February's monsoonal rainfall in northern Queensland has had a devastating effect on the Giant Clam Garden off James Cook University's Orpheus Island Research Station, with the reduced salinity in the water leading to a mass die off.
Up to 3000 Giant Clams were planted at the edge of Pioneer Bay by JCU researcher Drs John Lucas and Rick Braley in 1986 but preliminary surveys have shown a 100 percent mortality rate at the renowned snorkeling site.
Extensive rain and flooding caused by the recent tropical low and subsequent fresh water and groundwater run-off from a creek at Orpheus Island Research Station into the Clam Garden saw salinity levels at one third of the normal range for saltwater on the Great Barrier Reef.
Orpheus Island Research Station Manager and marine ecologist Jenni Calcraft has lamented the loss of thousands of the live giant clams at the well-loved site.
"These giant clams are in shallow water and at times completely exposed, so they basically just had a freshwater lens over them for two to three weeks during this heavy rainfall event," she said.
"OIRS staff took some readings after the weather event and noted the water quality in the area was severely reduced. The salinity of the water was about a third of what normal salt water is supposed to be in the Great Barrier Reef.
"During normal conditions the salinity level is around 35 ppt (parts per thousand) however we recorded levels of 10 to 11. Salinity levels are now back to normal however."
Like coral, Giant Clams are a marine saltwater animal that filter feed off plankton in the water column. They are also 'solar powered' and have symbiotic process which provides extra nutrients through photosynthesis.
"They basically would have just suffocated with the freshwater lens, and then they probably would have starved as well," Jenni Calcraft said.
"Extra sediment in the water column would have reduced their access to sunlight and subsequently their ability to photosynthesise.
"Any supplemental feeding on plankton in the water column probably would have been reduced as well as those critters would have moved to a different area or themselves died off with such a prolonged influx of freshwater. Unfortunately, once settled, Giant Clams cannot move across the reef."
Investigations into smaller giant clam populations in the deeper (5-6m) waters of Orpheus Island are far more positive and while there haven't been die-offs several of them are showing signs of bleaching stress.
"These climate change driven severe weather events over the years dramatically impact our local ecosystems. Corals and coral bleaching are a hot topic, but many other organisms like the giant clam are often overlooked," Jenni Calcraft said.
The Clam Garden was popular for snorkelling at mid to high tides. During low tides when the clams were exposed it was possible to walk among them and witness them expelling water in a geyser-like effect, as part of their seawater exchange process.
The last time this area experienced a rainfall event to this extent was during the 1967 floods in Ingham.
"This population was about 40 years old and would never have experienced something like this in their lives," Jenni Calcraft said.
"This species can live up to 100 years so to have these animals' lifespans cut in half is really devastating."
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