NUCLEAR TASK FORCE
PART I
Some background concepts of an analytical nature will assist in understanding the materials presented throughout the report.
LATITUDE ADJUSTMENT TECHNIQUES FOR NUCLIDES IN FALLOUT
In the early work on deposition of radioactive particles from the atmosphere to the surface of the earth, a simplified model was developed and used by atmospheric scientists and exploited by UNSCEAR. This model treats earth as a sphere (actually it is more of a prolated ellipsoid because of the configuration at the North and South poles) of radius 12,800 km. The surface of the sphere is divided into latitude bands of 10° latitude width from North Pole to South Pole. The designation of latitude bands is north or south of the Equator, which by convention is designated 0° latitude. The Equator represents the latitude of maximum spherical circumference. The numerical designations of either a north latitude or south latitude range from 0° to 90°.
Nuclear scientists have determined that deposition of atmospheric radioactive material, because of factors ranging from the locations of nuclear sources to the axial tilt of the earth relative to the sun, that 10° latitude bands are the smallest statistically useful scale to resolve the deposition data. The Great Lakes occupy parts of the latitude bands from 40°N to 60°N. Their surface area for deposition is 24.6 X 10 4 km 2 , the world-wide area in that latitude band for deposition based on the spherical assumption is 67.2 X 10 6 km 2 . The UNSCEAR model assumes uniform deposition over the sphere; therefore, the fraction of the global deposition falling on the Great Lakes surface available area is 0.00366. The drainage basin is 49.6 X 10 4 km 2 , but it extends below 40°N and above 60°N. Because the increased area for deposition is small compared with the area of global deposition, one need not adjust for the extended area outside of the two boundary latitude bands. Thus one can use a simple proportionality to obtain the increased deposition associated with the drainage basin relative to the Lakes. The fraction of the global deposition falling on the Great Lakes drainage basin is approximately 0.0074. These deposition data are then corrected for radioactive decay of the isotopes.
STATISTICAL APPROACHES -- COMPOSITIONAL DATA
Statistical data for substrates or media that contain, receive, or export multiple isotopes require multivariate statistical techniques. The data do not represent the entire chemical composition of the substrate or media but, rather, are some unknown fraction of the same. This raises the issue of the applicability of what Aitchison (1986) called compositional data . While one assumes that the sum of all the elemental parts is 100% and the sum of fractions is unity, in the actual analytical situation the variability of one or more components may be artificially restrained by this model. Most data sets on isotopes or chemicals, especially those used to consider biological cycling of elements through biota and biotic communities are compositional data sets. They usually have, however, enough unspecified or unmeasured subsystems to permit a relaxation of any statistical restrictions associated with compositional data, especially when assessing correlation or analyzing variance.
Although many radionuclides occur in the discharges from various nuclear sources to the Great Lakes, not every element has radionuclides that will bioaccumulate and affect biota of the Great Lakes. The Task Force, in its work on the development of inventories of "all" radionuclides found in the Great Lakes, recognized the need to bound its inquiry. It addressed only those elements whose radionuclides require inventories for biological compartments. Such elements usually share one or more of the properties listed in Text Box 1. In reviewing the material in Text Box 1, the Task Force notes a special concern with respect to the half-life of a radionuclide: beyond the separate importance for studying 131 I (half-life 8 days), the Task Force has chosen to study radionuclides with half-lives greater than 14 days. The Task Force wishes to acknowledge some problems, encountered in the preparation of this report, which could not be resolved based on currently available data.
The Task Force addressed in a limited way the following elements: arsenic, cadmium, copper, and mercury. Data on the bioaccumulation of these elements exists from which to develop bioaccumulation factors, but radioactive isotopes of these elements are not major concerns in the Great Lakes. Given the known toxicity of these elements as well as the toxicities of osmium, tellurium, and thallium, one might consider it strange that the bioaccumulation tables in this report do not include many entries. The Task Force had no elemental data for osmium, thallium, and tellurium. Other unusual elements are tantalum and antimony. Tantalum has not been documented as present in aquatic biota . Antimony is toxic to aquatic organisms, and its radionuclides are known fission products, which have been detected in the Great Lakes, but antimony's presence is freshwater aquatic organisms has only very rarely been documented . Table 1 lists elements that were considered in this work.
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PROPERTIES OF RADIONUCLIDES THAT WOULD DIMINISH THE NEED TO ESTIMATE BIOACCUMULATION AND BIOMAGNIFICATION PARAMETERS |
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CLASSIFICATION OF ELEMENTS FOR REQUIRING ESTIMATES OF BIOACCUMULATION AND BIOMAGNIFICATION PARAMETERS FOR BIOTA OF THE GREAT LAKES |
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BIOACCUMULATION AND BIOMAGNIFICATION
The terms bioaccumulation and biomagnification refer to the uptake of chemicals by organisms and their retention in amounts that may exceed the levels present in the source materials. Bioaccumulation is a more general term, with biomagnification describing a special type of bioaccumulation usually associated with dynamics of organismal food webs and the cycling of chemicals through biotic assemblages and ecosystems. Furthermore, biomagnification has been used more often in discussions of organic chemicals than radionuclides.
Bioaccumulation can be assessed directly or indirectly. Direct measurements quantify the residue or elemental/chemical content of some substance in tissue, for example, analytical determination of the content of 40 K in a tissue. Indirect measurements examine some indicator of bioaccumulation, measure the indicator, and use statistical correlations or calibration equations to relate the measure to the accumulated material, for example, measurement of chlorophyll a in algae as a surrogate for productivity or biomass production.
It is sometimes possible to use mathematical models and simulations to predict bioaccumulation and biomagnification. The simulation models emphasize the cycling of chemicals through biotic assemblages (communities, trophic levels) and ecosystems in somewhat generalized form, usually as simplified foodwebs or energy flow systems.
Simulation techniques usually work better with organic chemicals than with radionuclides despite the fact that some simulations were originally developed for radionuclides. The success of the simulation techniques with chemicals mainly reflects their dependency of many bioaccumulation processes on the size of an organism's lipid reserves with the bioaccumulated chemicals depositing selectively in lipid tissues. Several physicochemical ( i.e., quasi-thermodynamic) properties of chemicals express this partitioning of a chemical between lipid and aqueous tissue reserves rather well, the best known one being the "octanol-water partition coefficient." Radionuclides that show an affinity toward lipids include those whose stable nuclides are parts of lipid compounds ( e.g., carbon) and those that form lipid-soluble compounds, notably metals which form organometallic compounds through such mechanisms as methylation ( e.g., mercury, tin, arsenic, selenium).
Simulation techniques also work well with the known metabolic pathways and mechanisms for specific compounds. Compounds of interest include the vitamins, hormones, and small molecules that act as cofactors in metabolism. These compounds either contain an unusual element ( e.g., vitamin B 12 has cobalt, thyroxine has iodine) or require the presence of a special element for activation ( e.g., magnesium, iron, manganese, molybdenum), and the unusual or special element can easily occur as a radionuclide.
