Revisiting inland hypoxia: diverse exceedances of dissolved oxygen thresholds for freshwater aquatic life

Teasing out the Effects of Natural Stressors at Chemically Contaminated Sites

Aquatic ecosystems are often impacted by a multitude of stressors, many of which are introduced by a combination of anthropogenic activities such as agricultural development, urbanization, damming, and industrial discharge. Determining the primary stressors responsible for ecological impairments at a site can be complex and challenging; however, it is crucial for making informed management decisions. Improper diagnosis of an impaired system can lead to misguided attempts at remediation, which can be both time consuming and costly. We focused on the development, implementation, and evaluation of methodologies that, in combination, allowed us to identify the primary stressors. These included a four-phase, weight-of-evidence (WOE) assessment including in situ Toxicity Identification and Evaluation (iTIE) testing, physicochemical and macrobenthos characterization, reciprocal sediment transplants, and laboratory and in situ toxicity testing. The contaminants of concern (COCs) at the site were elevated levels of ammonia, chloride, pH, and total dissolved solids in groundwater upwellings into a high-quality waterway. Reciprocal transplants of site sediments and nearby reference sediments and traditional benthic sampling showed impaired benthic indices and multiple stations around a contaminated industrial settling basin. Impaired stations had elevated COCs in groundwaters but exhibited a steep vertical concentration gradient, with concentrations decreasing near the sediment-surface water interface. We describe Phase 4 of the study, which focused on teasing out the role of dissolved oxygen sags in benthic macroinvertebrate responses. Extensive submerged and emergent macrophytes, algae, and cyanobacteria co-occurred at the impaired sites and increased throughout the summer. Laboratory testing suggested that ammonia and pH were possibly toxic at the sites, based on groundwater concentrations. The in situ toxicity testing, however, showed toxicity occurring even at stations with low levels of COCs concurrently with large diurnal fluxes in dissolved oxygen (DO). A final phase using a type of iTIE approach utilized limnocorrals with and without aeration and with in situ toxicity measures using Hyalella azteca. The Phase 4 assessment revealed that low DO levels were primarily responsible for impaired benthic communities, and COC upwellings were diluted at the sediment-water interface to nontoxic levels. These findings will allow for improved management decisions for more efficient and effective restoration activities. Environ Toxicol Chem 2024;43:1524-1536. © 2024 SETAC.

Comparative transcriptomics combined with physiological and functional analysis reveals the regulatory mechanism of rainbow trout (Oncorhynchus mykiss) under acute hypoxia stress

Hypoxia, largely triggered by global warming and water contamination, has become an environmental issue of great concern, posing a great threat to aquatic ecosystem. As one of the world's most economically important fish, rainbow trout (Oncorhynchus mykiss) is extremely intolerant of hypoxic environments, however, little is known about the roles of non-coding RNAs (ncRNAs) in the response of rainbow trout to hypoxia stress. Herein, effects of moderate (Tm12L) and severe hypoxia for 12 h (Ts12L) and 12 h reoxygenation on histology, biochemical parameters (antioxidant, metabolism and immunity) and transcriptome (lncRNA, miRNA and mRNA) in rainbow trout liver were investigated. We further validated the regulatory relationships between LOC110519952, novel-m0023-5p and glut1a via dual‑luciferase reporter, overexpression and silencing assays. Compared with Tm12L, the liver in Ts12L showed more severe oxidative damage. Anaerobic, lipid and protein metabolism was enhanced under hypoxia stress, especially in Ts12L. We also found that Tm12L could strengthen innate immune response, which was inhibited in Ts12L. Besides, several hypoxia-related genes (glut1a, vegfaa, hmox, epoa, foxo1a and igfbp1) and ceRNA networks were identified from 1824, 427 and 545 differentially expressed mRNAs, miRNAs and lncRNAs, including LOC118965299-novel-m0179-3p-epoa, LOC110519952-novel-m0023-5p-glut1a, MSTRG.7382.2-miR-184-y-hmox and LOC110520012-miR-206-y-vegfaa. Through in vitro and in vivo functional analysis, we demonstrated that glut1a is a target of novel-m0023-5p, and LOC110519952 can positively regulate glut1a by targeting novel-m0023-5p. Introduction of LOC110519952 could attenuate the promoting effects of novel-m0023-5p on rainbow trout liver cell viability and proliferation. This study highlights the differences in the regulatory mechanism of rainbow trout under different concentrations of hypoxia stress and provides valuable data for further research on the molecular mechanisms of fish adaptation to hypoxic environments.

