Effects of hypoxia on benthic organisms in Tokyo Bay, Japan: a review

Remarkable shifts in the megabenthic community structure over four decades in Tokyo Bay, Japan, in relation to environmental variations

We investigated long-term changes in the megabenthic community in Tokyo Bay, Japan, using data from fisheries-independent trawl surveys conducted from 1977 to 2023. In addition, we examined the potential relationship between changes in biotic communities and environmental conditions. The total abundance and biomass exhibited an increasing trend until 1987, followed by a substantial decline from the late 1980s to the 1990s due to a decrease in small to medium-sized fish and crustacean species. Meanwhile, a marked increase in the number of large fish (including elasmobranchs), mollusks, and echinoids, was observed in the 2000s. These shifts in the megabenthic community structure were correlated with an increase in water temperature and a decrease in nutrient concentrations and copepod densities. Cumulative evidence suggests that a remarkable shift in the megabenthic community structure occurred between the 1970s and the 2020s, which was possibly associated with variations in the environmental conditions in Tokyo Bay.

Long-term developments in seasonal hypoxia and response to climate change: A three-decade modeling study in the Ariake Sea, Japan

Hypoxia in the Ariake Sea, Japan, is steadily increasing in both duration and spatial coverage. Hypoxia, defined as dissolved oxygen (DO) below 3 mg/L, is strongly associated with the amplified frequency of extreme rainfall events driven by climate change, which poses a mounting threat to marine ecosystems on a global scale. In this study, we employed a general three-dimensional (3-D) hydrodynamic coastal model and a phytoplankton-based ecosystem model to identify the potential cause of seasonal hypoxic events over three decades. The results indicated a substantial decrease in bottom DO levels from 1992 to 2021, with the rate of increase in hypoxic area being 8 km2/yr (95 % CI: -0.38, 16.2) and the anoxic area increasing from almost non-existent to 100 km2. Notably, among various environmental drivers, increased river discharge was identified as a pivotal factor in the occurrence of hypoxia. Large-scale river discharge events can potentially increase water stratification, leading to the formation of hypoxia. River discharge volume and the duration of bottom hypoxia in the Ariake Sea were correlated. The duration of hypoxia was strongly associated with river discharge magnitude, with correlation coefficients ranging from 0.56 to 0.82 across six observational stations. Furthermore, analysis of varied simulated environmental factors over multiple years revealed diverse responses to climate change, indicating that the Ariake Sea is prone to experiencing a decline in its physical and water quality conditions.

Hypoxia in the Blue Mussel Mytilus chilensis Induces a Transcriptome Shift Associated with Endoplasmic Reticulum Stress, Metabolism, and Immune Response

The increase in hypoxia events, a result of climate change in coastal and fjord ecosystems, impacts the health and survival of mussels. These organisms deploy physiological and molecular responses as an adaptive mechanism to maintain cellular homeostasis under environmental stress. However, the specific effects of hypoxia on mussels of socioeconomic interest, such as Mytilus chilensis, are unknown. Using RNA-seq, we investigated the transcriptomic profiles of the gills, digestive gland, and adductor muscle of M. chilensis under hypoxia (10 days at 2 mg L-1) and reoxygenation (10 days at 6 mg L-1). There were 15,056 differentially expressed transcripts identified in gills, 11,864 in the digestive gland, and 9862 in the adductor muscle. The response varied among tissues, showing chromosomal changes in Chr1, Chr9, and Chr10 during hypoxia. Hypoxia regulated signaling genes in the Toll-like, mTOR, citrate cycle, and apoptosis pathways in gills, indicating metabolic and immunological alterations. These changes suggest that hypoxia induced a metabolic shift in mussels, reducing reliance on aerobic respiration and increasing reliance on anaerobic metabolism. Furthermore, hypoxia appeared to suppress the immune response, potentially increasing disease susceptibility, with negative implications for the mussel culture industry and natural bed populations. This study provides pivotal insights into metabolic and immunological adaptations to hypoxia in M. chilensis, offering candidate genes for adaptive traits.

