Using Zoobenthic Organisms as a measure of Stream Habitat in Mill Creek
By: Kristin Knight
Mill Creek is a valuable resource to the Walla Walla basin. In order to protect the ecosystem, we need to understand the health of the stream. One method of measuring stream health is by assessing zoobenthic organisms, which are visible animals that live on the bottom of aquatic environments. Such organisms are primarily insect larvae and aquatic worms but can include an entire array of life. Benthic communities are ideal for studying aquatic ecosystems because they are mostly sedentary and have long life cycles. The response of benthic life to different types of pollution has also been well documented (Resh et al. 1996).
An important tool to use with benthic monitoring is the river continuum concept (RCC), put forth by Vannote et al. (1980). The RCC attempts to create a single framework to describe a waterway from the source to mouth, including terrestrial changes (Allan 1995). For example, a low-order stream near the headwaters should have a large input of coarse particulate organic matter (CPOM). Most of the benthic organisms found in low-order streams are classified as shredders, scrapers and collectors that feed on terrestrial debris. The RCC predicts that the stream should then shift to a dependence on fine particulate organic matter (FPOM) as it travels downstream. Downstream communities tend to be dominated by organisms classified as collectors and predators.
The objective of this study was to compare the physical and chemical parameters and the macrobenthic communities at different elevational sites on Mill Creek. The stream is a changing continuum throughout its course. At the upstream site, abundant riparian vegetation and a strong velocity characterize the stream. Downstream, houses located next to the stream reduce the riparian vegetation. As Mill Creek nears Walla Walla, it passes through a flood-control dam and then a concrete channel with weirs. Increased stream width and minimal riparian vegetation are a result of the rip rap lined banks. Given these changes, the main hypothesis of this study was that benthic diversity would change according to the RCC from the upper reaches of Mill Creek to the lower reaches as it nears the city of Walla Walla.
The four sites beginning upstream were: 1) Whitman College’s Johnston Wilderness Campus, directly upstream of the bridge (JWC); 2) 0.6 km west of the Washington/ Oregon state line above Kooskooskie, WA, designated Upper Kooskooskie (UK); 3) 4.5 km west of the Washington/Oregon state line below Kooskooskie, WA, designated Lower Kooskooskie (LK) and 4) Walla Walla Community College, Walla Walla, Walla Walla County, WA, directly adjacent to the sports complex parking area (WWCC). All sampling occurred during the fall and winter of 2000. Mill Creek originates in the Blue Mountains and reaches an elevation of 680 m at the Johnston Wilderness Campus and descends to 370 m at the Walla Walla Community College site. The riparian habitat along this section of Mill Creek is dominated by Alder (Alnus incana), Douglas Fir (Pseudotsuga menziesii), and Grand Fir (Abies grandis). The four sites were chosen to be as equally free of shade as possible.
Data collected from the benthic sites support the hypothesis that diversity should decrease as more human influence takes place. The JWC site (Figure 1) had the highest diversity of the four sites using the Shannon Diversity Index and is classified as a stream of excellent water quality using the Family Biotic Index (FBI). This site had the greatest amount of Plecoptera (stone flies), which rate excellent on water quality values (Resh et al. 1996). The Upper Kooskooskie site (Figure 2) varied from the Johnston Wilderness Campus, but followed the changes predicted by the RCC; the two sites had a similarity index of 83% (Sorenson).
The Lower Kooskooskie (LK) site had fewer Plecoptera than JWC and a reduction in this taxon has been shown to be a sign of degradation at a site (Fore 1998). This site contained fewer families of trichopterans (caddiesflies), indicating that the community supported less diversity within the order. The LK site also contains larger quantities of oligochaetes and dipterans (flies) than found in the upper two sites. These soft-bodied organisms are more tolerant of pollutants and fluctuations in oxygen content and temperature. The invertebrates at LK also contained a greater percentage of collectors, suggesting more dependence on in-stream production, which follows the RCC’s predictions. The FBI classified LK as a site of very good water quality.