The correlation between echinoderms diversity and physicochemical parameters in marine pollution: A case study of the Persian Gulf coastline

This study was conducted with the aim of investigating the correlation between echinoderms diversity and physicochemical parameters in the Persian Gulf coastline in Bushehr province in 4 seasons from March to December 2017. The physicochemical parameters including water temperature, dissolved oxygen (DO), electrical conductivity (EC), salinity, pH and turbidity were measured at each sampling location. The results showed a significant correlation between echinoderms diversity and physicochemical parameters. The correlation coefficient of the Astropecten polyacanthus species with the parameters of temperature, DO, EC, salinity and turbidity was reported as -0.41, 0.64, -0.25, -0.44 and 0.60 respectively. This coefficient for the Ophiothrix fragilis species was reported as -0.68, 0.70, -0.21, -0.36 and -0.55 respectively. The results demonstrated that the most sensitive species were Astropecten polyacanthus and Ophiothrix fragilis respectively. The different species of echinoderms can be used as biological indicators of pollution in evaluating the physicochemical quality of marine environments.

The Multifaceted Effects of Short-Term Acute Hypoxia Stress: Insights into the Tolerance Mechanism of Propsilocerus akamusi (Diptera: Chironomidae)

Plenty of freshwater species, especially macroinvertebrates that are essential to the provision of numerous ecosystem functions, encounter higher mortality due to acute hypoxia. However, within the family Chironomidae, a wide range of tolerance to hypoxia/anoxia is displayed. Propsilocerus akamusi depends on this great tolerance to become a dominant species in eutrophic lakes. To further understand how P. akamusi responds to acute hypoxic stress, we used multi-omics analysis in combination with histomorphological characteristics and physiological indicators. Thus, we set up two groups-a control group (DO 8.4 mg/L) and a hypoxic group (DO 0.39 mg/L)-to evaluate enzyme activity and the transcriptome, metabolome, and histomorphological characteristics. With blue-black chromatin, cell tightness, cell membrane invagination, and the production of apoptotic vesicles, tissue cells displayed typical apoptotic features in the hypoxic group. Although lactate dehydrogenase (LDH), alcohol dehydrogenase (ADH), catalase (CAT), and Na+/K+ -ATPase (NKA) activities were dramatically enhanced under hypoxic stress, glycogen content, and superoxide dismutase (SOD) activities were significantly reduced compared to the control group. The combined analysis of the transcriptome and metabolome, which further demonstrated, in addition to carbohydrates, including glycogen, the involvement of energy metabolism pathways, including fatty acid, protein, trehalose, and glyoxylate cycles, provided additional support for the aforementioned findings. Lactate is the end product of glycogen degradation, and HIF-1 plays an important role in promoting glycogenolysis in acute hypoxic conditions. However, we discovered that the ethanol tested under hypoxic stress likely originates from the symbiodinium of P. akamusi. These results imply that some parameters related to energy metabolism, antioxidant enzyme activities, and histomorphological features may be used as biomarkers of eutrophic lakes in Chironomus riparius larvae. The study also provides a scientific reference for assessing toxicity and favoring policies to reduce their impact on the environment.

