8.3 Aquatic toxicology

Aquatic toxicology examines how pollutants affect organisms in water environments. It covers freshwater and marine systems, exploring contaminant sources, fate, and impacts on aquatic life. Understanding these processes is crucial for protecting water resources and ecosystems.

This field investigates toxicity testing methods, ecological risk assessment, and remediation strategies. It considers how pollutants move through food webs, disrupt habitats, and threaten biodiversity. Aquatic toxicology informs water quality management and conservation efforts worldwide.

Aquatic environments

• Aquatic environments encompass a diverse range of habitats, from freshwater systems to marine ecosystems, each with unique physical, chemical, and biological characteristics that influence the fate and effects of contaminants
• Understanding the differences between these environments is crucial for assessing the potential impacts of pollutants on aquatic organisms and developing appropriate management strategies

Freshwater vs saltwater

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• Freshwater environments have low salinity levels (typically <0.5 ppt) and include rivers, lakes, streams, and wetlands
• Saltwater environments have higher salinity levels (>30 ppt) and include oceans, seas, and estuaries
• Salinity influences the solubility, bioavailability, and toxicity of contaminants, as well as the physiology and adaptations of aquatic organisms
• Freshwater organisms are more susceptible to osmotic stress and ion imbalances caused by pollutants compared to saltwater organisms

Lentic vs lotic systems

• Lentic systems are still or slow-moving water bodies (lakes, ponds, and reservoirs) characterized by stratification, longer water retention times, and higher sedimentation rates
• Lotic systems are fast-moving water bodies (rivers and streams) characterized by turbulent flow, shorter water retention times, and higher oxygen levels
• Contaminants in lentic systems tend to accumulate in sediments and have longer residence times, while those in lotic systems are more rapidly dispersed and diluted
• Aquatic organisms in lentic systems are more exposed to sediment-bound pollutants, while those in lotic systems are more affected by dissolved contaminants

Estuaries and coastal zones

• Estuaries are transitional zones where freshwater rivers meet the ocean, resulting in gradients of salinity, temperature, and nutrients
• Coastal zones include nearshore marine environments influenced by land-based activities and freshwater inputs
• These areas are highly productive and support diverse communities of aquatic organisms, but are also vulnerable to pollution from urban, industrial, and agricultural sources
• Contaminants in estuaries and coastal zones can have complex interactions with changing salinity, pH, and organic matter, affecting their bioavailability and toxicity

Contaminants in aquatic systems

• Aquatic systems are exposed to a wide range of contaminants from natural and anthropogenic sources, which can have adverse effects on water quality, aquatic life, and human health
• Understanding the sources, types, and behavior of these pollutants is essential for assessing their ecological risks and developing effective management strategies

Sources of pollution

• Point sources are discrete and identifiable origins of pollution (wastewater treatment plants, industrial discharges, and accidental spills)
• Non-point sources are diffuse and widespread origins of pollution (agricultural runoff, urban stormwater, and atmospheric deposition)
• Legacy pollutants are persistent contaminants from historical activities that continue to impact aquatic environments (PCBs, DDT, and mercury)
• Emerging pollutants are newly recognized or increasing in prevalence (pharmaceuticals, personal care products, and microplastics)

Types of aquatic pollutants

• Nutrients (nitrogen and phosphorus) from agricultural runoff and wastewater discharges can cause eutrophication, algal blooms, and hypoxia
• Metals (lead, cadmium, and mercury) from mining, industrial activities, and atmospheric deposition can accumulate in sediments and biota, causing toxicity and bioaccumulation
• Organic contaminants (pesticides, PAHs, and PCBs) from agricultural, urban, and industrial sources can persist in the environment and cause endocrine disruption, developmental abnormalities, and cancer in aquatic organisms
• Pathogens (bacteria, viruses, and protozoa) from sewage and animal waste can pose risks to human health through recreational water use and seafood consumption

