle composition of the sediment reduced the oxidation-reduction potential, stimulating H2S production. Use of a microsensor enabled measurement of H2S concentration profiles with submillimetre resolution, whereas the conventional sediment-sectioning method gave H2S measurements with a spatial resolution of 10 mm. Furthermore, microsensor measurements revealed H2S consumption occurring at the sediment surface in both the microbial mat and the sediment anoxic layer, which were not observed with sectioning. This H2S consumption prevented H2S release into the overlying water. However, the microsensor measurements had the potential to underestimate H2S concentrations. We propose that a combination of several techniques to measure microbial activity and determine its relationships with physicochemical properties of the sediment is essential to understanding the sulfur cycle under hypoxic conditions in eutrophic sediments.

Effects of sedimentary sulfide on community structure, population dynamics, and colonization depth of macrozoobenthos in organic-rich estuarine sediments.

An annual field survey and in situ recolonization experiment revealed the effects of sedimentary sulfide (H2S) on macrozoobenthos in a eutrophic brackish lagoon. Species diversity was much lower throughout the year in muddy opportunist-dominant sulfidic areas. Mass mortality occurred during warmer months under elevated H2S levels. An enclosure experiment demonstrated that sedimentary H2S modified community composition, size structure, and colonization depth of macrozoobenthos. Species-specific responses to each sediment type (sand, sulfidic mud, and mud with H2S removed) resulted in changes in the established community structure. Dominant polychaetes (Hediste spp., Pseudopolydora spp., and Capitella teleta) occurred predominantly in a thin surface layer in the presence of H2S. On the other hand, organic-rich mud facilitated settlement of polychaete larvae if it does not contain H2S. These results demonstrate that sediment characteristics, including H2S level and organic content, were key structuring factors for the macrozoobenthic assemblage in organically polluted estuarine sediments.

Impacts of the 2011 Tsunami on Sediment Characteristics and Macrozoobenthic Assemblages in a Shallow Eutrophic Lagoon, Sendai Bay, Japan.

A huge tsunami is one of the greatest disturbance events in coastal benthic communities, although the ecological consequences are not fully understood. Here we examined the tsunami-induced changes in the sediment environment and macrozoobenthic assemblage in a eutrophic brackish lagoon in eastern Japan. The 7.2-m-high tsunami completely replaced muddy sediment with drifting sea sand throughout the lagoon, leading to the drastic changes in quantity and quality of sedimental organic matters, sulfide contents, and sediment redox condition. Intensive physical stress devastated the benthic community, but the disappearance of sulfidic muddy bottoms significantly improved the habitat quality for macrozoobenthos. The re-established macrozoobenthic community after 5 months was characterized by (1) a 2-fold higher total density, but sharp declines in species richness, diversity, and evenness; (2) an increased density of opportunistic taxa (e.g., polychaete Pseudopolydora spp. and amphipod Monocorophium uenoi) in newly created sandy bottoms; and (3) disappearance of several dominant taxa including bivalves and chironomid larvae. These findings indicate that the sensitivity and recovery potential of macrozoobenthos were highly taxa-specific, which was closely related to the taxa's ecological characteristics, including tolerance to physical disturbance, life-history traits, and life form. Our data revealed the rapid recolonization of opportunistic macrozoobenthos after a huge tsunami, which would contribute to the functional recovery of estuarine soft-bottom habitats shortly after a disturbance event.

Seasonal changes in infaunal community structure in a hypertrophic brackish canal: Effects of hypoxia, sulfide, and predator-prey interaction.

We conducted a one-year survey of macrozoobenthic community structure at 5 stations in a eutrophic canal in inner Tokyo Bay, focusing on the impacts of hypoxia, sediment H2S, and species interaction in the littoral soft-bottom habitats. Complete defaunation or decreasing density of less-tolerant taxa occurred under hypoxia during warmer months, especially at subtidal or sulfidic stations; this was followed by rapid recolonization by opportunistic polychaetes in fall-winter. Sedimentary H2S increased the mortality of macroinvertebrates under hypoxia or delayed population recovery during recolonization. The density of several polychaetes (e.g., Pseudopolydora reticulata) declined in winter, coincident with immigration of the predator Armandia lanceolata. This suggests that absence of A. lanceolata under moderate hypoxia enabled the proliferation of prey taxa. We conclude that oxygen concentration, sediment H2S, and hypoxia-induced changes in species interactions are potential drivers for spatiotemporal changes in macrozoobenthic assemblage structure in hypoxia-prone soft-bottom communities.

