A freshwater river report reinforces what Northland Regional Council already knows — ''Must do better''.

The multi-agency quality monitor Land, Air, Water Aotearoa (Lawa) annual summary of the previous 10 years of data and trends analyses from 1456 river sites around New Zealand shows that levels of harmful E. coli and nitrogen in many waterways have markedly worsened over the past decade.

The Lawa report draws on information from monitoring by regional and unitary councils and the National Institute of Water and Atmospheric Research (Niwa) who test for a range of water quality - chemical-physical and bacterial indicators - at 986 sites across New Zealand and macroinvertebrate indicators at 981 sites.

However, regional authorities have different criteria regarding where to test and for what, so they don't all provide information for all the categories Lawa then ''summarises''.

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According to Northland District Council's 10-year trend analysis of regional sites that returned sufficient data, about 23 per cent that were tested for E. coli have improved but 65 per cent have become worse, Northland Regional Council regulatory services group manager Colin Dall said.

The Lawa report indicates that about 45 per cent of the nationwide sites tested for E.coli had ''likely'' degraded and 21 per cent fell into the ''results weren't clear'' ledger.

E. coli is a bacteria linked to faeces that can cause vomiting, cramping, nausea and diarrhoea in swimmers.

Whangarei Falls on the Hatea River is a popular Northland swimming spot where regular water quality testing is carried out.
Whangarei Falls on the Hatea River is a popular Northland swimming spot where regular water quality testing is carried out.

''The results confirm that the council, together with Northland's communities, have a lot of work to do to improve water quality in Northland rivers, which is why the council has significantly increased investment in this area in recent years,'' Dall said.

''Stopping and reversing the decline in water quality in rivers will take time, but with the increased effort that Northlanders are putting into improving water quality, it is expected that future trend analyses will show water quality improving at more and more sites.''

Dall said that in Northland the difference in E. coli levels between urban and pastoral, or rural, areas compared with those of wilderness and forested areas was markedly less than the national average. Here, median E. coli levels are 17 per cent higher in urban and 21 per cent higher in pastoral sites compared with forested sites.

''Previous reports have shown E.coli levels 22 times higher in towns/cities than in wilderness areas, and 10 times higher in pastoral areas,'' Dall said.

Other trends analyses in the Lawa report concern life in the rivers. The macroinvertebrate communities, such as water insects, worms and snails, are improving at 20 per cent of Northland sites.

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But that improvement doesn't look so good against the trend where macroinvertebrate communities (MCI) are decreasing at 45 per cent of the sites.

Nationwide, the MCI in about 25 per cent of the 1456 river sites showed an improvement over the 10-year analyses.

As for testing for total nitrogen (TN), key nutrients linked to fertiliser, farm run-off and industrial waste that can fuel algae growth in rivers and lower light and oxygen, the national results were dismal.

Only a quarter of sites showed an improving trend, half were worsening and there wasn't enough data to make a call on another 17 per cent.

However, Northland river testing didn't contribute to the national TN picture because the NRC doesn't provide Lawa figures for it, Dall said.

Freshwater ecologist Dr Roger Young, from the Lawa study's lead agency, the Cawthron Institute, said while there were more encouraging signs for other indicators such as water clarity, ammonia, and phosphorus, the results for E. Coli, TN and MCI were a concerning mixed national picture.

''[It] suggests New Zealand has some way to go before meeting our aspirations to maintain or improve river water quality."