Fate and transport mechanisms

• Advection is the transport of contaminants by the bulk movement of water, influenced by currents, tides, and river flow
• Dispersion is the spreading of contaminants due to turbulence and mixing, resulting in dilution and reduced concentrations
• Sedimentation is the settling of particulate-bound contaminants from the water column to the bottom sediments, where they can accumulate and become a long-term source of exposure
• Volatilization is the transfer of contaminants from the water to the atmosphere, influenced by temperature, wind speed, and the chemical's vapor pressure

Bioaccumulation and biomagnification

• Bioaccumulation is the uptake and retention of contaminants in an organism's tissues at levels higher than in the surrounding environment or food
• Biomagnification is the increasing concentration of contaminants in organisms at higher trophic levels due to the transfer of accumulated contaminants through the food chain
• Lipophilic and persistent contaminants (PCBs, DDT, and mercury) are more likely to bioaccumulate and biomagnify in aquatic food webs
• Top predators (fish, birds, and mammals) are at greatest risk of exposure to high levels of contaminants due to biomagnification

Aquatic organisms as bioindicators

• Aquatic organisms can serve as bioindicators of environmental quality, providing valuable information on the presence, levels, and effects of contaminants in aquatic ecosystems
• Different groups of organisms have varying sensitivities, exposure routes, and ecological roles, making them useful for assessing the impacts of pollution at different scales and levels of biological organization

Algae and aquatic plants

• Algae and aquatic plants are primary producers that respond quickly to changes in water quality, particularly nutrient enrichment and herbicides
• Shifts in algal community composition (from diatoms to cyanobacteria) can indicate eutrophication and potential toxin production
• Macrophyte diversity and abundance can reflect long-term water quality conditions and habitat suitability for other aquatic organisms
• Bioassays using algae (Selenastrum capricornutum) and aquatic plants (Lemna minor) are used to assess the toxicity of water and sediment samples

Invertebrates and fish

• Invertebrates (insects, crustaceans, and mollusks) are diverse and abundant in aquatic ecosystems, occupying various trophic levels and habitats
• Changes in invertebrate community structure (species richness, diversity, and functional feeding groups) can indicate water quality impairment and ecosystem stress
• Fish are long-lived and mobile organisms that can accumulate contaminants in their tissues, reflecting exposure over time and space
• Fish health assessments (growth, reproduction, and histopathology) can reveal sublethal effects of chronic pollution exposure
• Biomarkers in fish (enzyme activities, DNA damage, and gene expression) can provide early warning signals of contaminant stress

Amphibians and reptiles

• Amphibians (frogs, toads, and salamanders) have permeable skin and complex life cycles that make them sensitive to both aquatic and terrestrial pollutants
• Malformations, reduced growth, and altered sex ratios in amphibian populations can indicate exposure to endocrine disrupting chemicals and other contaminants
• Reptiles (turtles and crocodilians) are long-lived and occupy high trophic levels, making them susceptible to bioaccumulation of persistent pollutants
• Eggshell thinning and reproductive impairment in reptiles can result from exposure to organochlorine pesticides and other contaminants

Aquatic birds and mammals

• Aquatic birds (waterfowl, shorebirds, and seabirds) feed on aquatic organisms and can accumulate contaminants from their prey
• Mass mortality events, reduced reproductive success, and behavioral abnormalities in aquatic birds can signal acute or chronic pollution exposure
• Aquatic mammals (otters, seals, and whales) are top predators that can accumulate high levels of persistent pollutants in their blubber and other tissues
• Population declines, increased disease susceptibility, and reproductive failure in aquatic mammals can result from exposure to PCBs, DDT, and other contaminants

Toxicity testing methods

• Toxicity testing methods are used to assess the adverse effects of contaminants on aquatic organisms under controlled laboratory conditions
• These methods provide quantitative data on the concentration-response relationships, toxicity thresholds, and relative sensitivities of different species to inform ecological risk assessments and regulatory decision-making

