8.4.3: Watershed Functions

Among the most essential functions that occur in healthy watersheds are:

• Transport and storage
• Cycling and transformation
• Ecological succession

Transport and storage (of water, energy, organisms, sediments, and other materials)

Because a watershed is an area that drains to a common body of water, one of its main functions is to temporarily store and transport water from the land surface to the water body and ultimately (for most watersheds) onward to the ocean. But, in addition to moving the water, watersheds and their water bodies also transport sediment and other materials (including pollutants), energy, and many types of organisms. It is important when recognizing the transport function to also recognize temporary retention or storage at different locations in the watershed

Figure \(\PageIndex{1}\): Sediment transport and deposition. USEPA, Introduction to Watershed Ecology, Public Domain

Transport and Storage. As matter physically moves through the watershed, there are a number of terms which arise relative to various stages of cycling. Availability refers not just to the presence of an element in a system, but also speaks to the usability of a given agent. For instance, nitrogen gas may be plentiful in and around dam spillways, but N2 is not a usable form for most aquatic organisms, and thus the availability of nitrogen is compromised. Detachment refers to the release of matter from an anchoring point, and its subsequent movement. Transport, a process most evident in stream channels, involves the movement of a material through a system. Deposition refers to a given endpoint within a cycle. Integration refers to the assimilation of matter into a site or organism following depositional processes (see Naiman and Bilby 1998). An example using these terms is included below.

Example: Seston Seston (suspended particulate matter) is an important food source for a few key aquatic organisms. In the context of the above continuum, seston’s availability within a given stream system may refer to the source. In a hypothetical example, nitrogen and phosphorus, both limiting in many freshwater ecosystems, are made available to a stream during a period of elevated discharge. Attached algae (periphyton) begins to grow on rocks and submerged surfaces (availability). During growth and maturation, algal cells begin to slough off from the rocks and logs (detachment) and the current carries these cells, rich in chlorophyll, downstream (transport). Hydropsychid caddisfly larvae, which feed on seston, spin elaborate nets (much like a spider’s web), anchoring these nets on cobble substrates or other submerged features. These nets catch the suspended particulates as they drift with the current (deposition). The caddisfly larvae then feed off particles captured in the net (assimilation); algal cells represent high quality food for these organisms and they may, as a population, exert an influence over the relative abundance of various particle-types.

Transport and storage of water. One can view a watershed as an enormous precipitation collecting and routing device, but transportation and storage of water actually involves a complicated mix of many smaller processes (which are bolded in the following text). Even before precipitation reaches the ground (Figure 28), it interacts with vegetation. Trees and other vegetation are responsible for interception and detention of some of the rainfall, leading to some evaporation and also slowing the amount reaching the ground via throughfall and giving it time for better infiltration to groundwater (one form of storage). Saturation of soils, occurring when precipitation exceeds infiltration, leads to overland flow and, over longer time frames, drainage network development (Figure \(\PageIndex{2}\)). The consistent flow of water in channels affects and shapes channel development and morphology in ways that seek dynamic equilibrium with the job to be done (moving water downstream). Recall also this module’s earlier discussion of the longitudinal profile development of rivers and streams, and how upper, middle and lower zones of streams generally have very different forms to handle very different sets of functions, many related to transport and storage of water.

Figure \(\PageIndex{2}\): Common drainage patterns USEPA, Introduction to Watershed Ecology, Public Domain

Figure \(\PageIndex{3}\): Dynamics of precipitation. USEPA, Introduction to Watershed Ecology, Public Domain

Transport and storage of sediments. Watersheds also collect and transport sediments as a major function. Sediment transport and storage is a complex network of smaller watershed processes, like the water processes described above, and actually is inseparable from water transport and storage. Sediment related processes mostly involve erosion and deposition, but sediment transport and storage also play a longer-term role in soil development.

The drainage network development and channel development (Figure \(\PageIndex{2}\) and \(\PageIndex{3}\)) discussed above appears to be dominated by erosion at first glance, but the redeposition of sediments on floodplains is an important function that rejuvenates soils and influences the productivity and diversity of stream corridor ecosystems.

Cycling and Transformation.

Cycling and transformation are another broad class of natural functions in watersheds. Various elements and materials (including water) are in constant cycle through watersheds, and their interactions drive countless other watershed functions. Figure 31, for example (overleaf), illustrates interactions of the carbon and nitrogen cycles with stream biota and the resulting influence on dissolved oxygen. Elements like carbon, nitrogen, and phosphorus comprise the watershed’s most important biogeochemical cycles. Cycling involves an element of interest’s transport and storage, change in form, chemical transformation and adsorption.

