Climate mediates hypoxic stress on fish diversity and nursery function at the land-sea interface

Mangrove Health Index, Community Structure and Canopy Cover in Small Islands of Bunaken National Park, Indonesia: Insights into Dominant Mangrove Species and Overall Mangrove Condition

Mangrove ecosystems are crucial for protecting littoral regions, preserving biodiversity and sequestering carbon. The implementation of effective conservation and management strategies requires a comprehensive understanding of mangrove community structure, canopy coverage and overall health. This investigation focused on four small islands located within the Bunaken National Park in Indonesia: Bunaken, Manado Tua, Mantehage and Nain. Utilising the line transect quadrant method and hemispherical photography, the investigation comprised a total of 12 observation stations. Nain had the greatest average canopy coverage at 76.09%, followed by Mantehage, Manado Tua and Bunaken at 75.82%, 71.83% and 70.01%, respectively. Mantehage had the maximum species density, with 770.83 ind/ha, followed by Bunaken, Nain and Manado Tua with 675 ind/ha, 616.67 ind/ha and 483.34 ind/ha, respectively. The predominant sediment type observed was sandy mud and the mangrove species identified were Avicennia officinalis (AO), Bruguiera gymnorrhiza (BG), Rhizophora apiculata (RA), R. mucronata (RM), and Sonneratia alba (SA). On the small islands, S. alba emerged as the dominant mangrove species based on the importance value index (IVI). In addition, the Mangrove Health Index revealed that only 6.79% of the region exhibited poor health values, while 50% of the region was categorised as being in outstanding condition. These findings indicate that the overall condition of mangroves on these islands was relatively favourable.

Integrated Transcriptomics and Metabolomics Reveal Changes in Cell Homeostasis and Energy Metabolism in Trachinotus ovatus in Response to Acute Hypoxic Stress

Trachinotus ovatus is an economically important mariculture fish, and hypoxia has become a critical threat to this hypoxia-sensitive species. However, the molecular adaptation mechanism of T. ovatus liver to hypoxia remains unclear. In this study, we investigated the effects of acute hypoxic stress (1.5 ± 0.1 mg·L-1 for 6 h) and re-oxygenation (5.8 ± 0.3 mg·L-1 for 12 h) in T. ovatus liver at both the transcriptomic and metabolic levels to elucidate hypoxia adaptation mechanism. Integrated transcriptomics and metabolomics analyses identified 36 genes and seven metabolites as key molecules that were highly related to signal transduction, cell growth and death, carbohydrate metabolism, amino acid metabolism, and lipid metabolism, and all played key roles in hypoxia adaptation. Of these, the hub genes FOS and JUN were pivotal hypoxia adaptation biomarkers for regulating cell growth and death. During hypoxia, up-regulation of GADD45B and CDKN1A genes induced cell cycle arrest. Enhancing intrinsic and extrinsic pathways in combination with glutathione metabolism triggered apoptosis; meanwhile, anti-apoptosis mechanism was activated after hypoxia. Expression of genes related to glycolysis, gluconeogenesis, amino acid metabolism, fat mobilization, and fatty acid biosynthesis were up-regulated after acute hypoxic stress, promoting energy supply. After re-oxygenation for 12 h, continuous apoptosis favored cellular function and tissue repair. Shifting from anaerobic metabolism (glycolysis) during hypoxia to aerobic metabolism (fatty acid β-oxidation and TCA cycle) after re-oxygenation was an important energy metabolism adaptation mechanism. Hypoxia 6 h was a critical period for metabolism alteration and cellular homeostasis, and re-oxygenation intervention should be implemented in a timely way. This study thoroughly examined the molecular response mechanism of T. ovatus under acute hypoxic stress, which contributes to the molecular breeding of hypoxia-tolerant cultivars.

