Seagrass balls

Mrs G (plus K), and I were intrigued recently to see dark, basketball-sized shapes floating in the ocean. We were in the middle of a Westernport Bay walk at Corinella.

A number had washed ashore at one location and closer inspection found them to be totally made up of seaweed. We had not seen this before and some homework ensued.

It seems the marine plant Posidonia australis, has this strange property of forming balls. I’ve so far been unable to find out why or how it does this; just the action of the tides and currents I suppose.

Wikipedia - “Posidonia australis is a species of seagrass that occurs in the southern waters of Australia. It is sometimes referred to as fibreball weed. It forms large meadows important to environmental conservation. Balls of decomposing detritus from the foliage are found along nearby shore-lines.”

Distribution of Posidonia australis. Credit: ala.org.au

Posidonia sp is slow growing and very long-lived; some plants are believed to be thousands of years old. It seems that global warming – rising sea temperatures – is having a devastating effect on colonies of seagrass species world-wide. Posidonia australis is on the Red List of Threatened Species of the International Union of Conservation of Nature.

Posidonia is considered to be an important organism within its ecosystem: it grows in large meadows, provides habitat for a variety of marine life, produces oxygen, sequesters carbon and mitigates seabed erosion. Some references mention that it is a bioindicator of a healthy ecosystem as it tends to be found growing best in unpolluted water.

Spanish research in 2018-19 indicated that balls of Neptune Grass, Posidonia oceanica, play an important role in ‘bundling up’ floating particles of plastic, helping to rid the ocean of a considerable amount of a growing pollutant. Other research found that a hectare of Posidonia, over a given time, can soak up 15 times more carbon dioxide than a similar sized piece of the Amazon rainforest.

Huh!