Wintertime Watersheds

Andrew Thurber retrieves an OsmoSampler from East River, CO during the wintertime. Photo by Laura Lapham.

Humans and the Hydrosphere

The Hydrosphere is the system of global water exchange between the atmosphere (vapor, clouds, and precipitation), biosphere (rivers, lakes, oceans), and geosphere (icebergs, glaciers, hot springs, groundwater).  Ecosystems all around the world rely on the hydrosphere in unique ways to access water for survival, from morning dew in arid deserts, to daily rainstorms in the tropics. Humans are not exempt from this cycle, and we have shared and accessed waters for thousands of years for drinking, farming, and industry.  Despite our reliance on the hydrosphere, we lack fundamental knowledge on the specific inner-workings inherent to hydrologic systems. Our modern world is experiencing disturbances in this precious resource through pollution, contamination, and droughts. Watersheds are areas of land where streams and rivers drain into lakes and ocean, and these ecological systems are particularly vulnerable to changes such as wildfires, erosion, and drought.  Our response to these changes in our water availability rely on our knowledge of how watersheds function, and how they are responding to climate change.

Human agriculture is known to be founded upon the banks of the mighty Tigris and Euphrates Rivers.  Civilizations spread along the Nile, Yellow, and Indus Rivers, as water is the crux and the heart of ancient and modern societies.  Present-day human establishments all reside on the banks of freshwater resources. These mighty rivers are the “trunks” to a network of branches of smaller streams, lakes, and tributaries all at a higher elevation draining into a single area.  This network of systems is known as a watershed. Over 30 million square kilometers, or 0.1% of the Earth’s surface is comprised of watersheds, equating to <0.5% of total global freshwater, emphasizing the importance of preserving these precious resources.  

Fragile Headwaters

The ecological structure of watersheds begins with headwater streams, or the very beginnings of a watershed.  Headwater streams may originate seasonally through snow and ice melt at high elevations, or are sustained year-round by bubbling springs of groundwater.  In US watersheds, headwater streams account for almost three quarters of stream channel length. Headwater streams represent the most intimate interaction of Earth and water, as the transitory point from a terrestrial system to aquatic, and are therefore disproportionately sensitive to disturbances.  In addition to being physically small and delicate, headwater streams are typically in high elevation or high latitude areas, exposing them to greater warming events and unpredictable weather compared to Equatorial and lowland regions. Alterations at the very start of a watershed has a snow-ball effect on water availability in low-lying areas, as evidenced by disappearing rivers and streams as glaciers die, or fluctuations in seasonal variability depending on when the snow melts in higher elevations.  Despite these major changes, scientists struggle to keep up with future predictions of these ecological behaviors due to a lack of knowledge. Nevertheless, current research in understanding watershed changes is a step towards critical stewardship of water supplies for humans and ecosystems.  

Surface-Subsurface Mediator: the Hyporheic Zone

When assessing these broad-scale watershed systems, our knowledge must take into account the physical area and three-dimensional space, and the time at which interactions occur (daily, annually, etc… ).  These interactions involve chemistry and biology of the surface (above ground) and the subsurface (below the ground) across time and space. Microorganisms are present in the surface and subsurface, yet the types of organisms vary, much like how giraffes are only native to Africa while polar bears are restricted to the Arctic.  The subsurface generally has little or no oxygen, which is a key determining factor for microbes present and how the chemistry behaves compared to the surface chemistry.  

The subsurface is unique in that it is “buffered” from surface interactions, which are rapid and dynamic.  The transition zone between the surface (river water) to the subsurface (below the ground) is known as the hyporheic zone, and may range from ~10 cm to several meters deep.  In watershed systems, the hyporheic zone is a slurry of mud and sediments that contains water which is moving (slowly) in the same direction as the above ground surface water.  Beneath the hyporheic zone is a much larger space of dirt and water where the water is not moving in any particular direction. Layered ecosystems of surface river water, to the hyporheic zone, to the subsurface have complex cross-talk of microbes, fluids, and chemistry that all change based on season, precipitation events (or lack thereof), and geographical location.  

Wintertime Isolation

For all the reasons that make headwater streams and hyporheic zones dynamic and fascinating also make it difficult for humans to physically attain seasonally.  Wintertime snowstorms and ice cover, and spring floods are too dangerous in these high elevation areas for researchers to access to understand the transport of chemistry and biology across the surface and subsurface.  In these cases, data may be collected using autonomous samplers that are able to continuously sample the environment for a long time (~1 year), while preserving and protecting the sample. The Colwell Geomicrobiology Lab and collaborators address this seasonal knowledge gap by employing the use of autonomous samplers known as OsmoSamplers.  We are able to prime, deploy, and retrieve OsmoSamplers in these headwater streams in high-elevation areas to continuously sample the chemistry of groundwater, hyporheic water, and river water during the wintertime.

Using Water to Collect Water

OsmoSamplers are long cylindrical tubes containing two fluid reservoirs separated by a plastic membrane.  The first reservoir (A) contains an incredibly salty water, while the second reservoir (B) contains fresh water. Water has many unique properties, one being an attraction to other compounds dissolved in water, such as table salt.  Individual water molecules will surround a salt molecule due to a polar attraction, similar to magnets. Reservoir A has so many salt molecules that there are not enough water molecules to surround each salt molecule. Reservoir B contains only water molecules, and they are able to pass through the membrane while the salt molecules may not since they are too large.  The movement of water molecules from Reservoir B to Reservoir A creates a flow of fluid from one end of the OsmoSampler to the other end.  

We are able to harness this fluid flow by attaching a coil of copper tubing full of fresh water to the end of the OsmoSampler.  We may sample an aquatic environment, such as a headwater stream, the ocean, lakes, wetlands, and rivers for a prolonged period of time, as the rate of the fluid flow is very slow and the copper coils are almost 1,000 ft long!  If we wish to capture microorganisms using OsmoSamplers, we modify the existing instrument to contain a second OsmoSampler that is pumping water in the opposite direction to pump a DNA preservative from a second coil into the inlet of the first coil.  This way a DNA preservative is added to the water sample, freezing the cells in time as we collect them.  

East River, Colorado, Research

The East River in Colorado is a high-altitude watershed in the Rocky Mountains that is well known to experience dynamic shifts in the surface and subsurface chemistry.  The soils and groundwater within this watershed are susceptible to warming weather patterns and vegetation shifts, causing changes in the timing and volume of drainage. As part of the Colorado River Basin, East River headwater streams are pivotal in the collection of water for Nevada, Utah, Colorado, Arizona, New Mexico, southern California, and northern Mexico (Baja California and Sonora).  The Department of Energy has a research station off East River where they collect chemistry of the surface and subsurface, yet it is unsafe to access these stations between November to March due to snow, ice, and flooding.  

Future Predictions

By using these OsmoSamplers and biological OsmoSamplers to collect water from inaccessible yet crucial areas of watersheds such as East River headwater streams, we will be able to model the chemical and biological dynamics between the surface and subsurface during the winter and early spring.  A mathematical model of an ecological system helps inform researchers about how specific systems and interactions will change based on different inputs. By collecting sufficient data to “capture” the goings-on of watersheds year-round in light of certain events such as precipitation, temperature, and location, we can begin to predict the behavior of a specific watershed in the future.  

More on this research at: lapham-u-maryland/

Wintertime Watersheds by Jessica Buser-Young

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