You are what you eat…and also where you fish

Article: Why productive lakes are larger mercury sedimentary sinks than oligotrophic brown water lakes

Authors: Martin Schütze, Philipp Gatz, Benjamin‐Silas Gilfedder, Harald Biester

Advisories and outreach campaigns have worked for years to help us understand how the fish we eat impacts the amount of hazardous mercury we consume. Mercury is present in the environment naturally in several forms, but consumption advisories warn against methyl-mercury. This substance not only moves throughout the aquatic ecosystem, but bio-accumulates, or increases in concentration, as it moves higher in the food chain.  But the size of the fish is not the only influence on its mercury levels – it may also matter where it lives. 

Most mercury in lakes is initially deposited from the atmosphere. These levels vary regionally, influenced by things like weather patterns and local industry. Mercury is also deposited on land, though, and it can eventually leach and erode from soils, moving through surface and groundwater into local lakes. Researchers have known for some time that the vegetation and soil types in the watershed can influence mercury influx to lakes; for example, coniferous trees generally take up more mercury from the atmosphere than deciduous trees, making the forest litter and, eventually, the organic rich layers of forest soils more concentrated in mercury in coniferous forests. 

A recent German study compared mercury levels in two sets of lakes, looking at everything from surrounding vegetation and topography to local weather patterns, and found that previously observed findings held up; mercury levels were higher in leaf litter and organic soils than other surrounding sediments, and higher in areas with more coniferous vegetation. However, when the authors undertook mathematical modelling to balance the input of mercury from atmospheric deposition and local erosion to the outflow, the numbers didn’t add up the same way in all the lakes.

The difference, they documented, was in the productivity of the lakes. Algae scavenge mercury from the water column and, when they die and sink, take the mercury along. This leads to mercury deposition in the lake sediments. By comparing measurements of mercury in the water column, in accumulated sediments on the lake floors, and in sediment ‘traps’ that collect sediment as it is falling through the water column, the researchers showed that large algal blooms significantly increase the transport of mercury from the water column into lake sediments. In a set of forested, alpine lakes that were low in nutrients and had few algal blooms, the monitoring data showed that most of the mercury inputs were eventually lost to a combination of river outflow, re-emission to the atmosphere, and sediment burial. In lakes with higher nutrient loads and more common algal blooms, a similar input of mercury was translated into a much higher flux to the lake sediments, which they traced to the concentration of mercury in the algal organic matter.

The high rate of mercury delivery to lake sediments, especially in very productive lakes, may be bad news for fishing. The high rate of organic matter input to the sediment also leads to a low-oxygen environment which can spur the bacterially-mediated chemical process that turns mercury into the methyl-mercury form. When it is released and recycled from the sediments, it works its way up the food chain. Lakes in many parts of the world are seeing increased algal growth from warmer temperatures and higher nutrient input, and the resultant highly-visible algal blooms may have a significant impact on the invisible movement of hazardous mercury to consumers at the top.


You are what you eat…and also where you fish by Avery Shinneman is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Tiny organisms’ race to the bottom of the ocean

Paper: Microbial dynamics of elevated carbon flux in the open ocean’s abyss

Authors: Kirsten Poff, Andy Leu, John Eppley, David Karl and Edward DeLong

Cells from blooms of phytoplankton, or tiny plants, can enhance carbon flux all the way down to the deepest parts of the ocean. The authors of this recent study measured the amount of carbon in sinking particles deep in the ocean at a station near Hawaii. Over the course of three years, scientists identified three time periods of unusually high carbon flux, or transport, at this depth, and found that the organisms that made up the sinking particles were significantly different between high flux events and the rest of the time period. The data showed higher abundances of surface-dwelling microorganisms, including phytoplankton, contributing to these particles during high-flux events.

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Muddy waters lead to decreased oxygen in Chesapeake Bay

Featured Image: Plumes of muddy, sediment-laden water at the Chesapeake Bay Bridge near Annapolis, MD. Photo courtesy of Jane Thomas/ IAN, UMCES.

Paper: Seabed Resuspension in the Chesapeake Bay: Implications for Biogeochemical Cycling and Hypoxia
Authors: Julia Moriarty, Marjorie Friedrichs, Courtney Harris

A memorable feature of the Chesapeake Bay, the largest estuary in the USA, is that the water is very murky and looks like chocolate milk. Former Senator Bernie Fowler has conducted public “wade-ins” over the past 50 years in one of the Bay’s tributaries, seeing how deep the water is before he can no longer see his white tennis shoes, and let’s just say it is never very deep. This is because of the high concentrations of sediment, or small particles of sand and organic material, in the water. Besides making it harder for seagrasses to grow and serving as food for the economically-important oyster, sediment impacts the biological processes that determine how much oxygen and nutrients are available in the water for algae and fish.

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Isotopes Begin to Unlock the Mystery of Methane Source in the Scheldt Estuary

Scheldt Estuary with tidal flats and water shown

Featured Image: Eastern Scheldt Estuary near Zeeland, Netherlands. Photo courtesy Wikimedia Commons/ Luka Peternel, CC BY-SA 4.0 license.

Paper: Carbon and Hydrogen Isotope Signatures of Dissolved Methane in the Scheldt Estuary

Authors: Caroline Jacques, Thanos Gkritzalis, Jean-Louis Tison, Thomas Hartley, Carina van der Veen, Thomas Röckmann, Jack J. Middelburg, André Cattrijsse, Matthias Egger, Frank Dehairs & Célia J. Sapart

Estuaries are dynamic coastal environments where freshwater and saltwater collide and mix. Across the world, estuaries regularly have higher methane concentrations in the water than would be expected from equilibrium with the atmosphere. If the water was in equilibrium, or at a happy balance, with the atmosphere, then there would be no net transfer of methane to the atmosphere. Because there is more methane than expected in the water, estuaries are a source of this potent greenhouse gas, methane (CH4), to the atmosphere. The problem is that the processes leading to the excess methane in the estuary’s surface water are not well known in many European estuaries.

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Do Microbes Release Fluorine from Rocks?

Image of soil microcosm

Featured Image used with permission of photographer (Cassi Wattenburger)

Paper: Indigenous microbes induced fluoride release from aquifer sediments

Authors: Xubo Gao, Wenting Luo, Xuesong Luo, Chengcheng Li, Xin Zhang, Yanxin Wang

My science textbook taught me that fluorine (F) was really important for dental health, and I’ve since learned that both excessive and insufficient amounts of fluoride in groundwater can cause health issues. While the chemistry behind the release of fluoride ions from rocks or sediments into groundwater is well understood, the microbiology of this process is not. Specifically, scientists have been wondering whether microbes could speed up the release of F from sediments into groundwater. 

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