Spatial changes in carbon and nitrogen stable isotope ratios of sludge and associated organisms in a biological sewage treatment system.

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹5N) have been utilized as powerful tools for tracing energy or material flows within food webs in a range of environmental studies. However, the techniques have rarely been applied to the study of biological wastewater treatment technologies. We report on the spatial changes in δ¹³C and δ¹5N in sludge and its associated biotic community in a wastewater treatment system. This system consisted of an upflow anaerobic sludge blanket (UASB) and a down-flow hanging sponge (DHS) which is a novel type of trickling filter. The results showed clear spatial changes in the δ¹³C and δ¹5N of suspended solids (SS), retained sludge, and macrofauna (oligochaetes and fly larvae) in the system. The δ¹³C and δ¹5N was used as a natural tracer to determine the SS dynamic throughout the system. The results imply that SS in the DHS effluent was mainly eluted from the retained sludge in the lower section of the DHS reactor. The δ¹5N of the retained sludge in the DHS reactor increased drastically from the inlet towards to the outlet, from -0.7‰ to 10.3‰. This phenomenon may be attributed to nitrogen conversion processes (i.e. nitrification and denitrification). The δ¹5N of oligochaetes also increased from the inlet to the outlet, which corresponded well to that of the retained sludge. Thus, the δ¹5N of the oligochaetes might simply mirror the δ¹5N of the retained sludge. On the other hand, the δ¹³C and δ¹5N of sympatric fly larvae differed from those of the oligochaetes sampled, indicating dietary differences between the taxa. Therefore δ¹³C and δ¹5N reflected both treatment and dietary characteristics. We concluded that δ¹³C and δ¹5N values are potentially useful as alternative indicators for investigating microbial ecosystems and treatment characteristics of biological wastewater treatment systems.

Recolonization of macrozoobenthos on defaunated sediments in a hypertrophic brackish lagoon: effects of sulfide removal and sediment grain size.

Influences of sediment types on recolonization of estuarine macrozoobenthos were tested using enclosures in a hypertrophic lagoon. Three types of azoic sediment, sand (S), sulfide-rich mud (M), and mud removed of sulfide through iron addition (MFe), were set in field for 35 days during a hypoxic period. A total of 14 taxa including opportunistic polychaetes and amphipods occurred. Infaunal community in S treatment was characterized by highest diversity, total density and biomass, and population density of five dominant taxa, while those parameters were lowest in M treatment. Sulfide removal in MFe treatment achieved much higher density, biomass, and population densities of several taxa in the sediment. Multivariate analyses demonstrated that the established community structure was unique to each treatment. These imply that dissolved sulfide level as well as sediment grain size is a key determinant for the community composition and recolonization speed of early colonists in estuarine soft-bottom habitats.

Temporal changes in carbon and nitrogen stable isotope ratios of macrozoobenthos on an artificial tidal flat facing a hypertrophic canal, inner Tokyo Bay.

Temporal changes in benthic food web structure were analyzed in an artificial tidal flat in inner Tokyo Bay, Japan, using carbon and nitrogen stable isotope ratios (δ(13)C and δ(15)N). Microphytobenthos were the most important food sources of macrozoobenthos, due to high microphytobenthic biomass on the tidal flat, while phytoplankton in canal water (canal POMPP), terrestrial materials from urban surface runoff (canal POMTM), and marsh plants were less important. Dietary contribution of microphytobenthos was highest in April to June, while decreased towards December owing to the supply of canal POMPP and canal POMTM following red tides and heavy rainfall events in summer to fall. Temporal changes in δ(15)N (Δδ(15)N) of consumer corresponded well to the (15)N-enrichment in canal POMPP in summer. A meta-analysis showed that the consumer-Δδ(15)N was considerably larger in inner Tokyo Bay than those in other estuaries, which may be a specific characteristic of benthic food web in highly urbanized estuaries.

Estimating isotope fractionation between cercariae and host snail with the use of isotope measurement designed for very small organisms.

Most studies have reported negative carbon and nitrogen isotope fractionations between hosts and parasites, but isotope values have not yet been determined for many parasite species, such as trematodes, due to their relatively small body sizes. We investigated the carbon and nitrogen isotope values of freshwater snails and trematode parasites by using a method for organisms with very small body sizes. We found negative isotope fractionation values between host snails and trematode parasites, similar to published values for other parasite groups with larger body sizes, which suggest that the mechanisms for determining isotope fractionations between hosts and parasites provide similar results.