Flood Redistributes Mercury in Grand Canyon Aquatic Food Webs

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Scientists coupled the concepts of energy flow through food webs with measurements of mercury in organic matter and animals to estimate mercury fluxes and fate during an experimental flood in the Colorado River. The flood redistributed mercury in simple, upstream food webs but not in more complex, downstream food webs. 

USGS scientists collect and analyze algae, invertebrates and fish on the Colorado River.

USGS scientists Kyle Hanus, Cheyenne Szydlo and Ted Kennedy collect and analyze algae, invertebrates and fish on the Colorado River. This is part of a study to learn more about how food web dynamics influence the movement of mercury throughout the Colorado River in the Grand Canyon.

(Credit: David Herasimtschuk, Freshwaters Illustrated. Copyright: David Herasimtschuk/Freshwaters Illustrated and U.S. Geological Survey)

Mercury in its organic form, methylmercury, is a neurotoxin that can biomagnify in food chains. Mercury is present in food webs throughout the world, even in seemingly remote locations such as Grand Canyon National Park, where aquatic animals in the Colorado River accumulate mercury at levels posing risks to fish and wildlife.

Floods are a regular feature of many ecosystems including the Colorado River, and contaminant fluxes during flooding events are poorly understood. It also is unclear how floods and their associated changes in community dynamics might alter pathways of mercury transfer among aquatic species.

Therefore, U.S. Geological Survey scientists and university partners conducted a study that provided an understanding of factors that drive mercury transfers in food webs and govern exposure before and after an experimental flood on the Colorado River. They were able to take advantage of periodic high-flow releases from the Glen Canyon Dam that simulate floods and supply sand for sand bar rebuilding in the Colorado River.

Invertebrates, organic matter, and fish were collected at six sites spanning 370 kilometers of the Colorado River in the year before and after the experimental flood. Scientists collected 18–20 benthic invertebrate samples per season and site and sampled the predominant habitats using multiple approaches. Organic matter collected included fine benthic organic matter, seston (suspended organic matter), epilithon (benthic biofilm), attached algae (Cladophora sp.), and epiphyton (diatoms attached to Cladophora). Scientists collected fish with various approaches, including angling, electrofishing, and seining. 

Only a few pathways dominated mercury flux in the Colorado River despite large spatial differences in food web complexity, and these fluxes were largely mediated by one critical invertebrate trait—resistance to predation.

New Zealand mudsnails

New Zealand mudsnails were analyzed as part of a study to evaluate how food web dynamics influence the movement of mercury throughout the Colorado River in the Grand Canyon.

(Credit: David Herasimtschuk, Freshwaters Illustrated. Copyright: David Herasimtschuk/Freshwaters Illustrated and U.S. Geological Survey)

For example, invasive New Zealand mudsnails (Potamopyrgus antipodarumare) accumulated available mercury because they are resistant to predation and created a trophic dead end for a substantial proportion of mercury moving through the food web; mudsnails in the habitat immediately below the dam (tailwater) decreased after flooding. Conversely, blackflies (Simulium spp.) increased after flooding and are preferred prey; consumption of blackflies accounted for 56–80 percent of the total mercury flux to fishes, even at sites where they constituted only a fraction of total invertebrate production.

The scientists concluded that the experimental flood redistributed mercury fluxes in the relatively simple, tailwater food web but not in more complex, downstream food webs. Recognizing that species traits, ecological interactions, and disturbance regimes mediate contaminant exposure can improve management of linked aquatic-terrestrial ecosystems.

This research provided a unique opportunity to explore how differences in food webs affect contaminant transfer, which ultimately governs exposure to fish, wildlife, and humans. This study was funded in part by the U.S. Geological Survey Environmental Health Program (Toxic Substances Hydrology and Contaminant Biology), which is part of the Ecosystems Mission Area.