An Overview of Dr. Edwards' Aquatic Invertebrate Study Analysis
By Rick Hafele
As recently reported, the DRA has just posted to its website a new report, by Dr. Patrick Edwards, that provides a detailed statistical analysis of the aquatic macroinvertebrates in the Deschutes River before and after the commencement of surface water releases from the Selective Water Withdrawal (SWW) tower at Round Butte Dam. Dr. Edwards’ report provides important confirmation that since the SWW Tower began operating, aquatic life in the lower Deschutes River (the 100 miles of river below the dams) has changed significantly for the worse.
Dr. Edwards’ report is actually a new analysis of data originally collected and analyzed for PGE by R2 Resource Consultants, as required under the Pelton Round Butte Project’s Clean Water Act certification. The R2 report and data were released to the public in April 2016. Unfortunately R2’s original analysis was flawed. As a result, the Oregon Department of Environmental Quality (ODEQ) requested that PGE have the data reanalyzed using proper methods. It has now been 19 months since that request by ODEQ, and PGE has yet to release a new analysis of the study.
To ensure that a new unbiased analysis would be completed, the DRA commissioned Dr. Edwards to reanalyze the data from the R2 study. To further ensure that the methods used by Dr. Edwards were correct and based on the best available statistical methods, the DRA had the report peer reviewed by one of the top environmental statisticians in the country.
While we invite all of our supporters to read the lower Deschutes River aquatic macroinvertebrate report by Dr. Edwards, the analysis relies on a number of complex statistical methods; unless you have a degree in statistics, it might leave you scratching your head. For that reason a less technical explanation of the analysis and its findings is provided here.
Why Aquatic Invertebrates and Algae?
You might first wonder why aquatic macroinvertebrates (this includes all aquatic insects, as well as other invertebrates like snails and worms) and algae were the only aquatic life forms sampled to assess the possible impacts of surface water withdrawal on the ecology of the lower Deschutes River. Aren’t trout, steelhead, and salmon much more important as a recreational resource and commercial commodity? Certainly, fish outweigh invertebrates and algae in recreational and economic importance, but in terms of ecosystem health, if the organisms at the bottom of the food chain aren’t healthy and sustainable then the rest of the species further up the food chain will suffer.
There are several reasons why these lower food chain communities, especially aquatic invertebrates, are often closely examined in stream health studies.
- Aquatic invertebrates can be sampled more effectively and at less cost than fish. This is particularly true in a big river like the lower Deschutes. This doesn’t mean that fish studies in the lower Deschutes aren’t possible or shouldn’t be done but, to get a relatively quick and accurate assessment of possible impacts to the aquatic ecosystem, aquatic invertebrates are a good choice.
- Because the life cycle of aquatic invertebrates is much shorter than fish (one year or less for most invertebrates compared to four to six years for most salmonids) they will show a response to environmental changes much faster than will fish. This is critical if one wants to identify ecosystem problems as soon as possible.
- There is a long history within the study of stream ecology of sampling aquatic invertebrate populations to assess stream health and function. This means there are well-established methods for sampling and analyzing the data, and for interpreting the results. For example, when certain invertebrate populations thrive while others are lost or diminished, prior experience on other rivers can help us understand what is happening on the lower Deschutes.
- Last, the number of species of aquatic invertebrates found in Western rivers and streams is much greater than the diversity of fish, giving researchers a broader, more robust community of organisms to study. For example, invertebrate studies often collect more than 100 different species from a single Western stream, compared to 3-6 species of fish. In addition, the sensitivity of these different invertebrates to altered water quality and habitat conditions have been well documented for a wide range of species, and the sensitivity of different species to changes in water quality varies over a wide range. As a result, changes in the species composition of invertebrates provide a sensitive indicator of impacts to the biological health of streams and rivers. For example, decades of studies have shown that stoneflies are more sensitive to poor water quality than most other species. Therefore, a decline in their diversity or abundance is one of the first signs of declining stream health.
Statistical Methods Used
The purpose of Dr. Edwards’ study was to determine if the aquatic invertebrate community sampled after surface withdrawal began had changed in a statistically significant way from the community present before surface withdrawal. To make this determination, Dr. Edwards used three statistical methods:
- Multivariate ordinations
- A measure of species diversity
- A measure of species pollution tolerance
Multivariate statistics is a powerful tool that you won’t find discussed in Statistics 101. This powerful and complex field of statistical analysis requires considerable experience to use and understand. Multivariate statistical methods like Non-metric Multi Dimensional Scaling (NMDS) are commonly used today partly because modern computing power makes it possible.
Basically, NMDS takes all 100+ invertebrate taxa from each sample and plots the relative abundance of each taxon in each sample in multidimensional space, and then compresses the multiple dimensions into a two-dimensional graph. The distance between dots on the plot indicate their degree of similarity; dots close together indicates a similar invertebrate community between samples, while dots farther apart indicates the communities present were different. Whether the distance between two groups of dots is statistically significant (meaning that the difference noted is very likely the result of actual differences and not due to random chance alone) is determined by performing other statistical tests.
