Biomagnification in marine systems: the perspective of an ecologist by John S. Gray Lyrics
Biomagnification in marine systems: the perspective of an ecologist
John S. Gray *
Department of Biology, University of Oslo, PB10634 Blindern, 0316 Oslo, Norway
Abstract
Biomagnification is the process where xenobiotic substances are transferred from food to an organism resulting in higher con- centrations compared with the source. It is widely believed that this is a general phenomenon for marine food webs. An analysis of 148 papers with biomagnification in the title shows that under half show biomagnification. Of studies on metals only organic mercury shows biomagnification and most metals are regulated and excreted and do not biomagnify. Of the studies on organic compounds 67% claimed to show biomagnification. However, bioconcentration (uptake from the surrounding water) is the most usual way that organic compounds are accumulated in organisms from invertebrates to and including fish. Only in sea-birds and marine mammals is food intake the major route and where biomagnification can be clearly shown. Body concentrations of organic compounds vary with lipid content and thus in order to compare across species normalisation to uniform lipid content should be done. Yet often this is not done so data purporting to show biomagnification merely relate to differing lipid content in the different species studied. Finally suggestions are made as to how data can be collected to better interpret the process of biomagnification in marine food webs.
1. Introduction
Rachel Carson’s book ‘‘Silent Spring’’ started an era of research into biomagnification up food chains. Gen- erations of students, myself included, were brought up knowing top predators, such as hawks and eagles, had accumulated so much dichlorodiphenyltrichloroethane (DDT) that eggs laid had shells so thin that chicks died and ultimately populations were affected. Following the lessons from this book it is widely assumed that bio- magnification up food webs is a general rule in all eco- logical systems. Yet in the 1970s Isaacs (1973) suggested that this was not the case in marine systems as marine food chains were more open than terrestrial ones. Isaacs postulated that terrestrial systems were characterised by tight predator–prey relationships where only one or at most a few species of prey were consumed. In marine systems predators consumed a wide range of prey that were smaller than the predator. As contaminant con- centrations vary between species and among age groups within species there was less likelihood of biomagnifi- cation in marine food chains.
Biomagnification is usually defined as the transfer of a xenobiotic chemical from food to an organism, re- sulting in a generally higher concentration within the organism than source, (Connell, 1989, 1990; Rand et al., 1995). However, not all authors use this definition and some define biomagnification as the increase in concentration between trophic levels, if the biomagnifica- tion factor (concentration in predator/concentration in prey)>1, then the compound is biomagnified. The problem with this definition is that the mechanism leading to the increase may be simply due to accumulation from the surrounding water whereas Connell’s definition restricts the term to concentration increases that result from food intake alone. I believe that restricting the definition solely to food intake is preferable as the mechanism of uptake is defined and follow this common usage of the term.
Yet it is not only via food intake that contaminants can accumulate within organisms, (McKay and Fraser, 2000). There are two other mechanisms for uptake of contaminants by biota, bioconcentration and bioaccumulation. Here I follow the definitions given byRand et al. (1995) where, bioconcentration is defined as the uptake of a chemical by an organism directly from the abiotic environment resulting in a higher concentration within the organism and bioaccumulation is defined as the uptake of a chemical by an organism from the abiotic and/or biotic (food) environment, that is from all sources. Once contaminants are within an organism they may be metabolised and/or excreted so that the concentration is a balance between intake, by whatever means, and regulation.
John S. Gray *
Department of Biology, University of Oslo, PB10634 Blindern, 0316 Oslo, Norway
Abstract
Biomagnification is the process where xenobiotic substances are transferred from food to an organism resulting in higher con- centrations compared with the source. It is widely believed that this is a general phenomenon for marine food webs. An analysis of 148 papers with biomagnification in the title shows that under half show biomagnification. Of studies on metals only organic mercury shows biomagnification and most metals are regulated and excreted and do not biomagnify. Of the studies on organic compounds 67% claimed to show biomagnification. However, bioconcentration (uptake from the surrounding water) is the most usual way that organic compounds are accumulated in organisms from invertebrates to and including fish. Only in sea-birds and marine mammals is food intake the major route and where biomagnification can be clearly shown. Body concentrations of organic compounds vary with lipid content and thus in order to compare across species normalisation to uniform lipid content should be done. Yet often this is not done so data purporting to show biomagnification merely relate to differing lipid content in the different species studied. Finally suggestions are made as to how data can be collected to better interpret the process of biomagnification in marine food webs.
1. Introduction
Rachel Carson’s book ‘‘Silent Spring’’ started an era of research into biomagnification up food chains. Gen- erations of students, myself included, were brought up knowing top predators, such as hawks and eagles, had accumulated so much dichlorodiphenyltrichloroethane (DDT) that eggs laid had shells so thin that chicks died and ultimately populations were affected. Following the lessons from this book it is widely assumed that bio- magnification up food webs is a general rule in all eco- logical systems. Yet in the 1970s Isaacs (1973) suggested that this was not the case in marine systems as marine food chains were more open than terrestrial ones. Isaacs postulated that terrestrial systems were characterised by tight predator–prey relationships where only one or at most a few species of prey were consumed. In marine systems predators consumed a wide range of prey that were smaller than the predator. As contaminant con- centrations vary between species and among age groups within species there was less likelihood of biomagnifi- cation in marine food chains.
Biomagnification is usually defined as the transfer of a xenobiotic chemical from food to an organism, re- sulting in a generally higher concentration within the organism than source, (Connell, 1989, 1990; Rand et al., 1995). However, not all authors use this definition and some define biomagnification as the increase in concentration between trophic levels, if the biomagnifica- tion factor (concentration in predator/concentration in prey)>1, then the compound is biomagnified. The problem with this definition is that the mechanism leading to the increase may be simply due to accumulation from the surrounding water whereas Connell’s definition restricts the term to concentration increases that result from food intake alone. I believe that restricting the definition solely to food intake is preferable as the mechanism of uptake is defined and follow this common usage of the term.
Yet it is not only via food intake that contaminants can accumulate within organisms, (McKay and Fraser, 2000). There are two other mechanisms for uptake of contaminants by biota, bioconcentration and bioaccumulation. Here I follow the definitions given byRand et al. (1995) where, bioconcentration is defined as the uptake of a chemical by an organism directly from the abiotic environment resulting in a higher concentration within the organism and bioaccumulation is defined as the uptake of a chemical by an organism from the abiotic and/or biotic (food) environment, that is from all sources. Once contaminants are within an organism they may be metabolised and/or excreted so that the concentration is a balance between intake, by whatever means, and regulation.