On 17th December 2023, the sea was flat calm. There was no wind, and partial to full sun, so water close to shore was surprisingly warm for the time of year. I didn’t feel cold even after almost 3 hours in the water and I wore only a 2mm wetsuit. I saw no dead or overtly heat stressed motile lifeforms.

David preparing to snorkel

I suspect, however, that if there’d been a dodge tide (prolonged lack of tidal exchange akin to “missing a tide”, being a phenomenon essentially unique to SA’s main gulfs), ,many of the shallowest subtidal and intertidal lifeforms would have suffered severely. This is known to occur occasionally on dodge tides during any warm season and, until recently acute die-offs of littoral zone reef lifeforms during dodge tides were considered natural phenomena. However, climate change will surely exacerbate the number and severity of such events.

Also worth noting is the trend in SA and other parts of southern Australia towards increased ‘monsoonal ‘ warm season rainfall. Along with lower cool season rainfall, average annual rainfall in SA is expected to decrease slightly, but the % occurring in the warm season will increase considerably, according to meteorological forecasts. The rapidity of these changes means they are largely anthropogenic.

So, inevitably we will experience dodge tides (normal) coinciding with abnormally big summer rain events.

David preparing to dive

This warm season trend (combinations of lower salinity due dilution via catchment outflows with higher water temperatures (without even mentioning anthropogenic nutrient excess!) means that catastrophic die-offs on intertidal and shallow subtidal reefs in the upper to mid Gulfs will become more frequent.

So monitoring of the shallowest parts of the Great Southern Reef is a high priority.

Currently in SA most of the modest funding available for monitoring and protecting coastal ecosystems focuses on seagrass retention, restoration of shellfish reefs, mitigation of beach sand loss, and maintenance of commercially important fish stocks. All hugely important facets, but not yet a holistic approach.

David preparing to dive

By David Muirhead

David is a long-serving member of the Marine Life Society of South Australia. He has dived and snorkeled in South Australian waters for around five decades and has a particular interest in bony fishes. David has made the greatest single contribution to the society's Photo Index of local marine species.

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