Nutrient Cycling in Forest and Riparian Ecosystems
Every organism requires the elements carbon, hydrogen, oxygen, nitrogen, and sulfur (C,
H, O, N, P, S), as well as other micronutrients. There is a finite availability of each of
these elements, particularly on a local scale. Therefore, the functioning of biota depends
on the flow or cycling of these necessary elements. Nutrient cycles have been wellstudied and characterized to help understand how ecosystems function across global
biotic environmental variation. Ecosystems include the community of interacting
organisms and their environment (abiotic, chemical, and geophysical characteristics).
The unique properties of each ecosystem control the flux of nutrients available to its
members and the nutrients’ potential for cycling and nutrient reuse.
Forests ecosystems are important in two main respects: 1) effects on the organisms within
and surrounding them (providing food, habitat, protection, etc), and 2) effects on the
transformation and movement of energy and elements (known as biogeochemistry).
Biogeochemistry can further be broken down into two main types of processes, those that
use and/or modify the atmosphere and those that use and/or modify through-flowing
water. The vegetation of the forest can contribute to cleansing the air of dust and
pollutants. Furthermore, vegetation also reduces surface run-off, erosion, and flooding,
while simultaneously limiting the movement of nutrients. These processes are
particularly evident in riparian habitats or areas of vegetation in close proximity to
above-ground flowing water such as streams or rivers.
Deforestation has removed or damaged the ability of forests to buffer the effects of
precipitation. Furthermore, excessive use of pesticides and fertilizers near streams has
major effects on the downstream communities by increasing nutrient concentrations in
these habitats. In urban areas, these two effects are often compounded, making the
effects even more pronounced downstream. The process of increasing nutrient
concentrations in aquatic habitats is called eutrophication.
Eutrophic waters have a large supply of nutrients and generally have high
concentrations of nitrogen and phosphorus stimulating high productivity levels of
plankton. The waters of such aquatic systems are usually murky, and lakes and coastal
marine systems may be oxygen-depleted.
By contrast, oligotrophic waters are poorly fed by nitrogen and phosphorus and have
low concentrations of these elements. Therefore, there is low production of organic
matter by photosynthesis in such waters.
The process of eutrophication is the increase in productivity resulting from increased
input of nutrients or organic matter into aquatic systems. Natural eutrophication occurs
as aquatic systems fill in with organic matter; it is distinct from anthropogenic
eutrophication, which is caused by human intervention. The latter is characteristic of
aquatic systems that have been artificially enriched by excess nutrients and organic
matter from sewage, agriculture, and industry. Eutrophication of water systems results in
the competitive dominance for some species, typically resulting in a reduction in
biological diversity.
Today’s lab will focus on understanding how riparian habitat can buffer nutrient
availability in a pristine setting. Secondarily, we will determine how human influences
(urbanization) have affected downstream communities.
Gills Creek is a small stream that drains a small watershed that starts on Fort
Jackson and includes a relatively pristine riparian buffer. Under these conditions, the
ecosystem surrounding the stream should minimize the amount of nutrients transported
through the surface runoff into the stream. In contrast, lower portions of the watershed,
which travel through Columbia before exiting on the other side of the city, have little
riparian vegetation left. Surface runoff from parking lots, roads, and lawns flows freely
into the stream, thereby providing little buffering action to limit nutrient transport.
We will be measuring the concentration of nitrates and phosphates along three
points of the stream: above, below, and about midway between the two sites, at Maxcy
Gregg Park. Your TA has collected water samples from above and below the city, but
you will collect the sample from the park. While collecting your sample, make notes
about the condition of the land adjacent to the stream, the stream bank, and streambed, as
well as notes about the water itself. Be sure to compare what you see to what you know
about the upper and lower portions of the creek, and how that should relate to the
buffering potential of the creek. By comparing the nutrient concentrations of each of the
three samples, we will determine what the natural effects of riparian habitats are, as well
as how urbanization affects the biota of downstream communities.
We will be using water quality test kits made by Hach Company. The directions
for their use are included in the kits. Be sure to read and follow them carefully. Each lab
section will make 6 measurements of nitrate and phosphorous levels for each site. We
will then pool the class data to create figures and run statistical analyses on the results.
References:
Kent, R. and Belitz, K. 2004. Concentrations of dissolved solids and nutrients in water
sources and selected streams of the Santa Anna Basin, California October 1998September 2001: U.S. Geological Survey. Water-Resources Investigations Report
03-4326
Likens, G.E. 2001. Biogeochemistry, the watershed approach: Some uses and limitations.
Marine and Freshwater Research 52: 5-12
Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S., and Johnson, N.M. 1977.
Biogeochemistry of a Forested Ecosystem. Springer-Verlag. New York, NY.
Perry, D.A. 1994. Forest Ecosystems. John Hopkins University Press. Baltimore, MD.