Rivers cleaner, but critters declining
Aquatic creatures have been included in our national river quality index for the first time, but, while water is getting cleaner, on some measures, they're declining, reports Eloise Gibson.
A survey of small aquatic critters has been included in national river quality measures for the first time – striking a depressing note amid water samples showing overall lessening pollution.
Every summer, staff from each regional council go out with specially-designed nets to scoop snails, sandflies, worms and other insects and their larvae from on or just below the stream beds.
The object is to see whether our most sensitive aquatic creatures are surviving and thriving in waterways, or merely the species most hardened to pollution.
These creepy-crawlies matter because they supply food for fish, and over time, their survival or otherwise can indicate what is happening in rivers outside councils’ monthly water sampling dates.
They can demonstrate effects that aren’t captured by a water pollution sample, such as stress from warming rivers caused by climate change.
The reason the creatures give valuable clues to river health is because they have to live there constantly – not only on days when council staff happen to be there, scooping.
Ecologists know which ones have a low tolerance for pollution: stoneflies, for example, can’t cope with high nutrient pollution levels, while aquatic worms “are perfectly happy in scummy farm drains”, as the Cawthron Institute’s Rob Holmes explains in this educational video. “They are like a black box recorder for waterways,” he says.
Macroinvertebrates in two out of every five monitored sites were assessed as likely or very likely to have been struggling harder over the past 10 years.
That is why ecologists wanted the critter surveys included in government-approved water quality indicators, and, today, for the first time, they are included.
Creepy crawlies have their own segment of the Land, Air, Water, Aotearoa (LAWA) National River Water Quality 10-year Trend Summary, which is available to the public in full here.
LAWA is a collaboration between the Ministry for the Environment, independent science research organisation the Cawthron Institute and 16 regional and unitary councils.
As well as the ‘Macroinvertebrate Community Index’ (MCI), as the snail, worm and bug index is known, the report includes eight long-standing chemical indicators gleaned from monthly water samples taken by regional council staff at hundreds of sites around the country.
These monthly water samples track measures such as E. Coli, water clarity, nitrogen, cloudiness or sediment, phosphorous and other pollution indicators at up to 673 sites, making it the biggest and newest trove of consistent data on river quality trends.
The report sorts the various indicators into five categories: very likely improving, likely improving, indeterminate, likely degrading and very likely degrading. The results are a mixed bag, as LAWA itself puts it.
The index of aquatic creatures covers the most sites out of any measure but is also the most depressing. Macroinvertebrates in two out of every five monitored sites were assessed as likely or very likely to have been struggling harder over the past 10 years. Only a small proportion of sites (173) were likely or very likely to be getting better as habitats for snails, insects and others, while 446 sites were likely or very likely to be getting worse. Results for the other 219 sites were indeterminate.
On the positive side, all of the other, water sample-based indicators of pollution were getting better in more places than they were getting worse. The other eight indicators that were measured were more likely to be improving than not, especially one most closely linked to dairy effluent and raw sewage, ammonium, suggesting efforts to clean up water are making a difference in some places.
Unfortunately, despite the heartening signs, every pollution indicator revealed many dozens of sites where things were getting worse – in some cases almost as many places were worsening as improving.
Some indicators of farming and urban run-off, for example, seemed to be improving at many sites. Ammonium, a potential toxin which is found in dairy shed effluent and raw sewage, was improving markedly – it was likely or very likely to be getting better at 308 sites, compared with 143 where things were likely or very likely to be worsening.
Ecologists believe hotter summers, lower water levels and, conversely, flushes of heavy rain worsened by climate change have been having an impact on species over the past decade.
Total nitrogen was likely or very likely to be improving at 185 sites, compared with 168 where things were likely or very likely to be worsening.
E.Coli, levels of which can indicate how likely people are to get sick from swimming, was improving at 197 sites, just edging out the 176 places where it was likely or very likely to be worsening.
Trying to reconcile the declining creature-survey with other, more hopeful, indicators, water quality experts said the signs of struggling macroinvertebrates were worrying and may indicate overall river health was worse than the chemical tests revealed.
But they also noted there might be a lag between water quality improving, and species recovering.
Professor Angus McIntosh, a professor of freshwater ecology at the University of Canterbury, said the MCI was “one of the best available measures of the life-supporting capacity of our rivers. 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.”
“The declining trends in MCI … is a real worry. It’s encouraging that other water quality measures are improving at many sites, but the MCI trends indicate that much more needs to be done to reverse negative trends,” McIntosh told the Science Media Centre.
Other experts acknowledged there might be a lag in aquatic animals’ recovery following efforts to improve waterways. For example, actions to clean up water by fencing stock out of streams may start lowering the chemical measures of water pollution before there are any signs of recovery in river snails, worms and insects.
“There’s a lag effect for MCI to recover, whereas chemical measures respond quicker. It will be interesting to see what next year’s trend is for MCI because it’s hard to know,” says Cawthron Freshwater Ecologist Katharina Doehring, who led the compilation of regional councils’ data for the LAWA report. “It (the MCI) is a valuable indicator because it responds to a lot of stresses and it’s a living biological indicator as opposed to just a chemical sample – these are living creatures,” she says. “It’s also quite sensitive to pressures that chemical indicators might not show, like climate change and (river) flow.”
Based on the trends in aquatic creatures, ecologists believe hotter summers, lower water levels and, conversely, flushes of heavy rain worsened by climate change have been having an impact on species over the past decade, says Doehring.
The measure of macroinvertebrates is not a simple count: researchers look at what species are present and compare this with what they know about the kind of conditions the creatures can tolerate to draw conclusions about waterway health, assigning different weights to species depending on how pollution-sensitive they are.
Doehring said in her experience councils were well aware of the need to improve so-called “red” sites, which were worsening, and she’d found them open and responsive when collating their data for the report.
After publishing an earlier water quality report in April, LAWA changed its methodology for assessing the results supplied by councils, she says.
The new system allowed trends to be identified at more sites, with fewer falling into the “no-trend” grey area.
Unfortunately, a lot of sites that used to be grey turned orange using the new methods, she says, tipping them into “likely to be degrading".
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