Characterization of polychaetes inhabiting estuaries and inner bays by composition analysis of amino acids and lactate enantiomers

In this study, we investigated the composition of free amino acids and lactate (Lac) in polychaetes in river estuaries and inner bays using chromatographic techniques. Both L-amino acids and D-amino acids (D-asparagine, D-alanine (D-Ala), D-serine, D-aspartic acid, and D-proline (D-Pro)) were detected, indicating that polychaetes contain some D-amino acids. Some polychaete species exhibited notable amino acid levels, such as glycine in Capitellidae sp. and Thelepus sp., D-Pro in Glycera sp., and β-Ala in Scoletoma nipponica and Scoletoma sp.. High D-Lac levels were detected in Tylorrhynchus osawai and Hediste diadroma, (691 and 797 μmol/100 g-wet, respectively), with the D-form exceeding 98%. T. osawai was dominant in the upper tidal-sensitive zone, wherein other organisms were less abundant because of low salinity (3-8 PSU). Seasonal differences in the concentrations of components in T. osawai were observed, particularly a significant increase in D-Lac in the reproductive period. Notably, the D-Lac concentrations of T. osawai were higher upstream than downstream. Thus, D-Lac might be involved in strategies underlying adaptations to low salinity and reproductive activity. These results suggest that both the D-form of Lac and amino acids may play certain physiological roles in the life of polychaetes.

Monitoring harmful microalgal species and their appearance in Tokyo Bay, Japan, using metabarcoding

During the recent decade, high-throughput sequencing (HTS) techniques, in particular, DNA metabarcoding, have facilitated increased detection of biodiversity, including harmful algal bloom (HAB) species. In this study, the presence of HAB species and their appearance patterns were investigated by employing molecular and light microscopy-based monitoring in Tokyo Bay, Japan. The potential co-appearance patterns between the HAB species, as well as with other eukaryotes and prokaryotes were investigated using correlation and association rule-based time-series analysis. In total, 40 unique HAB species were detected, including 12 toxin-producing HAB species previously not reported from the area. More than half of the HAB species were present throughout the sampling season (summer to autumn) and no structuring or succession patterns associated with the environmental conditions could be detected. Statistically significant (p < 0.05, rS ranging from −0.88 to 0.90) associations were found amongst the HAB species and other eukaryotic and prokaryotic species, including genera containing growth-limiting bacteria. However, significant correlations between species differed amongst the years, indicating that variability in environmental conditions between the years may have a stronger influence on the microalgal community structure and interspecies interactions than the variability during the sampling season. The association rule-based time-series analysis allowed the detection of a previously reported negative relationship between Synechococcus sp. and Skeletonema sp. in nature. Overall, the results support the applicability of metabarcoding and HTS-based microalgae monitoring, as it facilitates more precise species identification compared to light microscopy, as well as provides input for investigating potential interactions amongst different species/groups through simultaneous detection of multiple species/genera.

Similar fish species composition despite larger environmental heterogeneity during severe hypoxia in a coastal ecosystem

Environmental heterogeneity is one of the most influential factors that create compositional variation among local communities. Greater compositional variation is expected when an environmental gradient encompasses the most severe conditions where species sorting is more likely to operate. However, evidence for stronger species sorting at severer environment has typically been obtained for less mobile organisms and tests are scarce for those with higher dispersal ability that allows individuals to sensitively respond to environmental stress. Here, with the dynamics of fish communities in a Japanese bay revealed by environmental DNA metabarcoding analyses as a model case, we tested the hypothesis that larger environmental heterogeneity caused by severe seasonal hypoxia (lower concentration of oxygen in bottom waters in summer) leads to larger variation of species composition among communities. During summer, fish species richness was lower in the bottom layer, suggesting the severity of the hypoxic bottom water. In contrast to the prediction, we found that although the environmental parameters of bottom and surface water was clearly distinct in summer, fish species composition was more similar between the two layers. Our null model analysis suggested that the higher compositional similarity during hypoxia season was not a result of the sampling effect reflecting differences in the alpha or gamma diversity. Furthermore, a shift in the species occurrence from bottom to surface layers was observed during hypoxia season, which was consistent across species, suggesting that the severe condition in the bottom adversely affected fish species irrespective of their identity. These results suggest that larger environmental heterogeneity does not necessarily lead to higher compositional variation once the environmental gradient encompasses extremely severe conditions. This is most likely because individual organisms actively avoided the severity quasi‐neutrally, which induced mass effect‐like dispersal and lead to the mixing of species composition across habitats. By showing counter evidence against the prevailing view, we provide novel insights into how species sorting by environment acts in heterogeneous and severe conditions.