Flushing away the future: The effects of wastewater treatment plants on aquatic invertebrates

Wastewater treatment plants (WWTP) are essential infrastructure in our developing world. However, with the development and release of novel entities and without modern upgrades, they are ineffective at fully removing micropollutants before treated effluents are released back into aquatic environments. Thus, WWTPs may represent additional point source impacts to freshwater environments, further pressuring aquatic fauna and already vulnerable insect communities. Previous studies - mostly focusing on single WWTPs - have shown general trends of freshwater invertebrate communities becoming dominated by pollution tolerant taxa. To expand on these findings, the current study is the first to comprehensively investigate data on the effects of 170 WWTPs on invertebrate taxonomic composition. We compared data for several diversity and pollution indices, as well as the taxonomic composition both upstream and downstream of the WWTPs (366 sampling sites). In terms of abundance, the three most frequent and negatively impacted orders were the Plecoptera, Trichoptera and Gastropoda, while the Turbellaria, Hirudinea and Crustacea increased in abundance. Although strong changes in community composition were observed between upstream and downstream sites (mean species turnover of 61%), commonly used diversity indices were not sensitive to these changes, highlighting their potential inadequacy in accurately assessing ecological health. Our results indicate that WWTPs change downstream conditions in favour of pollution tolerant taxa to the detriment of sensitive taxa. Order-level taxonomic responses can be informative but should be interpreted with caution, since they can be driven by a few taxa, or opposing responses of species in the same group can result in an overall low order-level response. Upgrading WWTPs via additional treatment steps or merging may be beneficial, provided upstream sections are unimpacted and/or are in a good chemical and structural condition.

Impacts of Low Oxygen on Marine Life: Neglected, but a Crucial Priority for Research

AbstractGlobal ocean O₂ content has varied significantly across the eons, both shaping and being shaped by the evolutionary history of life on planet Earth. Indeed, past O₂ fluctuations have been associated with major extinctions and the reorganization of marine biota. Moreover, its most recent iteration-now anthropogenically driven-represents one of the most prominent challenges for both marine ecosystems and human societies, with ocean deoxygenation being regarded as one of the main drivers of global biodiversity loss. Yet ocean deoxygenation has received far less attention than concurrent environmental variables of marine climate change, namely, ocean warming and acidification, particularly in the field of experimental marine ecology. Together with the lack of consistent criteria defining gradual and acute changes in O₂ content, a general lack of multifactorial studies featuring all three drivers and their interactions prevents an adequate interpretation of the potential effects of extreme and gradual deoxygenation. We present a comprehensive overview of the interplay between O₂ and marine life across space and time and discuss the current knowledge gaps and future steps for deoxygenation research. This work may also contribute to the ongoing call for an integrative perspective on the combined effects of these three drivers of change for marine organisms and ecosystems worldwide.

Thresholds for Reduction in Fish Growth and Consumption Due to Hypoxia: Implications for Water Quality Guidelines to Protect Aquatic Life

Control of hypoxia is a key element of water quality management, and guidelines are usually based on qualitative reviews of hypoxia impacts. In this study we use segmented regression to identify both thresholds for growth reduction and rate of decline of fish growth and food consumption under hypoxia; and then evaluate whether current freshwater guidelines for dissolved oxygen based on qualitative reviews are consistent with the quantitative analysis of hypoxia thresholds. Segmented regressions were fit to data from published growth-hypoxia studies for freshwater (N = 17) and marine fishes (N = 13). To understand potential drivers of hypoxia tolerance, we also modelled thresholds as simple functions of environmental and ecological covariates for each species including trophic level, marine vs. freshwater environment, maximum fish length, fish weight, and maximum temperature tolerance. The average threshold for growth reduction (G_crit; 5.1 mg·l^-1 DO) and decreased food consumption (C_crit = 5.6 mg·l^-1 DO) were not significantly different, and did not differ between marine and freshwater taxa. However, salmonids showed a significantly steeper decline in growth with increasing hypoxia relative to other taxa. Growth declined by 22% for every mg·l^-1 reduction in DO below average G_crit, and significant regressions indicate that warmwater (R² = 0.25) and smaller-bodied (R² = 0.44) species are more likely to be hypoxia tolerant. Observed mean G_crit and C_crit in the range of 5-6 mg·l^-1 broadly match minimum water quality guidelines for the protection of aquatic life in freshwater in representative industrialized countries. However, this is much higher than the definition of hypoxia typically used in marine systems (2-2.5 mg·l^-1), indicating a need to reconcile definition of hypoxia in the marine environment with empirical data. The principal challenge in freshwater hypoxia management is now translating discretionary guidelines into effective regulatory frameworks to reduce the incidence and severity of hypoxia.

