Report: TR 2008/32

Author: Joanne Clapcott,  Roger Young (Cawthron Institute)

Abstract

The utility of different ecological monitoring approaches for assessing large river health has not been tested in New Zealand. This issue has particular relevance to the Waikato River where there is considerable interest in documenting current status and future trends brought about by management changes. Additionally, there is very limited information on the ecological function of large rivers, particularly with respect to sources of productivity. To address these issues the applicability of measuring functional indicators in large rivers as a measure of ecosystem health was tested in the Waikato River.

Ecosystem metabolism (the combination of primary production and ecosystem respiration) and rates of organic matter processing have been demonstrated as good functional indicators of ecosystem health. These indicators were measured at six sites within a 21 km reach of the Waikato River stretching from Hamilton Gardens downstream to Ngaruawahia. Site locations were chosen to represent an increasing gradient of catchment and reach pressures with the upper three sites close to urban river confluences and the lower three sites close to industrial discharges. Ecosystem metabolism was estimated using the single station open system method at nearshore and farshore locations at each site over two days. Similarly, organic matter processing was estimated at two locations at each site using a cotton strip assay which involved the deployment of cotton over seven days.

Both functional indicators showed a downstream response to disturbance. Gross primary productivity and respiration appeared to decrease downstream with consistent P/R ratios demonstrating the heterotrophic status of this reach of the Waikato River. Whilst there was no difference between nearshore and farshore locations, metabolic estimates were higher at sites closer to a point source impact. These results indicate that a single deployment per site is sufficient to characterise metabolism and suggest that, as long as it is not immediately downstream of a point source impact, a representative assessment of ecosystem health will be gained. Conversely, the effect of a point source impact may be readily assessed using this methodology. Organic matter processing increased downstream and concurrently was significantly higher at sites subject to industrial impacts compared to urban impacts. These results demonstrate the applicability of cotton strip assays to indicate potentially catchment, reach and local-scale disturbances.

In this survey, the functional assessment of the Waikato River was ‘healthy’ to ‘satisfactory’ based on reference values derived for smaller systems. However, these values need to be validated for larger systems. Temporal sampling would contribute to a better understanding of the functional dynamics of this reach of the Waikato and would lend greater confidence to ecosystem health assessments. The detectable response of functional indicators to small gradients in catchment change and larger gradients in point source impacts in the Waikato River suggest that these measures are likely to be good tools for assessing the ecosystem health of non-wadeable rivers.

Spatial Variation of Function Indicators in Waikato River [PDF, 741 KB]