What is the relationship between hypoxia, water chemistry and otolith manganese content?

Previous studies have shown an increase in otolith Mn caused by exposure to hypoxic water masses. The mechanism leading to the increases in otolith Mn is still unclear, but might possibly be due to the larger amount of available Mn left in the water column under hypoxia. Thus, this study aimed to examine the relationship between hypoxia, water Mn and otolith Mn through marbled flounder (Pseudopleuronectes yokohamae, Günther) captured from Tokyo Bay and reared under different water Mn at laboratory. Otoliths from the Bay showed a higher (Mn/Ca)_otolith than outside, together with a seasonal trend of high (Mn/Ca)_otolith at the start of translucent zones (which form in the summer), supporting the occurrence of summer hypoxia in Tokyo Bay. Nonetheless, juveniles reared under control (Mn 0.50 μmol l^-1 ), middle (Mn 6.94 μmol l^-1 ) and high (Mn 10.4 μmol l^-1 ) treatments of water Mn concentrations showed a disproportional smaller increase in (Mn/Ca)_otolith . Comparing the laboratory experiment with the field data, (Mn/Ca)_water under hypoxia in Tokyo Bay could reach a low level similar to control treatment, yet (Mn/Ca)_otolith of the Bay showed a higher value than the high treatment. These results revealed an elevated (Mn/Ca)_otolith towards hypoxia, but also suggested that changes in water Mn might not be directly recorded by otolith Mn.

Transcriptome analysis reveals differential immune related genes expression in Ruditapes philippinarum under hypoxia stress: potential HIF and NF-κB crosstalk in immune responses in clam

Background

Hypoxia is an important environmental stressor in aquatic ecosystems, with increasingly impacts on global biodiversity. Benthic communities are the most sensitive parts of the coastal ecosystem to eutrophication and resulting hypoxia. As a filter-feeding organism living in the seafloor sediment, Ruditapes philippinarum represents an excellent “sentinel” species to assess the quality of marine environment. In order to gain insight into the molecular response and acclimatization mechanisms to hypoxia stress in marine invertebrates, we examined hypoxia-induced changes in immune-related gene expression and gene pathways involved in hypoxia regulation of R. philippinarum.

Results

We investigated the response of the Manila clam R. philippinarum to hypoxia under experimental conditions and focused on the analysis of the differential expression patterns of specific genes associated with hypoxia response by RNA-seq and time course qPCR analysis. A total of 75 genes were captured significantly differentially expressed, and were categorized into antioxidant/oxidative stress response, chaperones/heat shock proteins, immune alteration, and cell proliferation/apoptosis. Fourteen hypoxia responsive genes were validated significantly up/down regulated at different time 0, 2, 5, and 8 d in gills of R. philippinarum in hypoxia challenged group. Functional enrichment analysis revealed the HIF signaling pathway and NF-κB signaling pathway play pivotal roles in hypoxia tolerance and resistance in R. philippinarum.

Conclusion

The HIF signaling pathway and NF-κB signaling pathway play a critical role in hypoxia tolerance and resistance in Manila clam. The immune and defense related genes and pathways obtained here gain a fundamental understanding of the hypoxia stress in marine bivalves and provide important insights into the physiological acclimation, immune response and defense activity under hypoxia challenge. The reduced metabolism is a consequence of counterbalancing investments in immune defense against other physiological processes.