Investigating the causes of reduced dissolved oxygen concentrations in Kickapoo Creek, TX

Excessive organic matter and nutrients can depress dissolved oxygen concentration (DOC) in streams. The aim of this study was to understand the depressed DOC of Kickapoo Creek in Texas, USA, which is an impaired stream; identify the possible sources causing the depressed DOC and elevated Escherichia coli levels; and identify possible remedial measures. Monthly grab water quality data was monitored for nine stations in the watershed. For three of the nine stations, 24-h DOC was also monitored for a comparison with the minimum and average DOC criteria. Correlational, graphical, spatial, and temporal analyses were carried out for DOC concentration with other water quality variables which have the potential to depress DOC in the stream. The correlational analyses show a weak to moderate correlation for DOC with nutrient and oxygen-demanding substance concentrations present in the stream. However, there are spatial and temporal trends in DOC data that can be attributed to the nutrient influx into the stream. A pattern of increasing nutrient concentrations from upstream to downstream partially explains the decreasing dissolved oxygen (DO) concentrations observed towards the lower reaches. Visual interpretations of riparian vegetation and sediment influx also support the spatial patterns in DO concentrations. The majority of the depressed DOC occurs in the summer months when streamflow is at a minimum. The depressed DOC during summer is partially explained by the increasing daily temperatures for the summer months as revealed by the trend analysis of the daily temperature data from 1981 to 2020 using the modified Mann-Kendall test, Pettitt test, and Sen's slope.

Interplay of geomorphology and hydrology drives macroinvertebrate assemblage responses to hydropeaking

Hydropeaking leads to major anthropogenic disturbance of river networks worldwide. Flow variations imposed by hydropeaking may significantly affect macroinvertebrate assemblages within the river network. As such, the responses of macroinvertebrate assemblages to hydropeaking are expected to be complex and vary across spatial and temporal scales as well as ecological organization levels. To unpack this complexity, we assessed the interplay of geomorphic and hydrological variables as drivers of the responses of macroinvertebrate assemblages to hydropeaking. Specifically, we studied different levels of ecological organization of macroinvertebrate assemblages in two functional process zones (FPZs; Sub-Andean and Central Valley Gravel Dominated) subjected to different flow management in two Chilean Andean river networks. Hydropeaking caused significant reduction of macroinvertebrate abundances in both FPZs and at all ecological organization levels with the exception of one feeding guild (scrapers). Furthermore, the response of macroinvertebrate assemblage variance was stronger in the Central Valley Gravel Dominated FPZ. Both geomorphic and hydrological variables influenced macroinvertebrate assemblage responses. However, the effects of the principal geomorphic variables operated at valley (meso) spatial scale and the main hydrological variables operated at the sub-daily (micro) temporal scale. Therefore, to minimise the effects of hydropeaking on macroinvertebrate assemblages, flow management should consider reduction of sub-daily variability. Furthermore, placement of new barriers should take into account not only their position within the river network but also their effects downstream that strongly depend on characteristics of river valley.

Integrating Exposure and Effect Distributions with the Ecotoxicity Risk Calculator: Case Studies with Crop Protection Products

Risk curves describe the relationship between cumulative probability and magnitude of effect and thus express far more information than risk quotients. However, their adoption has remained limited in ecological risk assessment. Therefore, we developed the Ecotoxicity Risk Calculator (ERC) to simplify the derivation of risk curves, which can be used to inform risk management decisions. Case studies are presented with crop protection products, highlighting the utility of the ERC at incorporating various data sources, including surface water modeling estimates, monitoring observations, and species sensitivity distributions. Integr Environ Assess Manag 2021;17:321-330. © 2020 Syngenta Crop Protection, LLC. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Seasonal variation in thermal tolerance of redside dace Clinostomus elongatus

