Cleaning rivers

Words by Nane Steinhoff
Photographs by AgriSea

In New Zealand, a new project seeks to restore waterways by using sea lettuce to soak up nutrients that freshwater plants are currently unable to absorb. The team of researchers behind the land-based trial has high hopes for the future.

Sea lettuce or ulva is a common seaweed that can be found in high and low intertidal zones in most parts of the world. While some people like to dry it and use it as a flavourful addition in their recipes, a team of researchers in New Zealand has a somewhat different idea. AgriSea, a seaweed innovation company based in Paeroa on the North Island, is currently working with the University of Waikato on a land-based sea lettuce growing trial at Kopu marine precinct. The cooperation hopes to help restore New Zealand’s waterways.

Damien O’Connor, New Zealand’s minister of agriculture said the trial is a nationwide first and seeks to address an important environmental issue by using the seaweed to soak up nutrients that freshwater plants are currently unable to absorb: “Currently the loss of nutrients not absorbed by plants enters the soil and drains into groundwater that leads straight to our waterways. This promotes algal blooms which reduces oxygen levels, which then threatens animals such as eels, freshwater mussels, freshwater crayfish, and whitebait. This bioremediation project is using Kiwi ingenuity to see if the seaweed can act as a sponge, soaking up excess nitrogen, phosphorus and helping clean our waterways.”

Three ponds totalling 60 square metres that draw water from the Waihou estuary will grow the locally present green seaweed species ulva for 12 months in the hope of providing data from real-world ambient conditions. “Sea lettuce will be cultivated and scaled up at the University of Waikato’s aquaculture facility. Researchers will use DNA barcoding to confirm its genetic identity. Species selection will be based on growth performance for bioremediation in conditions mimicking the Waihou river estuarine water,” says O’Connor and continues: “If successful, this will be an environmentally-friendly way to improve water quality, create jobs in the science sector, revitalise our waterways and improve our on-land farming systems.”

The team of researchers argue that the seaweed can be turned into high-value products while the process of growing it also has a multitude of benefits. AgriSea’s managing director Tane Bradley says that the seaweed species doesn’t have root systems and therefore is able to grow by pulling nutrients from the water around it: “In this case, as sea lettuce grows it pulls Nitrogen from the water and incorporates it into its tissue.”

Known as ‘bioremediation’, the project will use seaweed in an attempt to clean excess nutrients from the Waihou estuary. “The seaweed in tanks will act as a sponge and filter feed on excess minerals like Nitrogen, Phosphorous and other heavy metals, which is then returned to the sea, filtered and clean,” explains Bradley and continues: “Ulva is particularly suitable for land-based aquaculture in ponds or raceways as you can typically maintain growth for most of the year, meaning year-round nutrient recovery and also year-round biomass product delivery. Many sea lettuce species have bloom-forming traits; they take up nutrients quickly and convert this to seaweed tissue and tolerate growing as really dense populations across a broad environmental range. For a raceway-system, this means we can grow a lot of seaweed on a smaller footprint which is really helpful from a land-use and availability perspective too.”

“In terms of biomass uses, there is a plethora of options for sea lettuce. This contributes to why it is so good for nutrient recovery applications as it means there is always something we can use the biomass for. And that means we are not removing nutrients in one system, the estuary, just to harvest and dispose of the resulting biomass as a waste product,” Bradley adds.

In terms of products, it is estimated that up to 50 tonnes of dry ulva per hectare could be produced from a scaled-up facility. The sea lettuce species also has about 20 per cent protein which is one of the higher percentages for seaweed. AgriSea is partnered with Farm Source already helping farmers improve soil and water quality using AgriSea’s seaweed bio stimulants as an alternative to chemical fertilisers. Bio stimulants made from seaweed are high-value macro-algal products used on dairy, sheep and beef farms as well as in the apiculture, horticulture and viticulture industries. “There’s a growing desire from science institutes with assistance from government funding agencies to explore innovative projects such as this, not only for their environmental benefits but as potential commercial harvest of seaweed for food, bio stimulants and high value bio actives,” explains Bradley.

Since securing the site, AgriSea has managed to get funding through a grant as well as project approval. Designing the plant is now at the forefront of their operations. As the company has been provided with access to council land at the local treatment plant and next to the new Kopu Marine precinct, AgriSea has been working closely with the managers. “Once the design is finalised, site preparations, installation, and commissioning of the plant will take another couple of months, and then we’re up and running. Shovels should be in the ground by mid-April,” smiles Bradley. In the meantime, AgriSea are scaling up the local seaweed cultivar at their research aquaculture facility at the University of Waikato. Once operational, they will inoculate the pilot raceway ponds with the scaled up, local strain of sea lettuce biomass and start quantifying productivity and nutrient removal over time, along with water and biomass quality sampling. This will allow them to model the capacity for a scaled-up plant in terms of amount of seaweed produced per hectare per year, and the amounts of nutrients recovered from the environment.

“We will also be analysing the composition of the biomass and how this changes over the year to inform end-use potential. Together, this data will underpin the environmental and economic value proposition for land-based seaweed aquaculture systems like this one to recover diffuse source dissolved nutrients from our estuaries and provide alternative uses for marginal land affected by coastal flooding and salinity. We are also looking to use the produced biomass for further joint research trials, but the target will be developed further down the track when we have more information regarding the composition and quality of the biomass,” explains Bradley and adds: “AgriSea estimates the potential value as up to $219,000 per hectare of production per annum. This is based on nitrogen credits coming into effect in New Zealand and the value of sea lettuce based products which are on AgriSea’s innovation pipeline.”

While New Zealand doesn’t have a nitrogen trading scheme yet, if one was introduced the biomass produced in one year from one hectare of ponds would be worth NZ$82,000 in nitrogen credits, based on international values for nitrogen trading.

Alongside the scientific objectives of operating the pilot plant and collecting data, AgriSea will put special emphasis on community awareness by inviting local schools and marine biology students to their facilities. “There is nothing quite like seeing a pond full of bright green sea lettuce to change your perspective on seaweed. We are hoping to build increased community engagement with environmental issues, remediation technologies, and the role of science and research in providing solutions together with industry,” adds Bradley.

In the future, AgriSea hopes that the pilot project can be transferred to the Waikato River, a freshwater river, to see which freshwater seaweed species could replicate the findings from the pilot project within this freshwater scenario. “We would also like to see hectares worth of these systems up and down the country, cleaning and revitalising our waterways,” explains Bradley. “Agrisea look forward to sharing the results of this sea lettuce pilot with as many people as possible to enable these systems and methods to really add value to our planet, people and health,” he condludes.

Photographs by AgriSea