Food Web Model Application

Food-web bioaccumulation models have been applied in a number of different ways. However, in terms of assessing ecotoxicological risks, two main methods of application should be emphasized. The first method, referred to as the 'forward calculation', uses observed distributions in measured chemical concentrations in the water and sediments as the starting point of the model (i.e., the external variable or forcing function) to calculate the anticipated corresponding concentration in the wildlife species of interest. The resulting distribution can then be compared to tissue residue guidelines or toxicological threshold values to derive the fraction of the affected wildlife population that contains chemical concentrations greater than or below the reference value of interest. This is illustrated in Figure 6, which illustrates the application of the model to the calculation of PCBs concentrations in harbor seal pups as a result of exposure to PCB concentration distributions in water and sediments. In this particular example, the distribution of PCB concentrations in water and sediments of the affected water body can be expected to produce PCB concentrations that vary substantially as described by the distribution with a large percentage of the concentrations exceeding the threshold effects concentration.

The second method, referred to as the 'backward calculation', applies the model to back-calculate what distribution of PCB concentrations in water and sediments can be expected to produce a particular distribution of concentrations in the target species of ecotoxicological concern or interest. The application of the model is illustrated in Figure 7 and refers to a management goal to ensure that 95% of the population of a particular target species (e.g., seal pups) contains chemical (e.g., PCBs) concentrations less than a threshold effects concentration (e.g., 5000ng/gww). The food-web

Forward calculation

Forward calculation

Concentration in water

Figure 6 Illustration of the application of the food-web model to assess the ecotoxicological risk of a contaminant in a higher-trophic-level organism (seal). Observed chemical concentrations in sediment and water (presented as statistical distributions) are entered in the model to derive the chemical concentration distribution in a resident seal population and the incidence of concentrations greater than the toxicological threshold effect concentration (TEC).

Concentration in water

Figure 6 Illustration of the application of the food-web model to assess the ecotoxicological risk of a contaminant in a higher-trophic-level organism (seal). Observed chemical concentrations in sediment and water (presented as statistical distributions) are entered in the model to derive the chemical concentration distribution in a resident seal population and the incidence of concentrations greater than the toxicological threshold effect concentration (TEC).

Backward calculation

Backward calculation

Concentration in sediment

Figure 7 Illustration of the application of the food-web model to derive the system-wide sediment concentration distribution that can be expected to meet an acceptable risk level, set at 5% of the target population of seals exceeding the toxicological threshold effect concentration (TEC).

Concentration in sediment

Figure 7 Illustration of the application of the food-web model to derive the system-wide sediment concentration distribution that can be expected to meet an acceptable risk level, set at 5% of the target population of seals exceeding the toxicological threshold effect concentration (TEC).

bioaccumulation model is then used to calculate what distribution in PCB concentrations in the sediments of the system can be expected to produce this distribution. The relationship between PCB concentrations in water and sediments, determined from monitoring programs or estimated using models, needs to be known for this purpose. The calculated distribution can then serve as a remediation or pollution control objective or a sediment quality criterion for the protection of wildlife species.

Examples of the application of the food-web bioaccumulation for ecotoxicological risk assessment, including the use of forward and backward calculations, include the San Francisco Bay food-web bioaccumulation model. The model is documented in a Gobas and Arnot report listed on the website of the San Francisco Bay Clean Estuary Partnership (CEP), and can be downloaded in the form of a Microsoft EXCEL® workbook from http://www.rem.sfu.ca. The purpose of this model is to estimate concentrations of PCBs in a set of key species that reside in the Bay, including double-crested cormorants, the Forster's tern, the harbor seal, and three fish species that are frequently caught by fishermen in the bay, as a result of PCB concentrations in sediments and water in the bay. The model can be used to determine what concentrations of PCBs in the water and sediments of the bay need to be reached to achieve an adequate margin of safety in wildlife and humans exposed to PCBs in the bay area. This information can be used as part of a TMDL characterization to formulate remedial actions to achieve desired water quality goals. The management module includes a simple worksheet to conduct two types of calculations, viz. 'forward' calculations to estimate the concentrations of PCBs in biota of the bay from PCB concentrations in the sediments of the bay and 'backward' calculations to calculate the PCB concentrations in the sediments of the bay that are required to meet PCB concentration based criteria in fish and wildlife for the bay. The backward calculation is designed to determine target PCB concentrations in sediments that meet ecological and/or human health criteria.

Other applications of the food-web bioaccumulation include the estimation of the BAF and BCF for fish species in lower, middle, and upper trophic levels of aquatic food webs. The model predictions can include the effect of metabolic transformation and trophic dilution on the BAF if a reliable estimate of the chemical's metabolic transformation rate in fish is available. The model is named BAF-QSAR vl.1 and is coded in a Microsoft EXCEL workbook, is freely available for download, and can be run for a large number of chemicals.

Food-web bioaccumulation models have also been used for the derivation of water quality guidelines. For example, the Gobas 1993 model, which was originally published for application to the Lake Ontario ecosystem and has since been applied to many other ecosystems by several authors, has been reviewed and adopted by the US Environmental Protection Agency for developing water quality criteria and waste load allocations in the US under the Great Lakes Water Quality Initiative (EPA-822-R-94-002). This model has been updated and is now referred to as AQUAWEB v1.1. It provides site-specific estimates of chemical concentrations in organisms of aquatic food webs from chemical concentrations in the water and the sediment. Key revisions included new equations for the partitioning of chemicals into organisms, new kinetic models for predicting chemical concentrations in algae, phytoplankton, and zooplankton, new allometric relationships for predicting gill ventilation rates in a wide range of aquatic species, and a novel mechanistic model for predicting gastrointestinal magnification of organic chemicals in a range of species. The model has been evaluated using empirical data from three different fresh water ecosystems involving 1019 observations for 35 species and 64 chemicals. Both models are coded in one Microsoft EXCEL workbook and can be downloaded from http://www.rem.sfu.catoxicology.

See also: Bioaccumulation; Biomagnification; Food Chains and Food Webs.

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