Evaluation of the effect of water type on the toxicity of nitrate to aquatic organisms

Derivation of site-specific guideline values for nitrate toxicity in Pilbara receiving waters with high hardness

The current study aimed to derive site-specific guideline values (SSGVs) for nitrate toxicity that are relevant to high hardness surface waters of the Pilbara region, north-western Australia, many of which receive nitrate-rich mine water discharges. The approach involved deriving SSGVs from a species sensitivity distribution (SSD) based on candidate data sets comprising toxicity data for local Pilbara species tested in local waters and nonlocal species tested under water quality conditions similar to those of local Pilbara waters. Water hardness was identified as the primary toxicity-modifying factor for nitrate that needed to be accounted for, with temperature and pH identified as supporting variables. Using ~10 years of local water quality data, primary and secondary criteria for hardness, temperature and pH were developed and used to select the most relevant toxicity data for the derivation. The selected toxicity data, which included data for four local species tested in local water and 10 nonlocal species tested under representative water quality conditions, were categorized according to the primary and secondary criteria. Using this categorization, four candidate nitrate toxicity data sets (n = 5, 10, 12, and 14) were assessed for their suitability to derive the SSGVs. The SSDs for all data sets yielded similar protective concentration (PC) values. Based on the best balance between the relevance of the toxicity data set to the local water quality conditions and the confidence in the PC values, the PC values based on data set 3 (12 species, six taxonomic groups) were identified as being the most appropriate for the SSGVs. The SSGVs for 99%, 95%, 90%, and 80% species protection were 7.6, 15, 23, and 39 mg/L NO₃ -N, respectively. An assessment of the appropriateness of the SSGVs indicated that they were likely to be appropriately protective of nitrate toxicity for the high hardness (i.e., ≥160 mg/L as CaCO₃ ) Pilbara receiving waters. Integr Environ Assess Manag 2022;18:1035-1046. © 2021 SETAC.

Development of Fluoride Protective Values for Aquatic Life Using Empirical Bioavailability Models

The derivation of protective values for aquatic life can be enhanced by the development and use of bioavailability models. Recent advances to metals bioavailability modeling are applicable to other analyte groups and should be widely considered. We conducted a meta-analysis of the available aquatic toxicity literature for fluoride to evaluate the utility of hardness, alkalinity, and chloride as toxicity-modifying factors (TMFs) in empirical bioavailability models of freshwater taxa. The resulting optimal multiple linear regression model predicting acute fluoride toxicity to the invertebrate Hyalella azteca included all three TMFs (observed vs. predicted 50% lethal concentrations, R²  = 0.88) and the optimal model predicting toxicity to the fish Oncorhynchus mykiss included alkalinity and hardness (R²  = 0.37). At >20 mg/L chloride, the preliminary final acute values for fluoride were within 1 order of magnitude and ranged from approximately 18.1 to 56.3 mg/L, depending on water chemistry. Sensitivity of H. azteca to low-chloride conditions increased model uncertainty when chloride was <20 mg/L. Because of limited toxicity data, chronic bioavailability models were not developed, and final chronic values were derived using an acute-to-chronic ratio (ACR) approach. Accounting for TMFs, the geometric mean ACR was 5.4 for fish and invertebrate taxa (n = 6). The present assessment highlights the need to expand bioavailability modeling to include inorganic anions, particularly fluoride, and demonstrates that existing promulgated protective values for fluoride are likely overly conservative. More toxicological studies are recommended to further refine multivariate empirical bioavailability models for inorganic anions. Environ Toxicol Chem 2022;41:396-409. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Nitrates in the environment: A critical review of their distribution, sensing techniques, ecological effects and remediation

Nitrate pollution is eminent in almost all the developing nations as a result of increased natural activities apart from anthropogenic pollution. The release of nitrates in more than critical quantities into the water bodies causes accretion impacts on living creatures, environmental receptors, and human vigour by accumulation through the food chain. Nitrates have recently acquired researchers' huge attention and extend their roots in environmental contamination of surface and groundwater systems. The presence of nitrate in high concentrations in surface and groundwater triggers several health problems, for instance, methemoglobinemia, diabetes, eruption of infectious disorders, harmfully influence aquatic organisms. Sensing nitrate is an alternate option for monitoring the distribution of nitrate in different water bodies. Here we review electrochemical, spectroscopic, and electrical modes of nitrate sensing. It is concluded that, among the various sensors discussed in this review, FET sensors are the most desirable choice. Their sensitivity, ease of use and scope for miniaturisation are exceptional. Advanced functional materials need to be designed to satiate the growing need for environmental monitoring. Different sources of nitrate contamination in ground and surface water can be estimated using different techniques such as nitrate isotopic composition, co contaminants, water tracers, and other specialized techniques. This review intends to explore the research work on remediation of nitrate from wastewater and soil using different processes such as reverse osmosis, chemical denitrification, biological denitrification, ion exchange, electrodialysis, and adsorption. Denitrification proves as a promising alternative over previously reported techniques in terms of their nitrate removal because of its high cost-effectiveness.

