Publications: Research reports and publications

A trial of wood decomposition rates as an ecological assessment tool in large rivers

1 August, 2007
Cawthron Report 1339. Prepared for West Coast Regional Council.

EXECUTIVE SUMMARY

Traditional approaches to assessing river ecosystem health have tended to focus on structural components of the ecosystem such as the abundance and composition of invertebrate and periphyton communities. However, the value of incorporating measurements of ecosystem function (i.e. the ratesof certain ecological processes) into regular monitoring programmes is increasingly being recognised. One of the potential advantages of using functional measures is that they allow some flexibility in the types of habitats that can be assessed and may be helpful in situations where habitat differences between control and impact sites make interpretations of invertebrate community data problematic. This report describes the results of a test/demonstration of the use of wood decomposition rates as a functional indicator of river ecosystem health in the lower reaches of the Hokitika River in the vicinity of a dairy effluent discharge. Monitoring of invertebrate community composition and periphyton growth is regularly conducted above and below this discharge. However, the results of these monitoring efforts are sometimes difficult to interpret due to tidal fluctuations influencing the river just downstream of the discharge, thus making it hard to separate any effects of the discharge from effects associated with tidal fluctuations.

Replicate sets of birchwood sticks were deployed at one site upstream of the dairy factory and two sites downstream of the dairy factory for three months during off-season and production season periods. A temperature logger was also deployed throughout the study to allow decay rates to be standardised between the two deployment periods. Any differences in decomposition rates among upstream and downstream sites, or in the pattern of decay rates at the sites between production season and off-season could be associated with an effect of the milk factory discharge.

Only two sets of sticks out of 24 were lost during the three months deployment periods, which demonstrates that with careful site selection and deployment it is possible to use this technique in large rivers with variable flow regimes. During the off-season, wood decay rates were similar at all the sites. However, during the production season wood decay rates at the site immediately downstream of the discharge were significantly lower than at the other two sites. When considering all the data together after temperature compensation, however, there was no interaction between site and season which suggests that inherent differences among sites were larger than any effect occurring during the production season at the site just downstream of the discharge. Organic pollution generally increases microbial activity and was expected to increase wood decay rates. Therefore, the pattern of decay rates among sites that was observed in the production season was the opposite of what was expected. The lower decay rates at the site downstream of the discharge during the production season were probably related to the sticks being buried by deposited sediment during the deployment period rather than a direct effect of the discharge.

It appears that wood decomposition is affected by site-specific habitat factors, such as sediment deposition, and so doesn't overcome the issues that are faced by invertebrate community analysis. Nevertheless, this method does provide insight into aspects of river ecosystem health that are not considered by invertebrate sampling alone. Wood decomposition should be considered as a tool alongside others for assessing river ecosystem health, especially if effects on microbial community activity are suspected.