Formatted Title:

Terrestrial Food Web Model to Predict Biomonitoring Data for Eagles

Background/Objectives:

Observed PCB concentrations in plasma based on biomonitoring data for PCB concentrations in nestling eagles demonstrate spatial heterogeneity relative to potential PCB sources in prey. A challenge with biomonitoring data is to better understand potential exposure sources, that is, what are the exposure concentrations that would be required to result in observed plasma levels. Nestling eagles consume prey items brought to the nest. From a site management perspective, questions arise around the contribution of contaminated areas given that eagles can have wide-ranging foraging areas depending on prey availability within a feeding range. The goal is to identify ranges of concentrations of selected PCBs in fish that would lead to the concentrations observed in nestling eagle plasma. These fish concentrations can then be compared to fish data from localized areas that may be associated with specific contaminant sources.

Ecological risk assessments typically rely on dietary dose models to quantify ecological receptor exposures, and these are compared to dietary doses at which effects have been observed. In this case, there are data on PCB concentrations in eagle nestling plasma. Short of a full toxicodynamic model, which requires significantly more resources, time, and data, a simple dietary dose model can serve as a first approximation of body burdens in eagles by assuming absorption rates and assimilation efficiencies in these organisms. Rearranging these equations allows us to solve for the fish concentration that would result in observed avian concentrations under specific assumptions.

Approach/Activities:

We developed several complementary dietary dose models for nestling eagles assuming a 100% fish diet. Assuming a linear relationship between dose and fish concentration based on the dietary dose models, it is straightforward to predict a dose or, conversely, to back calculate a fish concentration associated with a specific dose. The simplest relationship between dose and plasma concentration is based on the half-life of the PCB congener or mixture and the volume of distribution (V_d) in nestlings. As there were no Vd data specific to nestlings (or eagles of any kind) and PCBs, we conducted a literature review to identify relevant studies in related organisms and constituents to establish uncertainty bounds for these inputs. Finally, combining the dietary dose models with the plasma model, we calculated proportionality constants between plasma concentrations in nestling eagles and concentrations in fish prey for specific individual PCB congeners.

Results/Lessons Learned:

Dietary dose models using different assumptions and mathematical formulations show excellent concordance. Linear regressions provide central tendency estimates and associated uncertainty. Significant data gaps exist around V_d and half-lives of PCBs in eagle plasma; nonetheless, it is possible to bound these inputs and using a probabilistic framework and uncertainty in these underlying inputs can be captured. Here we present the approach and the calculated proportionality constants. We compare back-calculated fish concentrations to data from localized areas in the vicinity of the nestling eagle biomonitoring data to evaluate whether specific contaminant sources are likely to have contributed to observed data.

Heather Thurston

Affiliation

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Terrestrial Food Web Model to Predict Biomonitoring Data for Eagles