Stable Isotopes & Aquatic Ecosystems 101
By Oriana Silva
Oriana Silva is the President and Founder of the Sustainable Age - Student Journal. She studies Chemistry and French at the University of North Texas and participates in a research group studying food webs in aquatic ecosystems. The following is a summary of the team’s findings over the last few months. -Ramsey Cook
I commenced a couple of months ago working for Dr. Compson’s Lab at the University of North Texas (UNT), where he is leading a research project about food webs in aquatic ecosystems in order to understand how biodiversity is structured, how energy flows, and how intrinsic and extrinsic factors influence the structure and function of these systems.
An aquatic ecosystem is defined as a community of organisms that live and interact within a particular environment; in this case, it can be freshwater or saltwater. The type of water determines the species of animals and plants that can live there. Protecting these ecosystems is an important challenge since humans rely on them for food, drinking water, energy, and recreation.
In Dr. Compson’s lab, we are studying freshwater ecosystems in the United States. These ecosystems are composed of rivers, streams, lakes, ponds, wetlands, and even groundwater. Because every habitat depends on altitude, temperature, and humidity, it is possible to find an extensive variety of animals and plants in each of them. In addition to these variables, the amount of dissolved oxygen (DO) found in freshwater can determine what type of species live in these systems. Adequate DO saturation is vital for plant life and phytoplankton, which require these dissociated particles for respiration when there is no light for photosynthesis. Therefore, DO levels in water are also considered strong indicators of biodiversity.
One of the methodologies used in this research is stable isotope analysis to track aquatic food webs. Isotopes differ from the original element itself because they possess in their nucleus a different number of neutrons; therefore, they also possess different masses. Lighter isotopes are the most common and usually the most stable ones. These tiny differences in mass from the base elements make isotopes form weaker bonds than the heavier isotopes. Therefore, lighter isotopes produce faster reactions.
Stable isotopes are calculated as isotopic deviations from international standards and are expressed as delta (δ) values as parts per thousand. The formula is the following:
δ X = [(Rsample/Rstandard) – 1] x 1000
δ is the value of quotient of the ratios in the sample relative to the standard
X is the element (For example 13C)
R is the corresponding isotope ratio (For example 13C/12C; Carbon 12 has 6 neutrons and 6 protons in its nucleus which is the normal state of this element, while Carbon 13 has an extra neutron in its nucleus )
Why do we use stable isotopes to track aquatic food webs?
When we add stable isotopes to the diet of any animal from a specific food web, these isotopes are incorporated into the animal’s tissue and it makes it easier to track the web. In our lab, we isotopically labeled leaf litters with the purpose of creating tracers for carbon and nitrogen that will help determine how energy moves from leaf litter to higher trophic levels. These leaf litter packs were located in different creeks and streams across the country with the support of National Ecological Observatory Network (NEON) scientists. NEON is “a continental-scale observation facility operated by Battelle (an independent not-for-profit organization that advances science and technology to have the greatest impact on our society and economy), and designed to collect long-term, open-access ecological data to better understand how ecosystems in the United States are changing.”
Why do we use leaf litter and not another type of tissue?
Some tissues can differ in how metabolically active they are. If we select tissues that are metabolically inert after synthesis, like leaf litter, we will be able to examine long-term movements. If we select metabolically active tissues, like blood plasma, they can provide dietary and source information for a short period.
Slowly Decomposing Litter vs Fast Decomposing litter
When we use rapidly decomposing leaf litter to track the aquatic food web, we perceive a loss of carbon and nitrogen isotopes from the samples, a fact that helps to increase microbial respiration. Therefore, rapidly decomposing leaf litter is more accessible for consumption by microbial organisms. On the other side, slowly decomposing leaf litter possesses more complex forms of carbon bonded to nitrogen, which helps to last longer in the streams, making it more accessible for macroscopic food webs.
Detritus vs Leaf litter packs
Detritus is a component made of organic material that usually deposits on the floor of any aquatic ecosystem. This material affects trophic dynamics, species interactions, and ecosystem functioning. Plant-derived and animal-derived detritus are also used to track aquatic food webs because they help to increase O2 consumption by micro-organisms. However, in ecosystems where the invertebrate carcasses are predominant, we can perceive a greater amount of nitrogen, reactive phosphorus, and higher pH, than plant-based ecosystems. Therefore, this type of detritus decays faster than plant-derived ones.
Why do we use carbon(13C) and nitrogen (15N)?
Carbon isotopes have been used by scientists to reconstruct migratory routes of species and to study how these correspond to dietary changes over time. On the other hand, nitrogen isotopes can determine the trophic level of a species and any dietary shifts because nitrogen increases their isotope enrichment between 2 and 4 percent with each trophic level. Using both of them allows us to define biodiversity structure and determine energy flows through the trophic levels.
Freshwater ecosystems are a vital source for a wide variety of animals, plants, and humans. These systems are home to at least 45,000 fish species, billions of insects, and amphibians. They provide water for agricultural and industrial use, sanitation, and the most important, drinking water. Studying the aquatic food webs will allow us to understand better biodiversity structures, and to visualize extrinsic (like human disturbance or climate change) and intrinsic (like plant and insect genetics or genetic diversity) factors that influence the function in these systems. Hopefully, the results obtained in this research will provide a clear path for how we can positively impact aquatic ecosystems and what we must do to protect them. As world citizens, it is our duty to make environmentally friendly decisions for aquatic ecosystems in order to preserve our only home: Earth.
Compson, Zacchaeus. Ecolab. “Research” https://compsonlab.org/research-1
Fondriest Environmental, Inc. “Dissolved Oxygen.” Fundamentals of Environmental Measurements. 19 November 2013.
Gast, Cynthia. Sciencing. “Definition of an Aquatic Ecosystem.” 30 September, 2012.
National Ecological Observatory Network https://www.neonscience.org/about
Yee, Donald, and Juliano, Steven. Freshw Biol.“Consequences of detritus type in an aquatic microsystem: effects on water quality, micro-organisms and performance of the dominant consumer.” March 2006.
Zimmo, Sahar; Blanco, Jake & Nebel, Silke. Nature Education Knowledge. “The Use of Stable Isotopes in the Study of Animal Migration.” 2012.