Publications: Research reports and publications

Seasonal patterns in ecosystem metabolism of rivers in the Auckland Region 2003-2009

1 May, 2010
Cawthron Report 1795. Prepared for Auckland Regional Council

EXECUTIVE SUMMARY

Auckland Regional Council (ARC) has been collecting continuous records of dissolved oxygen (DO)concentration at 13 sites around the Auckland region since 2003. These data can be used to calculate rates of ecosystem metabolism (the combination of gross primary production and ecosystem respiration) and thus provide a picture of the ecosystem health of these river systems. The sites used for this analysis were Ararimu, Hoteo, Kaipara, Kaukapakapa, Kumeu, Mahurangi, Ngakaroa, Puhinui, Rangitopuni, Vaughan, Wairoa, Waitangi and West Hoe.

The data quality control system used by the ARC generally provided raw data that was suitable for analysis. However, data from some sites were unrealistically high and needed to be adjusted before they could be used for metabolism calculation. A 5-10 day period within each season was extracted from the dataset available for each river and used to calculate daily rates of metabolism. Ecosystem metabolism was calculated successfully during all seasons for all sites, where data were available. However, there were difficulties with the calculations at West Hoe, during winter, that appear to be related to the extremely low rates of metabolism at this site.

Between 2003 and 2009, average gross primary production (GPP) rates generally were low for most sites and indicative of healthy conditions. West Hoe and Rangitopuni showed particularly low GPP rates, indicating very healthy stream systems. However, rates of GPP at Kumeu, Puhinui and Waitangi were considerably higher and indicative of satisfactory or poor ecosystem health. Rates of ecosystem respiration (ER) at Hoteo, Mahurangi, West Hoe and Rangitopuni were relatively low and indicative of healthy ecosystems. In contrast, rates of ER were high at Kumeu, Puhinui, Vaughan and Waitangi and consistently indicative of poor ecosystem health. Rates of ER at Ararimu, Kaipara, Kaukapakapa, Ngakaroa and Wairoa were in between these extremes and indicative of poor health at times and satisfactory health at other times.

West Hoe has a catchment with more than 95% native forest and is considered a reference site. The metabolism values measured at West Hoe were similar to those measured in other healthy river systems in New Zealand and overseas, which suggests that the criteria for interpreting metabolism measurements presented in Young et al. (2008) are relevant to the Auckland region. Using these criteria, Kumeu, Puhinui, Vaughan and Waitangi regularly have metabolism measurements that are indicative of poor ecosystem health, while Rangitopuni, Mahurangi, and Wairoa are relatively healthy. These results reflected broad differences in catchment land cover among these sites, however, the ranking for the remaining sites would have been difficult to predict based on land cover percentages.