🐠Ecotoxicology Unit 9 – Aquatic Ecotoxicology: Fresh & Marine Waters

Key Concepts and Terminology

• Ecotoxicology studies the effects of toxicants on ecosystems, including aquatic environments (freshwater and marine)
• Toxicants are substances that have adverse effects on living organisms, such as heavy metals, pesticides, and pharmaceuticals
• Bioaccumulation occurs when an organism absorbs a toxicant at a rate faster than it can eliminate it, leading to an increase in concentration over time
  • Biomagnification is the process by which toxicants increase in concentration as they move up the food chain
• Bioassays are standardized tests used to determine the effects of toxicants on living organisms (Daphnia magna, Selenastrum capricornutum)
• Ecological risk assessment evaluates the likelihood and severity of adverse effects on ecosystems due to exposure to toxicants
• Remediation strategies aim to remove or neutralize toxicants from contaminated aquatic environments (bioremediation, phytoremediation)

Aquatic Ecosystems: Freshwater vs Marine

• Freshwater ecosystems include rivers, lakes, streams, and wetlands, characterized by low salinity (<0.5 ppt)
• Marine ecosystems encompass oceans, seas, and estuaries, with higher salinity (>30 ppt)
• Differences in physical and chemical properties (temperature, pH, dissolved oxygen) influence the distribution and sensitivity of aquatic organisms to toxicants
• Freshwater ecosystems are more susceptible to pollution due to limited water exchange and dilution compared to marine environments
  • However, marine ecosystems can be impacted by persistent and bioaccumulative toxicants (PCBs, mercury)
• Estuaries are transitional zones between freshwater and marine environments, supporting diverse and productive ecosystems (mangroves, salt marshes)
• Coastal marine ecosystems (coral reefs, seagrass beds) are particularly vulnerable to pollution and climate change impacts

Sources of Aquatic Pollution

• Point sources are identifiable and localized, such as industrial discharges, wastewater treatment plants, and oil spills
• Non-point sources are diffuse and widespread, including agricultural runoff, urban stormwater, and atmospheric deposition
• Industrial effluents contain a variety of toxicants (heavy metals, organic compounds) that can have acute and chronic effects on aquatic organisms
• Agricultural runoff carries pesticides, fertilizers, and sediments, leading to eutrophication and hypoxia in aquatic ecosystems
  • Eutrophication is the excessive growth of algae and aquatic plants due to nutrient enrichment, which can lead to oxygen depletion and fish kills
• Sewage and wastewater discharges introduce pathogens, nutrients, and emerging contaminants (pharmaceuticals, personal care products) into aquatic environments
• Marine debris, particularly plastic pollution, poses a significant threat to aquatic life through ingestion, entanglement, and habitat degradation

Toxicants and Their Effects

• Heavy metals (mercury, lead, cadmium) can bioaccumulate in aquatic organisms and cause neurotoxicity, reproductive impairment, and developmental abnormalities
• Pesticides (organochlorines, organophosphates, pyrethroids) can disrupt the nervous system, endocrine function, and immune response in aquatic species
  • DDT, a persistent organochlorine pesticide, caused eggshell thinning in birds and declines in bird populations before its ban in many countries
• Polychlorinated biphenyls (PCBs) are persistent organic pollutants that can biomagnify in aquatic food webs and cause reproductive and developmental effects
• Polycyclic aromatic hydrocarbons (PAHs) from oil spills and combustion can cause DNA damage, cancer, and immunosuppression in aquatic organisms
• Endocrine-disrupting chemicals (EDCs) interfere with hormone signaling and can lead to reproductive disorders, developmental abnormalities, and population declines (tributyltin, nonylphenol)
• Microplastics can adsorb and concentrate toxicants, providing a pathway for their transfer and bioaccumulation in aquatic food webs

Bioaccumulation and Biomagnification

• Bioaccumulation is the net uptake and retention of a toxicant in an organism's tissues over time
• Factors influencing bioaccumulation include the toxicant's lipophilicity, persistence, and the organism's metabolic rate and excretion capacity
• Biomagnification is the progressive increase in toxicant concentration at successive trophic levels in a food chain
  • Persistent and lipophilic toxicants (PCBs, mercury) are more likely to biomagnify due to their slow elimination and transfer through dietary exposure
• Bioaccumulation and biomagnification can lead to high toxicant concentrations in top predators (fish, birds, mammals), posing risks to their health and reproduction
• Bioaccumulation factors (BAFs) and biomagnification factors (BMFs) are used to quantify the extent of toxicant accumulation and transfer in aquatic food webs
• Monitoring programs often focus on measuring toxicant levels in top predators and long-lived species as indicators of ecosystem contamination

Testing Methods and Bioassays

• Acute toxicity tests measure the short-term effects of toxicants on survival, growth, or behavior of aquatic organisms (LC50, EC50)
• Chronic toxicity tests assess the long-term effects of toxicants on reproduction, development, and sublethal endpoints (NOEC, LOEC)
• Standardized test species are used to ensure comparability and reproducibility of results (Daphnia magna, Pimephales promelas)
  • Species selection considers ecological relevance, sensitivity, and ease of culture and maintenance
• Microcosm and mesocosm studies simulate natural conditions to evaluate the effects of toxicants on aquatic communities and ecosystem processes
• In vitro assays (cell cultures, enzyme assays) provide rapid and cost-effective screening tools for assessing the toxicity and mode of action of chemicals
• Biomarkers (molecular, biochemical, physiological) are used to detect early signs of toxicant exposure and effects in aquatic organisms (metallothionein, vitellogenin)

Ecological Risk Assessment

• Problem formulation defines the goals, scope, and endpoints of the assessment, considering the characteristics of the toxicant and the ecosystem at risk
• Exposure assessment estimates the concentration and duration of toxicant exposure to aquatic organisms based on environmental monitoring and modeling
• Effects assessment determines the relationship between toxicant exposure and adverse effects on aquatic organisms using dose-response curves and threshold values
• Risk characterization integrates exposure and effects information to estimate the likelihood and magnitude of adverse ecological effects
  • Hazard quotients (HQs) compare the predicted or measured exposure concentrations to the toxicity thresholds (PEC/PNEC ratio)
• Uncertainty analysis identifies and quantifies the sources of variability and uncertainty in the risk assessment process (data gaps, extrapolation methods)
• Ecological risk assessment informs decision-making and prioritization of management actions to protect aquatic ecosystems from the impacts of toxicants

Management and Remediation Strategies

• Source control measures aim to reduce or eliminate the release of toxicants into aquatic environments (effluent treatment, best management practices)
• Environmental quality standards and discharge limits are established based on the results of ecological risk assessments and regulatory guidelines
• Monitoring programs track the spatial and temporal trends of toxicant concentrations and their effects on aquatic ecosystems to assess the effectiveness of management actions
• Remediation techniques remove or immobilize toxicants from contaminated sediments and water (dredging, capping, activated carbon)
  • Bioremediation uses microorganisms to degrade or transform organic pollutants into less toxic compounds
  • Phytoremediation employs plants to absorb, accumulate, or detoxify pollutants from water and sediments
• Habitat restoration and enhancement promote the recovery of aquatic ecosystems by improving water quality, increasing biodiversity, and restoring ecological functions (wetland creation, riparian buffers)
• Integrated watershed management addresses the multiple stressors and cumulative effects of toxicants on aquatic ecosystems, considering land use, climate change, and socioeconomic factors
• Public education and stakeholder engagement are crucial for raising awareness, changing behaviors, and building support for the protection and sustainable management of aquatic resources