Bioaccumulation processes are dynamic. They may occur over time periods that appear to be essentially "infinite" relative to the expected life-span of an organism. However, the limitations of organismal size specify the number of tissue sites or of tissue capacity to carry chemicals coupled with physiological processes of detoxification and internal processes of tissue repair. That assures that bioaccumulation would reach a steady state. This "steady state" may be an academic concept, since very few bioaccumulation processes have received sufficient study to show that the steady state has been reached. Many factors can confound the observation of a steady state and are presented in Text Box 2 and discussed individually.
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FACTORS AFFECTING OBSERVATION OF STEADY STATES IN BIOACCUMULATION PROCESSES |
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Organismal growth increases capacity to bioaccumulate a given nuclide. Depending on past exposure, organismal growth may also include adaptation to an element not previously experienced. DeFilippis and Pallaghy (1994) refer to "a considerable induction of tolerance" in algae chronically exposed to heavy metals and the observations of reduced levels of toxicity under certain conditions of metabolic activity, notably in selective carbon and nitrogen assimilation processes. Organism reproduction also provides a mechanism for a parent to pass some of its accumulated material through food to an offspring, lower its own body burden, and free up existing or create more capacity to accumulate more material.
Is a tissue residue for a substance, even if a lethal level, the "steady-state" level or was passive uptake the method of bioaccumulation despite organismal death? Biological variability in susceptibility among organisms both intraspecifically and interspecifically creates a range of observed or measured "fatal" levels, which may not clearly indicate that some maximum observed "lethal" level is the steady-state level. Emerson and Hesslein (1973) added 226 Ra to Lake 227 of the Experimental Lakes Area (ELA) at Kenora, Ontario, and followed the distribution of both the radium and radon gas daughter nuclide in various components of the lake. They reported that the uptake of radium did not differ between live and dead algae, suggesting at least for this radionuclide, the importance of passive adsorption mechanisms in the biouptake processes. Further, their experiment suggests the importance of the particulate matter from dead organisms as a binding material for radionuclides and, thus, the potential to observe continued biouptake long after an organism has died.
By incorporating accumulated material into a special matrix that either renders the material inert in storage or hastens detoxification, the organism may add capacity to accumulate. Pentreath et al. (1980) studied the accumulation of 237 Pu by seaweeds such as Fucus vesiculosus and Fucus serratus. They showed that accumulation occurred mainly by surface adsorption with the bulk of the plutonium found in a very thin outer surface layer of the laminae. Early studies by Harvey and Patrick (1967) on the uptake of 137 Cs and 65 Zn by diatoms showed that the surface to volume ratios of the cells correlated with the bioaccumulation factors to a greater degree than the taxonomic characteristics of the diatoms studied. The matrix of mucoproteins and glycosidic residues on the organism surfaces provide the storage mechanism. Since these materials are water soluble, the investigators noted that the radionuclides washed off the organisms rather easily.
The background concentration of the substance in the source materials may dictate the extent of accumulation. Titanium usually has a very low uptake in biological organisms. Yan et al . (1989) found very low residues of titanium in the zooplankton species of Bat Lake and very high residues of titanium in the zooplankton of Horn-2 Lake. These lakes, in the Canadian Shield region of northwestern Ontario, had respectively, very low and very high titanium content of source material of the lakes' basins: the trap rock and water. In fact, Bat Lake had such a low mineral content in the source materials that most of the elements accumulated in the zooplankton species reflected the rather impoverished environment. By contrast, the zooplankton in Horn-2 Lake, a rather alkaline lake, had considerably elevated levels of calcium, copper, aluminum, magnesium, strontium, barium, zinc, and cadmium, reflecting the nature of the source materials.
The accumulation of an element very often depends on another element or suite of other elements, as well as its own levels in various environmental compartments. Hutchinson (1975), in his Treatise on Limnology, Volume III , noted that one of the more interesting problems in understanding the chemical ecology of lake macrophytes is the "unusual" array of accumulated elements that are sometimes observed. Does the accumulation of and value of the steady-state residue for some unusual element, such as titanium, depend on the presence of calcium, magnesium, aluminum, copper, etc. It is known that uptake of barium depends on calcium, but the unknown nature of the mechanisms for such "assisted" or "dependent" uptake complicates the problem.
Bioaccumulation factors
The fundamental parameter expressing bioaccumulation is the bioaccumulation factor, a dimensionless ratio of the concentration, activity level, or similar entity of a chemical within a given tissue to its comparable concentration, activity level, etc., in the source material to which the tissue is exposed. The bioaccumulation factor can be extended to describe relative distribution between two biological materials, a biotic source material and a second biological substrate exposed to the source material. If this ratio exceeds unity, one has "biomagnification" and the bioaccumulation factor becomes a "biomagnification factor." The usual situation compares the ratio of bioaccumulation factors for a nuclide for two species, a consumer species and a consumed species.
Reference elements
Geochemists sometimes choose a "reference" element for comparison when studying the geochemical relationships of chemicals and elements to each other. The technique often works well with bioaccumulation, but does not have a significant history of use for biomagnification. In fact, the Task Force does not recommend its use for biomagnification, unless there is evidence of the biomagnification of several nuclides by the same or very closely related mechanisms, and that the biomagnified nuclides are geochemically "coherent." "Coherence" is a geochemical term for some entity accompanying something else in a mechanistic manner such that inferences about the thing and that which it accompanies treat both entities identically. Such coherence can often overrule some of the obvious behavioral differences that would be ascribed to the entities based on chemistry, biology, or physics alone, if they were treated separately under the circumstances described.
There are many choices of reference element, but most currently used choices fit one of two categories: another element in the same family of the Periodic Table as the element of interest, or an element that reflects knowledge of the source materials for the nuclides being accumulated. In the first choice, the comparison to the reference element examines the chemistry of the family of the Periodic Table to understand the extent to which a given element follows the behavior of the reference element. In the second choice, the comparison examines the extent to which the element of concern follows the known behavior of the reference element in various types of source materials and substrates.
For the second choice consider the nature of soils. Elements that have limited biological mobility and would likely remain in soil in preference to uptake by an organism might show a certain mutual correlation. Titanium and aluminum tend to have limited biological mobility, although the Task Force has noted data that significantly challenge this "common wisdom" and have served as reference elements when examining the bioaccumulation of elements from mineral, soil, and related materials. When relating radionuclides to each other from source materials, the primordial natural radionuclide of potassium, 40 K, sometimes serves as a reference element.
A third choice, pointed out by Dr. Ursula Cowgill (personal communication) examines an element not known to be present in any of the substrates, media, or source materials as the reference. Such an element has complete accountability in all materials because its levels are those that have been quantitatively added to the system by the investigator for reference purposes . The comparisons are for material-balance calculations with quality-assurance checks inherent in the knowledge that all sources and amounts of the reference element are under the investigator's control and quantitatively known.