Public contestation over agricultural pollution: a discourse network analysis on narrative strategies in the policy process

The overuse of fertilizers in agriculture and their entry into freshwater has many negative impacts on biodiversity and poses problems for drinking water resources in Germany. In response to exceeding levels of nitrate concentrations in groundwater in parts of the country, an intense public dispute evolved and a significant policy change in fertilizer regulation occurred in 2020. Based on the German case of agricultural water pollution, this study demonstrates in an innovative way how discourse network analysis is a fruitful method for the integrated study of actor coalitions and their use of narrative strategies in public debate. Theoretically, the study draws on the narrative policy framework (NPF) to explain how actor coalitions use narrative strategies to attempt to influence policymaking on water pollution by agricultural activities. The empirical analysis builds on newspaper articles and press releases disseminated between 2010 and 2020. The results demonstrate how two opposing actor coalitions with congruent policy beliefs formed in the struggle over fertilizer regulation. These not only diverged in their policy beliefs but also differed in their use of narrative strategies to try to expand or contain the policy issue. More precisely, the coalitions adapted their narratives over time in response to changes in the likelihood to win or lose. Furthermore, the results suggest the coalition in favor of stricter fertilizer regulation was more sophisticated in its effort to mobilize specific target groups. Overall, the article provides a valuable contribution to the literature on the NPF by combining research on coalition formation and policy narratives.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11077-021-09439-x.

Chronic nitrate exposure cause alteration of blood physiological parameters, redox status and apoptosis of juvenile turbot (Scophthalmus maximus)

Nitrate (NO₃^-) is one of the common inorganic nitrogen compound pollutants in natural ecosystems, which may have serious risks for aquatic organisms. However, its toxicological mechanism remains unclear. In the current study, juvenile turbot (Scophthalmus maximus) were exposed to different concentrations of NO₃^- (CK- 3.57 ± 0.16, LN - 60.80 ± 1.21, MN - 203.13 ± 10.97 and HN - 414.16 ± 15.22 mg/L NO₃-N) for 60 d. The blood biochemical assays results revealed that elevated NO₃^- exposure significantly increased the concentrations of plasma NO₃^-, NO₂^-, MetHb, K⁺, cortisol, glucose, triglyceride, lactate, while significantly decreased the concentrations of plasma Hb, Na⁺ and Cl^-, which meant that NO₃^- caused hypoxic stress and further affected the osmoregulation and metabolism in fish. Besides, exposure to MN and HN induced a significant decrease in the level of antioxidants, including SOD (Point: 60th day, MN, HN v.s. CK: 258.36, 203.73 v.s. 326.95 U/mL), CAT (1.97, 1.17 v.s. 2.37 U/mL), GSH (25.38, 20.74 v.s. 37.00 μmol/L), and GPx (85.32, 71.46 v.s. 129.36 U/mL), and a significant increase of MDA (7.54, 9.73 v.s. 5.27 nmol/L), suggesting that NO₃^- exposure leading to a disruption of the redox status in fish. Also, further research revealed that NO₃^- exposure altered the mRNA levels of p53 (HN: up to 4.28 folds) and p53-regulated downstream genes such as Bcl-2 (inferior to 0.44 folds), caspase-3 (up to 2.90 folds) and caspase-7 (up to 3.49 folds), indicating that NO₃^- exposure induced abnormal apoptosis in the fish gills. Moreover, IBRv2 analysis showed that the toxicity of NO₃^- exposure to turbot was dose-dependent, and the toxicity peaked on the 15th day. In short, NO₃^- is an environmental toxicological factor that cannot be ignored, because its toxic effects are long-term and could cause irreversible damage to fish. These results would be beneficial to improve our understanding of the toxicity mechanism of NO₃^- to fish, which provides baseline evidence for the risk assessment of environmental NO₃^- in aquatic ecosystems.

