Ancient groundwater brings a new lease of life to arid ecosystems
Reference: Mazumder, D., Saintilan, N., Hollins, S., Meredith, K., Jacobsen, G., Kobayashi, T., & Wen, L. (2019). Carbon uptake in surface water food webs fed by palaeogroundwater. Journal of Geophysical Research: Biogeosciences, 124. https://doi.org/10.1029/2018JG004925
Featured image source: Lock the Gate Alliance
How does a living fish have a radiocarbon age of ~9,000 years BC? A recent study revealed that very old carbon from ancient groundwater actually provides a life-sustaining source of energy to the contemporary food web of wetlands in dry (“arid”) climates.
The miracle of ancient groundwater
Groundwater that is stored in underground aquifers and rises up to the surface makes it possible for many aquatic ecosystems in arid zones to exist. Aquatic systems like this are known as groundwater dependent ecosystems (GDEs). This ancient (“palaeolithic”) groundwater can be millions of years old due to the slow rate at which it travels through the subsurface, but is far from long gone. In addition to providing a more reliable source of water to small standing-water ecosystems than arid climate rainfall patterns allow, the very survival of keystone species depend on this source of water and its nutrients.
Researchers conducted a study looking at how old carbon from groundwater was integrated into the modern foodweb of a stream and wetland systems fed directly from a groundwater spring. Carbon exists in groundwater due to the infiltration of surface water and rainfall through the soil, which collects carbon dioxide and dissolved organic carbon compounds on it’s way down to the aquifer. The site was located in the iconic Great Artesian Basin of Australia, the largest artesian basin in the world. An artesian basin is an underground water reservoir that is trapped between layers of rock formed over different geologic time periods. The researcher’s findings shed new light on the importance of ancient groundwater carbon in semi-arid landscapes, which until now, had mostly been explored in cold regions of the world (e.g. melting peatlands).
Tracking an invisible source
Tracking the source of groundwater and determining how it contributes to the amount of carbon available in local ecosystems can be difficult to do due to the invisible nature of groundwater. In this study, the authors employed the use of radiocarbon tracers to determine both the source and contribution of groundwater carbon to the local food web. The age of stream, groundwater, and biotia samples were determined by measuring carbon-14 (C14), which is the radioactive isotope of carbon. Carbon-14 is an unstable isotope and gradually decays to carbon-12, the stable carbon isotope. All living organisms contain a little bit of carbon-14, and the older the organism is the lower the C14 relative to C12. From the ratio of C14 to C12, scientists can determine the age of any material that contains carbon.
However, old carbon in the form of dissolved inorganic and organic compounds can be taken up by primary producers such as algae as an energy source. In the studied ecosystem, algal carbon is consumed by organisms low on the food chain such as shrimp and eventually makes it into the diets of predatory fish. This is known as trophic transfer and is how old groundwater carbon can enter the modern food chain in aquatic ecosystems. The authors took samples of different organisms living in the stream and wetland and took note of the distance from the groundwater spring. Spatial sampling allowed the researchers to gather insight into how far the aged carbon influences aquatic food webs once it enters the surface.
Old carbon has a far reaching influence
The study found a surprisingly large amount of aged carbon within different trophic levels of the food web, with only slightly less contemporary carbon than they would expect. The dissolved organic carbon was some of the oldest ever recorded: 24,000 years old, and via uptake through the food web, resulted in finding a living fish with a radiocarbon age equivalent to 9,000 years! The finding that such aged carbon is readily incorporated into the bodies of fish also disputed the commonly held view that aged carbon matter is recalcitrant, meaning it can’t be used by organisms. Based on this recent finding, groundwater contributions of old carbon were the dominant source of energy for the entire food web.
The spatial findings showed that the contribution of aged groundwater carbon lessens as you travel further from the spring source. However, the food web remained strongly influenced by ancient carbon, with algae recording ages up to 23,000 years 300 meters away from the spring. In addition, complementary results using nitrogen isotopes instead of carbon isotopes revealed that the isotopic “fingerprint” between fish and their immediate surroundings where almost identical. This showed that the fish do not swim very far from the groundwater source habitat. Clearly they are happy enough surviving in one place, with a consistent food source provided by ancient groundwater.
A precious resource under threat
The undeniable influence of aged groundwater carbon on the arid-zone ecosystem studied here ultimately underscores the fragility of groundwater dependent ecosystems and the importance of groundwater. This critical water source from the Great Artesian Basin has been facing pressure from a changing climate as well as interactions with agricultural and mining industries. Long residence time of groundwater renders this basin’s water an unsustainable resource based on modern day extraction methods for human use, which could see the collapse of fragile aquatic ecosystems. The distinct signal of fossil carbon released when this groundwater is disrupted due to mining, such as fracking, could provide a way of tracking the legacy impact of mining operations on nearby groundwater dependent ecosystems. It is likely that other arid zone groundwater dependent ecosystems around the world are ecologically supported by artesian groundwater and warrants attention to better understand ecosystem responses to contemporary pressures. This demonstrated ecological importance highlights a further need to conserve and carefully monitor changes in groundwater dynamics of arid zone landscapes.