The use of a reference element changes the calculations and interpretations of bioaccumulation and biomagnification factors. First, the reference element approach converts the calculation of a bioaccumulation or biomagnification factor from a single geochemical ratio of a given element in two different source materials to a ratio of two geochemical ratios of the given element to reference element in each of the two different source materials. This new calculation procedure changes the interpretation of the bioaccumulation factor from the relative enrichment or rejection of the given element between the two different source materials to relative enrichment or rejection of a coherent pair of elements in the two different source materials. The incorporation of a reference element into the calculation may suggest the presence or absence of a commonality of the environmental distribution of the element being studied with the known environmental distribution of the reference element. The resulting bioaccumulation factor is sometimes referred to as the enrichment factor , because it compares behavior of an element with respect to a reference element in the uptake process to show the pattern of "enrichment" (increasing uptake relative to the reference) or "rejection," sometimes referred to as "depauperation" (failure to transfer to a new phase relative to the source materials).
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BIOACCUMULATION FACTOR AND BIOMAGNIFICATION FACTOR FORMULAS |
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The formulas for the various types of bioaccumulation factors are given in Text Box 3. The formulas do not depend on the stability of the nuclide. Obviously "activity" data will apply to radioactive materials, but the formulas are generic, since specific activity is defined as proportional to concentration.
Reference elements and Great Lakes data
When the source material of a nuclide is water, minerals, soils, or sediments, the reference element approach sometimes gives a greater bioaccumulation factor by suggesting possible geochemical relationships between elements to improve the understanding of an organism's uptake, rejection, or retention of a given element. Some marine geochemists used this approach to separate bioaccumulation rates and mechanisms for elements found in surface layers of the oceans from those in deeper layers of the oceans. The studies the Great Lakes usually do not have sufficiently rich data collection protocols to exploit the more advanced ideas in the reference element approach, but the Task Force has attempted this approach in a few instances.
Quality assurance provisions for biouptake data
Because of the variability in quality and form of the data for bioaccumulation and biomagnification, and because the bioaccumulation and biomagnification factors can be used for estimating the nuclide inventory of a biological compartment, the Task Force has listed a set of criteria for rating the quality of the data used. Text Box 4 summarizes this information. The most reliable data would fit a profile of all "Yes" answers to the questions, although the listing is not exclusive. Those data that a fit a profile of only a few "Yes" responses would engender much less confidence as to quality and usefulness.
A special note about the fifth question in Text Box 4. The Task Force does not expect that the information provided has associated with it a legal "chain of custody," as sometimes prescribed in protocols for research and monitoring that must meet legal or regulatory requirements. Rather the Task Force wishes to know enough about the collection, processing, handling, and analysis of materials and the subsequent reporting of results to assure confidence that the data can meet some tests of quality in later analyses of bioaccumulation and biomagnification processes and the development and assessment of radionuclide inventories.
The sixth question in the Text Box 4 on chemical and biological matrices refers to a practical problem of available reference samples of biological materials for quality assurance purposes. Some reference material is supplied by the National Institute of Technology (formerly: United States Bureau of Standards). The matrix problem in general requires careful review before using elemental data obtained from many substrates. Other reference samples have been developed cooperatively with the United States Environmental Protection Agency on radionuclides in a variety of substrates, some of which are of biological origin.
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QUESTIONS TO ASSESS THE QUALITY OF DATA USED TO EVALUATE BIOACCUMULATION AND BIOMAGNIFICATION |
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Biotic needs for bioaccumulation data
There are several data needs related to bioaccumulation. First is a need for biological productivity data. Productivity data converts concentrations and activities of nuclides into mass balance parameters and allows one to address whole-organism data as well as ecosystem and community data by considering the absolute rather relative cycling of selected isotopes.
For lakes, one needs data for plankton species (phytoplankton and zooplankton), benthos, and pelagic fishes. For rivers and tributaries that enter, connect, or leave lakes, one needs the comparable data for benthic algae, macrophytes, and macroinvertebrates. Finally, there are terrestrial species that provide a link between elements that cycle through aquatic biota and terrestrial biota. These include fish- and invertebrate-consuming birds, reptiles, and mammals (herein considered as "wildlife"), which spend part of their time in lake and river habitats but are not totally aquatic species.
The major data deficiencies occur for lake macroinvertebrates, both benthic and planktonic, river plants and animals, and the terrestrial wildlife species. Since 1985, several government research laboratories, mainly in Canada, have sought to close these data gaps. In many instances the focus of the research was on Arctic species or biological assemblages in national parks or remote areas, not specifically the Great Lakes. Nevertheless, the climate zones, latitude bands, species distributions, and often a fortuitous coverage of radionuclides from residual atmospheric fallout and weapons testing have enabled of the Task Force to address Great Lakes biotic compartments. Of particular value are data for species of Daphnia , which are important in the Great Lakes.
Limitations of elemental composition data
The simplest measures of bioaccumulation derive from the elemental composition of a tissue or organism and compared with the elemental composition of its food, water, or host environment. However, the comparisons of data on elemental composition of several biological samples or substrates do not always quantify the bioaccumulation processes.
One consideration is that very few residue measurements are taken as functions of time, and therefore, the state of bioaccumulation does not infer the rate of bioaccumulation . Further, the bioaccumulation data may reflect a history of organismal exposure, which is unknown to the investigator. This occurs routinely in migratory species or organisms that experience rapidly changing environmental conditions.
Organisms also can try to control much of their physiology and behavior to maintain homeostasis (an operational range of states that permit the organism to function successfully and minimize stress). This requires that an organism exert some control over its elemental composition despite fluctuating conditions of elemental scarcity or excess and changes in external physical environment ( i.e., temperature, pH, alkalinity, salinity, flow, and dilution). In maintaining homeostasis, organisms may modify considerably their own exposure history to the elements which they accumulate.
Assimilation efficiency
Calow and Fletcher (1972) pioneered a technique called the "assimilation efficiency," which relates the uptake of a chemical to the assimilation of carbon (a measure of new protoplasm production). The method designates the carbon assimilated from food sources as the reference element to which all other accumulated elements are compared. Note that the method only considers that part of the geochemical pool of carbon that goes into protoplasm. Other reference element methods do not make this distinction but, rather, make their comparisons with respect to the total pool of the element in a given medium, not a specific fraction of that element in a given medium. The choice of the element's entire geochemical pool is sometimes unavoidable because the studies cited had no methods for measurements of the various fractions of the element's pool that would allow a more refined choice. Many transfer factors , which relate elements found in certain organisms to the amount retained in consumer species, assume a 50% utilization of available carbon . This assumption occurs in models using marine species, since that data base is large.
14 C is a tracer of assimilated carbon. The method assumes a constant ratio of radioactive carbon to total carbon (complete mixing of the isotope pool), thus assuring the ratio of radioactive carbon to assimilated carbon is a constant. For a second element, a radionuclide version is used under other circumstances, which assures that element is also a constant fraction of its geochemical pool. The ratio of the activities of the two reference radionuclides equals the ratio of the elemental masses in the appropriate geochemical pool of the substrate being studied. The second isotope is typically 51 Cr, which is assumed to have no assimilation or protoplasm purpose. The retention of the radionuclide of chromium thus presumably measures some passive biouptake and storage mechanism. The geochemical ratios of 14 C/ 51 Cr in organismal food and fecal materials are determined and the assimilation efficiency obtained by difference.
