Northland has over 400 freshwater lakes, with many of these of national and international significance. Northland has the greatest number of dune lakes nationally and represents a large proportion of warm, lowland New Zealand lakes with relatively good water quality. These lakes and their surrounding wetland margins support a range of endemic endangered species providing the only known habitat, or national strongholds for a range of biota. Perhaps the most outstanding character of these lakes is the currently limited impact of invasive species on their biota, which is unparalleled in any other region of mainland New Zealand.
Most of Northland’s lakes are situated along the west coast, having been formed between stabilised sand dunes. The dune lakes are in four main groups situated on the Aupouri Peninsula, Karikari Peninsula, Kai Iwi lakes and Pouto Peninsula. They generally range in size between one and 35 hectares and are usually less than 15 metres deep.
Lake Taharoa of the Kai Iwi group is one of the largest and deepest dune lakes in the country, covering an area of 237 ha and being 37 metres deep. It has the deepest growing vegetation in the North Island growing at a depth of 24 metres. Dune lakes usually have little or no continuous surface inflows or outflows being fed primarily by direct rainfall or from surrounding wetlands. As a result, water levels fluctuate considerably with climatic patterns.
Because most of these lakes are relatively small and shallow they have limited capacity to assimilate any contaminants. They are prone to nutrient enrichment from stock and fertilizer particularly where lakeside vegetation has been grazed or removed and where there is direct stock access to the lake.
There are also several shallow inland lakes which originated through the damming of valleys by lava flows such as Lakes Owhareiti and Omapere near Kaikohe. Lake Omapere appears to have formed some 80,000 years ago. The Lake reformed 600 to 700 years ago which is attributed to siltation following deforestation of the area.
Further to the north are two large man-made lakes associated with the Kerikeri irrigation scheme, which are a major water resource for that area. The Northland Lake Water Quality Monitoring Network (LWQMN) was established in 2005 to collect water quality data at a selection of Northland’s lakes. Further to this NIWA carries out an annual Biodiversity Assessment of 86 Northland lakes on a rotating basis including those within the LWQMN. The data from these two programmes is used to assess the monitored lakes environmental condition, and their response to potential impacts such as surrounding land use change and weed infestations. This information is used to prioritise future surveying and management work in order to maintain/restore these valuable ecosystems.