The effect of hypoxia and hydrocarbons on the anti-predator performance of European sea bass (Dicentrarchus labrax)

Hydrocarbons contamination and hypoxia are two stressors that can coexist in coastal ecosystems. At present, few studies evaluated the combined impact of these stressors on fish physiology and behavior. Here, we tested the effect of the combination of hypoxia and petrogenic hydrocarbons on the anti-predator locomotor performance of fish. Specifically, two groups of European sea bass (Dicentrarchus labrax) were exposed to clean water (Ctrl) or oil-contaminated water (Oil). Subsequently, fish of both groups were placed in normoxic (norx) or hypoxic (hyp) experimental tanks (i.e. four groups of fish were formed: Ctrl norx, Ctrl hyp, Oil norx, Oil hyp). In these tanks, escape response was elicited by a mechano-acoustic stimulus and recorded with a high speed camera. Several variables were analyzed: escape response duration, responsiveness (percentage of fish responding to the stimulation), latency (time taken by the fish to initiate a response), directionality (defined as away or toward the stimulus), distance-time variables (such as speed and acceleration), maneuverability variables (such as turning rate), escape trajectory (angle of flight) and distancing of the fish from the stimulus. Results revealed (i) effects of stressors (Ctrl hyp, Oil norx and Oil hyp) on the directionality; (ii) effects of Oil norx and Oil hyp on maneuverability and (iii) effects of Oil hyp on distancing. These results suggest that individual stressors could alter the escape response of fish and that their combination could strengthen these effects. Such an impact could decrease the probability of prey escape success. By investigating the effects of hydrocarbons (and the interaction with hypoxia) on the anti-predator behavior of fish, this work increases our understanding of the biological impact of oil spill. Additionally, the results of this study are of interest for oil spill impact evaluation and also for developing new ecotoxicological tools of ecological significance.

Ontogenetic and temperature-dependent changes in tolerance to hypoxia and hydrogen sulfide during the early life stages of the Manila clam Ruditapes philippinarum

Wind-induced upwelling of hypoxic waters containing hydrogen sulfide (H₂S) sometimes causes mass mortalities of aquatic organisms inhabiting coastal areas, including the hypoxia-tolerant Manila clam Ruditapes philippinarum. We examined the tolerance of Manila clam to H₂S under controlled laboratory conditions. Larvae and juveniles obtained by artificial fertilization or from a wild population were exposed to normoxic or to hypoxic water with or without un-ionized H₂S (concentrations, 0.2-52.2 mg/L). Twenty-four-hour exposure experiments revealed ontogenetic changes in the clam's tolerance to H₂S exposure: tolerance was enhanced from the larval stages to juveniles just after settlement but was attenuated as juveniles grew. Tolerance of larvae and juveniles to H₂S exposure weakened as the water temperature rose from 20 to 28 °C. Prolonged 48-h exposure to H₂S attenuated the tolerance of juveniles to H₂S. Temporary suspension of H₂S exposure by 24-h reoxygenation improved the ability of juveniles to withstand repeated H₂S exposure.

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.

Duration rather than frequency of hypoxia causes mass mortality in ark shells (Anadara kagoshimensis)

Hypoxia is associated with mass mortality in estuaries, but a direct causal relationship has not been proven to date. This study aimed to demonstrate this relationship and to evaluate how the duration of hypoxia affects the survival of ark shells (Anadara kagoshimensis) using mathematical modeling. The dissolved oxygen concentration was monitored at two stations in the innermost area of Ariake Bay, Japan, to calculate the duration of hypoxia. This was then included in a mathematical model to simulate the population density with sequential computation. The population density decreased with prolonged hypoxia, reaching a value close to the observed population density, indicating that hypoxia is the main cause of mass mortality in ark shells. Furthermore, the ark shell population disappeared in 8days with constant hypoxia but persisted when hypoxia was alternated with normoxia every 6 h. Therefore, mass mortality is caused by the duration rather than the frequency of hypoxia.

Ventilatory and behavioural responses of the marbled sole Pseudopleuronectes yokohamae to progressive hypoxia

This study identified ventilatory and behavioural responses in the marbled sole Pseudopleuronectes yokohamae under experimentally induced progressive decreases in dissolved oxygen (DO) levels. Ventilation frequency showed an increase with decreasing DO levels from normoxia to 2·75 mg O₂  l^-1 , followed by a decrease in ventilation frequency at decreased DO levels from 2·00 to 0·75 mg O₂ l^-1 . At DO levels below 2·00 mg l^-1 , behaviours at the bottom were suppressed, whereas avoidance behaviours increased. A decrease in avoidance behaviours was observed from 1·00 to 0·75 mg O₂ l^-1 . Upside-down reversal and incapacitation at DO levels of 1·00-0·75 mg O₂ l^-1 suggested that sublethal effects on P. yokohamae were induced. The responses observed before the sublethal DO level could be interpreted as an effort to maintain oxygen uptake, reduce routine activities and facilitate avoidance. The observed DO level thresholds that induce behavioural responses, in addition to sublethal effects, indicate hypoxia-tolerance that is important for understanding the effects of hypoxia on coastal ecosystems.