Organisms living in environments with oscillating temperatures may rely on plastic traits to sustain thermal tolerance during high temperature periods. Phenotypic plasticity in critical thermal maximum (CTmax) is a powerful thermoregulative strategy that enables organisms to adjust CTmax when ambient temperatures do not match thermal preference. Given that global temperatures are increasing at an unprecedented rate, identifying factors that affect the plastic response in CTmax can help predict how organisms are likely to respond to changes in their thermal landscape. Using an experimental thermal chamber in the field, we investigated the effect of short-term acclimation on the CTmax and thermal safety margin (TSM) of wild-caught redside dace, Clinostomus elongatus, (n = 197) in a northern population in Two Tree River, Ontario. Streamside CTmax trials were used to identify the maximum temperature at which redside dace maintain equilibrium, providing a powerful tool for understanding how thermal stress affects individual performance. CTmax and TSM of redside dace were sensitive to changes in temperature, regardless of season, suggesting that temperature pulses caused by climate change or urban activities can impose negative fitness consequences year round. Interestingly, an individual's recent thermal history was more influential to its thermal tolerance than the current ambient water temperature. While the CTmax of redside dace increased with body size, the effect of body size on TSM remains unclear based on our models. The results provide insight into the thermal performance of redside dace that, to date, has been difficult to assess due to the species' rarity and lack of suitable streamside protocols.

Biological index based on epiphytic diatom assemblages is more restrictive than the physicochemical index in water assessment on an Amazon floodplain, Brazil

Canadian Water Quality Index (CWQI) provides protection for freshwater life promoting healthy ecosystems and safeguarding human health. Biological Diatom Index (BDI) was developed to indicate the ecological status and water quality of freshwater systems. This paper evaluates the relations between the two different indices. During rising and falling, water samples were taken in the Curuai Floodplain, Brazil. CWQI was calculated using 14 physicochemical parameters and 1 microbiological parameter. The limits were established according to freshwater quality conditions and standards based on water use classes 1 and 2 determined in CONAMA 357 legislation and British Columbia. Canadian Water Quality Index categorization ranged from "marginal" to "excellent," most sampling units were "good" (71%), followed by "fair" (12%) and "excellent" (12%) water quality. Total phosphorus (38 times), chlorophyll a (20), dissolved oxygen (10), and total organic carbon (10) were the parameters that presented the most non-compliance values. Encyonema silesiacum (14%), Gomphonema parvulum (13%), and Navicula cryptotenella (12%) were the main taxa in the rising period, while G. lagenula, E. silesiacum, and Fragilaria capucina were the main taxa during the falling period. BDI ranges from I to V water quality classes. We observed "poor" to "very good" ecological status, with most sampling units "moderate" (52%) and "good" (29%). Water quality for class 2 was better than water quality for class 1, as the limits of the parameters evaluated were more restrictive in class 1 than in class 2 and the predominant uses of water require a higher degree of water purity. The biological index based on diatoms was the most restrictive index whose water classes and categorizations have shown an ecological status that could threaten the protection of aquatic communities on the Curuai floodplain. We suggest the combined use of both indices-physicochemical and biological for water quality assessment in this type of environment.

Relations of dissolved-oxygen variability, selected field constituents, and metabolism estimates to land use and nutrients in high-gradient Boston Mountain streams, Arkansas

Continuous monitoring data can be extremely useful for assessing water-quality conditions particularly for variables, such as dissolved oxygen, that exhibit dynamic diel swings. As a means of evaluating stream dissolved oxygen criteria used by the Arkansas Department of Environmental Quality (ADEQ), we compared continuous dissolved oxygen (DO) data collected at five small- to moderate-sized (watersheds 10-100 mi²), high-gradient streams in the Boston Mountains distributed across a land-use and nutrient condition gradient. The sampled streams exhibit a general pattern established for other aquatic systems (e.g., larger streams, low-gradient streams, and lakes) where increasing land-use intensity results in increased nutrient concentrations, stream eutrophication, and increased DO variability. DO concentrations were < 6 mg/L for fewer than 4% of measurements at the two sites identified "a priori" as least disturbed by nutrient and land-use indices, while concentrations at the three sites identified as moderately and most disturbed were < 6 mg/L for 20 to 33% of measurements. These findings demonstrate that the current criterion (10% of the DO measurements are < 6 mg/L) employed by ADEQ was effective at identifying various degrees of DO impairment in Boston Mountain streams. Our analysis also demonstrated that continuous pH and specific conductance data and estimates of stream metabolism were helpful for attributing DO variability to anthropogenic or natural origins. Considerations that were useful for examining these relationships and evaluating ADEQ's DO criterion should be applicable to DO studies in other locations where stream and geologic characteristics are similar to those of the Boston Mountains.