Nicholas Fisher and his coworkers (Fisher et al. 1983; Luoma et al. 1992; Wang and Fisher 1996) have explored this method extensively with 51 Cr, 57 Co, 65 Zn, 75 Se, and 110 Ag with marine molluscs ( e.g. , Mya arenaria , Mytilus edulis ) and various species of marine algae. Fisher could derive regression equations between 14 C and some radionuclides accumulated in selected species, but in a few cases, especially 110 Ag, such regression equations could not be derived. Regression equations enhance an understanding of the biouptake processes in that one can see which elements correlate with carbon assimilation, but the absence of such equations offers equally important information on elements that do not correlate with carbon . For example, the environmental movement of silver is not well understood; thus, it is useful to know that the cycling of this element does not show an association with the assimilation and use of carbon in the nutrient pool.
One advantage of the assimilation-efficiency method is its consideration of growth and production of new protoplasm into the analysis, while treating dynamically the uptake, excretion, and retention of a radionuclide. The mathematical equations describing the method can treat several kinds of experimental data, including data from static and dynamic experiments and absolute concentration data as well as kinetic data on rates of change of concentration with time. The mathematical equations appear in Text Box 5.
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RELATIONSHIP BETWEEN ASSIMILATION EFFICIENCY AND BIOACCUMULATION FACTORS |
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The assimilation-efficiency method addresses an aspect of the transport process of fecal pellets of organisms excreted into the water column and moving to lake sediments. The settling of particles in the water column, of which the fecal pellets of organisms are an important component of the particulate matter, is a major transport mechanism of many elements to lake and ocean sediments. The marine studies far exceed in number and scope the studies of the freshwater systems. Most of the studies of vertical transport of particulate matter in lakes examines diurnal variations in plankton movement. The estimates of element fluxes to sediments are far more advanced for marine programs than for lake programs, but the method can contribute to an understanding of the settling of particulate matter in lakes.
The choice of chromium as a reference element in the excretion process, because chromium ostensibly has no role in the production of protoplasm, is questionable. Chromium has a role in vertebrate metabolism (see Chromium (1975) , a report of a committee of the United States National Academy of Sciences), but its role in invertebrate and plant metabolism is unknown. In the literature on assimilation efficiency, statistical arguments suggest that one can use chromium data to cross check the incorporation of carbon from nutrient materials from a chromium to carbon ratio.
A lack of comprehensive data sets
Hutchinson (1975) noted the wealth of data on the elemental composition of various species, but the completeness of the data was another issue. Most of the available radionuclide data refer to agricultural crops and products because of the requirement to monitor the food supply for direct contamination by radionuclides and concerns about the transfer of nuclides from the atmospheric fallout associated with atmospheric testing of nuclear weapons during the 1950s and 1960s.
Two kinds of data typify the radionuclide studies of foodstuffs: (1) radioactivity measurements of crops in farms near the site of a nuclide discharge to the environment, especially grass used in animal feeds, the milk content of local dairy cattle, and sometimes even the farm animals themselves; and (2) radioactivity measurements on crops that have been deliberately grown either on radioactively contaminated or amended soil or treated with radioactively contaminated or amended fertilizers. Periodically, selected government agencies screen the produce and meats at selected stores at locations either near areas receiving nuclide discharges or that receive their produce from areas near places of nuclide discharges.
A third kind of data comes from "geobotanical prospecting." Certain plant species are called "accumulators" because they selectively accumulate certain elements and enables a person to infer a geological resource by virtue of the presence of the plants. Geobotanical prospecting was used successfully to locate exploitable sources of uranium, lithium, gold, and silver.
Prior to Hutchinson's treatise (1975), there were few comprehensive data sets for inorganic elements in freshwater aquatic organisms, and none of them were for Great Lakes biota. The most comprehensive data came from studies by Cowgill (1970, 1973 a , 1973 b , 1974 a , 1974 b ), which Hutchinson cited extensively. Most of the data sets for the elemental composition of aquatic species cited in the literature of radiochemistry and radiobiology were for marine organisms. From a radiochemical perspective, the situation logically followed from the knowledge that the oceans were the main repositories for many of the nuclides, especially those from atmospheric fallout or discharged to rivers and estuaries and carried downstream to the oceans. The need for risk assessments for radionuclides to humans required data on food stuffs, and this caused an emphasis on those marine fishes and shellfishes harvested for human consumption. Thus, the early investigators of aquatic systems ( e.g., Vinogradov 1953; Bowen et al. 1971) focused on nuclide uptake by marine biota.
Concern for freshwater species has recently increased, but the motivation comes from the desire to understand the toxicology of the elements and their environmental effects and not from any interest in bioaccumulation processes. Bioaccumulation processes became a research priority with the discovery that several types of organic compounds became biomagnified as they cycled through ecosystems via foodwebs. These compounds were listed under the Great Lakes Water Quality Agreement of 1978 as "persistent toxic chemicals." A few inorganic elements belong to the category of persistent toxic chemicals: mercury, cadmium, and arsenic.
Pre-1970 data on bioaccumulation and biomagnification
Studies of marine biota have the produced most of the high-quality bioaccumulation data that are given in the tables and compilations of bioaccumulation and biomagnification factors found in the technical literature. Marine data are also among the oldest data, with many citations to two of the most widely accepted and used references: the volume by the United States National Academy of Sciences (1971), Radioactivity in the Marine Environment and the report by Chapman et al. (1977), Concentration Factors of Chemical Elements in Edible Aquatic Organisms (UCRL-50564). Table 2 summarizes the data of Chapman and coworkers.
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FRESHWATER BIOACCUMULATION FACTORS FOR VARIOUS ELEMENTS (based on Chapman et al . (1977) -- UCRL-50564) |
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| Notes: The factors are based on radionuclide uptake relative to the water. Bioaccumulation factors based on the consumption of plants and invertebrates, or on organisms found in benthic or sediment environments relative to the sediments are not given. Bioaccumulation factors are referred directly to water, not to the lower trophic level of an implied food chain. Therefore, to estimate biomagnification factors, one must take the ratio of the data in a given column to the ratio of the column which is at its left (the preceding column). |
The Task Force emphasizes the following important caveats and commentaries accompanying the tables in Radioactivity in the Marine Environment (1971): (1) bioaccumulation factors typically have an order of magnitude uncertainty; (2) the tables do not consider differences between nearshore and open-ocean environments, surface waters and bottom waters or gradients of chemicals in the water column, nor the environmental gradients of salinity in estuaries; and (3) data do not account for different elemental sensitivities between life stages of organisms. The bioaccumulation factors in these tables carry an inherent uncertainty of 1000% or more, which may propagate errors when these factors are used in mathematical models of ecological uptake, transport, distribution, and species interactions.
The use of marine bioaccumulation data to develop inventories for radionuclides in freshwater biota of the Great Lakes radionuclides has severe limitations. Oceans differ from lakes in their hydrodynamic regimes, chemical compositions (especially with regard to salinity), species array, and food web dynamics. The bioaccumulation and biomagnification patterns for marine food webs tend to show a strong uptake of chemicals at the primary producer level (algae), further uptake in first-level consumers (most zooplankton in the form of Euphausiads and calanoid copepods, and benthos in the form of molluscs and marine worms), and further uptake and accumulation in secondary consumers (fishes, birds, and sea mammals). Lakes often show a different pattern of uptake and accumulation at the first consumer level of zooplankton crustacea, protozoa, and insects.
