Sunday, 20 July 2014

Queensland Mangroves Die from Volcanic Eruption

To be precise I have found a creek choked full of pumice that comes from an undersea eruption. I am investigating the range of environmental insults that mangrove trees are subject to and being buried in pumice is one of the strangest.

An undersea volcano to the north of New Zealand and about 2900 km from Cooktown is the source of the pumice. A small fraction of the reported 20 000 square kilometres of floating pumice arrived in North Queensland after a journey lasting more than 9 months. Large 'rafts' of pumice were reported near Cooktown in August-Sept 2013 and bands of pumice were present on Bramston Beach in November. The Cairn's Northern Beaches which lie in the middle did not get any pumice until several weeks later. Out at sea, there were widely scattered blocks of pumice and I did not see any of the rafts personally. Most of the larger blocks had goose barnacles attached and and the beach was on the nose for a few days after the pumice washed up as the goose barnacles putrefied.

pumice washed up on beach
40 m wide drift of pumice at the mouth of the inlet
Some scientist postulated that all sorts of marine organisms are rafted across the ocean on floating blocks of pumice. All I could find was a thin coat of filamentous algae and goose barnacles so I suspect that the sea mount was too remote from reefs or life on a floating block of pumice is too harsh for most organisms.

The pumice on Cairn's beaches has long since disappeared. Most is buried within the beach I suppose. It gets blown inland by cyclones and covered. Pumice is also supposed to eventually become waterlogged and sink but this must be a slow process as a creek on the southern side of Archer Point is totally clogged with the stuff.

Archer Point is one of the windiest places in Australia and it has a south-facing beach with a small inlet that is a natural pumice trap. At the mouth of the inlet is a trapped raft of pumice some 40 m wide. Flowing into the inlet is a tiny creek with a channel approximately 100 m long and 5 m wide. The headwaters of the creek lie in a very small catchment which appears to be feed mainly with seepage rather than overland flow. Seepage fed systems do not flood violently like normal creeks so the pumice in the creek is not likely to be washed out. Small creeks like this are common where hilly land meets the sea.

pumice in north Queensland near Cooktown
Pumice within the mangroves, creek in in the middle
Now that the surface of the creek has been choked with pumice for almost a full year, has there been any effect. Several of the mangroves on the bank of the creek have expired including a myrtle mangrove and some stilt mangroves. The dead mangroves appear to be in the minority and live healthy mangroves dominate in back swamps away from the channel. It seems the impacts are limited to the channel and channel margins, which is not surprising as backswamps often act like normal forests rather than tidal forests. The pumice raft did not cover the ground in these areas.
Dead mangroves on creek margin
Several dead mangroves on the margin of the creek
The channel however appears to have been devastated. The thick raft of pumice would have cut off the air supply to the creek and the organic matter with the raft of pumice or previously deposited within the creek would be decomposing and releasing hydrogen sulfide. I poked my underwater camera into the small opening in the pumice created by the strong wind and attempted to record the amazing bacterial films that were coating every surface. There was a definite structure to the strands, quite like the root system of a plant. When I lifted my hand from the water, there was a powerful stench of hydrogen sulfide. It is likely that all of the crabs and fish in this little system perished when the pumice raft blocked the sun and air (see previous posts). I prefer crabs and fish to bacterial slimes however bacterial slime are also a topic of great scientific interest as the bacterial colony which consists of several species is self-organising to organs that resemble multi-cellular life. The size and structure of these slimes is the best I have ever seen.

feathery lace of bacterial slime
Bacterial slime on mangrove leaves
long filaments of bacterial slime with white colonies
Gap in pumice raft created by the wind
Small gap in pumice through which photos were taken
This is not the first pumice raft I have seen. They are semi regular occurring every decade or so. If the volcano is closer then raft can be very thick. In some of the Pacific Islands, they have even used pumice from similar creeks to make light-weight floating concrete that can be cut with a saw!

No comments:

Post a Comment