New Zealand's freshwater quality appears to be improving, but there is strong new evidence not all is well.
The number of macroinvertebrate species - snails, worms and insects, for example - are declining at more than 40 percent of monitored freshwater sites, says Land Air Water Aotearoa (LAWA).
"Macroinvertebrates are a good indicator of the wider health of waterways and have a high ecological value, so it's disappointing to see they're under pressure," said LAWA river water quality lead Tim Davie.
LAWA's National River Water Quality 10-year Trend Summary report, released Thursday, found a "mixed bag" of trends across the country's freshwater ecosystems. While the decline in macroinvertebrates is a concern, for every other parameter LAWA measures there were more sites improving than declining.
"It's positive to see improving trends for the eight chemical-physical water quality indicators as we know these are quicker to respond to change," said Dr Davie.
"It is particularly encouraging to see ammoniacal nitrogen improving at many sites given the work of councils in reducing point source discharge and farmers keeping stock out of waterways."
He credited community initiatives and changing farmer behaviour for the positive trend.
"They've cleaned up waterways, they've taken rubbish out, they've improved the banks, put a lot of planting in, made sure that it's well fenced-off so dogs can't get near the streams."
LAWA is a collaboration between the Ministry for the Environment, independent science research organisation the Cawthron Institute and 16 regional and unitary councils. The report looked at up to 673 sites across New Zealand for each indicator.
The eight indicators are water clarity, turbidity (cloudiness), E. coli, nitrogen, oxidised nitrogen, ammoniacal nitrogen, dissolved reactive phosphorus and the macroinvertebrate community index (MCI).
The first seven measure substances in the water, while the MCI looks at the "direct response of those ecosystems to changing water quality", says Prof Jenny Webster-Brown at the Waterways Centre for Freshwater Management.
"We are not just looking at the physical and chemical parameters which have the potential to affect ecosystems, but also the direct response of those ecosystems to changing water quality."
Prof Angus McIntosh of the University of Canterbury said the LAWA report shows even if water quality has stabilised or is improving, "legacy effects" are still wreaking havoc on the ecosystem.
"The insects and other small animals that the MCI is based on must actually live in the river during the aquatic phase of their life cycle, so they represent the cumulative effects of conditions over time. Moreover, many sensitive insects have relatively long life cycles, so will only be present if conditions are good over a longer period of time.
"Thus, the declining trends in MCI, whereby two out of five monitored sites are likely or very likely degrading is a real worry."
It's the first time MCI has been included in the data. Dr Mike Joy, whose campaigning on freshwater issues drew the ire of former Prime Minister John Key, said MCI is a "superior and more holistic and robust measure" of water quality than the other indicators.
"The MCI results do not support LAWA's previously claimed improvements in water quality," he said. "The MCI trends do however support my earlier criticism of claims of improvements using flawed measures."
He said reduced nitrogen levels could be the result of excess algae growth, a "completely opposite result", and other indicators fluctuate on a daily basis depending on influences like rainfall.
Prof Troy Baisden of the University of Waikato said MCI "measures the health of the little critters that are the food for iconic fish and taonga species".
"It has the greatest coverage of sites of all the parameters reported. It also shows the worst trends. That's concerning."
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Scott Larned, a researcher at NIWA, said the MCI data might also not be completely reliable however.
"They are based on the presence/absence of species, not their abundances or proportions," and - like the other indicators - can fluctuate depending on outside influences.
Results for each site are available on the LAWA website.