Gen Kanaya

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.

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.xa0lanceolata 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.

Genetic Structures of Laguncula pulchella Metapopulations Along the Northeast Coast of Japan After the Tsunamis Caused by the Great East Japan Earthquake

The carnivorous snail (Laguncula pulchella) was once considered an endangered species in Japan because its habitat was limited to western Japan. However, alien L. pulchella populations were transported from China and the Korean Peninsula to the Pacific coast of northeast Japan, presumably via anthropogenic activities. This study examined the genetic structures of the invasive L. pulchella populations in various coastal estuaries of Sendai Bay and Sanriku Ria areas to determine how the metapopulations were affected by the tsunamis caused by the Great East Japan Earthquake. The genetic compositions of these carnivorous snails significantly varied between the Sendai Bay and Sanriku areas. The result suggests that local L. pulchella populations were relatively isolated, and their genetic structure was minimally affected by the tsunamis. In addition, their genetic compositions were locally fixed. These results suggest that difference in the genetic composition among L. pulchella populations reflects that of artificial transport source.

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.

Ecological Consequences of the Tsunamis Caused by the Great East Japan Earthquake and Subsequent Disturbance Events in a Shallow Brackish Lagoon in Sendai Bay, Japan

The impacts of the tsunami caused by the Great East Japan Earthquake and subsequent disturbance events were examined in Gamo Lagoon, a shallow brackish lagoon in Sendai Bay, Japan. A major tsunami with a height of 7.2 m struck the lagoon on 11 March 2011, followed by a 2-month estuary closing and a typhoon-induced flood within a year. These events induced drastic changes in the salinity and sediment characteristics as well as a washout of vegetation. After the tsunami, the plant community was characterized chiefly by the proliferation of “pioneer species.” The annual helophyte Suaeda maritima became dominant in the bare high-tide zone created through the disappearance of the reed marsh. Sand dune vegetation (SDV) had recovered only in patchily after 4 years, and the community was characterized by the invasive species Cakile edentula. The density and species richness of macrozoobenthos changed after each disturbance event (i.e., the tsunami, estuary closing, and typhoon). Macrozoobenthic density recovered within 6 months after the tsunami and typhoon, chiefly due to the density overshoots of several opportunistic taxa. Though tsunami- and typhoon-induced changes in macrozoobenthic community structure were significant, by 3 years post-tsunami, the community had begun to recover toward pre-tsunami conditions. These results emphasize both the vulnerability and resiliency of estuarine biotic communities against large pulsed disturbance events.

Ecological thresholds of hypoxia and sedimentary H 2 S 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 8.5u202fmM). 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 2.4-3.3u202fmgu202fl-1 DO and 1.8-2.7u202fmM 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.

Impacts of Fuel Spills Caused by the Great East Japan Earthquake and Tsunami on the Subtidal Soft-Bottom Communities of a Semi-enclosed Bay Located on the Sanriku Coast

We assessed the ecological consequences of the fuel spills and the subsequent conflagration that resulted from the tsunamis caused by the Great East Japan Earthquake. Analyses were performed at a range of subtidal stations (water depth: 8.2–37.2 m) located in Kesennuma Bay, Japan, during the period 2011–2014. The sediments contained high concentrations of polycyclic aromatic hydrocarbons (PAHs) (mean total PAH concentrations: 18,023–89,197 μg kg−1 dry weight). Most PAHs in the sediment of the inner bay (57–82 %) were derived largely from petrogenic sources, but at other stations most (55–86 %) had pyrogenic origins. Levels of petrogenic PAHs decreased more rapidly than those of pyrogenic PAHs over 4 years, possibly because of microbial degradation. PAH contents in the sediments peaked at 8–15 cm depth and were much reduced near the sediment/water interface. The macrozoobenthic community was largely dominated by opportunistic polychaetes; other taxa, including amphipods and bivalves (but not Theora fragilis), were less abundant. The PAHs often exceeded the effects range median (ERM) in US Sediment Quality Guidelines, indicating elevated ecotoxicological risk levels for the benthic invertebrates. Nevertheless, negative impacts of PAHs were not detected in the dominant macrozoobenthos. Thus, the dominant taxa were potentially tolerant to PAH pollution. Sediment in the bay currently contains high levels of PAHs, especially in thesubsurface layer. Therefore, long-term monitoring is required to fully understand the ecological consequences (for the coastal soft-bottom communities) of the 2011 fuel spill and subsequent conflagration.

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 710u202fmg/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.

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.