Artificial Seawalls and Rubbish Habitats

For the past couple of months, opposite the overhang on the Ellon Bridge which crosses Thurso River there has been a discarded Tesco trolley which is covered and uncovered by the tide each day. After reading this recent article (available in abstract form), with Dr. Angus Jackson as lead author, and seeing this photograph by Biting Midge of Sunshine Coast Daily Photo, I am inclined to go and pull it under the bridge out of view.

Supermarket Trolley With Barnacle-Like Crustaceans C. Biting Midge

Jackson et al. examined urbanized coastal areas around Sydney, Australia and the adaptations of intertidal species such as whelks (Moruba marginalba) and Sydney rock oysters (Saccostrea glomerata) in establishing a habitat. Whelks serve as predators in intertidal habitats; although they may feed on immature oysters, they most receive indirect resources such as food and shelter. The more oysters there are, the greater in number and larger the whelks appear to be.

Not all human development is bad for species, and oysters are known to prefer artificial seawalls, such as concrete harbour walls, to natural surfaces such as rocky shores. On the other hand, exposure on uniform artificial seawalls to distrubances such as wakes from passing vessels will dislodge growing oysters.  Increasing human construction around densely populated coastal areas is presenting oysters with this double-edged sword.

In examining artificial seawalls and rocky shores around Sydney, Jackson et al. did note that on those artificial seawalls which were protected distrubances such as the wakes of passing vessels, oyster sizes and numbers were greater than on even the more optimal natural rocky shores; with accompanied increased in whelk size and numbers.  And vice versa on exposed artificial seawalls.

With human population increases naturally tending towards the coastal areas, adaptations to the shore-line may present another indirect threat to an integral member of local marine ecology.

Back to the haunting image of urban decay displayed above (taken further up the Australian coast).  With a then-PhD student now Dr. Brianna Clynick, one of Jackson’s co-authors, Dr. Gee Chapman also authored a study into the colonization by marine life of human ribbish such as discarded trolleys (this featured in the Sydney Morning Herald in 2005).  And so it goes.

FURTHER READING: Chapman and Clynick (2006) Experiments testing the use of waste material in estuaries as habitat for subtidal organisms. Journal of Experimental Marine Biology and Ecology 338 164-78

Jackson et al. (2008) Ecological interactions in the provision of habitat by urban development: whelks and engineering by oysters on artificial seawalls. Australian Ecology 33 3 307-16

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