Coastal and Estuarine Risk Assessment - Chapter 7 pptx

©2002 CRC Press LLC

Dietary Metals Exposure

and Toxicity to Aquatic

Organisms: Implications

for Ecological Risk

Assessment

Christian E. Schlekat, Byeong-Gweon Lee,

and Samuel N. Luoma

CONTENTS

7.1 Introduction

7.2 Current Status of Regulatory Approaches for Metals

in Aquatic Systems

7.2.1 The Importance of Phase and Speciation in Metal

Risk Assessment

7.2.2 Incorporation of Metal Speciation into Risk Assessment

7.2.3 The Biotic Ligand Model

7.2.3.1 Mechanisms of Metal Toxicity at the Gill

7.2.3.2 Model Assumptions and Components

7.2.4 Limitations of Current and Projected Risk Assessment Practices

7.3 Processes Affecting Dietary Metal Exposure

7.3.1 Metal Partitioning

7.3.2 Biological Mechanisms

7.3.2.1 Food Selection

7.3.2.2 Feeding Rates

7.3.2.3 Mechanisms of Dietary Metal Absorption

7.3.2.3.1 pH

7.3.2.3.2 Amino Acid–Rich Digestive Fluids

7.3.2.3.3 Surfactants

7.3.2.3.4 Intracellular Digestion

7.3.3 Experimental Designs for Laboratory Exposures via Diet

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7.4 The Relative Importance of Dietary vs. Dissolved Metal Uptake

for Bioaccumulation and Toxicity

7.4.1 Mass Balance Approach

7.4.1.1 Deposit and Suspension Feeders

7.4.1.2 Predators

7.4.2 The Use of Mathematical Models in Metals Risk Assessment

7.4.2.1 Background

7.4.2.2 Equilibrium Models

7.4.2.3 Dynamic Multipathway Bioaccumulation Model

7.4.2.3.1 DYMBAM Structure

7.4.2.4 Application of Models

7.4.2.4.1 DYMBAM Case Study: Selenium

in San Francisco Bay

7.4.3 Comparisons among Metals and Organisms

7.5 Toxicological Significance of Dietary Metals Exposure

7.5.1 Examples of Dietary Metals Toxicity

7.5.2 Why is Dietary Toxicity Difficult to Measure?

7.5.3 How Are These Subtle Effects To Be Handled in a Risk

Assessment Framework?

7.6 Conclusions/Recommendations

References

7.1 INTRODUCTION

Effects of trace element contamination on coastal and estuarine ecosystems have

received considerable attention over the past 50 to 60 years.1 Risk assessment frame￾works offer a means to quantify these effects, and to develop management alternatives

for dealing with historical and ongoing trace element contamination. Quantifying the

risk of metals to aquatic systems is now an established practice, but important uncer￾tainties remain about specific components of the metals risk assessment process.

In both the United States and Europe, ecological risk assessments that address

metal contamination in aquatic systems are conducted in accordance with the

National Research Council Risk Assessment (NRC) paradigm.2 After contaminants

of concern and relevant ecological communities have been identified, the risk assess￾ment paradigm calls for parallel characterizations of contaminant exposure and effect

(see Chapter 1 for more detail). A key element of exposure characterization is

estimating the dose of contaminant to which the organisms of interest is exposed in

situ. The effects characterization, or toxicity assessment, includes a dose–response

assessment, which is the dose necessary to elicit adverse effects to exposed organ￾isms. Both dose estimation and dose–response assessment typically assume that

adverse effects are caused by exposure to dissolved metals only.

The assumption that dissolved metals are responsible for toxicity has simplified

the risk assessment approach. Determinations of exposure require only consideration

of dissolved metal concentrations at the site, and knowing dose–response relation￾ships for dissolved metals. Assessing risks of individual contaminants typically

©2002 CRC Press LLC

involves the risk characterization ratio (RCR), which is the ratio of exposure con￾centration to a dose–response toxicity criterion:

RCR = DMC/DEC (7.1)

where DMC is the dissolved metal concentration (g/l) and DEC is an effects

concentration (g/l) derived from the response of aquatic organisms to dissolved

metal concentrations (e.g., ambient water quality criteria). When RCR < 1, adverse

effects are not expected.

Recently, several independent lines of research have challenged the underlying

assumptions supporting the “dissolved only” approach by highlighting the impor￾tance of dietary metals exposure. A growing body of work demonstrates that, in

conditions similar to nature, dietary exposure to metals associated with food items

is at least as important as exposure to dissolved metals.3–5 This generalization holds

for most metals and metalloids, and for organisms living within different trophic

levels. The findings that dietary exposures are important have implications for risk

assessment. The most important is that the dissolved only assumption may lead to

underestimates of metal exposure under natural conditions if animals are exposed

to both dietary and dissolved sources. If dietary exposure causes adverse biological

effects, the RCR needs modification to reflect the additional dietary dose (i.e., the

numerator in Equation 7.1) and its toxicological concentration threshold (i.e., the

denominator in Equation 7.1). The recognition of the importance of dietary metals

exposure emphasizes the need to conduct effects assessments in a way that more

closely approximates exposure conditions in nature. Specifically, metal concentra￾tions in food items that are representative of the system in question need to be

measured and included in estimates of dose. Similarly, the relationship between

organismal response and dietary metal dose must be better understood.

This chapter discusses the current state of knowledge concerning exposure and

some aspects of effects of metals and metalloids in estuarine and coastal systems.

The review will be organized to address the specific questions:

1. What is the current status of regulatory approaches for metals? Are there

significant limitations to these approaches?

2. What geochemical and physiological factors determine the importance of

dietary metals exposure?

3. What is the relative importance of dietary metals exposure compared with

dissolved metals exposure?

4. If dietary metals exposure is important at the organismal level, does this

exposure result in toxicity?

5. What are the implications for risk assessment when dietary exposure is

at least as important as dissolved exposure in eliciting dose effects?

We will provide geochemical and organismal evidence to demonstrate the quan￾titative importance of dietary metal exposure to aquatic organisms, and we will show

that it is likely that such exposures can have toxicological consequences. We will

also highlight the biological and geochemical uncertainties that must be addressed