Trace metal elements and organic contaminants are differently related to the growth and body condition of wild European sea bass juveniles

Chemical contaminants are one of the causes of the ongoing degradation of coastal and estuarine nurseries, key functional habitats in which the juveniles of many marine species grow. As chemical contaminants can cause a decrease in the energy available and induce defence mechanisms reducing the amount of energy allocated to life history traits, quantifying their effect on the fitness of juvenile fish is key to understand their population-level consequences. However, these effects are primarily estimated experimentally or in the wild but on a limited number of contaminants or congeners that do not reflect the wide variety of chemical contaminants to which juvenile fish are exposed. To address this issue, we measured concentrations of 14 trace metal elements (TMEs) and bioaccumulative organic contaminants (OCs) in European sea bass juveniles (1-year-old) from three major French nurseries (Seine, Loire and Gironde estuaries). We tested the hypotheses that (i) levels and profiles of contaminants differed among studied nurseries, and ii) fish growth and body condition (based on morphometric measurements and muscle C:N ratio) were lower in individuals with higher contaminant concentrations. Multivariate analyses showed that each nursery had distinct contaminant profiles for both TMEs and OCs, confirming the specific contamination of each estuary, and the large array of contaminants accumulated by sea bass juveniles. Increasing concentrations in some TMEs were associated to decreased growth, and TMEs were consistently related to lower fish body condition. The effect of OCs was more difficult to pinpoint possibly due to operational constraints (i.e., analyses on pooled fish) with contrasting results (i.e., higher growth and decreased body condition). Overall, this study shows that chemical contaminants are related to lower fish growth and body condition at an early life stage in the wild, an effect that can have major consequences if sustained in subsequent ages and associated with a decline in survival and/or reproductive success.

Rapid enhancement of multiple ecosystem services following the restoration of a coastal foundation species

The global decline of marine foundation species (kelp forests, mangroves, salt marshes, and seagrasses) has contributed to the degradation of the coastal zone and threatens the loss of critical ecosystem services and functions. Restoration of marine foundation species has had variable success, especially for seagrasses, where a majority of restoration efforts have failed. While most seagrass restorations track structural attributes over time, rarely do restorations assess the suite of ecological functions that may be affected by restoration. Here we report on the results of two small-scale experimental seagrass restoration efforts in a central California estuary where we transplanted 117 0.25-m² plots (2,340 shoots) of the seagrass species Zostera marina. We quantified restoration success relative to persistent reference beds, and in comparison to unrestored, unvegetated areas. Within three years, our restored plots expanded ~8,500%, from a total initial area of 29 to 2,513 m² . The restored beds rapidly began to resemble the reference beds in (1) seagrass structural attributes (canopy height, shoot density, biomass), (2) ecological functions (macrofaunal species richness and abundance, epifaunal species richness, nursery function), and (3) biogeochemical functions (modulation of water quality). We also developed a multifunctionality index to assess cumulative functional performance, which revealed restored plots are intermediate between reference and unvegetated habitats, illustrating how rapidly multiple functions recovered over a short time period. Our comprehensive study is one of few published studies to quantify how seagrass restoration can enhance both biological and biogeochemical functions. Our study serves as a model for quantifying ecosystem services associated with the restoration of a foundation species and demonstrates the potential for rapid functional recovery that can be achieved through targeted restoration of fast-growing foundation species under suitable conditions.

Severe hypoxia exposure inhibits larval brain development but does not affect the capacity to mount a cortisol stress response in zebrafish

Fish nursery habitats are increasingly hypoxic and the brain is recognized as highly hypoxia-sensitive, yet there is a lack of information on the effects of hypoxia on the development and function of the larval fish brain. Here, we tested the hypothesis that by inhibiting brain development, larval exposure to severe hypoxia has persistent functional effects on the cortisol stress response in zebrafish (Danio rerio). Exposing 5 days post-fertilization (dpf) larvae to 10% dissolved O2 (DO) for 16 h only marginally reduced survival, but it decreased forebrain neural proliferation by 55%, and reduced the expression of neurod1, gfap, and mbpa, markers of determined neurons, glia, and oligodendrocytes, respectively. The 5 dpf hypoxic exposure also elicited transient increases in whole body cortisol and in crf, uts1, and hsd20b2 expression, key regulators of the endocrine stress response. Hypoxia exposure at 5 dpf also inhibited the cortisol stress response to hypoxia in 10 dpf larvae and increased hypoxia tolerance. However, 10% DO exposure at 5 dpf for 16h did not affect the cortisol stress response to a novel stressor in 10 dpf larvae or the cortisol stress response to hypoxia in adult fish. Therefore, while larval exposure to severe hypoxia can inhibit brain development, it also increases hypoxia tolerance. These effects may transiently reduce the impact of hypoxia on the cortisol stress response but not its functional capacity to respond to novel stressors. We conclude that the larval cortisol stress response in zebrafish has a high capacity to cope with severe hypoxia-induced neurogenic impairment.

