Discussion

 

Water quality state

The water quality state in Northland rivers is highly variable. All rivers in the network have poor water quality on occasion and some consistently have poor water quality. Overall compliance with the ANZECC guidelines has improved for dissolved reactive phosphorus, total phosphorus, ammoniacal nitrogen, total nitrogen and turbidity when compared to the previous year's results. Compliance rates have notably improved in a number of catchments, which include the Awanui, Manganui, Mangere, Punakitere and Waipoua catchments. Significant changes were identified in the Waiarohia catchment with compliance rates improving in most parameters at the upper site (Waiarohia at Whau Valley) but dropping in three parameters at the lower site (Waiarohia at Second Ave). Macroinvertebrate trends also show similar results with an improving trend occurring at the upper site and a deteriorating trend at the lower site (Pohe, 2010)

Compliance with the dissolved oxygen ANZECC guideline were particularly poor throughout Northland this year with the majority of sites failing to meet the guideline on more than half of the sampling occasions. The drought likely influenced this year's results as most river flows were below average.

Water quality can be strongly influenced by its surrounding land use. Compliance is generally consistently poor at sites in intensive pastoral farming catchments, which includes the Ruakaka, Paparoa, Utakura and Awanui at Waihue Channel river sites. These sites generally have low water clarity and higher nutrient concentrations. These results are consistent with other national findings (Ballantine et al, 2010; Ballantine & Davies-Colley, 2009; Larned et al, 2004). Compliance rates were higher in native or exotic forest catchments and include the Waipoua, Waipapa, Opouteke and Mangakahia at Twin Bridges river sites. These sites meet the ANZECC clarity, turbidity and nutrient guidelines on the majority of sampling events.

Water quality trends

The trend analysis indicates that there have been several positive changes in water quality over the past 15 years at most river network sites. There were decreasing trends in both dissolved reactive phosphorus and total phosphorus at 11 and 13 sites, respectively. These trends were recorded across all land use classes; native forest, exotic forest, urban and pastoral. Seven of these sites are predominately pastoral catchments (>50% of the catchment) and include the Mangere, Punakitere, Wairua and Whakapara river sites.

Significant decreasing trends were observed in all nitrogen species across nine sites; three of these sites are in significant dairying catchments (Wairua, Manganui and Mangere rivers). Decreasing ammoniacal nitrogen trends were recorded at five sites that suggest there has been improvements in point source discharges. This is consistent with the national picture of (Scarsbrook, 2006) and research into the impacts of dairy farming (Wilcock et al, 2006)

Improvements in water clarity were detected at seven sites, with the majority of these located in intensive pastoral farming catchments (Mangahahuru, Mangere, Punakitere and Wairua catchments). This could also be related to the reduction of point source discharges in these catchments.

Both increasing and decreasing trends were recorded in dissolved oxygen. Eight out of the ten dissolved oxygen trends were deemed degrading trends as they were falling further away from the ANZECC guideline range. Thirteen of the fourteen pH trends were increasing trends. Scarsbrook (2006) also found mainly increasing trends in pH in the Auckland region, however decreasing trends in pH were recorded in other regions such as Waikato (Vant and Smith, 2004) and Bay of Plenty (Scholes and McIntosh, 2009).

Overall there were fewer negative trends than positive trends detected in water quality this year. Positive trends were observed in water clarity, turbidity and all phosphorus and nitrogen species. The majority of these trends were at sites located in intensive pastoral catchments. Eleven improving trends were recorded in the Mangere catchment, which has a large proportion of dairying in the catchment. NRC has been actively working alongside dairy farmers in the catchment to minimize the impact of discharges.