Seagrass species are dying back in some parts of Port Phillip bay and Western Port but growing back in others, a major study into marine health has found.
While water quality has been improving, a project that is monitoring the health of seagrass beds at seven sites has found mixed results.
Janty Taylor, Monitoring, Evaluation and Reporting Coordinator with the Port Phillip and Westernport Catchment Management Authority (PPWCMA) said that seagrasses are critical to the health of the marine ecosystem.
‘Seagrasses not only provide breeding grounds, food and shelter for marine life, but they trap and stabilise sediments to help reduce water pollution from suspended solids,’ Ms Taylor said.
The PPWCMA’s recent Catchment Condition Report found sediments have smothered seagrasses in some parts of the bays while mangroves are thriving in other areas.
‘This suggests that the dynamics of a bay are as important as water quality to the health of seagrasses,’ Ms Taylor said.
In Port Phillip Bay, four sites were monitored by the Department of Primary Industries (DPI) over several years: Point Richards, Kirk Pont, Swan Bay and Blairgowrie.
A major decline in seagrass health was identified at Blairgowrie and Point Richards between 2000 and 2005, whereas major improvement was recorded at Kirk Point, and Swan Bay is stable.
It is a similarly mixed story in Western Port, where the western half of the bay around French Island and Crib Point is stable, but the eastern side has been in continual decline since the 1980s.
Sediment and erosion are believed to be the culprit of continued seagrass loss around Coronet Bay.
Four years ago, over 2,100 seagrass plugs were planted at Coronet Bay, Phillip Island and Long Island, but all of the plantings have since died.
Another 1,000 seagrass plugs were planted at two-metre intervals in a one kilometre strip around Coronet Bay to discover where Seagrass might survive – but all of these plantings also died.
Cliff erosion in the Lang Lang area is thought to be responsible for smothering seagrass while high nutrient levels has smothered seagrass beds at Reef Island.
However, in contrast to seagrass mangroves have been spreading in Western Port in recent years and are proving easy to propagate.
‘Where there were previously solitary trees in Coronet Bay there are now young Mangroves laden with seeds in the summer,’ Ms Taylor said.
Bass Coast Primary Schools have been involved in germinating mangrove seedlings, which are growing well in Newhaven on Phillip Island and Grantville.
At Lang Lang, over 2,000 mangrove seedlings were planted and while some were washed away, others have doubled in size.
DPI has been monitoring the health and extent of seagrass beds and mangroves in Port Phillip and Western Port in a three-year project that ends later this year.
The project aims to address the lack of baseline data, which has been a barrier to identifying the causes of seagrass loss.
Ms Taylor said the PPWCMA has set a target of increasing the extent and quality of seagrass communities by 2020, to improve the marine health of the region.
While this would be a challenge, Ms Taylor said it was pleasing to see that the broad marine habitat in the region was relatively stable.
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