The Walla Walla Community College (WWCC) site is dominated by collectors, predators and scrapers and had the lowest diversity index of all four sites. The site is in accordance with the river continuum concept (RCC), as the stream is more dependent on FPOM. However, the stream changed more drastically than expected in the RCC, suggesting a strong correlation between the channelization of Mill Creek and a low diversity of organisms. Also, the removal of riparian vegetation has had a significant impact on types of organisms living at the WWCC site. The organisms found here represent only a few taxa in large numbers suggesting poor water quality (Fore 1998). This is largely due to the presence of soft-bodied individuals such as Oligochaetes (aquatic worms) and Sphaeridae (fingernail clams), both indicators of very poor water quality, and chironmids (midges), representatives of poor water quality (Resh et al. 1996). In the case of the WWCC site, modifications of riparian zone has altered the predictions of the RCC, resulting in significant changes in the composition of benthic life.
The chemical analysis done on the four sites was inconclusive in providing any strong trends in non-point pollutants in the stream. However, a large fluctuation in levels of nitrates occurred at the JWC and LK sites from September to November, suggesting that substantial changes in the concentrations of inorganic nutrients do occur. Perrin and Richardson (1997) found an increase in N and P caused a rise in the number of chironomids present. This could explain the increased amount of chironomids at the LK site as compared to the upper two sites. The type of organisms found at LK and WWCC suggests an increased nutrient load but further chemical analysis is needed to confirm this finding.
The physical dimensions of the stream were altered as the water moved downstream. The stream doubled in width and tripled in depth from JWC to WWCC. The stream velocity at the WWCC site was reduced to 8 cm/s, significantly slower stream velocity than at the other sites. These drastic changes in physical conditions correlate to the immense differences in benthic communities. The JWC and the WWCC have a similarity index of 25%, indicating that these two benthic communities are not supportive of each other (Resh et al. 1996). The site directly above WWCC, the LK site, also has a similarity index of 25% with WWCC. These differences indicate that physical alterations, such as channelization and loss of riparian vegetation, can significantly influence benthic communities.
After recognizing the problems in the Mill Creek watershed, the next step is to consider actions to improve such conditions. One possibility is restoring the stream to its natural conditions. However, the science of stream restoration is still an uncertain one. Different individuals are still hypothesizing about what the most expedient path to stream health is. As seen in Walla Walla, many rivers and streams have been altered in such a way that it would take drastic efforts to return the ecosystem to its former state. For example, one suggestion is to increase the allowances of floods within the watershed. This should raise the nutrient levels in riparian zones as well as bring in terrestrial food sources into the stream (Pelley 2000). However, stream restoration also faces economic and social challenges. People do not like to see existing structures such as dams or channelization simply destroyed. The potential for increased flooding could also endanger human lives, property and economic well-being. It is also uncertain whether these changes could truly restore the stream to natural conditions. More research is needed to better understand the successful methods for stream restoration.
The most feasible and readily available method for improvement is the prevention of continued damage. More work is needed in the area of agricultural run-off and the possible contaminants that pollute the Mill Creek watershed. Another method for improvement would be to work on improving overall riparian vegetation along the stream with native species. This could include areas as small as residential properties or could incorporate a whole stretch of land along the stream. Increased riparian vegetation should contribute shade and detritus to Mill Creek as well as provide the protective properties characteristic of riparian zones. Individuals can also attempt to have less of an impact on the stream by building houses away from the stream and following environmental regulations. These changes will not transform the stream into its natural state, but should prevent further degradation of the stream. Public land and water agencies also need to consider how their actions may alter the stream. Species such as the threatened Bull Trout require that the habitat be preserved for the health of the fish population. Habitat protection should not only help the bull trout, but other organisms in the stream as well. Overall, an increased consciousness of the sensitivity of Mill Creek to environmental change is needed in order to preserve the habitat.
Further study could include more investigations in the local area. A comparative study could be done with a stream of similar size and elevational change in the Blue Mountains. Another study could investigate Mill Creek below the city of Walla Walla and analyze the benthic organisms found there in comparison to those in this study. More extensive studies could be completed being more precise with seasonal changes as well. New information about local non-point pollutants would also aid in assessing the results of the zoobenthic study on Mill Creek.