32 Contaminant behaviour, exposure, uptake and impact of Marine Pollution

All contaminants, whether inorganic or organic, will ultimately be distributed through ecosystems and stored in various compartments (e.g. sediments and body tissues). The environmental fate of contaminants results from their chemical properties, and it is the fugacity (sometimes described as the potential to move between media) of a substance that determines its likely distribution after its release into the environment (e.g. water or lipid solubility, vapour pressure) as well as the hydrodynamic and physiological processes occurring in those ecosystems (e.g. winds, currents, flow rate, upwelling, and sedimentation).

‘Jellyfish Floating Amid Ocean Debris’ by Nadejda Bostanova is used under a Pexels License

We tend to focus our sampling on the various compartments of water, biota, and sediment. Once compartmentalised, the duration of storage will depend on the stability of the conditions in that compartment. For example, when sediments are disturbed, stored contaminants can be remobilised back into the water column, or if an organism dies the contaminants that were taken up and stored in its body will become available to detritivores and through trophic levels thereafter.

Ecotoxicological studies help us to understand how an organism interacts with a contaminant and identify measurable stress responses.

This text from “Marine Pollution in Context” In: Reichelt-Brushett, A. (eds) “Marine Pollution – Monitoring, Management and Mitigation” 2023, by Amanda Reichelt-Brushett, which is licensed under CC BY 4.0

Exposure and uptake pathways

Bioconcentration: The process whereby chemicals enter aquatic organisms through the gills or epithelial tissue directly from the water (or surrounding environmental medium) and become more concentrated in the organism than in the surrounding environmental medium (water, soil, etc.).

Bioaccumulation: Chemical uptake by an organism, attributable to both bioconcentration and dietary accumulation. Bioaccumulation is related to organism-specific rates of uptake, metabolism, and elimination, and occurs when a substance is absorbed at a faster rate than it is lost.

Biomagnification: The process whereby tissue concentrations of a bioaccumulated chemical increase with successively higher levels in a food chain (at least two trophic levels). This phenomenon is rare; however, those specific contaminants that do biomagnify pose serious environmental problems, and consequently, receive great publicity. Contaminants known to biomagnify include:

• Chlorinated hydrocarbons;
• Insecticides (e.g. DDT group and cyclodienes—dieldrin);
• Non-insecticides (e.g. PCBs—hydraulic fluids, heat-transfer fluids); and
• Some organometallic compounds (e.g. methylmercury [MeHg], tributyltin [TBT]).

These contaminants characteristically have a high lipid solubility and therefore, a strong affinity to accumulate in biological tissue with high-fat content (including adipose tissue/blubber, white muscle, and some organs). They are stable and persistent within the organism, and accumulation can lead to disease and mortality in higher order predators.

Bioavailability is another common term that refers to the fraction of an element (or compound) that is available to be taken up by an organism. The entry pathway of a contaminant into an organism (e.g. via water, sediment, food, etc.) will also influence the bioavailability. Metals that are weakly bound to sediment particles are bioavailable. However, some contaminants are very strongly adsorbed to sediment particles or bound within the lattice structure of the particle: they will not be reactive within the organism and will likely pass through the organism with the particle.

This text is from “Collecting, Measuring, and Understanding Contaminant Concentrations in the Marine Environment” In: Reichelt-Brushett, A. (eds) “Marine Pollution – Monitoring, Management and Mitigation.” 2023, by Reichelt-Brushett, A, which is licensed under CC BY 4.0.

Factors influencing toxicity

The concentration of a chemical is one of the more obvious factors that will affect its toxicity. You may have heard the historic quote from Paracelsus (1493‒1541), who expressed:

However, many other environmental, physiological, biological, and chemical factors influence the toxicity of a chemical (e.g. Charition et al. 2010; de Almeida Rodrigues et al. 2022). More specifically:

Temperature: the toxicity of many substances differs with temperature, however, there is no general rule as to whether a higher or lower temperature will elicit a higher toxicity. For example, some metals are more toxic in tropical marine ecosystems than in temperate or polar regions, whereas for other metals this is reversed (Chapman et al. 2006). This makes the extrapolation of toxicity data from different climatic regions unreliable. This is one reason that some countries such as Canada and the USA have requirements to use toxicity data for endemic species. This approach is possible for North American and European countries because most of the test organisms used in toxicity tests originated from those regions. However, in other countries with smaller populations, different climates, and/ or different ecosystems and species, this approach is not logistically possible, and instead toxicity data generated using non-endemic species must be relied upon.

pH: affects solubility, and therefore bioavailability, of contaminants such as metals. Generally, a decrease in pH increases the toxicity of metals as they become more bioavailable. Since marine waters are buffered, they resist changes in pH (to a point).

Salinity: can decrease the toxicity of chemicals by decreasing their aqueous solubility or changing their chemical form, although this depends on the type and concentration of other competing substances such as dissolved organic compounds (e.g. Hall and Anderson 2008).

