{"id":121454,"date":"2019-01-17T15:43:44","date_gmt":"2019-01-17T20:43:44","guid":{"rendered":"https:\/\/www.bates.edu\/news\/?p=121454"},"modified":"2024-07-01T15:56:48","modified_gmt":"2024-07-01T19:56:48","slug":"amidst-sharks-and-coral-reefs-kelton-mcmahon-05-unravels-a-paradox-as-old-as-darwin","status":"publish","type":"post","link":"https:\/\/www.bates.edu\/news\/2019\/01\/17\/amidst-sharks-and-coral-reefs-kelton-mcmahon-05-unravels-a-paradox-as-old-as-darwin\/","title":{"rendered":"Amidst sharks and coral reefs, Kelton McMahon ’05 unravels a paradox as old as Darwin"},"content":{"rendered":"

For a moment, the camera shows an inflatable boat, bobbing in the water near a remote Pacific coral reef where Kelton McMahon \u201905 does fieldwork.<\/p>\n

Then the camera drops into the clear blue water, revealing the subject of his research: hundreds of grey reef sharks and blacktip reef sharks, each five to six feet long, swimming together in massive schools around the reef.<\/p>\n

“How can coral reefs be so productive and biodiverse with a backdrop of such low nutrients?\u201d<\/p><\/blockquote>\n

McMahon, an assistant professor of biological oceanography at the University of Rhode Island, showed the footage during his Jan. 14 talk at Bates because it was awesome, yes, but also because it represents a paradox first noticed by Charles Darwin aboard the HMS Beagle<\/em> in the 1830s.<\/p>\n

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Kelton McMahon gestures while speaking to a junior seminar taught by Larissa Williams, associate professor of biology, on Jan. 14. A Purposeful Work infusion course, the seminar features speakers who help students make connections between coursework and the types of work they’d like to pursue after graduation. (Phyllis Graber Jensen\/Bates College)<\/p><\/div>\n

The paradox: Since Pacific coral reefs are in the nutrient-poor open ocean, there shouldn\u2019t be enough food for two species of apex predator to coexist in such big numbers.<\/p>\n

In other words, \u201cHow can coral reefs be so productive and biodiverse with a backdrop of such low nutrients?\u201d McMahon said.<\/p>\n

McMahon was the first of several scientists Associate Professor of Biology Larissa Williams invited to campus this semester, to speak with the students in her junior seminar and then give a public talk. Williams\u2019 class is a Purposeful Work infusion course, where students learn about ways they can take course material into the real world.<\/p>\n

McMahon is an ocean ecogeochemist, a title that suggests the depth of his interdisciplinary work. Over the course of his career, he and his colleagues have been developing and applying a suite of geochemistry tools, collectively called compound-specific stable isotope analysis (CSIA), that help scientists understand food webs better than ever before.<\/p>\n

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Kelton McMahon \u201905 takes a tissue sample from a grey reef shark off the Phoenix Islands, home to pristine coral reefs. (Mark Priest)<\/p><\/div>\n

McMahon told his Bates audience about two of his current projects, which apply CSIA to real-world problems.<\/p>\n

But first, a few terms:<\/p>\n

Trophic level<\/strong>: A living thing\u2019s place in the food chain. For example, plants are at trophic level 1, herbivores are at trophic level 2, carnivores that eat herbivores are at trophic level 3, and so on.<\/p>\n

Producers\/consumers<\/strong>: Producers are organisms that make their own food, as plants do through photosynthesis. Consumers get their food by eating other organisms.<\/p>\n

Amino acids<\/strong>: The compounds that make up proteins \u2014 essential for life.<\/p>\n

Isotope<\/strong>: Atoms of a given element with the same number of protons and electrons, but different numbers of neutrons. An atom of carbon-12 has six protons and six neutrons (light carbon), while carbon-13 has six protons and seven neutrons (heavy carbon).<\/p>\n

Isotope analysis<\/strong>: Measuring the ratio of heavy to light isotopes in a sample, which provides a chemical \u201cfingerprint\u201d of physical, chemical, and biological processes that have impacted those atoms. For example, by measuring the isotope ratios of a piece of shark muscle, we can understand how energy moves through a food web, from coral at the bottom to shark at the top.<\/p>\n

CSAI gets specific<\/h5>\n

Scientists have used conventional \u201cbulk\u201d isotope analysis for decades, but it has its limitations, McMahon said. The bulk method gives you the average isotope value of all the atoms in an element in an organism. It doesn\u2019t distinguish whether an isotope of carbon, for example, came from one or another compound in the body of a shark.<\/p>\n

That means a lot of detail gets lost.<\/p>\n

\u201cAll the different compounds in the body tell a different piece of the story about how organic matter is made by primary producers and passed on to upper trophic level consumers through the food web,\u201d McMahon said.<\/p>\n

As a result, if you sample two animals and get back different isotope ratios, it\u2019s difficult to know why. It could be because the animals have a different trophic position \u2014 they feed at different levels of the food chain.<\/p>\n

Or it could be because different producers are at the bottom of each animal\u2019s food web. Or it could be a combination of both.<\/p>\n

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Sharks! Watch this video clip of sharks swarming in the waters near the Phoenix Islands in the South Pacific Ocean. Video by Camrin Braun. <\/i><\/span><\/p>\n