Freshwater bioremediation using native mussels - focussed on shallow eutrophic lakes

MBIE funded project C01X1815

The issue

Freshwater mussels (known as kaaeo, kākahi or torewai) are considered a taonga. They were an important mahinga/hauaanga kai resource – and are still collected today.

There are indications that mussel populations are declining in many streams and lakes, including Lake Ohinewai (a degraded Waikato lake), but thriving in other locations, including Lakes Karaapiro and Taupō.

Ecological degradation of shallow lakes is a widespread problem in New Zealand, due to nutrient enrichment, establishment of pest fish and other environmental changes. In extreme cases, degraded shallow lakes “flip” from clear water and aquatic plant dominance, to turbid water and algae dominance. Flipped lakes are very resistant to restoration and can remain degraded despite conventional management.

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.

The people of Matahuru Marae supported this project at Lake Ohinewai and were involved in providing advice about the lake, surveys, monitoring and raft deployment. They also inspired this project with their long-held desire to reintroduce mussels to Lake Waikare, and their early work on mussel culture.

Approach

The project had four workstreams:

Understanding the ecophysiology of adult mussels We used laboratory and lake side studies to examine mussel responses, such as filtration rate, to a range of environmental conditions (e.g., high temperatures, suspended sediments) that they are likely to experience on rafts in shallow lakes. This workstream provided design parameters for our raft construction and increased the chances of the mussels thriving on the rafts.
Field Trials Different raft and basket designs were trialled at Lake Ohinewai. Mussel survival and growth on the rafts was monitored, alongside lake water quality. The localised effects of mussels on water quality were examined in lake side studies.
Modelling We improved an existing mathematical model of Lake Ohinewai using parameters from workstreams 1 & 2. The model was used to explore questions such as “how many mussels are needed per raft, and how many rafts will it take to have a significant impact on the water quality of Lake Ohinewai”?
Juvenile mussels and genetic relationships Declining mussel populations are often comprised of older animals with no evidence of juveniles, and these populations are at risk of dying out. Our research focussed on producing and growing juvenile mussels in the laboratory, because lab-reared mussels could potentially be used for stocking rafts, or for re-stocking lakes where there is no longer natural recruitment. Improvements were trialled to our laboratory culture methods to reduce the high (95%) mortality at the age of 1-2 months. Focussing on Waikato lakes, mussel populations were sampled for genetic analyses, to better understand how closely populations are related, and what those genetic relationships mean in situations where donor populations may be needed to restock lakes where mussels have died out.

Key findings

How much water do mussels filter?

Mussels feed by filtering large quantities of water at an approximate rate of one and a half litres per mussel per hour.

Lab-based studies with mussels examined their filtration rates and bio-deposits (i.e., faeces and pseudofaeces) under different concentrations of suspended sediment. These results indicate that high total suspended sediment, or cyanobacteria concentrations cause mussels to become inefficient and use energy to process particles that they are getting no nutritional value from.

What were the rafts design requirements?

Design requirements included raft transportability and durability, adequate water movement for food supply for the mussels and prevention of exposure to extreme temperature or low dissolved oxygen events.

Three designs with two types of baskets were evaluated. All rafts were designed to raise and lower the mussels with water level changes/fluctuations:

Designs one and two were circular with offset baskets to maximise water exchange, but only raft two rotated with water/wind movement.
Design three was a series of baskets in parallel.
Half of the baskets had a layer of lake sediment and half had none.

Can mussels survive on rafts?

Mussel survival rates were high (ca. 97%) on all raft types and reference sites on the bottom of the lake over the study period (15 months). However, there were some indications (worn shells) that longer periods on rafts may lead to shell damage if there is insufficient protection for mussels from damage on hard surfaces (e.g., plastic edges). Protection of mussels on the rafts, in the form of lake-sediment, hessian matting or felting, or less exposed raft sites requires consideration for long term deployment.

Now that we know mussels can survive and grow on the rafts, raft design and stocking rates could easily be improved through modelling.

How many mussels are needed to influence water quality?

Model simulations of varying densities of mussels, show impacts on lake water quality, generally reducing nutrient concentrations and algae. The largest impacts were simulated when the mussel density was greater than 40 individuals per square metre.

What did the lakeside studies show?

In the lakeside studies at Ohinewai, we found that mussels can reduce levels of Escherichia coli (the indicator bacteria for faecal microbial contamination of freshwaters) from lake water. Although mussel filtering rates can be highly variable, filtering can play a significant role in clearing lake water of particulate contaminants and improving water quality.

Can mussels be grown in the laboratory for use on rafts?

Culturing mussels in the lab is not a simple process, because the mussel life cycle requires a fish host, for its larvae to transform into a juvenile mussel. Our laboratory culture method skips the fish host step, but there has been high morality of the juvenile mussels. Lab culture methods that have focussed on a natural diet (e.g., river water) from an early stage has improved survival rates with juvenile mussels reared to seven months of age (640 μm in size).

Laboratory grown mussels bred from remnant lake populations could potentially be used to stock rafts.