Insects exhibit the most discrimination in their uptake and retention of chemicals from their food. Insects also dominate the invertebrate fauna of most lakes and rivers. Except for certain Lepidoptera and estuarine Corixidae, insects are very rare or absent from marine systems. Other benthic species express food preferences and discriminatory behaviors in bioaccumulation and retention of various elements and compounds. Hutchinson's (1995) cautions and comments on the vegetative preferences of freshwater gastropods are noted here.
The uptake, retention, and bioaccumulation of individual elements within tissues do not occur independently of one another. Elements move coherently with the uptake, retention, metabolism, and excretion of one element dependent on other elements. When an element's basic chemistry depends on or is affected by levels of sodium, calcium, chloride, carbonate, and sulfate ions, the biouptake processes will show major differences and gradients among freshwater, estuarine, brackish, and heavily saline or marine waters.
Calcium control paradigm
The Task Force examined data on the distribution of elements in both the environment and within biota for both radioactive and stable isotopes of many elements and noted their behavior in high- and low-calcium environments. These environments may be external (lake waters or lake sediments) or internal (cell contents or tissue distributions). Concentrations of calcium appear to correlate and interpret many of the observed situations and results. This applies to elements varying from barium and phosphorus to cerium and lanthanum. It appears that calcium controls the limnological behavior of many other elements, especially with regard to bioaccumulation and cycling through biotic communities and ecosystems. This led the Task Force to propose a calcium control paradigm .
Calcium within biota interacts with hormones, and because it usually exists in ionic form it can activate or interact with ionic channels. Other elements that can activate ionic channels are sodium, potassium, magnesium, and chlorine as the chloride ion.
The impoverished sodium content of fresh waters precludes this element from exerting the kind of widespread control needed because organism adaptations for low sodium have emphasized cell osmotic pressure. The single proton charge limits sodium's impact on cell systems relative to the calcium ion's divalent charge. Sodium also interacts with very few biomolecules. In marine systems, the environmental excess of sodium induced evolutionary changes to reject the element from many cellular systems to maintain cellular osmotic pressure.
Potassium occurs in relatively limited quantities in the natural lithosphere, although about 40% of the radioactivity of biota comes from the naturally occurring radionuclide, 40 K. Any consideration of potassium should be undertaken only after elimination of other control element possibilities.
The chloride ion is the major negative ion but has little effect on cellular pH. The other major negative ions (hydroxide, bicarbonate, and phosphate), through complex equilibrium relationships in cell fluids, mainly affect cell pH. Chloride serves mainly to provide the appropriate electroneutrality of biological systems. In certain marine teleost fishes, the chloride ion works with the bicarbonate ion to maintain osmotic pressure.
Magnesium plays an important biochemical role, but again, its limited geochemical occurrence makes it a lesser candidate for exerting widespread biological controls. Magnesium is a cofactor in metabolic reactions rather than through the operation of hormonal cascades. Magnesium also forms many covalent organo-magnesium compounds ( e.g. , the classical Grignard reagents) more than does calcium.
Calcium is the most widespread inorganic ion in the lithosphere. It, along with magnesium, sets the "hardness" of the freshwater environment through its presence as a carbonate and prescribes the "alkalinity" of the marine environment through its presence as a carbonate or sulfate. Its widespread presence in the marine environment provides the major cation to neutralize a divalent sulfate anion.
The early history of water-quality criteria noted that "hard" waters often offered organisms greater protection from the toxicity of heavy metals than soft waters. Calcium blocks the toxicity of nickel, copper, zinc, iron, and cadmium by increasing the pH of the medium to the point where these metals remain in precipitated form as sulfate, carbonate, or other salts. Calcium ion provides the major buffering capacity of lakes against acidic precipitation. Calcium also blocks the receptor sites that are easily attacked by these heavy metals in a manner comparable with the way potassium iodide tablets block sites in the thyroid against radioactive iodine -- through saturation by a competitive atom, a "mass action law" effect. Thus, calcium outside and calcium inside the cellular environment protect biota.
In the early history of limnology, waters were classified based on their hardness. As Balment and Henderson (1987) noted, the number of species often found in freshwater systems and their biomass correlate with the calcium levels of the environment, providing selected adverse factors are missing.
Also, the uptake of a radioactive form of an element often depends on the existing accumulation of a nonradioactive form of the same element (known as the "carrier" effect), a common example being radionuclides of iodine. The consumption of potassium iodide by humans to protect against uptake of radioactive iodine is a standard practice following a nuclear incident or upon exposure to radionuclides as radioiodine is a thyroid carcinogen.
Based on the previous discussion, the following elements would likely behave differently in their uptake by freshwater lake biota relative to marine biota: iodine, bromine, silver, zinc, cesium, rubidium, barium, strontium, manganese, cobalt, iron, molybdenum, sulfur, selenium, and lanthanum. Complicating the picture is the large-scale chemical pollution of the oceans and the Great Lakes. The documentation from nuclear power plants on the presumed behavior of nuclides following discharge into the environment has usually ignored the existing pollution problems and used estimates based on mathematical models with the bioaccumulation factors from tables derived from studies of the marine organisms.
Nuclides "in" and nuclides "on" organisms
An interesting problem in studying bioaccumulation of radionuclides is to distinguish between nuclides "on" the organism and nuclides "in" the organism. On means on the surface; in means inside the organism, either in tissue as a free nuclide or bound to some protein or receptor. Nuclides on an organism might be"loose" and easily washed off, or they may be bound to the surface because of the secretion of some adhesive type of biological material. The following are some specific examples.
The uptake of some elements is a two-step process: first a surface contact process that often can be described in terms of the area of contact or exposure, then a cell uptake process, which may require a diffusional mechanism, an active transport mechanism, or a chemical reaction. For some plants, the first process might mean entry through stomata or roots. For other species it may entail transport across a biological membrane or into a vacuole directly from the external medium. When nuclides on the organism are a surface contamination of the sample, the chemical analyses overestimate the uptake. If the nuclide levels on an organism quantify fallout of the nuclide, the information has separate value and importance. Nuclear power plants monitor the radiation levels at nearby farms, agricultural, and forestry tracts to assure that their nuclide emissions do not contaminate food supplies. The problem described has raised challenges to the quality of considerable data on nuclide bioaccumulation and biomonitoring in terrestrial systems.
Biomagnification of metals attached to organic molecules
According to DeFilippis and Pallaghy (1994), heavy metals do not typically follow the biomagnification patterns and processes of organic chemicals cycling through aquatic food webs. Thus for nuclides of heavy metals, algae would typically have greater concentrations of the nuclides than zooplankton predators and fish. This does not appear to be the case for metals attached to organic molecules, notably methylated compounds of mercury, tin, arsenic, lead, and cadmium. Therefore, radionuclides that can attach to hydrophobic molecules, and thus become lipid soluble, will likely biomagnify as they cycle through aquatic food webs.