Spatial and temporal changes of dissolved oxygen in waters of the Pajarales complex, Ciénaga Grande de Santa Marta: Two decades of monitoring

Ciénaga Grande de Santa Marta lagoon complex, located in the Colombian Caribbean, is a highly degraded estuarine system, in which massive deaths of organisms have occurred since the 1990s, causing socioeconomic effects on the inhabitants, who are mostly artisanal fishermen. These deaths have been attributed to the deoxygenation of the water at night, as a result of the eutrophication of the system. To understand the variability of dissolved oxygen and its relationship with other water quality variables, the monthly time series collected between 2001 and 2019, in seven stations of the Pajarales Complex (western side of the estuarine complex), were analyzed. Analyzes showed that there are significant differences between stations, as well as between the surface and the bottom of the water, indicating that the behavior of oxygen in the system is not homogeneous, a product of the hydrodynamics of the system. Also, temporal differences were found related to the periods of rain and drought, even with larger-scale climatic events such as El Niño and La Niña, with the lowest concentrations being recorded during the rainy seasons. On the other hand, the analysis of the time series of the average surface temperature of the seven stations analyzed showed a slight tendency to increase over time. Results indicate that the system is very dynamic and its oxygenation conditions are determined by climatic factors that promote changes in water chemistry, such as variations in salinity, temperature, and pH, and biological activity, determined by the abundance of the organisms. Analysis of this information becomes a tool to propose an alert system that allows reducing the impact of deaths.

Toxic responses of liver in Lateolabrax maculatus during hypoxia and re-oxygenation

Estuarine environmental have been reported to undergo significant fluctuations in oxygen concentrations with hypoxic conditions and subsequent re-oxygenation events being of significant concern for resident fish populations. In this study we assessed the toxicological effects of hypoxia and re-oxygenation on the liver of hypoxia-sensitive spotted sea bass (Lateolabrax maculatus) that were exposed to hypoxia (1.17 mg/L dissolved oxygen) for 12 h and then re-oxygenated for 12 h. The activities of glutamic-pyruvic transaminase and glutamic-oxalacetic transaminase in serum significantly increased under hypoxia (p < 0.05) and continued to increase during re-oxygenation (p < 0.05), indicating that normal liver function might be disrupted by hypoxia and might become worse during re-oxygenation for 12h. Total protein, albumin, and globulin levels in serum decreased under hypoxia but began to return to normal during re-oxygenation, showing that protein synthesis in the liver decreased during hypoxia but could be restored by re-oxygenation. We also used RNA-Seq technology to identify changes in gene expression in the liver during hypoxia and re-oxygenation. Transcriptome sequencing revealed that the hypoxia-inducible factor (HIF-1) signaling pathway, apoptosis, and purine metabolism transcripts were significantly enriched under hypoxia and re-oxygenation conditions. A total of 15 and 16 apoptosis-related genes were induced by hypoxia and re-oxygenation stress, respectively. The apoptosis index increased from the normal to the hypoxic condition and was highest under re-oxygenation. Additionally, 19 and 29 genes, that are involved in purine metabolism in the liver of L. maculatus during hypoxia and re-oxygenation, respectively, were dysregulated. Unexpectedly, the serum uric acid level significantly increased during hypoxia and significantly decreased under re-oxygenation, indicating the presence of purine metabolic disorder in the liver of L. maculatus. These results illustrate that hypoxia poses a pronounced threat to hepatocyte function in L. maculatus and that liver damage is difficult to reverse with 12 h of re-oxygenation, and it may actually become worse when re-oxygenation is established.

