Southern Bloom

Last week, incredible satellite imagery captured an expansive phytoplankton bloom off South Australia's south-east coast.

The blue swirls were easily visible from space, giving an indication of how thick the bloom was. In total, the bloom extended approximately 100km from north-east to south-west, yet the cause of the bloom isn't what most would expect.

Phytoplankton bloom off South Australia's South East

Though the Murray River has been flowing freely into the ocean nearby, it isn't the flood waters that are the cause for this bloom.

Instead it's an annual occurrence thanks to the Bonney Coast upwelling system.

Similar to how strong, persistent days of north-east winds upwell cold, nutrient-rich water along the East Coast, we have numerous other upwelling hot spots around the country.

During the South Australian summer, persistent south-east winds blow adjacent to the coast, with a net surface current (known as Ekman Transport) moving the warm top waters to the left, and therefore out to sea. This causes a slight lowering of the sea level at the coast, with cold, nutrient rich water upwelled from the deep to try and balance out this difference.

The closer the shelf break is to the coast, the more enhanced the upwelling, thanks to easier access to that deeper, colder water.

There are three main regions for upwelling across the South Australian coast, that being the Bonney Coast, south-western Kangaroo Island and the southern Eyre Peninsula.

The largest of these three is the Bonney Coast upwelling system which stretches from Portland in Victoria all the way to Robe. The shelf comes within 30km of the coast, with submarine canyons helping to funnel water from the depths up onto the shelf.

Thanks to the Bonney Coast upwelling system, inshore sea surface temperatures are always chilly in summer across the South East, with them currently being 13° - 14°C. Anyone who's surfed in the region during summer would not forget the juxtaposition of surfing in a scorching 40°C summer offshore wind while wearing a 4mm wetsuit.

The sea surface temperature chart (below left) shows the upwelling clearly, with the cold signal spreading north-west. While at right is the chlorophyll concentration, which matches up with visual observations from space.

Sea surface temperature chart (left) with chlorophyll concentration (right) for the 22nd of January

So where does the chlorophyll come from?

All the nutrients upwelled from the deep effectively fertilise the surface water, encouraging plant growth, that being phytoplankton. Phytoplankton are single-celled plants and contain chlorophyll to capture sunlight and convert it to energy. They form the basis of the marine food web and are the primary producers, while also absorbing carbon and releasing oxygen.

Thus the greater the concentration of phytoplankton, the more chlorophyll which appears as a blue/green bloom.

Phytoplankton bloom stretching for over 100km

Comparing the satellite imagery from last week to the chlorophyll concentration chart taken around the same time, we can see the source was from the upwelled cold water, not from the flood waters exiting the Murray Mouth.

The swirls are caused by the interaction of the colder, denser, upwelled water mixing with the warmer, ambient water along the density front.

While the bloom is quite striking from the air during the day time, whether or not the bloom was bioluminescent depends on whether it contains dinoflagellates (the most common phytoplankton to emit light). Seeing as dinoflagellates appear brown in colour, and the satellite imagery has a blue signal, it seems unlikely that the bloom was bioluminescent.

Regardless, with advances in technology, scientists can use such imagery among other tools to further their understanding of the oceans.