Active Treatment of Contaminants of Emerging Concern in Cold Mine Water Using Advanced Oxidation and Membrane-Related Processes: A Review

Responsible use and effective treatment of mine water are prerequisites of sustainable mining. The behavior of contaminants in mine water evolves in relation to the metastable characteristics of some species, changes related to the mine life cycle, and mixing processes at various scales. In cold climates, water treatment requires adaptation to site-specific conditions, including high flow rates, salinity, low temperatures, remoteness, and sensitivity of receiving waterbodies. Contaminants of emerging concern (CECs) represent a newer issue in mine water treatment. This paper reviews recent research on the challenges and opportunities related to CECs in mine water treatment, with a focus on advanced oxidation and membrane-based processes on mine sites operating in cold climates. Finally, the paper identifies research needs in mine water treatment.

Investigating the effect of nitrate on juvenile turbot (Scophthalmus maximus) growth performance, health status, and endocrine function in marine recirculation aquaculture systems

Nitrate (NO₃^-), a potential toxic nitrogenous compound to aquatic animals, is distributed in aquatic ecosystems worldwide. The aim of this study was to investigate the effects of different NO₃^- levels on growth performance, health status, and endocrine function of juvenile turbot (Scophthalmus maximus) in recirculating aquaculture systems (RAS). Fish were exposed to 0 mg/L (control, CK), 50 mg/L (low nitrate, LN), 200 mg/L (medium nitrate, MN), and 400 mg/L (high nitrate, HN) NO₃-N for 60 d in experimental RAS. Cumulative survival (CS) was significantly decreased with increasing NO₃^- levels in LN, MN, and HN. The lowest CS was 35% in the HN group. Growth parameters, including absolute growth rate, specific growth rate, and feed conversion rate, were significantly different in HN compared with that in the CK. Histological survey of gills and liver revealed dose-dependent histopathological damage induced by NO₃^- exposure and significant differences in glutamate pyruvate transaminase and glutamate oxalate transaminase in MN and HN compared with that in the CK. The hepatosomatic index in HN was significantly higher than that in the CK. Additionally, NO₃^- significantly increased bioaccumulation in plasma in LN, MN, and HN compared to that in the CK. Significant decreases in hemoglobin and increases in methemoglobin levels indicated reduced oxygen-carrying capacity in HN. Additionally, qRT-PCR and enzyme-linked immunosorbent assay (ELISA) were developed to investigate key biomarkers involved in the GH/IGF-1, HPT, and HPI axes. Compared with that in the CK, the abundance of GH, GHRb, and IGF-1 was significantly lower in HN, whereas GHRa did not differ between treatments. The plasma T₃ level significantly decreased in LN, MN, and HN and T₄ significantly decreased in HN. The CRH, ACTH, and plasma cortisol levels were significantly upregulated in HN compared with that in the CK. We conclude that elevated NO₃^- exposure leads to growth retardation, impaired health status, and endocrine disorders in turbot and the NO₃^- level for juvenile turbot culture should not exceed 50 mg/L NO₃-N in RAS. Our findings indicate that endocrine dysfunction of the GH/IGF-1, HPT, and HPI axes might be responsible for growth inhibition induced by NO₃^- exposure.

Acute Toxicity of Sodium Chloride, Nitrates, Ortho-Phosphates, Cadmium, Arsenic and Aluminum for Juveniles of the Freshwater Pearl Mussel: Margaritifera Margaritifera (L.1758)

The freshwater pearl mussel (FWPM) Margaritifera margaritifera (L.1758) is critically endangered in Europe and ecotoxicological studies on the species are scares. Here, 96 h acute toxicity tests were conducted at 16 °C with sodium chloride (NaCl), nitrates (NO3−), ortho-phosphates (PO43−), cadmium (Cd), aluminum (Al) and arsenic (As) on 13- to 28-month-old post-parasitic juveniles. The experimental protocol was developed according to conditions described in a standard guide and was modified in order to assess toxicity thresholds for the Dronne River. Results showed that juveniles were tolerant to concentrations of Al, Cd, As, PO43−, NO3− and NaCl, largely higher than those found in the Dronne river, since 96 h EC50s (effective concentrations) were >954 µg/L for Al; >110 µg/L for Cd; >127 µg/L for As; >5.01 mg/L for PO43−; between 1000 and 1500 mg/L for NO3− and were 1.19 and 1.33 g/L for NaCl. Moreover, the use of a substrate in experiments was found not to affect juvenile responses and younger juveniles seemed more sensitive than older individuals. This study thus provides new data about the tolerance of FWPM to environmental pollution and suggests that pollutant concentrations in the river are significantly lower than levels affecting them.