Other biomagnification processes are not exclusively the result of predator-prey (consumer-food) interactions. Mercury in the water or sediment may equally have provided the sources of mercury observed in the tissue of organisms. Further complicating the situation is the possibility that a previously biologically unavailable source of a nuclide became available in the digestive system of an organism through methylation reactions in vivo.
The biochemistry of the processes by which metals attach to organic molecules, other than an ion-exchange process (simple binding replacement of H + ), usually involves the presence of one or more of the following: Fe, Mn, Co, pyridoxine, NADP, ATP, or a sulfhydryl group. Their presence suggests interactions among elements. Since the processes are often microbially mediated in the environment rather than in the tissues of plants and animals, the half-life of the radionuclides is the major determining radiological factor, and appropriate populations of microorganisms and available food supplies are the major determining ecological factors. These processes will occur if radiation does not have a sterilizing effect.
Use of nuisance species for bioaccumulation studies
If nuisance species dominate or represent major components of the biotic assemblage of a region, then it would seem only natural to study their elemental composition and cycling as part of the intensive understanding of their metabolism, physiology, ecology, and behavior. This approach has found favor in selected European studies and recently in selected studies in North America. Particular useful are the recent studies of the Asiatic clam, Corbicula flumatilis , in San Francisco Bay, connecting tributaries, and lakes. Corbicula has spread rapidly eastward through North America since its introduction into California some 20 years ago and now occurs in several tributaries that feed the Great Lakes.
The sea lamprey, Petromyzon marinus , which entered the Great Lakes through the Saint Lawrence Seaway, has devastated the lake trout, Salvelinus namaycush . The lamprey has been the focus of an intensive binational effort through the Great Lakes Fisheries Commission to develop selected chemicals for its destruction, yet the Task Force finds it somewhat surprising that its elemental composition has gone unreported.
At one time a well known statement was "Lake Erie was dying." The alga, Cladophora glomerata , formed huge mats of floating and decaying vegetation that washed up on the beaches of Ontario and New York. Cladophora has received considerable attention for its elemental bioaccumulation because it is a known concentrator or accumulator of heavy metals, but the physiological factors that might enhance biouptake and the use of Cladophora as a mechanism for detoxification of elements and radionuclides in the Great Lakes were never really effectively explored. Because of its rapid growth under nuisance conditions, Whitton (1984) has shown that the young tips can be used to differentiate between recently accumulated pollution and older or long-term pollution. The species has a relatively high thermal tolerance so that its peak growth periods are late spring through summer and early fall. Further, Cladophora seems to have a special obligate requirement for sodium. Levels of sodium have increased in the Great Lakes because of human activities, but appropriate correlations with Cladophora were not reported in the literature. Rather, the emphasis on nutrient control (mainly nitrogen and phosphorus), sought to remove the basic building blocks from pollution sources for excessive algal growth. That strategy has worked well, and it no longer appears that Cladophora glomerata has nuisance abundance in the Great Lakes. Nevertheless, the species does exist and coexist with the biotic assemblages and could still serve as a sentinel in tracking the cycling of radionuclides if such work were included in existing research and monitoring studies.
Another nuisance species introduced into the Great Lakes through is the zebra mussel, Dresseina polymorpha . This species has caused a major shift in the plankton. The mussel's radionuclide content will reflect not only direct uptake from the aquatic environment but nuclides from the filtered phytoplankton species which it consumes. The species has received some attention for its elemental bioaccumulation in European studies, but the Task Force is not aware of comparable studies for North America.
BIOUPTAKE DIRECTLY FROM ATMOSPHERIC DEPOSITION
Many organisms can draw chemical substances directly from the atmosphere. Since the work of the Task Force emphasizes the Great Lakes, the concern will focus on plants, fungi, and related species that can provide insight into cycling of elements through biocompartments of the Great Lakes ecosystems.
The direct utilization of atmospheric components usually depends on the state of matter of the chemical species. Chemical substances in the gaseous or vapor phase can penetrate the air breathing apparatus most easily. If those chemicals readily dissolve in water, the contact of the substances with water on an organism may further facilitate penetration of the organism. Aerosols and particulate matter have greatest difficulty, usually requiring some preliminary processes simply to make the substances available to the organism.
Two important groups of organisms that can bioaccumulate materials from the atmosphere are lichens and mosses. These organisms have played a very important role in studying the deposition of atmospheric substances, both radioactive and nonradioactive, especially those chemicals in particulate and aerosol form. Lichens are emphasized in the materials. It is simply that lichens have a better studied and understood ecological context with respect to the uptake of radionuclides and their transfer to other species.
For chemicals in particulate form, lichens generally accumulate substances by a two-stage process: a contact or binding stage, which is well described thermodynamically as an ion-exchange process, followed by a tissue absorption and retention phase. The second phase typically uses a carrier molecule.
From a kinetic perspective, the relative time scales of the two stages are important in quantifying rates of bioaccumulation. Yet, the experimental and statistical methods used to study bioaccumulation processes rarely address the differences in time scales for the tissue absorption and retention steps.
For many of ecosystems, lichens are the major food source for selected mammals, notably deer and caribou. Further, sometimes lichens are the only or major food source for these mammals. Mammalian consumption of lichens is direct route of transfer and exposure of radionuclides regardless of how radionuclides may occur in the lichens. The mammalian digestive system of these species does not differentiate between the binding modes in the lichens because the system breaks down the lichen cell matter and makes nuclide absorption readily available in the mammal's stomach and intestines.
The ion exchange nature of the binding is easily confirmed statistically. A simple model of ion exchange allows for equal and independent competition of ions for sites among possible binding nuclides. The particulate matter and aerosol forms of the metallic element radionuclides of the following elements are usually oxides: Na, K, Rb, Cs, Ag, Be, Ca, Ba, Sr, Ra, Ce, La, Fe, and Ru. Contact with water releases these metals as cations which can exchange for replaceable ( sensu : ionizable) hydrogens on the lichen. The univalent Na, K, Cs, and Ag cations would each replace a single ionizable hydrogen; the divalent Be, Ca, Ba, Sr, and Ra would replace two ionizable hydrogens; and the trivalent Ce and La would replace three ionizable hydrogens.
Since the geochemical ratio of 90 Sr/ 137 Cs is important in evaluating depositional data of radionuclides, the behavior of an appropriate geochemical ratio with respect to bioaccumulation in lichens is derivable from the ion-exchange model. The model would suggest a strong correlation of the geochemical ratio for the two nuclides in atmospheric particulate matter and that bound to the lichens. Small deviations of the geochemical ratio reflect the reality that the binding process is not 100% efficient and that any two-stage processes of absorption and tissue retention would tend to favor strontium over cesium in most species. The two-stage model basically assumes that the uptake process is divided into separate binding and uptake steps. The first stage is a Langmuir isotherm equilibrium process for an ion in the atmosphere attaching to a suitable site on the surface of a plant. Once this binding has occurred, the plant may secrete carrier molecules that can bind to the ion and transport into the cell, or openings in the cell (stomata) allow the movement by diffusion of the ion into the tissue, or some similar process. The rates at which the two stages occur differs, and this difference in rates determines the extent to which the two-stage model correlates the available kinetic data. If the first stage is very fast, the slower second stage will determine the uptake. Materials still in the first stage could theoretically be washed off the surface binding sites. If both stages have process rates within the same numerical order of magnitude, both surface leaching and tissue uptake will be observed. If the second stage is much faster than the first, only tissue-based biouptake will be observed. Similarly, radionuclides of Br, Cl, I, Al, Mn, Tc, and Ru, which exist as anions, would require a different mechanism of tissue uptake. If a lichen had anionic sites, a second model of ion exchange would should behave similarly for anions as the previously described one for cations. Again, such a model would also permit assessment of the statistical behavior of appropriate geochemical ratios.
