9.4 Sediment toxicity and benthic communities
3 min read•august 7, 2024
Sediment toxicity and benthic communities are crucial in aquatic ecotoxicology. Contaminants in sediments can harm bottom-dwelling organisms and impact entire ecosystems. Understanding these interactions helps assess environmental risks and develop effective management strategies.
Sediment dynamics, contaminant interactions, and toxicity assessment are key areas of study. Processes like bioturbation and resuspension affect contaminant distribution, while sorption and desorption influence bioavailability. Toxicity tests on benthic organisms provide insights into ecological impacts of contaminated sediments.
Sediment Dynamics
Sediment-Water Interface and Pore Water
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- Sediment-water interface represents the boundary between the water column and sediment where important physical, chemical, and biological processes occur
- Pore water fills the spaces between sediment particles and can contain dissolved contaminants
- Contaminants in pore water are more bioavailable to benthic organisms compared to those bound to sediment particles
- Diffusion and advection processes transport contaminants between the water column and pore water
- Diffusion is driven by concentration gradients (high to low concentrations)
- Advection is caused by water movement or pressure differences
Bioturbation and Resuspension
- Bioturbation is the mixing of sediments by benthic organisms through activities such as burrowing, feeding, and locomotion
- Bioturbation can redistribute contaminants within sediments and enhance their release into the water column (polychaete worms, bivalves)
- Resuspension is the process by which settled sediments are re-entrained into the water column due to physical disturbances
- Resuspension can be caused by waves, currents, tides, or human activities (dredging, bottom trawling)
- Resuspension events can increase the exposure of aquatic organisms to sediment-bound contaminants by making them more bioavailable in the water column
Contaminant Interactions
Sorption and Desorption Processes
- Sorption is the process by which contaminants bind to sediment particles, reducing their bioavailability and mobility
- Contaminants can sorb to sediments through various mechanisms (electrostatic interactions, hydrophobic partitioning, complexation)
- Desorption is the release of contaminants from sediment particles back into the water or pore water
- Desorption can occur when environmental conditions change (pH, salinity, redox potential) or due to competition with other compounds
- Factors influencing sorption and desorption include sediment properties (organic matter content, clay content, surface area), contaminant properties (hydrophobicity, charge), and environmental conditions (temperature, pH)
Equilibrium Partitioning
- Equilibrium partitioning describes the distribution of a contaminant between the sediment and water phases at steady-state conditions
- The partitioning coefficient (Kd) is the ratio of the contaminant concentration in the sediment to that in the water at equilibrium
- Equilibrium partitioning models can be used to predict the distribution of contaminants between sediments and water based on their physicochemical properties
- These models assume that the system is at equilibrium and that sorption is reversible and linear
- Limitations of equilibrium partitioning models include the presence of multiple contaminant species, non-linear sorption, and kinetic limitations to reaching equilibrium
Toxicity Assessment
Benthic Organisms and Exposure Pathways
- Benthic organisms live in or on the sediment and are exposed to contaminants through multiple pathways
- Direct contact with contaminated sediments and pore water (infaunal organisms, epibenthic organisms)
- Ingestion of contaminated sediment particles or prey (deposit feeders, predators)
- Benthic organisms can accumulate contaminants in their tissues, leading to potential toxicity and transfer through the food web
- is the uptake and retention of contaminants in an organism's tissues over time
- Biomagnification is the increasing concentration of contaminants in organisms at higher trophic levels due to the consumption of contaminated prey
Whole Sediment Toxicity Tests
- Whole sediment toxicity tests assess the toxicity of contaminated sediments to benthic organisms under controlled laboratory conditions
- These tests expose organisms to field-collected or spiked sediments and measure endpoints such as survival, growth, or reproduction
- Common test organisms include amphipods (Hyalella azteca, Leptocheirus plumulosus), midges (Chironomus dilutus), and polychaetes (Neanthes arenaceodentata)
- Test organisms are selected based on their ecological relevance, sensitivity to contaminants, and ease of culture in the laboratory
- Whole sediment toxicity tests provide a direct measure of the potential impacts of contaminated sediments on benthic communities
- Results can be used to assess the need for sediment remediation, establish cleanup goals, and monitor the effectiveness of remediation efforts
Key Terms to Review (16)
Benthic invertebrates: Benthic invertebrates are organisms without a backbone that inhabit the bottom substrates of aquatic environments, including freshwater and marine ecosystems. These organisms play a crucial role in sediment toxicity assessments and benthic community health, acting as indicators of environmental conditions and the effects of pollutants.
