According to the MfE Environment-Aotearoa 2022 report, many of our lakes and rivers have unnaturally high levels of nutrients due to leaching and run-off from urban or agricultural sources. We used both measured and modelled data to assess the water quality of our rivers and lakes. Computer models estimate that of 3,813 lakes in Aotearoa, 46 percent rated poor or very poor in terms of nutrient enrichment (as measured by Trophic Level Index) between 2016 and 2020.
Modelled data for 2016–20 indicated risk of environmental impairment (based on comparison with reference conditions) from at least one form of phosphorus (total phosphorus or dissolved reactive phosphorus) in 64 percent of the river length of Aotearoa, and 69 percent for at least one form of nitrogen (total nitrogen, nitratenitrite-nitrogen, or ammoniacal nitrogen).
Water quality varies according to how much the catchment has been modified. Freshwater river quality tends to be poorest near areas with a high proportion of human modification and is highest quality in areas that have had the least modification. Monitored sites with higher proportions of human modified land cover in the upstream catchment area had higher concentrations of all forms of nitrogen and phosphorus, compared to sites with lower proportions of human modified land cover for 2016–20.
Between 2001 and 2020, trends were improving for nitrate-nitrite-nitrogen at 38 percent of monitoring sites, and for ammoniacal nitrogen at 61 percent of monitoring sites. (In this and the following section, improving and worsening trends include both likely and very likely statistical trends. See relevant indicators for trend assessment detail.) For the same period, 67 percent of sites had improving trends for dissolved reactive phosphorus.