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Bioaccumulation and Trophic Transfer of Hg in Aquatic Food Webs
How do metals build up in fish?
Health advisories warning people to limit the amount of freshwater fish they eat reflect the vulnerability of humans as participants in an ecosystem contaminated by harmful concentrations of metals. Both wildlife and humans are susceptible to the toxic effects of metals such as mercury, and fish are a primary source of exposure. But knowing when fish from a water body contain dangerous concentrations of metals is not a simple task. Metal concentrations in fish are the result of complex interactions between the environment and living organisms. Differences in water chemistry and in the numbers and types of organisms present in a lake can produce significant lake-to-lake variation in the amount of metal that moves from the water through the food chain to fish. Hence, fish in different lakes frequently carry different metal burdens even when lakes are located within the same geographic region. Eliminating the risk is not as simple as avoiding fish from "polluted" lakes, as high levels of mercury and other toxins are routinely measured in fish from lakes that appear “pristine” by other criteria. Dartmouth ecologists Carol Folt, Celia Chen and colleagues are developing ways to track and predict the movement of potentially toxic metals through aquatic ecosystems to lake fish. Their previous studies show that a large part of the variation in metal accumulation in fish may be attributable to differences among organisms in the lower levels of the food web, particularly the tiny algae and other foods that fish eat. These foods, which also include smaller fish, zooplankton and insects, can accumulate different quantities of metals due to metabolic differences and environmental conditions in which they live. The studies show that some species appear to be better able to concentrate metal in their bodies than many other zooplankton species. The researchers have identified one such species, a large-bodied cladoceran species of zooplankton called Daphnia, or water fleas. Daphnia are favorite foods for many lake fish. In field studies, the investigators found that fish from lakes where large-bodied species of zooplankton predominate tend to have more metal in their tissue than fish from lakes where small-bodied zooplankton predominate. Following up on these observations, the researchers are developing Daphnia as a model sentinel species to identify molecular biomarkers, or patterns of gene expression, that indicate when an organism is being exposed to potentially harmful levels of a toxin. The scientists also are conducting a series of field studies to track five potentially toxic metals — arsenic, mercury, zinc, cadmium and lead — as they move through various types of aquatic food webs into fish. By studying these metals across a range of lakes types and habitat types, the scientists seek to better understand how environmental and biological factors interact with each other to influence the bioaccumulation of metals in individual species and their fate in the overall ecosystem. From these studies, the scientists hope to identify other sentinel species that can be used to predict the exposure of aquatic communities to toxic concentrations of metals. The goal of their research is to develop methods for predicting the lakes and fish species that are likely to carry the greatest amount of metals as well as methods for detecting detrimental exposure to metals. This information could eventually lead to site-specific consumption advisories that are more useful to the public, municipal planners and regulators.
The concept of “biomagnification in a food chain" is often used to explain the way contaminants such as mercury or other contaminants build up in living things. The process begins when metals from natural sources such as rocks and from man-made sources such as industrial processes or agricultural practices are transported by wind or rain and deposited directly on watersheds. Elemental mercury in the air, for example, may fall to the ground with precipitation and enter the water where it is transformed into methylmercury. Tiny animals and plants known as plankton then take up the methylmercury in the water. Zooplankton, the small animals that fish feed on, eat large quantities of algae over time. In turn, these small fish are eaten by larger predatory fish, accumulating methylmercury in their tissues. In this "chain" of linked processes plants and animals are connected by who eats what. But the image of a vertical "chain" oversimplifies the complex structure of an ecosystem. Ecologists use the metaphor of a "food web" to suggest a horizontal dimension of the food chain. Food webs can be relatively simple or quite complex. Food web complexity refers to the connections between the numbers and types of organisms in the community; complex webs have more species at each level. Aquatic food webs are also described by other attributes such as species densities or biomass.
In previous studies, the Folt group discovered an important link between the amount of algae in the water and the amount of mercury going up the food chain. The researchers tracked mercury as it moved from the water and was taken up by algae. Eventually, the mercury found its way into zooplankton called Daphnia pulex, which eat the algae. Daphnia, in turn, serve as a food source for many species of fish. The study found that, in lakes with high algal concentrations, mercury is dispersed widely through the single-celled algae, in essence, diluting the mercury. Consequently, Daphnia that eat the algae in these lakes are not exposed to high levels of mercury. In systems with less algae, the mercury was found to be more highly concentrated on each plant, thus packing more mercury per meal for the Daphnia. These findings showed how biological systems can work either to suppress or to promote the transfer of metal from water to fish, and helped explain why levels of mercury in the water don’t always indicate corresponding levels in fish. The investigators are also conducting field studies to see how various other metals, such as arsenic, zinc, cadmium and lead, interact as they move through food webs, from water to fish. Many of their current studies focus on arsenic, a metal that occurs from both natural and anthropogenic sources in high levels within the Northeast region of the United States. The researchers are also investigating how seasonal changes affect the dissipation of metals through the food web. In a previous study, the investigators found strong seasonal patterns in the arsenic and lead content of plants and other species living in Upper Mystic Lake in Massachusetts. The results suggest that seasonal changes in metal bioaccumulation may be more pronounced in lake systems where there is an abundance of plant life. The group is now comparing seasonal variation across different lake types to better understand the nature and importance of seasonality to metal fate.
Determining the ecological fate and impact of metals in aquatic systems involves tracking the movement of metal through food webs to fish, and identifying the toxic effects on individuals and populations. But all lake systems must cope with exposure to multiple stressors. Most past studies on the transport of metals through ecosystems have focused on a single metal within a single species. Despite the common occurrence of metal contaminants, the biological effects of metal mixtures are poorly understood and difficult to predict. Even in laboratory studies using algae, zooplankton, and fish, the interactive effects of metals often deviate from predictions. Some studies suggest that a combination of various metals can interact with one another to enhance the metal effect. Folt, Chen and their colleagues are studying the factors that affect the way potentially toxic metals interact with each other, and with other environmental stressors, as they move through food webs into fish. Their aim is to determine the toxic effects of multiple metals and environmental stressors on key prey species. Prior research shows that interactions among multiple stressors can produce different effects from those predicted for single stressors, and that dose, exposure duration and stressor combinations greatly influence toxicity. By analyzing the interactive effects of multiple stressors — including combinations of metals and variations in water chemistry — on individual species, the researchers aim to develop models that could be used to better predict exposures and indicate when aquatic organisms in a lake are being exposed to metals at levels that may, over time, become hazardous. The researchers are also working to determine the toxic effects of multiple stressors on key indicator species. Their current studies focus on identifying patterns of gene expression in Daphnia that have been exposed to various combinations of arsenic, cadmium, and zinc. To develop a faster and broader screening technique for metal response, the scientists are creating cDNA microarrays capable of monitoring gene expression of thousands of genes simultaneously. Their premise is that genomic responses will differ between different metals, exposure times, and doses. The scientists are also analyzing
the effects of various exposures on the population’s
reproduction and survival rates. By tying the genetic responses
to these population level effects,
the scientists hope to provide a series of biomarkers that can
be used in the field for rapid, sensitive and metal-specific
screening in natural populations of Daphnia. The biomarkers could
then serve as an early warning system for metal exposure in field
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