Variable coastal hypoxia exposure and drivers across the southern California Current

Declining oxygen is one of the most drastic changes in the ocean, and this trend is expected to worsen under future climate change scenarios. Spatial variability in dissolved oxygen dynamics and hypoxia exposures can drive differences in vulnerabilities of coastal ecosystems and resources, but documentation of variability at regional scales is rare in open-coast systems. Using a regional collaborative network of dissolved oxygen and temperature sensors maintained by scientists and fishing cooperatives from California, USA, and Baja California, Mexico, we characterize spatial and temporal variability in dissolved oxygen and seawater temperature dynamics in kelp forest ecosystems across 13° of latitude in the productive California Current upwelling system. We find distinct latitudinal patterns of hypoxia exposure and evidence for upwelling and respiration as regional drivers of oxygen dynamics, as well as more localized effects. This regional and small-scale spatial variability in dissolved oxygen dynamics supports the use of adaptive management at local scales, and highlights the value of collaborative, large-scale coastal monitoring networks for informing effective adaptation strategies for coastal communities and fisheries in a changing climate.

Impacts of hypoxic events surpass those of future ocean warming and acidification

Over the past decades, three major challenges to marine life have emerged as a consequence of anthropogenic emissions: ocean warming, acidification and oxygen loss. While most experimental research has targeted the first two stressors, the last remains comparatively neglected. Here, we implemented sequential hierarchical mixed-model meta-analyses (721 control-treatment comparisons) to compare the impacts of oxygen conditions associated with the current and continuously intensifying hypoxic events (1-3.5 O₂ mg l^-1) with those experimentally yielded by ocean warming (+4 °C) and acidification (-0.4 units) conditions on the basis of IPCC projections (RCP 8.5) for 2100. In contrast to warming and acidification, hypoxic events elicited consistent negative effects relative to control biological performance-survival (-33%), abundance (-65%), development (-51%), metabolism (-33%), growth (-24%) and reproduction (-39%)-across the taxonomic groups (mollusks, crustaceans and fish), ontogenetic stages and climate regions studied. Our findings call for a refocus of global change experimental studies, integrating oxygen concentration drivers as a key factor of ocean change. Given potential combined effects, multistressor designs including gradual and extreme changes are further warranted to fully disclose the future impacts of ocean oxygen loss, warming and acidification.

Fish ecotyping based on machine learning and inferred network analysis of chemical and physical properties

Functional diversity rather than species richness is critical for the understanding of ecological patterns and processes. This study aimed to develop novel integrated analytical strategies for the functional characterization of fish diversity based on the quantification, prediction and integration of the chemical and physical features in fish muscles. Machine learning models with an improved random forest algorithm applied on 1867 muscle nuclear magnetic resonance spectra belonging to 249 fish species successfully predicted the mobility patterns of fishes into four categories (migratory, territorial, rockfish, and demersal) with accuracies of 90.3-95.4%. Markov blanket-based feature selection method with an ecological-chemical-physical integrated network based on the Bayesian network inference algorithm highlighted the importance of nitrogen metabolism, which is critical for environmental adaptability of fishes in nutrient-rich environments, in the functional characterization of fish biodiversity. Our study provides valuable information and analytical strategies for fish home-range assessment on the basis of the chemical and physical characterization of fish muscle, which can serve as an ecological indicator for fish ecotyping and human impact monitoring.

Combined effects of moderate hypoxia, pesticides and PCBs upon crucian carp fish, Carassius carassius, from a freshwater lake- in situ ecophysiological approach

Nowadays, depletion of oxygen or hypoxia has become a real concerning problem worldwide in freshwater, marine, and estuarine ecosystems and very often co-occurs with xenobiotics. Even though the acute and severe hypoxia is heavily studied in environment and laboratory studies, the in situ combined effects of these stressors on freshwater lake organisms are poorly understood. The current study sought to understand how the combined effects of moderate hypoxia, pesticides and PCBs affect the biochemistry, physiology and organ morphology of Carassius carassius, residing in the Lake Seferani, Dumrea region (Elbasan, Albania), a natural karst freshwater system declared as Nature Monument situated in central Albania. Crucian carp is used as a model organism, because of its residency and ecological relevance to the Lake, as well as for its amenability for the environmental toxicology studies. For this purpose, blood, liver and kidney samples of fish were processed for hematological, biochemical and histopathological analysis. We found a significant increase of blood glucose (GLU), cortisol levels, hematocrit (PCV) and hemoglobin (Hb) which clearly indicate the presence of stress in fish. Based on the histopathological evaluation and organ index results, liver and kidney organs displayed moderate-to-heavy histological-architecture changes. Our results provide a strong evidence that both, hypoxia and the presence of pesticides and PCB congeners found in Seferani Lake, put a heavy load on C. carassius energy metabolism and endocrine system, leading to an elevation of the biochemical and physiological parameters (hemoglobin level, hematocrit, glucose and cortisol), as well as the histopathological alterations. Additionally, in the presence of moderate hypoxia, the toxic effects of pesticides and PCBs on C. carassius are exacerbated. Further studies are needed to evaluate possible effects of pesticide and PCBs toxicity in human health, since crucian carp has an economic value for the population of the zone and it is used often as food sustenance. Elucidation of these kinds of responses can better improve our understanding of response of highly tolerant species, like Carassius carassius, to multiple stressors interactions, helping us to better predict and manage the consequences of the exposure of the freshwater biota to complex stressors in an environment that changes rapidly.