Comparative Toxicity of Nitrate to Common and Imperiled Freshwater Mussel Glochidia and Larval Fishes

Growing human populations and increasingly intensive agriculture have resulted in widespread aquatic nitrate pollution. Freshwater mussel populations have been in decline for decades but often are underrepresented in data used for the development of ambient water quality criteria and acute toxicity of nitrate to mussel glochidia has not yet been established. Additionally, toxicity testing with aquatic species often is limited to a few model species; however, relatively little is known about how representative model species are of imperiled species. Therefore, to better define the acute toxicity of nitrate to common and rare aquatic species, we conducted 24-h nitrate acute toxicity tests with glochidia of four species of freshwater mussels, including a federally threatened species (Hamiota altilis) and 7-day tests with larval fish of three species: fathead minnow (Pimephales promelas), tricolor shiner (Cyprinella trichroistia), and tilapia (Oreochromis spp.), across a range of water hardness. Median effective concentrations (EC₅₀s) in freshwater mussel glochidia ranged from 524 to 904 mg/L NO₃-N in moderately hard water. In fish, median lethal concentrations (LC₅₀s) ranged from 228 to 1725 mg/L NO₃-N and varied with water hardness. Of the species tested, generally sensitivity of the common species was similar to the rare species, although relative sensitivity varied with water hardness. Based on these results, we can conclude that acute lethal effects are unlikely for the fish and mussel species considered here at current environmental levels, but the results of these standardized tests are useful for the development of ambient water quality criteria and other regulatory and management decisions regarding acute nitrate exposures.

Acute and Chronic Toxicity of Sodium Nitrate and Sodium Sulfate to Several Freshwater Organisms in Water-Only Exposures

Elevated nitrate (NO₃ ) and sulfate (SO₄ ) in surface water are of global concern, and studies are needed to generate toxicity data to develop environmental guideline values for NO₃ and SO₄ . The present study was designed to fill existing gaps in toxicity databases by determining the acute and/or chronic toxicity of NO₃ (tested as NaNO₃ ) to a unionid mussel (Lampsilis siliquoidea), a midge (Chironomus dilutus), a fish (rainbow trout, Oncorhynchus mykiss), and 2 amphibians (Hyla versicolor and Lithobates sylvaticus), and to determine the acute and/or chronic toxicity of SO₄ (tested as Na₂ SO₄ ) to 2 unionid mussels (L. siliquoidea and Villosa iris), an amphipod (Hyalella azteca), and 2 fish species (fathead minnow, Pimephales promelas and O. mykiss). Among the different test species, acute NO₃ median effect concentrations (EC50s) ranged from 189 to >883 mg NO₃ -N/L, and chronic NO₃ 20% effect concentrations (EC20s) based on the most sensitive endpoint ranged from 9.6 to 47 mg NO₃ -N/L. The midge was the most sensitive species, and the trout was the least sensitive species in both acute and chronic NO₃ exposures. Acute SO₄ EC50s for the 2 mussel species (2071 and 2064 mg SO₄ /L) were similar to the EC50 for the amphipod (2689 mg SO₄ /L), whereas chronic EC20s for the 2 mussels (438 and 384 mg SO₄ /L) were >2-fold lower than the EC20 of the amphipod (1111 mg SO₄ /L), indicating the high sensitivity of mussels in chronic SO₄ exposures. However, the fathead minnow, with an EC20 of 374 mg SO₄ /L, was the most sensitive species in chronic SO₄ exposures whereas the rainbow trout was the least sensitive species (EC20 > 3240 mg SO₄ /L). The high sensitivity of fathead minnow was consistent with the finding in a previous chronic Na₂ SO₄ study. However, the EC20 values from the present study conducted in test water containing a higher potassium concentration (3 mg K/L) were >2-fold greater than those in the previous study at a lower potassium concentration (1 mg K/L), which confirmed the influence of potassium on chronic Na₂ SO₄ toxicity to the minnow. Environ Toxicol Chem 2020;39:1071-1085. © 2020 SETAC.

