Publications: Research reports and publications

Spatial and temporal variation of functional indicators in Waikato Rivers

1 November, 2009
Cawthron Report 1693. Prepared for Environment Waikato.

EXECUTIVE SUMMARY

There is considerable interest in documenting current status and future trends brought about by management changes in Waikato rivers. An increasing number of studies have shown that assessing the ecological function of large rivers can be achieved using functional indicators. The applicability of functional indicators in large rivers as a measure of ecosystem health was tested in the Waikato River and in other non-wadeable rivers throughout the Waikato region.

Ecosystem metabolism (the combination of primary production and ecosystem respiration) and rates of organic matter processing have been demonstrated as effective functional indicators of ecosystem health. Ecosystem metabolism was measured at six sites within a 21 km reach of the Waikato River stretching from Hamilton Gardens downstream to Ngaruawahia in October 2008. Ecosystem metabolism was estimated using the single station open-system method at each site over a 24-hr period. Rates of gross primary production and ecosystem respiration indicated the Waikato River had mostly healthy to satisfactory ecosystem health, based on the balance of processes affecting dissolved oxygen levels. A comparison to rates measured in April 2008 suggested temporal variation in ecosystem metabolism unrelated to expected seasonal trends.

An earlier study had identified a potential downstream response in functional indicators correlated with disturbance intensity. To test this hypothesis, ecosystem metabolism was measured above and at three sites at increasing distances below a thermal discharge at Huntly power station and a sewerage treatment point discharge at Pukete in April 2009. Results were inconclusive due to high ecosystem metabolism rates above Pukete sewerage treatment plant and suppressed rates above and below the Huntly power station. This suggests a one-off measure of metabolism is insufficient to assess the effects of these point-source discharges on river function. However, organic matter processing was estimated at each site using a cotton strip assay which involved the deployment of cotton over seven days. Cotton decay rates clearly showed accelerated organic matter processing below Pukete compared to above and suppressed organic matter processing below Huntly compared to above.

Spatial variation in the ecological function of large rivers throughout the Waikato region was examined in a survey of 10 randomly selected sites, including reaches of the Waipa, Puniu, Waitoa, Tongariro, Mokau and Waikato rivers in December 2008. There was a wide range in ecosystem metabolism values for the 10 river reaches, indicative of 'healthy' to 'poor' conditions based on reference values from national and international datasets. There was no apparent relationship between rates of ecosystem metabolism and catchment land-use. However, significantly greater cotton decay rates were observed in reaches with catchments containing less than 50% native vegetation cover, compared to lower rates at sites with more than 60% native vegetation cover.

The three surveys of ecosystem function in Waikato rivers, as well as an earlier survey in April 2008, show that ecosystem metabolism can be highly spatially and temporally diverse and more temporal sampling is probably necessary to consistently relate measures to impacts. By contrast, measures of organic matter processing consistently correlate to point-source and catchment-scale impacts and may provide a simple assessment of ecosystem function in non-wadeable streams.