Acute vs chronic toxicity

• Acute toxicity tests measure the short-term effects of high concentrations of a contaminant on organism survival, typically over a period of 24 to 96 hours
• Chronic toxicity tests measure the long-term effects of low concentrations of a contaminant on organism growth, reproduction, and survival, typically over a period of weeks to months
• Acute tests are useful for screening the relative toxicity of different chemicals and determining the lethal concentration (LC50) for a species
• Chronic tests are more ecologically relevant for assessing the impacts of persistent pollutants and sublethal effects that can affect population dynamics

Lethal concentration (LC50)

• The LC50 is the concentration of a contaminant that is lethal to 50% of the test organisms over a specified exposure period (usually 96 hours)
• LC50 values are used to compare the acute toxicity of different chemicals and to rank the relative sensitivities of different species
• The lower the LC50 value, the more toxic the chemical is to the test organism
• LC50 values are often used to derive water quality criteria and effluent discharge limits, with safety factors applied to account for species differences and environmental variability

Sublethal effects assessment

• Sublethal effects are adverse impacts on organism growth, reproduction, behavior, and physiology that occur at concentrations below the lethal threshold
• These effects can have significant ecological consequences by reducing organism fitness, altering population dynamics, and disrupting ecosystem functions
• Sublethal endpoints measured in toxicity tests include growth rate, fecundity, fertilization success, and developmental abnormalities
• Behavioral responses (avoidance, feeding rate, and predator evasion) can also be sensitive indicators of sublethal toxicity

Biomarkers of exposure

• Biomarkers are measurable biochemical, physiological, or histological changes in an organism that indicate exposure to a contaminant or its effects
• Biomarkers of exposure reflect the internal dose of a contaminant and can provide evidence of exposure even when the chemical is no longer detectable in the environment
• Examples of exposure biomarkers include enzyme induction (cytochrome P450 and metallothionein), DNA adducts, and bile metabolites
• Biomarkers of effect reflect the adverse impacts of a contaminant on an organism's health and can provide early warning signals of potential ecological harm
• Examples of effect biomarkers include oxidative stress, immunosuppression, endocrine disruption, and genotoxicity

Ecological risk assessment

• Ecological risk assessment is a process for evaluating the likelihood and magnitude of adverse effects on ecosystems and their components resulting from exposure to environmental stressors, such as contaminants
• The process involves four main steps: hazard identification, exposure assessment, dose-response assessment, and risk characterization, which are used to inform risk management decisions and regulatory actions

Hazard identification

• Hazard identification is the process of determining whether a contaminant has the potential to cause adverse ecological effects based on its inherent toxicity and mode of action
• This step involves reviewing available toxicity data from laboratory and field studies, as well as considering the chemical's physical and chemical properties that influence its environmental fate and transport
• Contaminants are prioritized for further assessment based on their toxicity, persistence, bioaccumulation potential, and likelihood of exposure in the environment
• Hazard identification also considers the potential for interactions among multiple contaminants and other stressors that may enhance or mitigate their effects

Exposure assessment

• Exposure assessment is the process of estimating the magnitude, frequency, and duration of organism exposure to a contaminant in the environment
• This step involves measuring or modeling the concentrations of the contaminant in various environmental media (water, sediment, and biota) over space and time
• Exposure pathways (ingestion, inhalation, and dermal absorption) and routes (water, food, and sediment) are identified for different organisms based on their life history, habitat use, and feeding ecology
• Bioaccumulation and biomagnification potential are considered for contaminants that are persistent and lipophilic, as they can result in higher exposures for organisms at higher trophic levels

Dose-response assessment

• Dose-response assessment is the process of characterizing the relationship between the dose of a contaminant and the incidence or severity of an adverse effect in exposed organisms
• This step involves analyzing data from toxicity tests to determine the concentrations that cause different levels of effects (e.g., LC50, EC50, and NOAEL) and to derive dose-response curves
• Dose-response relationships are used to estimate the likelihood and magnitude of effects at different exposure levels and to identify thresholds below which adverse effects are not expected to occur
• Interspecies and intraspecies differences in sensitivity are considered, as well as the potential for cumulative effects from multiple contaminants or stressors