Chronic Toxicity of Surface Water from a Canadian Oil Sands End Pit Lake to the Freshwater Invertebrates Chironomus dilutus and Ceriodaphnia dubia

Permanent reclamation of tailings generated by surface mining in the Canadian oil sands may be achieved through the creation of end pit lakes (EPLs) in which tailings are stored in mined-out pits and capped with water. However, these tailings contain high concentrations of dissolved organics, metals, and salts, and thus surface water quality of EPLs is a significant concern. This is the first study to investigate the chronic toxicity of surface water from Base Mine Lake (BML), the Canadian oil sands first large-scale EPL, to aquatic invertebrates that play a vital role in the early development of aquatic ecosystems (Chironomus dilutus and Ceriodaphnia dubia). After exposure of C. dilutus larvae for 23 days and C. dubia neonates for 8 days, no mortality was observed in any treatment with whole BML surface water. However, the emergence of C. dilutus adults was delayed by nearly 1 week, and their survival was significantly reduced (36%) compared with the controls. Reproduction (fecundity) of C. dubia was reduced by 20% after exposure to 2014 BML surface water; however, the effect was not observed after exposure to BML surface water collected a year later in 2015. Despite some adverse effects, the results of this study indicate that BML surface water quality is improving over time and is able to support certain salt-tolerant aquatic organisms. Because salinity within BML will persist for decades without manual intervention, the ecological development of the lake will likely resemble that of a brackish or estuarine ecosystem with reduced diversity.

Salient to Whom? The Positioning of German Political Parties on Agricultural Pollutants in Water Bodies

Scholars have increasingly argued for an integration of policies on agriculture and water due to their strong interlinkage. The entry of agricultural pollutants into water represents one of the main pressures on Europe’s ground and surface waters. This not only poses a risk to the environment and human health but also jeopardizes meeting the targets set by the EU Water Framework Directive. Research on the political agenda setting has shown that issue salience is key for triggering policy change. Nevertheless, Germany has repeatedly failed to adopt adequate policy measures despite the salience of the issue among the German public and increasing pressure by the EU. In this study, I shed light on the positioning of political parties in Germany on agricultural pollutants to explain the absence of policy change. More specifically, I ask whether there is an ideological division between political parties that hampers the adoption of effective, integrated policy measures. A qualitative content analysis of election manifestos published between 1998 and 2018 finds that political parties’ policy positions are predominantly influenced by their placement on an environmental and an economic ideological dimension. As a result, political parties in Germany advocate conflictive policy approaches, which is detrimental to the adoption of effective policy measures.

Genome-wide gene expression analysis reveals novel insights into the response to nitrite stress in gills of Branchiostoma belcheri

Amphioxus has been widely used as a model for the comparative immunology of vertebrates. Studies have reported that gene expression changes in the amphioxus gill in response to biotic stress, such as microbial and their mimic challenge, but little is known about how gene expression is affected by abiotic stress in the marine environment, such as nitrite. A lack of information regarding gene expression response to abiotic stress hinders a comprehensive understanding of gill defense response in amphioxus. Here, RNA sequencing was used to carry out gene expression profiling analyses of Branchiostoma belcheri gills under nitrite stress. Six libraries were created for the control and treatment groups, including three biological replicates. In total, 2416 differently expressed genes (DEGs) were detected in response to nitrite stress, of which 1522 DEGs were up-regulated in the treatment group in comparison to the control, while the remaining 894 DEGs were down-regulated genes. Functional enrichment revealed that these DEGs are primarily involved in disease, innate immunity, xenobiotic biodegradation and metabolism, and biomolecular processes and apoptosis. We screened 11 key nitrite-responsive DEGs to detect their expression responses to nitrite stress at different time points, and validate the sequencing data using real time quantitative PCR. The results indicated that the expression of gene encoding CYP3A, POD, CASPR1, GST, MAO, DDH, and XDH/XO were induced, while those encoding MRC, GT, DNASE1L, and RIPK5 were reduced, to participate in the anti-nitrite response. This study provides a useful resource for research of molecular toxicology in amphioxus under environmental stress.

