Manganui River

The Manganui is a major tributary into the Wairoa River, flowing from the western fringes of the Marertu forest to join the Wairoa just east of Dargaville. The river runs through extremely low gradient predominantly agricultural pastures, with some forestry blocks scattered through the catchment. The underlying geology at the site is soft sediments.

The 2005-2006 results for the Manganui River are summarised in the table below including the median, range and percentage of sampling occasions that comply with relevant guidelines for the 12 sampling occasions. Medians shown in red are outside the recommended guidelines.

Parameter Median Range % comply with guideline
Temperature (deg. cel.) 16.9 10.7 - 23.8
Dissolved oxygen (mg/L) 8.1 6.2 - 9.9 100
Dissolved oxygen (% Sat.) 81.5 57.6 - 109.2 0
Conductivity (mSm) 20.6 15.3 - 22.4
Water clarity (m) 0.83 0.33 - 1.26 82
Turbidity (NTU) 5.7 3.4 - 49 50
E. coli (n/100mL) 164 40 - 2613 42
Dissolved reactive phosphorus (mg/L) 0.058 0.044 - 0.082 0
Total phosphorus (mg/L) 0.102 0.061 - 0.167 0
Ammoniacal nitrogen (mg/L) 0.005 0.005 - 0.07 83
Total nitrogen (mg/L) 0.646 0.301 - 1.16 50
pH 7.2 6.4 - 8.7 50


The Manganui River has been heavily impacted on and in turn has relatively poor water quality. The medians of six parameters did not meet their respective guideline values. Similarly to last year, none of the dissolved oxygen saturation results for 2005-2006 were within the optimum range for the protection of aquatic ecosystems. These low dissolved oxygen results are not surprising with the sluggish nature of the extremely low gradient Manganui River.

High levels of all nutrients but particularly dissolved reactive and total phosphorus suggest that agricultural run-off is a problem in the Manganui catchment. All DRP and TP results were above their respective guideline trigger values. Elevated phosphorous in particular is a sign that a significant amount of the fertilisers applied to paddocks are not being absorbed, but washing into streams and rivers instead. Better riparian management and fertiliser application practices would help prevent such run-off.

With five years data for the Manganui River site, there is now enough data for trend analysis. The only significant trend with five years of data is a deceasing trend in nitrite/nitrate nitrogen (NNN) levels at 0.032 mg/L per year. This is a positive trend, as the NNN levels usually contribute about half of the total nitrogen at this site.