This column was written with expert advice from freshwater scientists at the National Institute of Water and Atmospheric Research (NIWA): Sandy Elliott, Paul Franklin and Jennifer Gadd.
OPINION: There’s a lot of slash anger about. It’s directed at forestry companies and the regulators that allow them to leave behind large quantities of tree trimmings, which are washed down rivers to the coast during severe weather events, causing devastation.
Forestry has been described as the only sector that gets to internalise the benefit and socialise the cost. But that tactic underlies the profitability of several sectors. One externalised cost that’s rife is freshwater pollution. It harms the water’s occupants – native freshwater fish, insects, kākahi (mussels) and kōura (crayfish) – mostly slowly, but sometimes catastrophically.
When land is cloaked with trees, even pine, waterways are largely protected. But when trees are fertilised, and during harvest and for a few years afterwards, waterways can receive harmful runoff. Agriculture inflicts the same problems on many more waterways. Urban runoff is also an intense polluter, although fewer waterways are affected.
Sediment pollutes. Some erosion is inevitable, but when it rains, sediment is most likely to run off unclothed land: clear-felled forests, eroding stream banks and gullies, earthworks, ploughed fields, and forage-crop paddocks that are intensively winter-grazed.
Sediment from urban regions can be loaded with heavy metals from vehicles, metal roofing and industrial activities. Rain washes it into streams. When it hangs suspended in water, it blocks light needed by native plants that produce oxygen.
Settled sediment fills up fishes’ hiding and breeding spots and smothers the insects they eat. If it includes organic matter such as faeces, bacteria feed upon it and multiply, all the while respiring and depleting the water’s oxygen.
Freshwater creatures breathe the oxygen dissolved in water, and a lack of it can be literally suffocating. It can also be depleted when nutrients – mostly nitrogen and phosphorus – leach into waterways from fertiliser, animal excreta and insufficiently treated sewage.
That’s because nutrients can fuel the proliferation of algae, and algal blooms consume oxygen during respiration.
There are ways to support nature’s capacity to fend off some fallout from modern land use. Planted waterways, preserved or restored wetlands, rain gardens and ponds slow the flow and filter water.
Riparian planting can trap sediment and potentially absorb nutrients. But planting can’t remove pollutants if runoff first soaks into groundwater, enters waterways via drains or stormwater pipes, or runs fast across the ground.
It still improves habitat, though, by binding banks and cooling streams, which were mostly well-shaded before land was cleared. Algal growth is slowed by shaded, cooler water. Shaded streams with little sediment are more resistant to invasive weeds, which can deplete oxygen from water.
Many of our freshwater-reliant native species need help, according to Stats NZ’s most recent data. Two-thirds of freshwater bird species and three-quarters of freshwater fish are in decline or threatened with extinction (other factors such as introduced species, altered water flow, and physical barriers to fish passage also contribute to this vulnerability).
Nearly half our lakes over 1ha are often murky, with high nutrient concentrations and frequent algal blooms, according to the state-of-the-nation report “Our Freshwater 2020″. Most are fed by water that runs through urban, pastoral or forestry land.
Pathogens from faeces make many rivers unsafe for swimming; during the past decade, that included nearly all river lengths flowing through predominantly urban areas, three-quarters running through pastoral land, and less than a third with mostly exotic forest in their catchments.
Shallow harbours, those shellfish havens, are contaminated by waterways flowing into them. It’s difficult to convincingly establish who’s responsible for water pollution, as multiple land uses often contribute.
Attempts to tighten water-quality rules are routinely pushed back against by some sectors. Many creatures, including people, pay for the way we use land. It’s hard to count the cost in dollars.
