“Silent Spring,” writer and ecologist Rachel Carson’s most famous work, focuses on a discussion of the detrimental effects of chemical pesticides on the environment, and on humanity. The book was published in 1962, but the issue of contaminants reverberating throughout the ecosystem is still pertinent today; though Carson’s writing inspired reforms, the problems she exposed are far from solved.
The authors of a study recently published in the journal Ecological Applications consider Carson’s seminal work relevant enough to cite it in the opening of their paper, an investigation of the pathways that mercury can take as it moves through a food web spanning an ecosystem that includes both aquatic and terrestrial organisms. (‘Boundaries’ between aquatic and terrestrial areas are often permeable to a reciprocal flow of food and energy resources.)
Mercury, a toxic heavy metal, is produced by both natural and anthropogenic sources; in aquatic environments, it’s easily converted to a bioavailable form that humans and other organisms can absorb, methyl mercury. Methyl mercury can reach high levels in predators like large fish as it biomagnifies, or bioaccumulates, instead of dissolving or metabolizing – as organisms consume contaminated prey, methyl mercury builds up in their flesh.
The scientists measured the methyl mercury concentrations of several species of terrestrial spiders, insects, and mites (all types of arthropods) around two Icelandic lakes; because of bioaccumulation, they expected the organisms that fed predominantly on aquatic food sources, which contain elevated methyl mercury levels, to display higher contamination levels than organisms that consumed terrestrial foods.
What they found instead was the opposite – arthropods that had an aquatic-based diet had much lower concentrations of methyl mercury in their bodies than those that ate terrestrial foods. The arthropods collected within a few feet of the lake with a large population of midges – an aquatic insect that was a likely food source for the spiders, insects, and mites – contained, on average, less than half the amount of methyl mercury found in arthropods collected far from the lake (and therefore, presumably, far from an aquatic-based food source like the midges).
The researchers also found that arthropods with an aquatic diet were at the top of a shorter food web than those with a terrestrial diet; with fewer steps in the food web, the scientists hypothesize, there was less bioaccumulation of methyl mercury, creating what they termed a ‘trophic bypass.’ As they write in their paper, “direct consumption of aquatic inputs result[ed] in a trophic bypass that create[d] a shorter terrestrial food web and reduced biomagnification of [methyl mercury] across the food web.”
The findings of this study were unexpected, and contrary to other research that revealed higher levels of terrestrial contamination in areas adjacent to polluted streams; as the authors of the study write, “our research highlights the fact that we still know little about the potential implications of [aquatic-terrestrial] linkages for terrestrial food webs and ecosystems, particularly with regard to societally important applied issues such as contaminant bioaccumulation.”
Contaminants and pollutants in the environment are a perennial problem; a problem that, in some ways, is just as obscure as it was more than half a century ago, when Rachel Carson first brought it to the attention of the public with “Silent Spring.”
Rachel Carson and colleague Bob Hines collecting marine samples in Florida in 1952.
(Image by U.S. Fish & Wildlife Service via USFWS National Digital Library)