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Trends in water quality

 

Long-term trend analysis was carried out using a Seasonal Kendall test, which eliminates any seasonal influences on the results. The table below shows water quality trends for 15 of Northland's river network sites, which are those that have five or more year's data. There is not yet enough data for Awanui at FNDC watertake, Kaihu and Waipoua Rivers, as well the three new sites added in 2005-06 (two Waiarohia sites and Mangahahuru at Main Road). The start of dataset for trend analysis is shown in brackets in the table below, and the end is June 2006 or May 2006 for the four NIWA sites.

Only significant trends are shown, which are those with at least a 95% confidence level i.e. 95% likelihood that the trend is "real". It must be highlighted that 5 years data is a minimum for carrying out long term trend analysis and that these trends could change when the analysis is carried out again next year, especially for the sites where there is only just five years data.

In the table an up arrow indicates an increasing trend, a down arrow a decreasing trend and a green arrow a desirable trend, while a red arrow indicates an undesirable trend. A blue arrow has been used where it is not yet known whether this is an improvement or deterioration.

There were no significant trends at a 95% confidence level for water temperature at any of the sites. Many of the sites could not have trend analysis done on ammoniacal nitrogen as too many results were below detection limit, shown as NA in the table below. Some sites had insufficient data for that parameter to carry out trend analysis, shown as ID in the table. Note: the decreasing trend marked with an asterisk for the four NIWA sites is for nitrate (NO3) only, not NNN.

Site Cond DO% DO pH Clarity Turb Ecoli TKN NH4 NNN TN DRP TP
Awanui
Waihue channel
(Oct 1996)
Victoria
(Oct 1996)
NA
Opouteke
(Oct 1996)
ID NA
Mangakahia
Twin Bridges
(Oct 1996)
ID NA
Mangahahuru
(Oct 1996)
Waiotu
(March 2000)
ID
Whakapara
(Oct 1996)
NA
Waitangi
Waimate North
(Aug 1999)
NA
Mangere
(Oct 1996)
Punakitere
(Aug 2001)
ID NA
Manganui
(Aug 2001)
ID NA
Waipapa
(Oct 1996)
ID ↓*
Wairua
(Oct 1996)
ID ↓*
Mangakahia
Titoki bridge
(Oct 1996)
ID ↓*
Waitangi
Watea
(Oct 1996)
ID ↓*

Conductivity

Conductivity (COND) is a measure of the amount of ions in the water column and can be used as in indicator of nutrient enrichment. There is an increasing trend in conductivity at only two sites; Awanui River at Waihue channel and Mangahahuru at Apotu Road. This is most likely an indication of increasing enrichment and therefore has been shown as an undesirable trend.

Dissolved oxygen

There is an increasing trend in dissolved oxygen (DO) at Mangahahuru at Apotu Road, Whakapara River, Waiotu at SH1, Waitangi at Waimate North and Wairua River sites. It is unknown whether this trend is beneficial or detrimental, because for all these sites dissolved oxygen is both above and below the optimum range on several occasions.

Although dissolved oxygen is required for freshwater fauna to survive, an extremely high DO during the day could indicate that DO lags during the night or early morning. All of these five sites are dominated by aquatic macrophytes such as Egeria densa (oxygen weed), which produce copious amounts of oxygen during the day while they are able to photosynthesis but not at night when lags in DO can occur. Diurnal patterns in dissolved oxygen will be investigated at these sites.

Water clarity

The trend analysis shows that water clarity has increased (improved) by 8 cm per year in Mangahahuru Stream and 6 cm/year in Mangere River. It is not obvious why water clarity has improved at these sites; it could be as a result of reduced land run off in conjunction with improved farming and forestry practices or better quality discharges from point source discharges. There has been a corresponding decrease in turbidity of 0.4 NTU/year at a 95% CI in Mangahahuru Stream and of 0.23 NTU per year at only 80% CI in Mangere River.

Bacterial contamination

There was only one site that showed a significant trend in bacterial quality (E. coli); Mangere stream in which E. coli has increased by 86 per 100 mL per year, which is seen as a deterioration. Similarly to water clarity, these trends cannot be attributed to anything in particular at this stage. This is the only detrimental trend significant at a 95% confidence level for this site.

Nutrients

In general nutrient levels are showing decreasing trends at the majority of sites, including decreasing dissolved reactive phosphorus, ammoniacal nitrogen, nitrite/nitrate nitrogen and in turn total nitrogen and phosphorus. This could be a result of overall improving farm management practices throughout the region, including increased stock exclusion from waterways, riparian planting and better fertiliser application procedures. It also could be a result of better quality point source discharges such as farm dairy effluent, oxidation ponds and other industrial discharges. However many of these sites still have very high nutrient levels and there is a long way to go until desirable levels are met.