The results of this analysis comparing the pre-tower to post-tower samples from the lower Deschutes River, showed that a statistically significant change occurred to the invertebrate community from the pre-tower to post-tower periods. What kind of change occurred is addressed with the other two analyses discussed below.
Measure of species diversity:
One of the most common measures of ecological or biological health is the diversity of species present. Healthy ecosystems are diverse ecosystems. In stream studies, healthier stream conditions are indicated by invertebrate communities with more species that are sensitive to poor water quality (higher temperature, lower dissolved oxygen or nutrient enrichment), relative to the number of species that are more tolerant of poor stream conditions. Mayflies, stoneflies, and caddisflies are the three groups of aquatic invertebrates with the most sensitive species to poor water quality. A decline in these sensitive species relative to species known to be more tolerant of degraded water is a sign that water quality is becoming degraded and constraining aquatic invertebrate populations. The metric EPTr refers to the percent of species of mayflies (E), stoneflies (P) and caddisflies (T) relative to the number of other species in the sample. In this study the metric EPTr was used to assess changes in the diversity of the sensitive taxa. The results show that at sites in the lower Deschutes River, EPTr declined in post-tower samples from pre-tower samples in both the spring and fall, and that the decline was statistically significant in the spring samples. A similar statistically significant decline was not observed at the three sites above the Round-Butte Dam Complex.
Measure of pollution tolerance:
As mentioned above, different species of aquatic invertebrates have different tolerance levels to water pollution. Years of researching the sensitivity of individual taxa to water quality conditions has produced a set of “tolerance” scores for each taxa. The metric used in this study is called RICHTOL, which calculates the mean tolerance score of all taxa present in a sample. Tolerance scores for individual taxa range from 0 to 10, with lower scores indicating more sensitivity to polluted water—species with these lower scores are more likely to decline in abundance as water quality declines. This analysis shows a statistically significant increase in the RICHTOL score in post-tower samples compared to pre-tower samples below the dam complex during both the spring and fall sample periods. An increase of this score indicates an increase in taxa present with greater tolerance to poor water quality, strongly suggesting that water quality has declined and this decline is having a negative affect on the aquatic invertebrate community. Again the sites above the dam complex did not show a similar significant increase in tolerant taxa.
In summary, here are the principal findings from Dr. Edwards’ statistical analysis:
- A multivariate statistical analysis, comparing the complete invertebrate community in the lower Deschutes River from before tower operations to after tower operations, found that a statistically significant change in the community occurred.
- Comparing pre-tower samples to post-tower samples showed that a decline in the percent of sensitive species of mayflies, stoneflies, and caddisflies occurred at sites in the lower Deschutes River.
- A comparison of pre-tower to post-tower samples also found that taxa tolerant to poor water quality conditions increased significantly at sites in the lower Deschutes River below the dams, but no significant increase occurred at sites above the dams.
These results confirm: 1) a significant change has occurred to the macroinvertebrate community in the lower Deschutes River after tower operations and surface water releases began, and 2) a significant decline in pollution sensitive species (mayflies, stoneflies and caddisflies) and a significant increase in pollution tolerant species (primarily worms and snails) has occurred in the lower Deschutes River following surface water releases at the SWW tower.
Decades of stream studies have documented similar impacts due to nutrient enrichment and the resulting changes in water chemistry and algal communities. For example, as long ago as the early 1970s stream ecologists understood that large dams and reservoirs can impact waters downstream, as shown in the following quote from the seminal book on stream ecology, The Ecology of Running Waters, by H.B.N. Hynes:
The great photosynthetic activity in large impoundments has marked effects upon the chemistry of the water, raising pH and oxygen content and reducing the hardness of the water. The influence of a large dam is therefore profound and it extends a long way downstream.
Anyone who has spent time on the lower Deschutes River after the SWW tower began operating knows there have been negative changes to water quality and the aquatic community. For example, if you have a house on the river, the simple fact that you no longer have to close your door at night to keep the bugs out when a porch light is on is a clear signal that something isn’t right. Observant anglers have seen crane fly numbers fall from very abundant to nearly non-existent. So why worry about statistics? Unfortunately those who might disagree with your porch light results or your onstream information on insect life may argue that your observations are anecdotal and don’t “prove” there is a biological impact from SWW operation. Such “proof” can be elusive, which is where the use of statistical analysis becomes important. The use of advanced statistical methods sets a standard for the level of confidence that the observed changes are real and not due to random variation.
Dr. Edwards’ analysis confirms what river users have been observing since the SWW tower began operating - the health of the river has declined. Fortunately, we know there is a simple way to reverse this decline in the river’s biological health: a significant increase in the amount of cooler, cleaner water discharged from the bottom of Lake Billy Chinook into the lower river.
For an introduction to Dr. Edwards' report, click here.
To read the full report, click here:
Deschutes River Alliance: Cooler, cleaner H2O for the lower Deschutes River.
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