Saturday, November 27

Plant rediscovered in Marlborough lagoon decades after last sighting

Seagrasses have been seen in a Marlborough lagoon for the first time in nearly four decades.

Brown algae grow among the beds of rupees in Marlborough's Wairau Lagoon.

Brown algae grow among the newly rediscovered rupee beds in Marlborough’s Wairau Lagoon.
Photo: Supplied – Salt ecology

Hundreds of hectares of ruppia, also known as horsehair grass, have been rediscovered in Wairau Lagoon, southeast of Blenheim.

The find was a “big surprise” for the team of scientists tasked with verifying the lagoon’s health in April. However, the presence of dense algae and abundant algae in other parts of the lagoon was worrisome.

Ruppia beds once dominated the lagoon, but mud, current changes, or competition from other plants were likely the causes of the population collapse. The last recorded sighting was in 1983.

But during the latest inspection of the lagoon by the Marlborough District Council, scientists found that 44.2 percent of the lagoon had some rupee coverage. About 16.2 percent, or 199 hectares, had moderate to heavy coverage.

“From a scientific perspective, this is really amazing … This shows that the lagoon system is quite healthy,” one of those scientists, Dr. Keryn Roberts, told advisers last week.

Ruppia, also known as horsehair grass, has been rediscovered in Wairau Lagoon, southeast of Blenheim, after nearly 40 years.

Dr Keryn Roberts suspects that the ruppia never left the pond, but that the number of plants had decreased, causing it to be overlooked during the pre-checks.
Photo: Supplied – Salt ecology

Ruppia added additional oxygen to the water and the lagoon bed, and provided food and shelter for wildlife. Its roots also helped hold the sediments together, preventing mud from churning on windy days and turning the waters brown.

Roberts suspected that the ruppia had never left the pond but that the number of plants had decreased, causing it to be overlooked in previous reviews that might not even have looked for it.

Its rediscovery was “regionally … and nationally significant.”

However, it was not a good state of health for the lagoon, as the presence of algae suggested that it was under stress.

Around the middle of the pond algae grew that resembled fine brown hair, covering the plant beds and mud in a dense mat.

Scientists monitor the health of the Wairau Lagoon every five to ten years.

Scientists weigh a dense cluster of algae that monitor the health of Marlborough’s Wairau Lagoon.
Photo: Supplied – Salt ecology

“This is really worrisome because it tends to grow when there are … too many nutrients in the pond,” Roberts said.

So did the algae. There were two types that grew in the Wairau lagoon, one was green and the other reddish brown, and both multiplied faster if there was extra food around.

The algae were so thick in some areas of the lagoon that a meter-by-meter space was worth six kilograms of material.

Multiple studies have found that seaweed began to have ‘bad’ effects on the environment at 1.45 kg per square meter, four times less than what was in the main channel of the lagoon.

Roberts wasn’t sure what was feeding the pond nutrients, but recommended that the council find it and “manage it properly.” Scientists had watched the Wairau Lagoon’s health slowly decline over decades, and if it degraded too much, it would be “very, very difficult” to reverse it.

The council agreed with the Roberts team’s findings and referred them to the next full council committee on October 28.

The Wairau Lagoon is actually three interconnected lagoons, separated from the sea by the Wairau / Te Pokohiwi bar. The lagoon and its surrounding estuary were important areas because they were home to rare and endangered species.

Waste from the Blenheim Sewage Treatment Plant was processed in filter ponds near the pond, and Council Member Gerald Hope suggested that the “nutrients” could be runoff from the pond.

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Local Democracy Reporting is a public interest news service supported by RNZ, the Association of News Publishers, and NZ On Air.


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