Oxidative stress response of caddisfly Stenopsyche marmorata larvae to abrupt hypoxia-normoxia shift

Natural and anthropogenic effects cause low dissolved oxygen conditions (hypoxia) and subsequent reoxygenated conditions (normoxia) in river systems. However, oxidative stress responses to hypoxia-normoxia shift in aquatic insects are still poorly understood. Here, we exposed caddisfly Stenopsyche marmorata larvae to 30-min hypoxic followed by 1-d normoxic exposure, with experiments being repeated at 14 °C (Exp.1) and 20 °C (Exp.2), respectively. Exp.1 was conducted in December 2016 using overwintering larvae, and Exp.2 was conducted in June 2016 using non-wintering larvae. The responses of superoxide dismutase (SOD) and catalase (CAT) activity, oxygen radical absorption capacity (ORAC), lipid peroxidation (LPO), and energy reserves were investigated. The hypoxia-normoxia shift considerably inhibited CAT and ORAC in Exp.1. In addition, the energy reserves were decreased in response to exposure to severe hypoxia-normoxia. However, LPO was not induced under these conditions. It is conceivable that regulating antioxidant defense enzymes and utilizing energy reserves may suppress the expected increases in LPO. In contrast, the hypoxia-normoxia shift in Exp.2 had almost no effect on oxidative stress response, with only ORAC being induced. Exp.1 had a lower dissolved oxygen partial pressure and a larger difference in the oxygen partial pressure between hypoxia and normoxia than Exp.2. The severity of hypoxia-normoxia shift and the differences in the life cycles (overwintering or non-wintering) may cause the difference in the response of ORAC in Exp.1 and Exp.2. This study revealed that the effect of the hypoxia-normoxia shift on oxidative stress response in aquatic insects and the strength of the impact of the shift on oxidative stress response may be influenced by water temperature and life cycles.

Influence of Diltiazem on Fathead Minnows Across Dissolved Oxygen Gradients

Water resources in many arid to semi-arid regions are stressed by population growth and drought. Growing populations and climatic changes are influencing contaminant and water chemistry dynamics in urban inland waters, where flows can be dominated by, or even dependent on, wastewater effluent discharge. In these watersheds, interacting stressors such as dissolved oxygen and environmental contaminants (e.g., pharmaceuticals) have the potential to affect fish physiology and populations. Recent field observations from our group identified the calcium channel blocker (CCB) diltiazem in fish plasma exceeding human therapeutic doses (e.g., C_min ) in aquatic systems impaired because of nonattainment of dissolved oxygen water quality standards. Therefore our study objectives examined: 1) standard acute and chronic effects of dissolved oxygen and diltiazem to fish, 2) influences of dissolved oxygen at criteria levels deemed protective of aquatic life on diltiazem toxicity to fish, and 3) whether sublethal effects occur at diltiazem water concentrations predicted to cause a human therapeutic level (therapeutic hazard value [THV]) in fish plasma. Dissolved oxygen × diltiazem co-exposures significantly decreased survival at typical stream, lake, and reservoir water quality standards of 5.0 and 3.0 mg dissolved oxygen/L. Dissolved oxygen and diltiazem growth effects were observed at 2 times and 10 times lower than median lethal concentration (LC50) values (1.7 and 28.2 mg/L, respectively). Larval fathead minnow (Pimephales promelas) swimming behavior following low dissolved oxygen and diltiazem exposure generally decreased and was significantly reduced in light-to-dark bursting distance traveled, number of movements, and duration at concentrations as low as the THV. Individual and population level consequences of such responses are not yet understood, particularly in older organisms or other species; however, these findings suggest that assessments with pharmaceuticals and other cardioactive contaminants may underestimate adverse outcomes in fish across dissolved oxygen levels considered protective of aquatic life. Environ Toxicol Chem 2018;37:2835-2850. © 2018 SETAC.