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The River Continuum Concept

I am quite surprised it took me this long to get around to writing about what is probably the most important and cited theory in stream ecology, the river continuum concept. The concept provides a model to understand the physical and biological changes that occur as you move from a streams headwaters to the ocean. As I write this post, it was been cited over 11,000 times in the scientific literature (that is a lot!) and there is hardly a student in aquatic or stream ecology that has not read the paper. In the stream ecology world, it is as famous an idea as there is.

Source: https://cdnsciencepub.com/doi/full/10.1139/cjfas-2020-0039


The main idea is that there are predictable physical changes along the continuum (as the stream flows downstream) that occur as we move from headwaters to large rivers. Because of these physical changes and different sources of energy, biological communities are structured to utilize the available food sources in different stream reaches. First, I think it is important to understand that the RCC was developed in the Pacific Northwest where headwaters occur in the mountains. As I will discuss, just a bit, there are places that deviate from the basic continuum set out in the paper but the idea that the form that energy is available in will dictate what types of "bugs" are there, holds constant. Rather obviously, Wisconsin lacks the mountains from where the RCC was developed but the general idea still works quite well as you will see.


Component of the Concept


I am going attack this by talking about the physical environment (stream and riparian zone), sources of energy (food), and biological communities - macroinvertebrate and fish will be handled separately.


Physical Environment


Obviously streams get larger as they flow downstream and capture water from a larger watershed. The amount of water (discharge) increases as does depth and width as we move downstream. Discharge is width x depth x current velocity and is measured in cubic meters or feet per second. More importantly, discharge is the best measure of stream size in almost all cases. And, as we move downstream not only does discharge increase but substrate (the rocks, sand, silt, and organic matter at the bottom of the stream) size decreases, temperature increases, and oxygen decreases due to temperature increasing.


While I did not spend a lot of time on this section - the physical environment is a critical determinant of the energy available to streams and the biota (living things) in the stream.


Sources of Energy


Increases in stream width mean that the riparian zone (the terrestrial areas adjacent to the stream) has a smaller influence on the stream. In headwater streams, riparian trees shade the stream much of the year - particularly during the time when the sun energy is high - which means that streams stay colder but it also means that they receive sunlight. Thus, these streams energy inputs is mostly through leaf fall (allochthonous energy = energy from "outside the system" which is terrestrial in the case of streams) which is then the food source for fungi, algae, and macroinvertebrates.

A heavily forested headwater stream that is largely dependent upon leaf fall as little sun hits the stream.

As we continue to move downstream and stream width increases, sunlight now becomes an important source of autoochthonous production (in-stream growth of algae and rooted vegetation). Scientists talk about the relationship between production (primary production = photosynthetic growth) and respiration (consumption of oxygen = heterotrophic organisms or consumers). In headwater streams, production is low due to shading and respiration is high (P < R) due to the decomposition and consumption of leaves. In mid-order streams production and respiration are about equal or production is greater than respiration (P ~ R or P > R). Lastly, in large streams, respiration is high because all of the organic matter from upstream is making its way downstream but in "smaller bundles" and production is low due largely to the high turbidity (low light penetration) of larger rivers (P < R).


Macroinvertebrate Communities


Essentially, what I had written in the previous section is that in headwater streams, the energy source is leaves which are broken down into smaller pieces as they are decomposed and consumed. We refer to leaves and other large sources of energy as coarse particulate organic matter (CPOM). CPOM is the food source of the macroinvertebrate functional feeding group, shredders. Though to be more accurate, they are often feeding on the organisms that decompose leaves. As CPOM is broken down, it becomes FPOM (F = fine) and now collector macroinvertebrates - like netspinning caddis - utilize this energy source. As the carbon that was in leaves continues to be moved downstream, it moves in smaller and smaller "bundles" until it is eventually referred to as dissolved organic matter. Scientists doing what they do, have a bunch of crazy categories for the sizes of different organic matter from coarse to dissolved organic matter.

Cheat River, West Virginia
West Virginia's Cheat River, a mid-order stream with high instream productivity.

In streams with high instream productivity of algae and aquatic macrophytes (large, rooted plants that grow in streams), macroinvertebrate grazers - the cows of the stream - are the dominant macroinvertebrate functional feeding group along with the collectors that are taking advantage of the broken down leaves and productivity produced instream. These occur not only in mid-order streams but also in stream reaches without many riparian trees.


