Energy flow

The Flow of Life Through the River


The amount of productivity (i.e., grams of biomass/square meter) among the numerous assemblages of plants and animals in a defined ecosystem is in large part determined by the annual input of energy (see energy flow) it receives and processes. The cycling and re-cycling of energy through those life forms typically defines the geographic limits of an ecosystem. Solar radiation is the energy source for all above ground ecosystems. The amount of solar energy reaching the surface of the earth is calculated at 1.94 calories/cm2/minute, and this number is referred to as the solar constant. A calorie is a unit of heat equivalent to the energy needed to raise one milliliter of distilled water one degree centigrade above its ambient temperature of 200 C.

Primary producers (i.e., all the green plants) convert about 1-3% of the sun’s calories into usable chemical energy. Primary consumers, herbivorous animals, eat the plants, converting their energy into biomass, on the order of 10% of the total amount consumed. The rest of the energy (90%) is used to maintain the animal’s body temperature at a constant level. Many of these animals become food for secondary consumers (carnivores and scavengers). Since meat contains about 10 times the calories per gram as vegetable matter, meat-eaters need to consume less biomass to get the same level of nutrition as herbivores. All trout species are strict carnivores, while some salmon species (e.g., Kokanee, a landlocked version of sockeye salmon) are filter feeders, consuming plankton as their main food source.

Solar energy can enter a river in one of two ways. It can stimulate the growth of in situ macrophytes and algae, providing a constant source of food for the in-stream herbivores, as is the case for limestone streams. It can also enter the stream in the form of bank-side foliage (e.g., dead leaves, branches, grasses, flowers, seeds, etc.) at times in the year corresponding to the stream-side plants cycles of growth and dormancy. Energy input from within a river is referred to as autochthonous, while energy that is imported from the stream banks is termed allocthanous. The later is essential for the health of the freestone river.

The amount and kind of energy input determines the biological nature of each river. The amount of energy also has great influence on the “slope” of the linear gradient (Figure 3) of life within rivers. Life stratifies downstream according to each species of plant and animal’s genetic blueprint, responding to a myriad of limiting factors, not the least of which is the amount of available food and dissolved oxygen. Tailwater fisheries are somewhat different, receiving much of their energy from the reservoir above. In this instance, sunlight penetrates the photic zone of the lake’s surface water, stimulating the growth of algae, which are then exported to the river below through the out flow tubes at the dam site. The lake serves as a primary source of nutrients (fine particulate organic matter or FPOM), boosting the river’s normal capacity to produce macro -invertebrates and salmonids to near theoretical capacity in some cases.

For the freestone river, the region of land bordering the water is critical to the flow of energy. This narrow strip of land dictates some of the more important physical conditions under which all the life forms of those rivers must live; bright sunlight or sun-dappled shade; abundant input of decaying plant material, or desert-like conditions. The ecotonal (see ecotone) characteristics of freestone rivers that wander through hardwood forests allow for a high degree of biodiversity among the life forms occupying the water’s edge throughout the seasons. As mentioned, limestone streams are less dependent upon the banks for energy in-put, and because of their proximity to civilization, most limestone streams throughout the world lie within fertile farm land that has been colonized and worked on for centuries by countless agricultural communities. As the result, biodiversity along their banks is more restricted than along freestone rivers.

Assemblages of macroinvertebrates are selected for life in cold, running waters according to the kinds and amounts of energy input (e.g., dead leaves, fine particulate organic matter, or even dissolved organic matter), the physical and chemical characteristics of the geological strata over which the water flows, and the presence or absence of keystone species. In both the north and south islands of New Zealand, prior to the introduction of trout, macroinvertebrate communities had lived undisturbed on the top, as well as on the bottom of rocks and debris in those crystal clear, freestone rivers. Lack of a top carnivore species in most of them allowed this behavior to evolve unimpeded by predation. After the introduction of trout, several entire genera of macroinvertebrates became extinct, much to chagrin of some aquatic entomologists, while others were selected for life under rocks, only. All of this evolution occurred within the span of just 50 years.        

Macroinvertebrates can be found wherever there is cold, running water, and the establishment of complex relationships among the numerous species (see Mayfly, Caddisfly and Stonefly) attests to the fact that these ecosystems are among the most intricate and inter-dependent to be found in nature. In most cases, fresh running water-based ecosystems defy detailed description, and seldom has anyone been able to identify all the macroinvertebrate species and their abundances in any river system. The fact that the environment constantly changes, and that aquatic insects go through complex developmental cycles in order to progress to adult flies that typically leave the river, makes for some fascinating ecological modeling, to say the least.

Energy flow is linked inexorably with macroinvertebrate developmental cycles and the resident salmonids must take advantage of each feeding opportunity, regardless of which species is hatching. No matter where in the world they find themselves, trout are able to identify and take advantage of suitable food items. The following essays describe the conditions under which energy flows through the major river types that support the growth and reproduction of salmonids.


Green River, Wyoming.

Tiger swallowtails.

False chanterelles.