Anyone who has dived Rapid Bay Jetty over the last 6 weeks would know how bad the visibility has been.

There have been days there when you could not see more than 50cm through the milky brine which always seems to follow a period of northerly fetch. I have heard many different opinions on why the visibility here is so changeable. Some people suggest that it has been ‘bad’ since the new jetty was built a few years ago due to disturbance of the sea bed however I can remember these ‘milky’ days from when I first started diving and the original jetty was still intact and in use.

My thoughts are that the ‘milk’ is cause by colloidal gypsum, a legacy of years of gypsum loading here and the the long term exposure of the area to gypsum dust both directly and through run-off.
I am sure the sea bed surrounding the area is loaded with this super fine gymsum. A prolonged northerly fetch creates a ground surge which lifts the dust back into the water making it milky blue.

The visibility here is very good at the moment at around 10-15m. It’s an odd kind of visibility with the the milkiness still there but at least now you can enjoy the old wives swimming among the pylons.

Photo: Robert Rath, ‘Day 681, Rapid Bay Looking Good’, 1/160s f/11 ISO640 22mm+2D