Lastly, large rivers receive the energy from all the streams that feed them largely as FPOM or smaller particles. Collectors are the predominant macroinvertebrate functional feeding group in large rivers. Predators, the last of the functional feeding groups, make up about the same percentage of the macroinvertebrate community in all stream types. This is because that there is a lot of energy required to sustain predators as that energy needs to assimilated by another organism to reach predators.

Mississippi River
A small part of the Mississippi River - Big Muddy. Most of the energy available is "upstream processing inefficiencies".

To sum up this very important section, macroinvertebrate communities are structured to take advantage of the energy available to them. Production occurs outside the stream (allochthonous; leaf fall and wood) or within the stream (autochthonous; algae and rooted aquatic plants (macrophytes)). All of this organic matter is consumed, decomposed, or transported downstream - and pooped out (yes, that is important). Thus the energy is in increasingly smaller particle sizes as it is transported by water downstream. The proportion of the different functional feeding groups - shredders, collectors, grazers, and predators - are predictable based on the energy available to them.


Fish Communities


Many people reading this will instantly understand this section. Trout live in cold headwater streams where there are generally few other fish species tolerant of the cold conditions. Few species are able to live in cold conditions largely because enzyme (catalyst) function for cold-blooded species is low in all species except for those relatively few species that have evolved in cold places. In the coldest of streams, only sculpin (Cottidae) and trout exist for fishes. Maybe you are familiar with some of the tailwaters trout fisheries which are low in aquatic insect diversity but high in productivity. Some species are able to thrive but most are excluded from these tailwater streams.


As we move downstream along the river continuum, the number of fish species increases - we pick up minnows (Cyprinidae), suckers (Catostomidae), darters (sub-family Etheostomatinae of the perch family), madtom catfishes (Ictaluridae; genus Noturus), and some of the sunfishes (Centrarchidae) like Bluegill and Smallmouth Bass. Eventually, as we move to large rivers, we expect to see sturgeon (Acipenseridae), gar (Lepisostidae), and some of the other families associated with large rivers (shad, Clupeidae; Mooneye and Goldeye, Hiodontidae; suckers, Catostomidae, minnows, Cyprinidae - and others. Again, more species are able to survive as the temperatures warms (to a point, of course).

Fishes from West Virginia's Cheat River drainage.
Mid-order stream fishes - minnows, suckers, and sculpin.

Wisconsin and Deviations from the RCC


The RCC model provides a framework to understand how the physical and and biological aspects of streams are interconnected. Vannote and colleagues wrote the paper with their Western rivers in mind and as such, there are places where streams do not fit this model. Certainly in Wisconsin, northern streams more closely follow the RCC model. Much of the state has been transformed by agriculture and in many places, prairies are the native vegetation community so in many places headwater streams are not forested. Instead, many small Wisconsin streams flow through prairies, pastures, and open agricultural areas which means that they will have more instream (autochthonous) production and derive less energy from less leaf fall (allochthonous). This means that the shredders of the forested streams are replaced by grazers in these more open stream reaches.

Driftless spring creek
A typical Driftless stream flowing through agricultural lands will receive a lot of sunlight.

I do not plan to get into many deviations and theories that describe how dams and other disturbances alter the RCC's expectations but I think it is important to understand that like all scientific ideas, the RCC has been tested, modified, and in some places, discarded for ideas that fit those streams better. No scientific ideas or models are perfect, however I will say that the RCC is the most important idea in stream ecology. It ties together the physical and the biological aspects of streams and gives us a starting point to better understand stream habitat, sources of energy, and biological communities. Certainly there are places that do not fit the montaine environments where the RCC was developed. However, they do fit into the physical/biological relationships that the RCC prescribes. The RCC sets our expectations and it tells us how the physical world, energy inputs, and "processing inefficiencies" shape biological communities. It is one hell of a useful idea - and probably one you knew about before reading this post - but I hope it gives you a little more insight into how streams change as we move from headwaters towards the ocean.


Resources:


https://www.shorelandmanagement.org/depth/rivers/02.html

https://en.wikipedia.org/wiki/River_Continuum_Concept

https://cdnsciencepub.com/doi/full/10.1139/cjfas-2020-0039

https://cdnsciencepub.com/doi/abs/10.1139/f80-017

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