Benthic microflora: Benthic microflora refers to the diverse community of microscopic plants and algae that inhabit the sediments at the bottom of aquatic environments. These microorganisms play a crucial role in nutrient cycling, sediment stability, and as a food source for higher trophic levels, making them integral to the health of benthic communities and ecosystems.
Bioaccumulation: Bioaccumulation is the process by which organisms accumulate contaminants in their bodies over time, often from their environment or food sources. This phenomenon can lead to higher concentrations of harmful substances in the tissues of an organism compared to the surrounding environment, significantly impacting health and ecological dynamics.
Biodiversity indicators: Biodiversity indicators are measurable features used to assess the health and stability of ecosystems, reflecting the variety of life forms within a particular environment. They help in evaluating the impacts of environmental changes, such as sediment toxicity, on species diversity and community structure, making them crucial for understanding the dynamics of benthic communities.
Biological toxicity: Biological toxicity refers to the harmful effects that certain substances or pollutants can have on living organisms, including plants, animals, and microorganisms. This concept is crucial in understanding how contaminants in the environment, especially sediments, can adversely impact benthic communities that play a vital role in aquatic ecosystems. Recognizing biological toxicity helps to assess the risks associated with pollutants and informs management strategies for preserving ecological balance.
Chemical toxicity: Chemical toxicity refers to the harmful effects of chemical substances on living organisms and ecosystems. It encompasses a wide range of effects, from acute poisoning to chronic health issues and ecological damage, making it crucial in understanding how contaminants impact both benthic communities and sediment quality in aquatic environments.
Community resilience: Community resilience refers to the ability of a community to withstand, adapt, and recover from environmental stressors and disturbances, including pollution and habitat degradation. This concept emphasizes the interconnectedness of ecosystems and human activities, highlighting how healthy ecosystems can better support community stability and sustainability in the face of challenges, such as sediment toxicity affecting benthic communities.
Ecological Restoration: Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed, aiming to return it to a stable and healthy state. This practice involves various techniques to improve the quality and functionality of ecosystems, promoting biodiversity and ecological balance. Successful restoration can enhance ecosystem services, such as water purification and habitat provision, which are vital for both wildlife and human communities.
Heavy metals: Heavy metals are a group of metallic elements that have a high density and are toxic at low concentrations, including elements like lead, mercury, cadmium, and arsenic. Their persistence in the environment and potential to accumulate in living organisms makes them a significant concern in ecotoxicology, influencing various ecological and health-related outcomes.
Linear bioaccumulation model: The linear bioaccumulation model is a mathematical representation that describes how organisms accumulate contaminants from their environment in a linear manner, meaning the concentration of the contaminant in the organism increases proportionally with the concentration of the contaminant in the surrounding environment. This model helps predict the levels of toxic substances in organisms and their potential impacts on ecosystems, especially concerning sediment toxicity and benthic communities.
Persistent Organic Pollutants (POPs): Persistent Organic Pollutants (POPs) are organic compounds that resist environmental degradation through chemical, biological, and photolytic processes. These pollutants can remain in the environment for long periods, bioaccumulate in the food chain, and pose serious health risks to both ecosystems and human health. Their persistence and potential for long-range transport make them critical concerns in aquatic ecosystems and benthic communities.
Risk Assessment Framework: A risk assessment framework is a structured approach to identify, evaluate, and manage risks associated with environmental hazards and their potential impacts on ecosystems and human health. It provides a systematic process that guides decision-making by considering the likelihood and severity of adverse effects, especially concerning pollution sources and their effects on sediment toxicity and benthic communities.
Sediment grain size: Sediment grain size refers to the diameter of individual particles in sediment, which can vary from very fine silt to coarse gravel. The size of these grains significantly influences the physical, chemical, and biological properties of sediment, impacting how contaminants interact with benthic organisms and the overall health of aquatic ecosystems.
Sediment Profiling: Sediment profiling is a technique used to analyze the vertical distribution of sediment layers and their properties in aquatic environments. This method helps to assess the quality and toxicity of sediments, providing insights into the ecological health of benthic communities that reside in or interact with these sediments.
Sediment quality guidelines: Sediment quality guidelines (SQGs) are benchmarks used to assess the health of sediment in aquatic environments and their potential effects on benthic organisms. These guidelines help determine acceptable levels of contaminants in sediments, ensuring the protection of benthic communities and overall ecosystem health. By comparing sediment contaminant concentrations to SQGs, scientists can identify pollution levels that pose risks to aquatic life and inform management decisions.
Toxicity bioassays: Toxicity bioassays are experimental tests that measure the harmful effects of substances on living organisms to assess the level of toxicity present in environmental samples. These assays help in understanding how pollutants affect aquatic life, especially within sediment environments, and can reveal the potential risks to benthic communities and their overall health.