Towards a meaningful assessment of marine ecological impacts in life cycle assessment (LCA)

Human demands on marine resources and space are currently unprecedented and concerns are rising over observed declines in marine biodiversity. A quantitative understanding of the impact of industrial activities on the marine environment is thus essential. Life cycle assessment (LCA) is a widely applied method for quantifying the environmental impact of products and processes. LCA was originally developed to assess the impacts of land-based industries on mainly terrestrial and freshwater ecosystems. As such, impact indicators for major drivers of marine biodiversity loss are currently lacking. We review quantitative approaches for cause-effect assessment of seven major drivers of marine biodiversity loss: climate change, ocean acidification, eutrophication-induced hypoxia, seabed damage, overexploitation of biotic resources, invasive species and marine plastic debris. Our review shows that impact indicators can be developed for all identified drivers, albeit at different levels of coverage of cause-effect pathways and variable levels of uncertainty and spatial coverage. Modeling approaches to predict the spatial distribution and intensity of human-driven interventions in the marine environment are relatively well-established and can be employed to develop spatially-explicit LCA fate factors. Modeling approaches to quantify the effects of these interventions on marine biodiversity are less well-developed. We highlight specific research challenges to facilitate a coherent incorporation of marine biodiversity loss in LCA, thereby making LCA a more comprehensive and robust environmental impact assessment tool. Research challenges of particular importance include i) incorporation of the non-linear behavior of global circulation models (GCMs) within an LCA framework and ii) improving spatial differentiation, especially the representation of coastal regions in GCMs and ocean-carbon cycle models.

Impaired gamete production and viability in Atlantic croaker collected throughout the 20,000 km(2) hypoxic region in the northern Gulf of Mexico

The long-term impacts of recent marked increases in the incidence and extent of hypoxia (dissolved oxygen <2 mg/L) in coastal regions worldwide on fisheries and ecosystems are unknown. Reproductive impairment was investigated in Atlantic croaker collected in 2010 from the extensive coastal hypoxic region in the northern Gulf of Mexico. Potential fecundity was significantly lower in croaker collected throughout the ~20,000 km(2) hypoxic region than in croaker from normoxic sites. In vitro bioassays of gamete viability showed reductions in oocyte maturation and sperm motility in croaker collected from the hypoxic sites in response to reproductive hormones which were accompanied by decreases in gonadal levels of membrane progestin receptor alpha, the receptor regulating these processes. The finding that environmental hypoxia exposure reduces oocyte viability in addition to decreasing oocyte production in croaker suggests that fecundity estimates need to be adjusted to account for the decrease in oocyte maturation.

Disturbance of recruitment success of mantis shrimp in Tokyo Bay associated with effects of hypoxia on the early life history

We investigated effects of severe hypoxia (dissolved oxygen <1 ml l(-1)) on recruitment of mantis shrimp Oratosquilla oratoria in Tokyo Bay. Ten-year field surveys were conducted to examine quantitative relationships in annual mean densities of larvae and juveniles, and spatial distribution of juveniles and severe hypoxia. There was no significant correlation between annual mean densities of larvae and juveniles, suggesting that mortality during larval or juvenile stages varies among years, which might have regulated abundance of young-of-the-year juveniles. Juvenile density was low in the severely hypoxic area, implying that hypoxia could affect survivals and spatial distribution of juveniles. Meanwhile, there are yearly fluctuations in juvenile density in normoxic areas of both northern and southern part of the bay. This evidence suggests that abundance of post-settled juveniles might have been determined by not only effects of hypoxia, but also other factors influencing mortality during the early life stages.

