Publications: Research reports and publications

Dissolved oxygen changes and ecosystem metabolism in the Motupipi catchment - Implications for river health assessment

1 November, 2006
Cawthron Report 1235. Prepared for Tasman District Council.

EXECUTIVE SUMMARY

A recent review of surface water quality and stream health throughout the Tasman District identified some concerns in the Motupipi Catchment. In an effort to determine the causes of these concerns, the Tasman District Council has been undertaking a series of further studies. One of these studies has included detailed measurements of dissolved oxygen fluctuations at five sites throughout the catchment in January/February 2006, and at two further sites in the neighbouring Te Kakau Stream. Dissolved oxygen is an important factor controlling the types of organisms that will be found at a site. Sensitive species will die if dissolved oxygen concentrations become too low. Dissolved oxygen concentrations can also be used to calculate the rates of plant growth and oxygen uptake that occur in rivers. Oxygen is produced during the daytime by algae and other aquatic plants as they photosynthesise, while oxygen is used up by all the organisms living in the river as they respire. These ecological processes can be used as an indicator of river health.

Daily changes in dissolved oxygen saturation at each site had characteristic patterns with some very large daily fluctuations at the Motupipi up-stream (u/s) Watercress, Motupipi at (@) Reilly's and Powell @ Reilly's sites. Powell @ Glenview was the only site where oxygen saturation was consistently high enough to ensure that sensitive biota would not be detrimentally affected. The pattern of oxygen saturation at the Te Kakau @ Haldane site was somewhat different to the other sites with complete anoxia (i.e. zero oxygen) occurring for part of the day. Rates of plant production (GPP) ranged from 0.2 gO2/m2/day at McConnell to 28 gO2/m2/day at Motupipi u/s Watercress. Rates of plant production were generally low at McConnell and Powell @ Glenview and indicative of healthy conditions. However, rates of plant production at Motupipi @ Reilly's, Motupipi u/s Watercress and Te Kakau @ Feary were considerably higher and indicative of poor ecosystem health.

Rates of oxygen uptake (ER) ranged from 1.8 gO2/m2/day at McConnell to 78 gO2/m2/day at Te Kakau @ Feary. Rates of oxygen uptake at McConnell, Powell @ Glenview and Powell @ Reilly's were indicative of healthy or satisfactory ecosystem health. Whereas, rates of oxygen uptake at Motupipi @ Reilly's, Motupipi u/s Watercress and Te Kakau @ Feary were among the highest recorded anywhere and indicative of poor ecosystem health.

The poor stream health that has been recorded previously in the Motupipi Catchment is likely related to the low concentrations of dissolved oxygen. However, other factors such as water temperature, pH and sediment may also be responsible for the lack of sensitive indicator species present in the catchment. The main reason for the low oxygen concentrations is the high rates of oxygen uptake and plant production that were observed, although inputs of groundwater containing low concentrations of dissolved oxygen and the low potential for oxygen exchange through the river surface are also contributing to this problem.

Plant production and oxygen uptake are driven by abundant nutrients, stable flows and relatively clear water allowing ample sunlight to reach the river bed. Given the complex hydrology of the Motupipi Catchment it is difficult to determine the source(s) of nutrients and faecal bacteria that are getting into the system, although several developing methods (stable isotope analyses and microbial source tracking) might be helpful.

The water quality problems currently faced in the Motupipi catchment are probably the legacy of activities in the catchment over many years. Initiatives involving the restriction of stock access to waterways, riparian plantings, and more careful nutrient management would be expected to improve stream health in the catchment. However, any improvements may take a considerable amount of time.

Further information on the sources of water in different parts of the catchment and the relative importance of 'old' nutrients from the aquifer versus 'recent' inputs from the land would be helpful for targeting restoration efforts.