Impact of multiple stressors on juvenile fish in estuaries of the northeast Pacific

A key step in identifying global change impacts on species and ecosystems is to quantify effects of multiple stressors. To date, the science of global change has been dominated by regional field studies, experimental manipulation, meta-analyses, conceptual models, reviews, and studies focusing on a single stressor or species over broad spatial and temporal scales. Here, we provide one of the first studies for coastal systems examining multiple stressor effects across broad scales, focused on the nursery function of 20 estuaries spanning 1,600 km of coastline, 25 years of monitoring, and seven fish and invertebrate species along the northeast Pacific coast. We hypothesized those species most estuarine dependent and negatively impacted by human activities would have lower presence and abundances in estuaries with greater anthropogenic land cover, pollution, and water flow stress. We found significant negative relationships between juveniles of two of seven species (Chinook salmon and English sole) and estuarine stressors. Chinook salmon were less likely to occur and were less abundant in estuaries with greater pollution stress. They were also less abundant in estuaries with greater flow stress, although this relationship was marginally insignificant. English sole were less abundant in estuaries with greater land cover stress. Together, we provide new empirical evidence that effects of stressors on two fish species culminate in detectable trends along the northeast Pacific coast, elevating the need for protection from pollution, land cover, and flow stressors to their habitats. Lack of response among the other five species could be related to differing resistance to specific stressors, type and precision of the stressor metrics, and limitations in catch data across estuaries and habitats. Acquiring improved measurements of impacts to species will guide future management actions, and help predict how estuarine nursery functions can be optimized given anthropogenic stressors and climate change scenarios.

Declining oxygen in the global ocean and coastal waters

Oxygen is fundamental to life. Not only is it essential for the survival of individual animals, but it regulates global cycles of major nutrients and carbon. The oxygen content of the open ocean and coastal waters has been declining for at least the past half-century, largely because of human activities that have increased global temperatures and nutrients discharged to coastal waters. These changes have accelerated consumption of oxygen by microbial respiration, reduced solubility of oxygen in water, and reduced the rate of oxygen resupply from the atmosphere to the ocean interior, with a wide range of biological and ecological consequences. Further research is needed to understand and predict long-term, global- and regional-scale oxygen changes and their effects on marine and estuarine fisheries and ecosystems.

Tracing the influence of land-use change on water quality and coral reefs using a Bayesian model

Coastal ecosystems can be degraded by poor water quality. Tracing the causes of poor water quality back to land-use change is necessary to target catchment management for coastal zone management. However, existing models for tracing the sources of pollution require extensive data-sets which are not available for many of the world’s coral reef regions that may have severe water quality issues. Here we develop a hierarchical Bayesian model that uses freely available satellite data to infer the connection between land-uses in catchments and water clarity in coastal oceans. We apply the model to estimate the influence of land-use change on water clarity in Fiji. We tested the model’s predictions against underwater surveys, finding that predictions of poor water quality are consistent with observations of high siltation and low coverage of sediment-sensitive coral genera. The model thus provides a means to link land-use change to declines in coastal water quality.