Risk characterization

• Risk characterization is the process of integrating the information from the hazard identification, exposure assessment, and dose-response assessment to estimate the overall risk to ecological receptors
• This step involves comparing the estimated exposure levels to the toxicity thresholds derived from the dose-response assessment to determine the likelihood and magnitude of adverse effects
• Risks are typically expressed as hazard quotients (HQs) or risk probabilities, which can be used to prioritize contaminants and sites for management actions
• Uncertainty analysis is conducted to identify the sources and magnitude of variability and uncertainty in the risk estimates and to guide further data collection and research needs
• Risk characterization also considers the ecological significance of the predicted effects, such as impacts on population viability, community structure, and ecosystem functions

Impacts on aquatic ecosystems

• Aquatic ecosystems are highly vulnerable to the impacts of contaminants due to their complex food webs, biogeochemical cycles, and habitat heterogeneity
• The effects of pollution can manifest at different levels of biological organization, from individual organisms to populations, communities, and ecosystems, with cascading consequences for the structure and function of aquatic environments

Population and community effects

• Contaminants can cause direct mortality, reduced growth, and reproductive impairment in exposed organisms, leading to population declines and local extinctions
• Sensitive species may be eliminated from contaminated areas, resulting in shifts in community composition towards more tolerant or opportunistic species
• Indirect effects can occur through trophic interactions, such as reduced prey availability or increased predation pressure, which can alter population dynamics and community structure
• Changes in species diversity, dominance, and functional traits can indicate the severity and extent of pollution impacts on aquatic communities

Trophic level disruptions

• Contaminants can disrupt energy flow and nutrient cycling in aquatic food webs by altering the abundance, productivity, and interactions of organisms at different trophic levels
• Bioaccumulation and biomagnification of persistent pollutants can lead to higher exposure and toxicity in top predators, such as fish, birds, and mammals
• Impacts on primary producers (algae and macrophytes) can cascade up the food web by reducing the quantity and quality of food resources for herbivores and detritivores
• Contaminant-induced changes in the behavior, physiology, and life history of organisms can alter predator-prey relationships and trophic transfer efficiency

Habitat degradation

• Pollution can degrade the physical and chemical properties of aquatic habitats, making them less suitable for the growth, reproduction, and survival of aquatic organisms
• Nutrient enrichment can cause eutrophication, leading to algal blooms, hypoxia, and changes in water clarity and temperature that affect the distribution and abundance of aquatic species
• Sedimentation and siltation from land-based activities can smother benthic habitats, clog the gills of fish and invertebrates, and reduce the availability of spawning and nursery areas
• Contaminants can alter the structure and function of microbial communities in sediments and biofilms, which play key roles in nutrient cycling and organic matter decomposition

Biodiversity loss

• Aquatic biodiversity is threatened by the cumulative impacts of pollution, habitat loss, invasive species, overharvesting, and climate change
• Contaminants can contribute to the decline and extinction of sensitive and endemic species, particularly those with limited geographic ranges or specialized habitat requirements
• The loss of biodiversity can reduce the resilience and stability of aquatic ecosystems, making them more vulnerable to future perturbations and environmental change
• Biodiversity loss can also have significant economic and social consequences, such as reduced fisheries productivity, decreased water quality, and loss of cultural and recreational values associated with aquatic environments

Remediation and restoration

• Remediation and restoration are strategies for mitigating the impacts of pollution on aquatic ecosystems and promoting their recovery and resilience
• These approaches involve a combination of source control measures, bioremediation techniques, habitat rehabilitation, and monitoring and assessment to evaluate the effectiveness of the interventions

Source control measures

• Source control measures aim to reduce or eliminate the input of contaminants into aquatic environments from point and non-point sources
• Examples include upgrading wastewater treatment plants, implementing best management practices for agricultural and urban runoff, and regulating industrial discharges and emissions
• Source control also involves the proper handling, storage, and disposal of hazardous materials to prevent accidental spills and leaks
• Stormwater management techniques, such as green infrastructure and low impact development, can help reduce the volume and pollutant load of urban runoff

Bioremediation techniques

• Bioremediation involves the use of microorganisms, plants, or other biological