Science of the Seasons: Riparian areas of real importance

By Dr. David Wartinbee, for the Redoubt Reporter

Photo courtesy of Dr. David Wartinbee. Natural vegetation surrounding Soldotna Creek secures the riparian area as well as provides nutrients to life in the water — namely, insects, which then feed young fish.

Photos courtesy of Dr. David Wartinbee. Natural vegetation surrounding Soldotna Creek secures the riparian area as well as provides nutrients to life in the water — namely, insects, which then feed young fish.

The transitional areas between a water body and the surrounding upland community are known as the riparian zones. Riparian zones are a transition of physical conditions, like moisture levels, light, temperatures and canopy.

As well, there is a distinct change of one plant community into another. Along with the changing vegetation, we see a change in the animal and microbial communities, too. The distance required for this change is highly variable depending on slope, geology and hydrology of the area. In constrained areas, meaning with steep-sloped edges, the riparian zone may be fairly small. However, in many cases, these riparian zones can be hundreds of feet wide.

Riparian zones are often species rich. They can contain more species of plants and animals than the starting aquatic habitats or the upland communities. The reason for this is that the transitional zones have plants or animals from both communities. It is well documented that plants in riparian zones grow more rapidly than similar plants in other areas. Some trees in riparian zones are known to grow at twice the rate of their siblings in upland areas.

Riparian zones play major roles in the biogeochemistry of the nearby water body. It is within the riparian areas that plant material and other debris gets trapped and stopped before entering the water body. Leaves, seeds, twigs, limbs and the like that fall to the ground are rapidly processed here. The transformation is from recognizable biotic debris to inorganic elements that are now usable by plants. These transformations are performed by a large suite of insects, worms and, most importantly, bacteria and fungi.

Leaf contributions to streams, like Quartz Creek, seen here, follow a natural schedule. The nutrients enter the stream in the fall and are utilized by insects and bacteria during the winter.

Leaf contributions to streams, like Quartz Creek, seen here, follow a natural schedule. The nutrients enter the stream in the fall and are utilized by insects and bacteria during the winter.

Research has shown that 67 percent to 89 percent of “upslope litter” is retained and processed in the riparian zone. The remaining elements, like nitrogen, phosphorous and potassium, are now in a form that can be utilized by riparian zone plants. (Notice the numbers on your bag of garden fertilizer — they stand for the percentage of nitrogen, phosphorous and potassium contained within.) This source of nutrients explains why riparian zone plants grow so rapidly.

A major benefit of the riparian zone is to remove incoming pollutants or extra nutrients that would otherwise enter the stream, river or lake. It has been known for years that removing or altering riparian zones results in decreased efficiency in the biogeochemical cycles.

Research has further shown that changes within the riparian zones will decrease the growth rates of community plants, as well as cause a marked decrease in the diversity within the community.

When areas are logged or deforested, it has been found that more debris and nutrients are able to wash into the aquatic habitat. After the initial removal of the riparian communities, additional nutrients and mineral silt continue to wash into the aquatic habitat for years to come. Recognizing this potential detriment to the aquatic habitat, minimal buffer areas are now mandated around all water bodies during logging operations.

Riparian zone plants play an important role in the aquatic habitat nearby. Leaves that enter a lake or stream become food sources for a variety of insects. Once the leaves are colonized by bacteria and fungi, they are food sources for many insects.

The insects, like crane fly larvae or some species of stonefly nymphs, will chop up leaves into very small particles. These particles continue to be colonized by fungi and bacteria, and then become food sources for a great many other aquatic invertebrates. Additionally, some larger leaf pieces and small sticks might be used for case construction by some of the resident caddisflies.

An important note needs to be made here about the timing and the form of input to our water bodies. When raw, elemental nutrients are allowed to enter a water body on a continual basis, such as from leaking septic systems or fertilizer runoff, it promotes growth of free-floating algae and other aquatic plants.

This radically changes the aquatic habitat and its overall productivity. Housing developments around once-clear lakes have had to wrestle with the green-colored water and massive emergences of vegetation clogging the lake.

The changes are easily traceable back to human alterations in the riparian zones. It seems that, while we love to live near water bodies, we tend to love them to death.

When nutrients enter the aquatic system as leaf fall, there is a natural seasonal timing of the input and the riparian community is prepared to process the leaves efficiently. The insects, fungi and bacteria are already prepared to process the leaves over the winter months. If the leaves were added at another time of year the biogeochemical processing community would not be able to handle the input. Any major increase in nutrient input that strains biogeochemical cycling abilities results in profound detriment to the system, ecologically and aesthetically.

In areas of the world like Alaska, where there are anadromous fish using the water bodies for periods of their life cycles, riparian habitats play another important role. The riparian vegetation provides seasonally important areas of shelter for growing fish. These nearshore areas are sought out by salmon fry, since their predators lurk in the deeper waters.

The nearby vegetation provides shade and a place for young fish to hide as well as a substrate for their predominant food — aquatic insects. Shaded stream and lake edges are nursery grounds for young anadromous fish, worthy of being protected.

Aquatic ecologist Robert Naiman has commented that, “Societies and cultures do not maintain what they do not value.” We now know that riparian habitats are critical for the effective functioning of all aquatic habitats and are certainly of great value.

Locally, as in the Kenai River watershed, riparian areas are where young salmon call home for the first couple years before heading to the oceans. These transitional areas between aquatic habitats and upland areas are keys to the long-term survival of the water bodies that we already consider so valuable.

For a thorough discussion on the role or riparian zones, consult “Riparia, Ecology, Conservation, and Management of Streamside Communities,” by Robert Naimen, Henri Decamps and Michael McCain, Elsevier Academic Press, 2005.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus.

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Filed under ecology, science of the seasons

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