Several elements occur environmentally in several different forms, making a model based on ion exchange processes inadequate. The two main examples are sulfur and nitrogen. Forms of sulfur require special treatment because they interact with metallic elements. The chemistry of nitrogen is also complex, but if the form of nitrogen present in the environment is inorganic, it can often be predicted from a knowledge of pH and oxygen levels through a consideration of the ammonia-nitrate-nitrite equilibria. Nitrogen, in ammonia, can bind ionically to lichens in the same manner as cations, but nitrite, and nitrate are anionic and would not bind to the ion exchange sites in lichens that are designed for cations.
Nitrogen is also the elemental basis for proteins. Radionuclides of nitrogen are not a problem with respect to the Great Lakes, although the element is intricately tied to the problems of acid precipitation and eutrophication. Nitrogen atoms are a source of cosmogenically derived radionuclides, but these nuclides have chemistries that can be treated on their own.
Several elements can exist in ionic and covalent forms. The covalent bond offers additional mechanisms of biouptake, including the solubilization in lipid materials and transport across certain types of hydrophobic membranes. Such elements include Hg, Cd, Sn, Te, Se, As, P, and, again, various forms of nitrogen.
The Task Force did not access data on lichens in the Great Lakes region. Most of the available lichen data refer to restricted and extreme climate and geographical zones: Arctic, high altitude, tropical regions in the vicinities of volcanic fumaroles, and selected forest regimes. Since lichens often dominate the flora in the area in which they reside, one need not consider many of the implications of species diversity on the reported values of elemental composition and bioaccumulation factors.
Table 3 describes a tropical lichen in the vicinity of a volcanic fumarole in Hawaii. The purpose of O'Connor's study (1979) was to track mercury from the volcanic emissions and its long range transport effects on nearby and distant vegetation. The usefulness of the data is in its coverage of a large number of elements. Radionuclide deposition data for the region for 90 Sr and 137 Cs are available, and the data array can be used to compare lichen behavior for bioaccumulation in a more generic manner.
Table 4 presents data for lichens and mosses which have been studied for their content of several cosmogenically produced radionuclides and two groups of radioactive decay series nuclides from transuranics. These data from Jenkins et al. (1972) are from one of the classic studies on natural background levels of radiation.
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CHEMICAL COMPOSITION OF A TROPICAL GROUND LICHEN ( Cladonia skottsbergii ) (from O'Connor 1979) |
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| Notes : Results are average of three samples except those marked with an asterisk (*), which are based on either one or two samples. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Notes : Enrichment factors are calculated relative to crustal material. Thus, values greater than 1 for enrichment factors means that organisms retain the elements to a greater extent than is found in natural soils or crustal materials. This suggests that bioaccumulation is important for barium, calcium, strontium, magnesium, iron, manganese, vanadium, cobalt, and nickel and of limited potential for titanium, zirconium, and zinc. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Notes : Enrichment factors are calculated relative to crustal material. Thus, values greater than 1 for enrichment factors means that organisms retain the elements to a greater extent than is found in natural soils or crustal materials. This suggests that bioaccumulation is important for molybdenum, selenium, and sulfur and of limited potential for scandium, yttrium, phosphorus, and lead. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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NATURALLY OCCURRING RADIONUCLIDES IN LICHENS AND MOSSES FROM A FOREST IN WASHINGTON STATE, 1966-1967 (from Jenkins et al . 1972) |
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Notes:
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Radionuclide data from Arctic ecosystems
The deposition of radionuclides of cesium from the atmosphere in the northern hemisphere favors the northernmost latitudes. Some of the ecological systems in these latitudes are rather limited because of the climate conditions. An important system in these regions is the lichen - reindeer/caribou - human food chain. Lichens, a plant-fungus symbiont, can readily accumulate metallic elements because some of the plant material behaves like an ion-exchange resin. Metallic elements in particulate matter and aerosol materials of atmospheric fallout will collect on the lichens and can displace bound hydrogen atoms. Cesium ions are positive and univalent and readily displace hydrogen in ion-exchange systems. Thus, the chemical behavior of radionuclides of cesium, as in 137 Cs, assures that, if it contacts lichens, it will be incorporated into the tissue through this ion-exchange mechanism. Reindeer and caribou eat the lichens, and these mammals are in turn the food of resident populations. This food chain pathway to man of the radionuclides of cesium has been studied in various Arctic regions, including northern Canada, Alaska, Denmark and the Faroë Islands, Sweden, Norway, Finland, and Commonwealth of Independent States (formerly Russia). Table 5 lists the concentrations of cesium and strontium in Arctic biota.
The studies of Arctic species and radioactive cesium loadings go back to 1969. UNSCEAR (1977) reported on the work through the early and middle 1970s. Following the accident at the nuclear power plant at Chernobyl (Ukraine) in 1986, researchers and Government agencies undertook new studies of atmospheric deposition in the Arctic. Many of the results of these studies appeared in 1995 in a special symposium on pollution in the Arctic (Volume 160/161, Science of the Total Environment ). The research findings updated important information on 137 Cs levels in Arctic species, including some species not previously considered. (Arctic Monitoring and Assessment Program, 1997.)
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RADIONUCLIDES IN ARCTIC BIOTA (LICHENS AND MOSSES) |
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ELEMENTAL ANALYSES (INCLUDING RADIONUCLIDES) OF ARCTIC BIOTA |
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| Notes : "R" is the geochemical ratio based on the activity measurements indicated in the columns to the left. The l3 lichens are those for which there are both Sr and Cs data. The 14 lichens are for the entire collection of lichens. The geochemical ratios are based on 13 lichens and 15 mosses. The combined geochemical ratio for lichens and mosses is based on 28 plant species. "EF" is the enrichment factor, which is the ratio of geochemical ratios of the two indicated nuclides relative to their respective media or substrates (thus, EF for lichens/fallout is the ratio of the geochemical ratios for each nuclide in lichens to the geochemical ratio of the two nuclides in fallout). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
BIOACCUMULATION IN SOIL MICROFLORA
There are several kinds of organisms important in the cycling of elements through ecosystems that can accumulate elements from air and soils. The soil layer microflora, especially the fungi, are very important, but this group of organisms is often overlooked in understanding the movement of elements through ecosystems. Because some fungi are edible ( e.g. , selected species of mushrooms), the interest in accumulated elements has applicability to risk assessment to ecosystems and humans. Further, the fungi provide a comparison group to the lichens and mosses, which have been previously discussed. However, data sets on bioaccumulation of elements were available mainly for European species not North American species.
