Report: TR 2006/39

Author: Roger Young (Cawthron Insitute), Kevin Collier (Environment Waikato)

Abstract

In order to maintain or improve river ecosystem health, tools for assessing the current ecological state of river ecosystems are needed so the causes of poor health, or the success of rehabilitation efforts, can be measured. In the past, the tools used to measure river ecosystem health have concentrated on structural aspects of the ecosystem (e.g. community composition). However, the value of incorporating measurements of ecosystem processes/functions (e.g. rates of organic matter decomposition) into regular monitoring programmes is increasingly being recognised.

Rates of organic matter decomposition and ecosystem metabolism (the combination of primary production and ecosystem respiration) appear to have promise as functional indicators of ecosystem health. These indicators were measured at five sites along a continuum of anticipated water quality impacts in the Mangaokewa Stream near Te Kuiti. Strips of cotton cloth and wooden sticks were deployed in three habitat types (edge, bottom and midwater) at each site to determine the amount of small-scale spatial variation in organic matter decay rates. Ecosystem metabolism was estimated using measurements of the natural change in dissolved oxygen concentration at each site over at least a 24 hour period. In addition, invertebrates were collected from wood and stony substrates at each site to determine any differences among sites in traditional stream health indices.

Rates of ecosystem metabolism were low upstream of Te Kuiti, but both gross primary productivity (GPP) and ecosystem respiration (ER) were higher downstream. Rates of GPP were within the range found at reference sites elsewhere. However, rates of ER at the three downstream sites exceeded the criteria that have been suggested for distinguishing between satisfactory and poor ecosystem health.

Rates of wooden stick and cotton cloth decomposition were also highest at the two downstream sites. There were also differences among habitats at individual sites, with slower decay occurring at the stream edge compared with midwater or the river bottom. This suggests that differences in habitat need to be considered when making comparisons of organic matter decomposition among contrasting sites.

Multivariate analyses indicated some differences in the invertebrate community between the upper and lower sites, but these differences were smaller than those found between different substrates (wood versus stone) at the same site. Traditional metrics such as %EPT and MCI did not differentiate between sites or indicate any pattern of cumulative ecological stress downstream.

Given the relatively low number of sites it was difficult to detect statistically significant relationships between the traditional invertebrate-based metrics and either metabolism or organic matter decomposition rates. However, there were trends indicating relationships between wood breakdown rates on the river bottom and the invertebrate metrics. Functional indicators measure a different component of the river ecosystem and relationships among the different types of indicators would not necessarily be expected.

Spatial Variation In Functional Indicators For Non-Wadeable Streams [PDF, 356 KB]