For centuries, the humble freshwater mussel has played a crucial role in the daily lives and traditions of the Māori people of New Zealand. Not only are mussels an easy-to-harvest, nutritious food source from local lakes, but the sharp-edged shell has had a variety of traditional uses, from a knife for cutting hair and the umbilical cords of new-borns to a noisy rattle trailing behind children’s kites. It is no wonder that the versatile shellfish has an important position in Māori culture—it is featured in songs, stories, and proverbs.
However, this beloved mollusc, known as kākahi in New Zealand, is playing perhaps one of its most important roles yet. Scientists are attempting to harness their incredible ability to steadily filter the impurities out of water to help clean up the country’s slowly suffocating lakes.
As it is with many of the world’s waterways, the poor water quality of New Zealand’s shallow lakes is a widespread problem. Data collected from 127 lake sites throughout the country by the research agency, Land Air and Water Aotearoa, showed that more than half of the lakes were categorized as either of poor or very poor quality. Such a dismal situation runs contrary to the country’s international image as a place of clean landscapes and unspoiled environments. This problem is especially apparent in the Waikato region in the upper area of the North Island.
A range of factors and environmental changes, from nutrients bleeding into the water from the land to the unwanted establishment of pest fish such as koi carp and catfish, have caused healthy lakes to “flip.” In some of the worst examples, shallow lakes have transformed from clear pools with plentiful aquatic plants to a state in which the water remains permanently muddy or turbid and is dominated by algae. Once a lake has flipped, it can be very difficult for it to be returned to its original state, despite the best attempts made through conventional water management.
"International marine biologists have long been aware of the cleansing power of mussels."
However, restoration could be at hand. International marine biologists have long been aware of the cleansing power of mussels, nicknaming the shellfish “The Hoovers of the Ocean,” in a reference to the powerful vacuum cleaners. "It's a super-filter in the marine world, filtering up to 25 liters [6.6 gal] of water a day," says marine biologist Leila Meistertzheim, who heads a study for France's Tara Ocean Foundation.
New Zealand already has a reputation for innovative approaches to improving its water quality, including the world’s first nitrogen cap for the Lake Taupō catchment over a decade ago. Scientists at the National Institute of Water and Atmospheric Research organization (NIWA) were intrigued to see the impact of a freshwater mussel population on the clarity of water in certain lakes and decided to investigate further. They stated, “Our project was inspired by the observation of abundant mussel populations in small shallow lakes with excellent water quality. This observation made us wonder if freshwater mussels could be part of the restoration process in flipped lakes. The project aim was to harness the filter-feeding capacity of native freshwater mussels on rafts to assist in lake restoration.”
Researchers selected Lake Ohinewai, a degraded Waikato lake, to run their large-scale natural filtering experiment. The local Matahuru Marae people welcomed the chance to have mussels reintroduced into their lake and were able to offer expert advice to the scientists. The research team, led by Dr. Deborah Hoftstra, decided to place the shellfish on “rafts,” which looked more like blue plastic bottle crates than the name suggests. These rafts raised the shellfish above the lakebed to a level where there is more oxygen; the additional oxygen helped the mussels to survive longer while also improving the efficiency of their filtration.
In pre-trial runs, the team of researchers used a combination oflaboratory and lakeside studies to examine mussel responses to environmental conditions that mussels were likely to experience on rafts in shallow lakes. This information influenced the final designs of the rafts and baskets.
In the actual lake filtration experiments, researchers monitored the mussels’ survival and growth aboard the rafts and conducted lakeside studies to examine the localized effects of mussels on water quality. The team used the data to mathematically model Lake Ohinewai and calculate the optimal number of mussels per raft and how many rafts would be needed to significantly impact the water quality of the lake.
The researchers did not use naturally born mollusks due to the diminishing stock. Instead, they focused on producing and growing juvenile mussels in a laboratory; these in-house manufactured mussels were used for stocking rafts or re-stocking lakes where the natural species had died out.
The team gradually improved the initial high mortality rates in which more than nine out of ten shellfish did not survive beyond two months. After these problems were resolved, the survival rate of mussels placed on all three types of rafts or at the bottom of the lake rose to 97% over the study period of fifteen months. Each experimental raft contained twelve mussel passengers in a basket, with up to eight baskets on a raft. The knowledge and techniques gained during this preparatory phase have aided the design and stocking rates for future trials.
By the end of the study period, the researchers discovered that the mussels, feeding on algae, zooplankton, and other microorganisms, were filtering an astonishing amount of water: an approximate flow rate of 1.5 L (0.39 gal) per hour per mussel. Simulations then modelled the positive impacts on the water quality of the lake derived from varying densities of mussels; these positive impacts generally reduced nutrient concentrations, chlorophyll A, and algae.
The greatest effect was simulated at more than forty individual shellfish per square meter, compared to the trial situation of two mussels per square meter in only one area of the lake. The study found that mussels can measurably reduce levels of E. coli bacteria from lake water, at a rate of 500 to 24,000 bacteria removed from the water per hour by one hard-working mussel. When one factors in a mussel’s average life expectancy of twelve to thirty years, it is obvious that their introduction could provide long-term natural water filtration with little human intervention.
There are now high hopes that future applications of the method can restore New Zealand’s lakes to their pristine glory. The scientists envisage fleets of rafts stocked with thousands of young, farmed mussels bred from surviving resident mussels placed in lakes around the nation.
According to Maori mythology, when their Polynesian ancestors made landfall on these uninhabited islands, they brought the mussels with them, trailing behind their boats on long ropes. It seems fitting that over 700 years later, the value of these hardy little shellfish is still recognized by the people of New Zealand.
*Gordon Cairns is a freelance journalist and a teacher of English and Forest Schools based in Scotland.