Sunday, March 13, 2011

Bioluminescence in the Gippsland Lakes

These pictures of bioluminescence in the Gippsland Lakes have proven quite popular, so it seems time to provide a story to accompany them. But this is not a short story, rather a convoluted one of fires and floods, of microscopic algae and the inspiring, remarkable and surprising beauty of nature.

The story begins with alpine bushfires in Victoria, which started on 1st December 2006 when over 70 fires were started by a band of thunderstorms and lightning strikes which moved across the state. This was an early start to the fire season following an extremely dry autumn and several years of drought conditions before that. The fires were in remote, rugged alpine areas, which combined with bone dry forest fuels to make them very hard to control. The fires burnt for 69 days, merging to become the ‘Great Divide Complex’ and ultimately covering an area of well over a million hectares.

Smoke from the Great Divide Complex fire spreading over south-east Australia

These fires burnt a vast area of the catchment for the Gippsland Lakes, a chain of large inland lakes in eastern Victoria. The fires affected the upper reaches of all the major rivers feeding into the Gippsland Lakes including the Latrobe, Thomson, Avon, Mitchell, Nicholson and Tambo Rivers. Such a large and in places severe fire through the catchment areas was always going to have an impact on the lakes themselves but a lot would depend on the intensity of rain events that followed the fires.

In the end, it was not any ordinary rainfall event that arrived that winter. A deep east coast low pressure system dumped more than 100mm of rain over many locations across Gippsland on 27th June 2007. The result was a 1 in a 100 year flood in the days and weeks that followed.

Floodwaters around Bairnsdale, East Gippsland in June 2007

The effect of the torrential rain over the over the vast area of recently burnt alpine forest was to wash ash and soil rich in nitrogen and other nutrients into the Gippsland Lakes. Counter intuitively, the rain and floods also increased salinity in the Lakes as the higher water level facilitated greater mixing with seawater at Lakes Entrance.

Floodwaters over Glenmaggie Weir

The fires and floods immediately led to concern about the impact on the health of the Gippsland Lakes, particularly whether it would lead to an outbreak of blue-green algae. Sure enough, the following summer, as the water in the lakes warmed up, the water across most of the Gippsland Lakes took on a disconcerting green tinge.

Bioluminescence is used as a lure to attract prey by several deep sea fish such as the anglerfish. A dangling appendage that extends from the head of the fish attracts small animals to within striking distance of the fish. Some fish, however, use a non-bioluminescent lure.

The cookiecutter shark uses bioluminescence for camouflage, but a small patch on its underbelly remains dark and appears as a small fish to large predatory fish like tuna and mackerel swimming beneath it. When these fish try to consume the "small fish", they are bitten by the shark, which gouges out small circular "cookie cutter" shaped chunks of flesh from its hosts.

Dinoflagellates have an interesting twist on this mechanism. When a predator of plankton is sensed through motion in the water, the dinoflagellate luminesces. This in turn attracts even larger predators which will consume the would-be predator of the dinoflagellate.

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