Changes in growth and reproductive traits of dragonet Callionymus valenciennei in Tokyo Bay, concurrent with decrease in stock size

We examined changes in the growth and reproductive traits of dragonet Callionymus valenciennei concurrent with decrease in stock size in Tokyo Bay. Stock size in mid 2000s decreased to 14% of that recorded in early 1990s. The minimum standard length at which dragonet attain gonadal maturation was smaller in 2000s (4.8 cm) compared to that in 1990s (6.0 cm). In addition, timing of the onset of the first spawning became earlier in 2000s (starting from spring) than that in 1990s (from summer). We also found significant changes in growth for both sexes from 1990s to 2000s; growth of dragonet after they attained sexual maturation showed a significant decrease in 2000s against that in 1990s. Changes in the life history traits may reflect trade-off for allocating available energy resource to reproduction rather than to somatic growth under limited prey abundance for enhancing a chance for stock recovery. However, these changes in the life history traits might have not led to the resilience of the population due to negative effects of hypoxia on settlement of larvae from the spring spawning season.

Hypoxia induces abnormal larval development and affects biofilm-larval interaction in the serpulid polychaete Hydroides elegans

Hydroides elegans, a worldwide fouling polychaete, can spawn throughout the year, but its recruitment drops during summer when hypoxia prevails. Here, the influence of hypoxia on larval development and settlement of H. elegans was investigated. Results showed that larval development was compromised at 1mg O2 l(-1) with a lower proportion of competent larvae and a higher proportion of malformed larvae, probably due to reduction in clearance rate. Regarding larval settlement, although most of the larvae were reluctant to settle at 1mg O2 l(-1), regardless of the biofilm nature, they settled quickly within 24h in response to the resumption of dissolved oxygen. Furthermore, only about 5% of the larvae settled on the biofilms developed under hypoxia, regardless of dissolved oxygen levels of the seawater. The delayed larval development and potential alteration of biofilm nature owing to hypoxia explained why the recruitment of H. elegans declines during summer.

Sediment contamination assessment in urban areas based on total suspended solids

Sediment represents an important compartment in surface waters. It constitutes a habitat or spawning site for many organisms and is an essential trophic resource for higher level organisms. It can be impacted by anthropogenic activities, particularly through urban wet-weather discharges like stormwater and combined sewer overflows. An approach was presented for assessing the risks caused by urban wet-weather discharges to the sediment compartment based on total suspended solids (TSS). TSS is routinely measured in field surveys and can be considered as a tracer for urban wet-weather contamination. Three assessment endpoints linked with TSS were proposed: a) siltation of the riverbed, b) oxygen demand due to organic matter degradation and c) accumulation of ecotoxic contaminants on the riverbed (heavy metals, PAHs). These criteria were translated in terms of the maximal TSS accumulation load and exposure time (percentage of time exceeding the accumulation criteria) to account for sediment accumulation dynamics and resuspension in streams impacted by urban wet-weather discharges. These assessment endpoints were implemented in a stochastic model that calculates TSS behavior in receiving waters and allows therefore an assessment of potential impacts. The approach was applied to three Swiss case studies. For each, good agreement was found between the risk predictions and the field measurements confirming the reliability of the approach.

A year in hypoxia: epibenthic community responses to severe oxygen deficit at a subsea observatory in a coastal inlet

Changes in ocean ventilation driven by climate change result in loss of oxygen in the open ocean that, in turn, affects coastal areas in upwelling zones such as the northeast Pacific. Saanich Inlet, on the west coast of Canada, is a natural seasonally hypoxic fjord where certain continental shelf species occur in extreme hypoxia. One study site on the VENUS cabled subsea network is located in the hypoxic zone at 104 m depth. Photographs of the same 5 m(2) area were taken with a remotely-controlled still camera every 2/3 days between October 6(th) 2009 and October 18(th) 2010 and examined for community composition, species behaviour and microbial mat features. Instruments located on a near-by platform provided high-resolution measurements of environmental variables. We applied multivariate ordination methods and a principal coordinate analysis of neighbour matrices to determine temporal structures in our dataset. Responses to seasonal hypoxia (0.1-1.27 ml/l) and its high variability on short time-scale (hours) varied among species, and their life stages. During extreme hypoxia, microbial mats developed then disappeared as a hippolytid shrimp, Spirontocaris sica, appeared in high densities (200 m(-2)) despite oxygen below 0.2 ml/l. The slender sole Lyopsetta exilis was abundant in severe hypoxia and diminished as oxygen increased in the summer. This planktivore may be responding to changes in the depth of the diurnal migration of zooplankton. While the squat lobster Munida quadrispina was common at all times, juveniles disappeared in fluctuating conditions. Despite low oxygen conditions, animal densities were high indicating that the risk from hypoxia is balanced by factors such as food availability and escape from less tolerant predators. As hypoxia increases on the continental shelf, we expect benthic communities to become dominated by low diversity, hypoxia-tolerant species of low commercial significance.

