Coastal and Estuarine Risk Assessment - Chapter 6 pdf

©2002 CRC Press LLC

The Bioaccumulation of

Mercury, Methylmercury,

and Other Toxic Elements

into Pelagic and Benthic

Organisms

Robert P. Mason

CONTENTS

6.1 Introduction

6.2 Bioaccumulation in Pelagic Food Webs

6.3 Bioaccumulation in Benthic Organisms

6.4 Membrane Transport Processes

6.5 Summary

Acknowledgments

References

6.1 INTRODUCTION

Many elements are toxic to organisms, but often only in a specific chemical form.

For example, although inorganic mercury (Hg) is toxic to organisms at low concen￾trations, it is the organic form of Hg, monomethylmercury (MMHg), that is highly

bioaccumulative and accounts for the wildlife and health concerns resulting from

the consumption of fish with elevated MMHg burdens.1 For other metals and met￾alloids, such as cadmium (Cd), lead (Pb), arsenic (As), and selenium (Se), it is also

often specific chemical forms, such as the free ion, e.g., Cd2+, or the methylated or

reduced species, e.g., mono- and dimethylAs or As(III), that are the most toxic.2

Thus, knowledge of the total concentration (i.e., the sum of all chemical forms) of

a potentially toxic element in the environment is insufficient to assess its toxicity

accurately. Furthermore, it is accepted that contaminants must be in solution to be

taken up directly from water3,4 and, as a result, it is the competitive binding of

6

©2002 CRC Press LLC

contaminants to dissolved organic and inorganic ligands, colloids, and to particulate

phases that ultimately controls the availability of an element in an aquatic system.4

In sediments, it is also the specific composition of the solid matrix, such as the

amount of organic carbon or sulfide (acid volatile sulfide, or AVS; or pyrite), that

determines the amount in solution in the sediment pore water,

4,5 as well as the

bioavailability of the contaminants in the solid phase. For example, Lawrence and

Mason6 showed that the MMHg bioaccumulation factor (BAF) for amphipods living

in sediment was a function of the sediment particulate organic content (POC).

Additionally, a number of studies have shown that the particulate–water distribution

coefficient (Kd) for Hg and MMHg is a function of POC.5,7,8 Metal concentrations

in sediments away from point source inputs are often strongly correlated with

sedimentary parameters such as POC, AVS, or Fe,4,5 and, as these parameters are

often co-correlated, it is difficult to determine the controlling phase. Nonetheless,

the binding strength of a metal or metalloid to the sediment influences its bioavail￾ability and bioaccumulation in benthic organisms. This has been shown for copper

(Cu), as well as other metals and organic contaminants.9,10 Recently, Lawrence et

al.11 also showed that the bioavailability of Hg and MMHg to benthic organisms

during digestion depended on the organic content of the sediment and further studies

have extended these ideas to other metals.12

Understanding the sources, fate, and bioaccumulation of Hg and MMHg in the

environment has received heightened attention primarily as a result of human and

wildlife concerns resulting from the consumption of fish with elevated Hg.1,13,14 In

the United States, the U.S. EPA has targeted anthropogenic sources of Hg for

regulation1,15 to reduce Hg inputs to the atmosphere. It is apparent that future

regulatory policies will focus on other metals and metalloids, such as As, Se, and

Cd, that are volatilized to the atmosphere during high temperature combustion

processes.16 Each element has a particular anthropogenic source inventory, e.g., coal

combustion and waste incineration (both medical and municipal) for Hg; coal com￾bustion for Se; waste incineration for Cd; and smelting and other industrial activities

for As.17 It has been estimated that the input of metals to the atmosphere as a result

of human activities has increased emissions by a factor of 5 for Cd, 1.6 for As, and

3 for Hg. Selenium anthropogenic inputs are about 60% of natural inputs.18–20

In addition to anthropogenic inputs to the atmosphere, metal and metalloids are

also introduced directly into the aquatic environment as a result of activities such

as mining and smelting and other industrial processes. These elements are typically

retained within watersheds,21,22 and postindustrialization activities have likely

resulted in a general increase in their burden in surface soils, lake sediments, and

other aquatic systems. For example, studies in contaminated environments such as

the Clark Fork Superfund Site in Montana23 have documented the bioaccumulation

of metals in stream biota and have documented the environmental perturbation

resulting from elevated metals in sediments and water.

The knowledge that chemical speciation controls bioavailability has become

the guiding principle for research into the toxicity and bioaccumulation of inor￾ganic contaminants in the environment.4,5,24 However, while recent research has

advanced the knowledge of the important differences in toxicity and fate of

inorganic species, corresponding environmental regulations, especially for coastal