BIOLOGICAL PRODUCTIVITY
Because of the importance of biological productivity measurements in an discussion of bioaccumulation and biomagnification of various nuclides, this section begins with a short discussion of the subject and a review of the available data on biological productivity for various locations within the Great Lakes Basin. A fundamental research problem in biology is how biological systems sustain themselves. Scientists want to quantify those sustaining properties, the most of important of which is the ability to produce new protoplasm to replace the aging, injured, diseased, dead, or lost (removed) protoplasm while assuring that the new protoplasm maintains the speciation and distribution to sustain the different kinds (diversity) of species that form the biological system. The production of protoplasm in biological assemblages and ecosystems is biological productivity . This inventory of radionuclide is the product of two terms: the radionuclide activity in a biological compartment and the mass of that compartment. The term usually known to the Task Force is the radionuclide activity for some medium, substrate, or even compartment. Because the second term in the product is usually absent, the available data can only show the relative distribution of radionuclides within compartments but no overall absolute estimate of their presence.
The "metabolism of lakes"
Limnologists desired a unifying concept to analyze the nutrient-induced excess or luxuriant biomass productivity of lakes (eutrophication). Wilhelm Rhode convened a seminar on the subject of "lake metabolism" ( Ergebniesse der Limnologie , 1979) in which he compared a lake to a living organism with a "metabolism" that can be studied much as the physicians study the metabolism of human patients.
Traditionally, the measurement of organismal metabolism tracks the oxygen supplied to the organism and used and the carbon dioxide returned. During periods of photosynthesis, plants partially reverse the process and consume carbon dioxide and return oxygen. However, photosynthesis rarely fixes all of the carbon dioxide produced through respiration and metabolism, and thus one typically observes diminished rates of carbon dioxide production during periods of photosynthetic activity relative to periods of nonphotosynthetic activity, rather than reversal of the direction of the metabolic measurements.
Since only plants and some photosynthetic protozoans can carry out photosynthesis in freshwater systems, productivity in consumer organisms moves in the same direction as metabolism; oxygen uptake and carbon dioxide release. Further, if there is organismal death or tissue destruction, the bacterial respiration release nutrients back to the pool, producing large amounts of carbon dioxide and creating oxygen deficits.
Whole-lake productivity sums the productivity of each biological compartment. Whole-lake metabolism sums the metabolism of each biological compartment. Depending on how one views the system, a biological compartment may consist of the population of a species or the population of a biological assemblage of species or the biomasses of the species that perform some collective ecosystem task of nutrient and energy cycling. Limnologists can usually measure some components of productivity and some components of metabolism, but not all of the components of each, and not always both components for a single biotic compartment. Therefore, limnologists seek predictive relationships between various kinds of productivity and metabolism data to obtain the whole-lake productivity and whole-lake metabolism as well as the productivity and metabolism of critical biotic parts of the lake ecosystem. Because of the complexity of many of these relationships, the prediction of the productivity components, especially in terms of speciation, is one of the great research problems guiding limnological investigation, and the motivation for the development of sensitive and widely applicable methods to measure the more difficult secondary productivity.
Because eutrophication complicates the relationships between metabolism and productivity, the Task Force cautions against extrapolating metabolic and productivity data from low nutrient systems to eutrophic systems, or even from one eutrophic system to another.
Direct measurements of the mass of an element in a biological compartment tend to be more difficult than direct measurements of the activity of a radionuclide, but the quantification of a radionuclide for purposes of establishing an inventory can be overwhelming. The difficulties begin with sampling of biological substrates, quality assurance on analytical methods and instrumentation, overcoming limitations at the "lower level of detection," the number of analyses required for a reasonable application of statistics, and the design of the protocols for the studies to obtain the nuclide data for an inventory.
Many of the problems mentioned in the previous paragraph require an iterative approach for their solution. Careful analyses and questioning enable refinement of sampling and analytical procedures. A consideration that strongly impacts the process of obtaining suitable data and calculating the inventories for nuclides and that presents a special barrier is that one must really know in advance how various chemical elements in the biological substrates in a given compartment relate or interact with each other as well as how different biological compartments relate and interact with each other. Such information is rarely known in advance of a study, but itself is gleaned from the work as a study progresses. That means one needs to begin with some rather educated guesses to minimize unproductive field and laboratory work.
Radioactivity adds to the problems encountered in an overall understanding of the processes of element uptake and cycling and associated observed phenomena in the following ways.
The "trophic cascade" concept
Biological productivity in the Great Lakes is a complicated set of processes and include the strategies of predation of the indigenous biota, the invasion of species from foreign habitats, shifts in habitat structure within the Great Lakes and the presence of many different kinds of pollutants, both radioactive and chemical. As used by Carpenter and Kitchell (1993), this model is one of a "trophic cascade" that attempts to "bridge ecosystem and population ecology." The former discipline tends to emphasize large scale processes of nutrient and energy cycling through complex compartments while the latter discipline examines more microscale processes within species or biotic assemblages without necessarily reflecting ecosystem organization. One begins with simple food-web relationships of primary productivity and detrital production, adds the consumption of these resources by invertebrates in plankton and benthos and the higher trophic level consumption of these resources by fishes, reptiles, birds, and mammals, as well as the feedback pathways which regenerate previously considered compartments.
A "cascade" implies some kind of "domino effect" or "amplification phenomenon" in a system. Each isolatable or definable food chain or food web subunit could manifest a possible cascade. By examining the relative rates of productivity of different elements of the food web as influenced by the population-level strategies of food choices, prey selection and acquisition, selective toxicity, and differential mortality of abiotic and external factors ( e.g. , harvesting, cropping, chemical toxicity, stochastic effects of climate change), one seeks a description of how the entire food web functions. If the effects of changes in one element of a food web propagate through the food web to produce a rapid and, sometimes, highly unpredictable realignment of the food web structure, such an effect has "cascaded."
The occurrence of a cascade requires that the food web have, as a minimum, either a point of unusual vulnerability or the interactions of various components of the food web during energy and chemical cycling have some basic nonlinear ( sensu: mathematical rather than biological) property to permit an amplification of some effect through an entire system. A point of vulnerability usually has the characteristic of a positive feedback element above some threshold system value. For example, some pathway becomes saturated and loses its stabilizing contribution to the food web, or the pathway becomes damaged and no longer contributes or is removed from the food web.
Most real ecosystems have both points of vulnerability and nonlinear properties, even if the particulars are unknown. However, an additional problem is that many of the mathematical and statistical techniques designed to elucidate cascade behavior have methodological limitations as to the number and type of such vulnerabilities or nonlinear properties they can handle simultaneously. These limitations give rise to production of mathematical artifacts or the predictions or behavior that are not confirmed by observations or measurements on the system being studied.
Because the cascade concept integrates biological and chemical behavior of species communities, it permits simultaneous consideration of the distribution of radionuclides in ecosystems with the effects on ecosystems as modified, if necessary, by the presence and quantities of other biotic and abiotic factors. For the Great Lakes, the cascade concept has been mainly applied to the dynamics of fisheries as influenced by changes in phytoplankton, zooplankton, and selective macroinvertebrate benthos compartments.