Response of the epibenthic macrofaunal community to a strong upwelling-driven hypoxic event in a shallow bay of the southern Humboldt Current System

In January 2008, most of the southern coastal zone of the Humboldt Current System was affected by an intense upwelling event. This caused an intrusion of equatorial sub-surface water into the coastal zone, generating severe hypoxic conditions (≤0.5 ml O(2) l(-1)) three days after the beginning of the event. A rapid, massive die-off of marine organisms occurred in Coliumo Bay on January 3rd, affecting zooplankton, mollusks, crustaceans and fishes. Normal oxygen concentrations were observed on January 10th, seven days after the hypoxic event. Here we analyze the response of the epibenthic macrofauna community using data spanning three years of sampling which encompass the short-term hypoxic disturbance in the bay. We found that (i) strong changes in total density, total biomass, and diversity occurred immediately after the hypoxic event, negatively affecting crustaceans and fishes, while gastropods were favored, (ii) initial changes were reverted over a period of three months, (iii) on an inter-annual time scale, species richness and diversity decreased following the hypoxic event. Total density increased strongly, but total biomass showed no clear inter-annual trend. These results show that, while initial recovery from hypoxia was fast, over longer time scales the community exhibited a shift to an alternative structure dominated principally by Nassariid scavenger species.

Spatio-temporal biodiversity of soft bottom macrofaunal assemblages in shallow coastal waters exposed to episodic hypoxic events

The Humboldt Current System (HCS) has one of the three most important oxygen minimum zones (OMZ) of the global ocean. Several studies have looked at the macrofaunal benthic assemblages inhabiting the continental shelf and shallow bays off central-southern Chile associated with low oxygen areas, but little is known about open coast macrofaunal communities within this zone, which are frequently subjected to the low oxygen conditions of Equatorial Subsurface Waters (ESSW). In order to assess local and mesoscale coastal macrofauna dynamics, the sampling area (ca. 40 linear km) was divided into seven local zones (Cobquecura, southern Cobquecura, northern Itata, Itata River mouth, external, southern Itata, and Coliumo). Eight oceanographic cruises were carried out between May 2006 and February 2008 covering 16 coastal sampling sites, between 36°07'S and 36°30'S. The macrofaunal assemblage was dominated by polychaetes, crustaceans, and mollusks. Our results suggest a high degree of temporal faunal stability on the mesoscale in soft bottom communities along the open coast, given the persistence of a faunal assemblage dominated by organisms tolerant of low oxygen conditions. While there is some local variability in community attributes, the main structuring factor for soft bottom communities in the shallow coastal area off central-southern Chile is the seasonal intrusion of low oxygen ESSW.

Upregulation of hypoxia-inducible factor (HIF)-1α and HIF-2α mRNA levels in dragonet Callionymus valenciennei exposed to environmental hypoxia in Tokyo Bay

There is an urgent need to develop reliable biomarkers of environmental hypoxia exposure in marine organisms in order to assess the ecological impacts of the marked increase in coastal hypoxia worldwide. Full-length hypoxia-inducible factor HIF-1α and HIF-2α cDNAs were cloned and characterized from dragonet Callionymus valenciennei, a representative epibenthic teleost inhabiting hypoxic areas in Tokyo Bay, to evaluate the potential utility of their transcript levels as biomarkers of environmental hypoxia exposure. HIF-1α and HIF-2α transcript levels in the livers of dragonet collected from hypoxic sites in Tokyo Bay were increased several-fold over those collected from nonhypoxic areas. Further, HIF-1α and HIF-2α mRNA levels were increased in dragonet livers after 7 days exposure to hypoxia in a controlled laboratory experiment, and they declined to control levels within 24h of restoration to normoxic conditions. Our findings suggest HIF-1α and HIF-2α transcript levels are potentially useful biomarkers of environmental hypoxia exposure.