Coast-wide recruitment dynamics of Olympia oysters reveal limited synchrony and multiple predictors of failure

Recruitment of new propagules into a population can be a critical determinant of adult density. We examined recruitment dynamics in the Olympia oyster (Ostrea lurida), a species occurring almost entirely in estuaries. We investigated spatial scales of interannual synchrony across 37 sites in eight estuaries along 2,500 km of Pacific North American coastline, predicting that high vs. low recruitment years would coincide among neighboring estuaries due to shared exposure to regional oceanographic factors. Such synchrony in recruitment has been found for many marine species and some migratory estuarine species, but has never been examined across estuaries in a species that can complete its entire life cycle within the same estuary. To inform ongoing restoration efforts for Olympia oysters, which have declined in abundance in many estuaries, we also investigated predictors of recruitment failure. We found striking contrasts in absolute recruitment rate and frequency of recruitment failure among sites, estuaries, and years. Although we found a positive relationship between upwelling and recruitment, there was little evidence of synchrony in recruitment among estuaries along the coast, and only limited synchrony of sites within estuaries, suggesting recruitment rates are affected more strongly by local dynamics within estuaries than by regional oceanographic factors operating at scales encompassing multiple estuaries. This highlights the importance of local wetland and watershed management for the demography of oysters, and perhaps other species that can complete their entire life cycle within estuaries. Estuaries with more homogeneous environmental conditions had greater synchrony among sites, and this led to the potential for estuary-wide failure when all sites had no recruitment in the same year. Environmental heterogeneity within estuaries may thus buffer against estuary-wide recruitment failure, analogous to the portfolio effect for diversity. Recruitment failure was correlated with lower summer water temperature, higher winter salinity, and shorter residence time: all indicators of stronger marine influence on estuaries. Recruitment failure was also more common in estuaries with limited networks of nearby adult oysters. Large existing oyster networks are thus of high conservation value, while estuaries that lack them would benefit from restoration efforts to increase the extent and connectivity of sites supporting oysters.

Behavioral and physiological responses of yellow perch (Perca flavescens) to moderate hypoxia

While severe hypoxia can be lethal and is usually avoided by mobile aquatic organisms, moderate hypoxic conditions are likely more prevalent and may affect organisms, such as fishes, in a variety of systems. However, fishes have the potential to adjust physiologically and behaviorally and thus reduce the negative effects of hypoxia. Quantifying such physiological responses may shed light on the ability of fishes to tolerate reduced oxygen concentrations. This study assessed how two different hatchery populations of yellow perch Perca flavescens, a fish that is likely to encounter moderate hypoxic conditions in a variety of systems, responded to moderate hypoxic exposure through three experiments: 1) a behavioral foraging experiment, 2) an acute exposure experiment, and 3) a chronic exposure experiment. No marked behavioral or physiological adjustments were observed in response to hypoxia (e.g., hemoglobin, feeding rate, movement frequency, gene expression did not change to a significant degree), possibly indicating a high tolerance level in this species. This may allow yellow perch to utilize areas of moderate hypoxia to continue foraging while avoiding predators that may be more sensitive to moderately low oxygen.

Trophic cascades on the edge: fostering seagrass resilience via a novel pathway

Despite widespread degradation, some coastal ecosystems display remarkable resilience. For seagrasses, a century-old paradigm has implicated macroalgal blooms stimulated by anthropogenic nutrient, loading as a primary driver of seagrass decline, yet relatively little attention has been given to drivers of seagrass resilience. In Elkhorn Slough, CA, an estuarine system characterized by extreme anthropogenic nutrient loading and macroalgal (Ulva spp.) blooms, seagrass (Zostera marina) beds have recovered concurrent with colonization of the estuary by top predators, sea otters (Enhydra lutris). Here, we follow up on the results of a previous experiment at the seagrass interior, showing how sea otters can generate a trophic cascade that promotes seagrass. We conducted an experiment and constructed structural equation models to determine how sea otters, through a trophic cascade, might affect the edge of seagrass beds where expansion occurs. We found that at the edge, sea otters promoted both seagrass and ephemeral macroalgae, with the latter contributing beneficial grazers to the seagrass. The surprising results that sea otters promote two potentially competing vegetation types, and a grazer assemblage at their boundary provides a mechanism by which seagrasses can expand in eutrophic environments, and contributes to a growing body of literature demonstrating that ephemeral macroalgae are not always negatively associated with seagrass. Our results highlight the potential for top predator recovery to enhance ecosystem resilience to anthropogenic alterations through several cascading mechanisms.