Suspended sediment: may increase toxicity (for example by providing a surface for contaminants to bind to, allowing uptake of contaminants by some organisms), but can also decrease toxicity (e.g. by decreasing the bioavailability of hydrophobic chemicals to aquatic organisms). This largely depends on the type of contaminant and the type of sediment. Suspended sediments may also act as a  stressor in their own right.

Dissolved organic carbon: may decrease toxicity by decreasing a chemical’s aqueous solubility, and its ability to pass through membranes or bioavailability.

Previous exposure/resistance: populations or strains of species may develop resistance to toxicants, so that considerable differences in sensitivity may exist within one species. For instance, species in areas with naturally elevated concentrations of metals may become more tolerant to those metals.

Organism life stage: generally the early and oldest life stages are the most sensitive to the harmful effects of chemicals.

Duration of exposure: generally a longer period of exposure will result in adverse effects at lower concentrations of a toxicant than the same effects measured after a short period of exposure.

This text is from “Assessing Organism and Community Responses” In: Reichelt-Brushett, A. (eds) “Marine Pollution – Monitoring, Management and Mitigation” 2023, by Amanda Reichelt-Brushett, Pelli L. Howe, Anthony A. Chariton, Michael St. J. Warne, which is licensed under CC BY 4.0

Contaminants in the marine environment: types, common sources, and general factors influencing impact and potential as pollutants

The table below summarises common contaminants in the marine environment: types, common sources, and general factors influencing impact and potential as pollutants.

This text and table are adapted from “Marine Pollution in Context” In: Reichelt-Brushett, A. (eds) “Marine Pollution – Monitoring, Management and Mitigation” 2023, by Amanda Reichelt-Brushett, which is licensed under CC BY 4.0.

Multiple stressors

Marine environments are rarely exposed to a single stressor, but rather experience a complex mix of many stressors. These stressors may be contaminants, such nutrients, chemicals, plastics as well as carbon dioxide, or they may be other stressors, such as invasive species, built infrastructure, aquaculture or fisheries, or climatic changes which themselves can contribute to contaminant stress, for example, nutrient loading is a well-known impact from aquaculture activities. All these stressors are ubiquitous in marine environments worldwide and have the potential to interact and have very different impacts compared to if they occurred singularly.

‘Pipe with Sewage’ by Joseph Russo is used under a Pexels License

Understanding how and when stressors interact is critical for predicting their effects and therefore establishing safe and realistic guidelines to protect the environment. The consequences of stressor interactions are complex and pose great challenges for researchers, practitioners and decision-makers.

Multiple stressor interactions

When more than one stressor occurs in an environment, they may or may not interact. If the effects of a certain stressor occur independently of any other stressor in the environment, then we consider that the stressors are not interacting (no interaction; Figure 1). This is defined as an additive (or cumulative) multiple stressor model where the overall total effects on a response variable are the sum of the single-stressor effects.

Conversely, the stressors may affect the biological system(s) in ways that are dependent on each other or are interacting. Therefore, the overall or combined effects of two or more stressors are different from the sum of the single-stressor effects. Interacting stressors are considered as either synergistic or antagonistic, which are defined as (Figure 1):

• Synergistic Interactions—the combined effects are greater than expected based on individual effects.
• Antagonistic Interactions—the combined effects are less than expected based on individual effects.

Examples of multiple stressors

Wastewater treatment plants and stormwater drains that discharge into coastal marine waters create multiple stressor conditions since they are sources of both nutrients and trace metals and metalloids. These contaminants have different modes of action, often leading to different types of ecological impacts. When acting separately, moderate levels of nutrients may cause an increase in a particular biological or ecological response (e.g. population growth or primary productivity) compared to control areas (e.g. Svensson et al. 2007), while trace metals may cause a relative decrease in these responses (Johnston and Roberts 2009; Mayer-Pinto et al. 2010). However, when these stressors occur simultaneously, they are likely to interact and can potentially result in no overall net effect on biological or ecological responses in the receiving environment (O’Brien et al. 2019).

Another multiple stressor situation occurs when built infrastructure (e.g. marinas, groynes or piers) is in proximity to toxic chemicals in the water column. Built infrastructure on its own can cause a significant negative effect on the diversity (i.e. number of species and abundance) of marine organisms (e.g. microbes, plankton, epibiota, and infauna) via loss or fragmentation of habitats (Bulleri and Chapman 2010; Dafforn et al. 2015; Bishop et al. 2017). This may be further exacerbated by toxic chemicals (e.g. metals; industrial wastes and agricultural runoff; McGee et al. 1995), with the overall effect of these stressors being potentially greater than if the two stressors occurred  separately (a synergistic effect).

This text, Figure 1 and its associated caption is from “Multiple Stressors” In: Reichelt-Brushett, A. (eds) “Marine Pollution – Monitoring, Management and Mitigation”  2023, by Allyson  L. O’Brien, Katherine Dafforn, Anthony Chariton, Laura Airoldi, Ralf B. Schäfer, Mariana Mayer-Pinto, which is licensed under CC BY 4.0