Two new water quality reports have illustrated the highly-degraded states of Wairarapa waterways.
Ministry for the Environment’s ‘Our Freshwater 2023’ report said that – between 2011 and 2020 – water quality at 36 per cent of lake monitoring sites nationwide improved while it worsened at 45 per cent of sites.
Findings were based on nutrient and algae measures that indicate ecosystem health.
A joint report produced by the Corthron Institute and GNS Science as part of the Lakes380 Research Programme showed that 83 per cent of lakes monitored for water quality in the Wellington region were in “poor or worse condition”.
Additionally, it has been estimated that 45 per cent of New Zealand’s total river length is not suitable for activities like swimming, according to models of campylobacter infection risk between 2016 and 2020, MtfE said.
Lake Wairarapa, the third largest lake in the North Island, is classified as “supertrophic”, meaning it has very high levels of nutrients such as nitrogen and phosphorus in its waters, Land Air Water Aotearoa said.
Corthron Institute and GNS Science’s data showed that 39 lakes in the Wellington region were supertrophic, while a further three were considered “hypertrophic” – a step up from supertrophic.
Supertrophic and hypertrophic lakes have lost so much of their dissolved oxygen that normal aquatic life has begun to die off. Excessive nutrients such as nitrogen and phosphorus cause this outcome.
MftE said analyses of national river water quality monitoring data from 2016 to 2020 showed that water quality was more degraded when there was more high-intensity pasture and horticultural land upstream.
It said that land-use intensification is one of the leading factors in degradation.
The majority of Wairarapa is used for primary production, according to Wairarapa Combined District Plan mapping.
The environment ministry’s report explained that excess nutrients from fertiliser and animal urine [such as phosphorus] that aren’t absorbed by plants can run off into freshwater, causing ecological harm.
Other causes of degradation include wastewater and stormwater runoff carrying heavy metals, plastic pollution, and introduced species.
New Zealand has experienced one of the highest rates of agricultural land intensification in the world over recent decades.
“Between 1996 and 2018, almost 60,000 hectares of exotic grassland was converted from low producing to high producing, compared with only 3500 hectares of exotic grassland converted from high to low producing.”
Dairy cattle numbers increased by 61 per cent between 1996 and 2014 before falling 5 per cent by 2018.
Meanwhile, the amount of irrigated land almost doubled between 2002 and 2019 from 384,000 hectares to 735,000 hectares – but only 9 per cent of irrigated land was used for sheep and beef farming, Wairarapa’s main land use.
“Research into soil physical properties suggests pasture irrigation can lead to soil compaction and less readily available water capacity, leading to increased nutrient leaching and run-off to waterways.”
The report said long-term degradation of river water quality measured nationwide was closely associated with the ratio of upstream land used for pastoral agriculture and plantation forestry.
Although on-farm mitigations like fertiliser management and protecting waterways from livestock reduced the amount of phosphorus and sediment that reached rivers between 1995 and 2015, it didn’t change much for nitrogen.
New Zealand Freshwater Sciences Society president Professor Jenny Webster-Brownsaid limitations of the report include a lack of a “call to action” on the issues facing freshwater.