Nutrient Spiraling. The flow of energy and nutrients in ecosystems are cyclic, but open-ended. True systems, in both an environmental and energetic context, are either “open” (meaning that there is some external input and/or output to the cyclic loop) or “closed” (meaning that the system is selfcontained). In watersheds, streams and rivers represent an open-system situation where energy and matter cycles, but due to the unidirectional flow, the matter does not return to the spot from whence it came. Also, nutrients “spiral” back and forth among the water column, the bodies of terrestrial and aquatic organisms, and the soil in the stream corridor en route downstream. Hence, the concept of nutrient “spiraling” implies both movement downstream and multiple exchanges between terrestrial and aquatic environment, as well as between biotic and abiotic components of the watershed.

The Cycling of Carbon and Energy. In food webs, carbon and the subsequent synthesized energy is cycled through trophic (food web) levels. Energy transfer is considered inefficient, with less than 1 percent of the usable solar radiation reaching a green plant being typically synthesized by consumers, and a mere 10 percent of energy being typically converted from trophic level to trophic level by consumers.

Figure \(\PageIndex{4}\): Carbon and nitrogen cycles affect stream biota. USEPA, Introduction to Watershed Ecology, Public Domain

Nitrogen and Phosphorus limitation. Most watershed systems (both the aquatic and terrestrial realms) are either N or P limited, in that these are the required elements which are at the lowest availability. As a general rule, the N:P ratio should be 15:1. A lower ratio would indicate that N is limiting, a higher ratio places P in that role. Commonly P is the limiting factor. Often, the slightest increase in P can trigger growth, as in algal blooms in an aquatic setting. In N and P limited systems, an input of either element above and beyond normal, “natural” levels may lead to eutrophication. The stream corridor is often a mediator of upland-terrestrial nutrient exchanges. As N and P move down through subsurface flow, riparian root systems often filter and utilize N and P, leaving less to reach the stream. This has a positive influence on those already nutrientoverloaded bodies of water, but would not necessarily be a positive influence on organisms struggling to find food in very clean, nutrient-limited headwaters streams. Microbes also denitrify significant amounts of N to the atmosphere . Still, N-fixers, like alder, may serve as sources of N for the stream channel, and groundwater pathways between the stream and the streamside forest may provide significant quantities of nitrogen.

Figure \(\PageIndex{4}\): A simplified decomposition process. USEPA, Introduction to Watershed Ecology, Public Domain

Decomposition (Figure \(\PageIndex{4}\)). Decomposition involves the reduction of energy-rich organic matter (detritus), mostly by microorganisms (fungi, bacteria, and protozoa) to CO2, H2O and inorganic nutrients. Through this process they both release nutrients available for other organisms and transform organic material into energy usable by other organisms. In lakes, much of the decomposition occurs in the waters prior to sedimentation. In the headwater reaches of streams, external sources of carbon from upland forests are a particularly important source of organic material for organisms and decomposition of microscopic particles occurs very rapidly. The bacteria and fungi modify the organic material through decomposition and make it an important food source for invertebrate and vertebrate detritivores, thereby reinserting these nutrients and materials into the watershed’s aquatic and terrestrial food webs.

Decomposition is influenced by moisture, temperature, exposure, type of microbial substrate, vegetation, etc. Specifically, temperature and moisture affect the metabolic activity on the decomposing substrate. Nutritional value (as well as palatability) of the decomposing structure will also affect the time involved in complete breakdown and mineralization. Decomposition involves the following processes:

• The leaching of soluble compounds from dead organic matter
• Fragmentation
• Bacterial and fungal breakdown
• Consumption of bacterial and fungal organisms by animals
• Excretion of organic and inorganic compounds by animals
• Clustering of colloidal organic matter into larger particles

The process of death and consumption, along with the leaching of soluble nutrients from the decomposing substrate, release minerals contained in the microbial and detrital biomass. This process is known as mineralization.

Excerpted from:

Thomas C. O'Keefe, Scott R. Elliott and Robert J. Naiman, Introduction to Watershed Ecology, USEPA Watershed Academy Web, Accessed on December 2023, https://cfpub.epa.gov/watertrain/module