Featured Image: Small headwater stream in Oregon’s Mountains. Image courtesy Jessica Buser-Young, used with permission.
Paper: The River Continuum Concept
Authors: Robin L. Vannote, G. Wayne Minshall, Kenneth W. Cummins, James R. Sedell, Colbert E. Cushing
Perhaps last time you went for a hike, you stumbled upon a burbling spring pushing its way up through the leaf litter after a heavy rainfall, creating a tiny rivulet of water crisscrossing over your path before plunging back into the forest. What a find! Excitedly, you squatted down and gently uncovered the spring to notice gnats lazily floating away, some nearby fruiting mushrooms, and great clumps of decomposing twigs and leaves which you assume harbor uncountable numbers of microorganisms. This unique little ecosystem is profiting from the nutrients and water being pushed from the ground, using the opportunity to have a feast. But what happens to the nutrients and carbon that gets past these plants and animals?
The carbon connection between upstream and downstream ecosystems wasn’t always well understood. In their seminal 1980 seminal work, Vannote and coworkers described the idea of a River Continuum Concept for the first time, unifying preexisting ideas and giving researchers a framework for understanding the factors affecting river ecosystems from headwaters to the delta. Their River Continuum Concept defines strict relationships between organic matter (such as plant detritus), availability of sunlight, and other predictable events such as temperature, how quickly the water is flowing, and water depth, in order to characterize and even predict ecosystem dynamics along a river continuum from the burbling headwaters to massive delta.
The central aspect to this unbroken gradient revolves solely around carbon and how it is transported, used, stored, and passed through each ecologically unique section of the continuum. Carbon is essential to all life, as it provides the necessary energy for cells to function, just as carbs the night before a big race give a marathoner an energy boost. In aquatic and terrestrial ecosystems, microorganisms can find carbon in the form of organic matter. At a spring, the organic matter of the system is largely sourced from the earth, which gives it a specific microscopic structure unique to terrestrial systems. Any organisms in the spring water will need to be able to crack open this terrestrial organic matter for consumption, requiring a specific set of skills. Once the terrestrial organic matter is broken open, the structure will take a new form that is more available to other aquatic organisms up the food chain. However, this whole process is incredibly inefficient, so copious amounts of different forms of carbon escape downstream. The lost carbon moving downstream along the continuum is eagerly awaited by a new ecosystem shaped specifically by this relationship. Typically, a trip downstream leads to a larger, more prolific area as many tributaries combine and widen the river. This change in river size is accompanied by a decrease in shade and an increase in plants, algae, and lichens as the sun becomes more available and total carbon is less abundant as it is consumed upstream.
Transfer of carbon along a river continuum is dynamic and complex. However, the exchange of carbon is finely balanced despite the movement of water and mobility of certain organisms such as microbes floating away to a new section of river, insects flying to and from the river, birds scratching for worms at the riverbanks and carrying them away, or the passage of time and seasonality. This balance is known as “dynamic equilibrium” as the whole system is balanced and stable yet inherently mobile. For example, as the worm near the river’s edge consumes algae and plant matter from the water, a nearby bird may snatch the worm away taking with it all the carbon from the consumed plants. Simultaneously, a different bird may fly over the river, dropping a feather into the river shallows, providing a new and different source of carbon to the river ecosystem. A system in dynamic equilibrium is able to adapt and adjust to permutations in organic matter availability, temperature, and other events.
The River Continuum Concept defines the dynamic equilibrium between the transfer of carbon along a river system from the spring to delta. River ecosystems along the continuum hinge around the presence and type of carbon as the original terrestrial source is munched and changed as it flows downstream. As Vannote and coworkers outlined these specific parameters, they altered the way modern science approaches research around river systems.
Mosaics of Life Along a River by Jessica Buser-Young is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.