Ecologically relevant biomarkers reveal that chronic effects of nitrate depend on sex and life stage in the invasive fish Gambusia holbrooki

Agricultural intensification and shifts in precipitation regimes due to global climate change are expected to increase nutrient concentrations in aquatic ecosystems. However, the direct effects of nutrients widely present in wastewaters, such as nitrate, are poorly studied. Here, we use multiple indicators of fish health to experimentally test the effects of three ecologically relevant nitrate concentrations (<10, 50 and 250 mg NO₃^-/l) on wild-collected mosquitofish (Gambusia holbrooki), a species widely introduced for mosquito biocontrol in often eutrophic waters. Overall, biomarkers (histopathology, feeding assays, growth and caloric content and stable isotopes as indicators of energy content) did not detect overt signs of serious disease in juveniles, males or females of mosquitofish. However, males reduced food intake at the highest nitrate concentration compared to the controls and females. Similarly, juveniles reduced energy reserves without significant changes in growth or food intake. Calorimetry was positively associated with the number of perivisceral fat cells in juveniles, and the growth rate of females was negatively associated with δ¹⁵N signature in muscle. This study shows that females are more tolerant to nitrate than males and juveniles and illustrates the advantages of combing short- and long-term biomarkers in environmental risk assessment, including when testing for the adequacy of legal thresholds for pollutants.

Effects of nitrate on freshwater mussel glochidia attachment and metamorphosis success to the juvenile stage

Water quality and contaminants have been frequently identified as critical stressors for freshwater mussels, many species of which are highly imperiled throughout North America and the world. Nutrient pollution, specifically nitrate, has become one of the most prevalent causes of water quality degradation globally, with increasing anthropogenic input from suburban and agricultural runoff, municipal wastewater, and industrial waste. Nitrate acute toxicity is generally low for aquatic species, but the potential effects of nitrate exposure are largely unknown for freshwater mussels, particularly during the parasitic stage of their complex lifecycle. Therefore, this study was designed to determine the effects of short-term nitrate exposure at environmentally relevant concentrations on juvenile production in two freshwater mussel species. Lampsilis siliquoidea and L. fasciola glochidia were exposed to nitrate (0, 11, or 56 mg NO₃-N/L) for 24 h before inoculation on a primary host, Largemouth Bass (Micropterus salmoides). Glochidia attachment, metamorphosis success, and total number of juveniles produced were monitored on individual fish. Exposure of L. siliquoidea glochidia to 56 mg NO₃-N/L nitrate resulted in a significant (p = 0.02) 35% reduction of total juveniles produced, a combined result of moderate decreases in both glochidia attachment and metamorphosis success. A similar trend (28% reduction; p = 0.06) was evident with 11 mg NO₃-N/L. No effects were apparent for L. fasciola, suggesting species-specific differences in responses even among closely related species. These results are the first to suggest that glochidia exposure to nitrate may adversely affect juvenile recruitment in some species. Findings from these studies are important for improving characterization of the hazards of nitrate pollution to aquatic life and this work will help better define the role of water quality in assessing habitat suitability for mussel conservation efforts.

Reproductive endocrinology of environmental nitrate

Nitrate is a widespread contaminant of aquatic ecosystems and drinking water. It is also broadly active in organismal physiology, and as such, has the potential to both enhance and disrupt normal physiological function. In animals, nitrate is a proposed endocrine disrupter that is converted in vivo to nitrite and nitric oxide. Nitric oxide, in particular, is a potent cell signaling molecule that participates in diverse biological pathways and events. Here, we review in vivo nitrate cycling and downstream mechanistic physiology, with an emphasis on reproductive outcomes. However, in many cases, the research produces contradictory results, in part because there is good evidence that nitrate follows a non-monotonic dose-response curve. This conundrum highlights an array of opportunities for scientists from different fields to collaborate for a full understanding of nitrate physiology. Opposing conclusions are especially likely when in vivo/in vitro, long term/short term, high dose/low dose, or hypoxia/normoxia studies are compared. We conclude that in vivo studies are most appropriate for testing an organism's integrated endocrine response to nitrate. Based on the limited available studies, there is a generalized trend that shorter term studies (less than 1 month) or studies involving low doses (≤5 mg/L NO₃-N) cause steroid hormone levels to decline. Studies that last more than a month and/or involve higher, but still environmentally relevant, exposures (>50-100 mg/L NO₃-N) cause steroid hormone levels to increase. Very high nitrate doses (>500 mg/L NO₃-N) are cytotoxic in many species. Hypoxia and acidity are likely to intensify the effects of nitrate. For study design, degree of study animal reproductive maturity or activity is important, with immature/reproductively quiescent animals responding to nitrate differently, compared with reproductively active animals. A detailed table of studies is presented.