Coastal resource degradation in the tropics: does the tragedy of the commons apply for coral reefs, mangrove forests and seagrass beds

The keynote paper by Garrett Hardin 44 years ago introduced the term 'tragedy of the commons' into our language (Hardin, 1968); this term is now used widely, but it is neither universally accepted nor fully understood. Irrespective, the 'tragedy of the commons' is an increasing reality for more than 500 million people that rely on the biodiversity resources and services of tropical coral reefs, mangrove forests, seagrass beds and associated fisheries. These natural resources continue to decline despite major advances in our scientific understanding of how ecosystems and human populations interact, and the application of considerable conservation and management efforts at scales from local user communities to oceans. Greater effort will be required to avert increasing damage from over-exploitation, pollution and global climate change; all deriving from increasing exploitation driven by poverty and progress i.e. continuing to expand development indefinitely and extraction of resources at industrial scales. However, the 'tragedy' concept has been widely criticized as a simple metaphor for a much larger set of problems and solutions. We argue that the 'tragedy' is essentially real and will continue to threaten the lives of millions of people unless there are some major moral and policy shifts to reverse increasing damage to coastal habitats and resources. We agree with the conclusion by Hardin that the solution to the tragedy will not be through the application of natural sciences, but via implementing exceedingly difficult and controversial moral decisions. An extreme example of a moral and controversial direction suggested by Hardin was in re-examining the 'freedom to breed' as an inherent human value. The need for 'moral decisions' is even greater in 2012.

Disturbance of benthic macrofauna in relation to hypoxia and organic enrichment in a eutrophic coastal bay

This study demonstrated the spatiotemporal patterns of the environmental conditions and benthic macrofauna in Tokyo Bay, Japan, and investigated the factors causing disturbances in the assemblage structure. In the north-central areas, the density and species diversity of the macrobenthos was low. Although hypoxia appeared in July, defaunation occurred in August. The delayed defaunation and recolonization soon after the abatement of hypoxia were attributed to several polychaete and bivalve species that were tolerant to the hypoxic environment. In the southeastern areas, however, the density and species diversity of the macrobenthos was high throughout the year, and no defaunation was recorded. Multivariate analyses showed that the disturbance in the macrofauna correlated with organic enrichment in the sediment and bottom-water hypoxia. There is a concern about further impairment of the macrofauna in the bay due to the expansion of sediment with high levels of organic matter towards the southern regions that could cause hypoxia and subsequent defaunation.

Assessment of hypoxia-inducible factor-1α mRNA expression in mantis shrimp as a biomarker of environmental hypoxia exposure

Efforts to assess the ecological impacts of the marked increase in coastal hypoxia worldwide have been hampered by a lack of biomarkers of hypoxia exposure in marine benthic organisms. Here, we show that hypoxia-inducible factor-1α (HIF-1α) transcript levels in the heart and cerebral ganglion of mantis shrimp (Oratosquilla oratoria) collected from hypoxic sites in Tokyo Bay are elevated several-fold over those in shrimp collected from normoxic sites. Upregulation of HIF-1α mRNA levels in the heart after exposure to sub-lethal hypoxia was confirmed in controlled laboratory experiments. HIF-1α transcript levels were increased at approximately threefold after 7 and 14 days of hypoxia exposure and declined to control levels within 24 h of restoration to normoxic conditions. The results provide the first evidence for upregulation of HIF-1α transcript levels in two hypoxia-sensitive organs, heart and cerebral ganglion, in a marine invertebrate exposed to environmental hypoxia. These results suggest that upregulation of HIF-1α transcript levels is an important component in adaptation of mantis shrimp to chronic hypoxia and is a potentially useful biomarker of environmental hypoxia exposure.