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Why so biodiverse? Rich pickings in the Roebuck Bay mud

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Professor Theunis Piersma (left) from the University of Groningen and the Royal Netherlands Institute for Sea Research is one of many scientists who are regularly attracted to Roebuck Bay in Western Australia. In this interview he talks about his decade of research surveys there and reveals why Roebuck Bay is such a unique place.   

Subject to enormous tides, the bay is home to very dynamic tropical tidal mudflats stretching over an area far larger than Sydney Harbour. Roebuck Bay is a very rich biogeographical area with varied sediments, rocks and mangroves that harbour different communities.

First begun in 1997, during each of their research expeditions, Theunis and his colleagues’ goal is to map out the benthos of Roebuck Bay. Benthos is the myriad of animals living in the top layer of the sediments of the bay in the mud and the sand.

Above: Theunis Piersma presenting some of the group's findings during a community seminar in Broome.
Photo from Jan Drent.

Right: With the more remote areas only exposed for a few hours at a time even during the very low tides (for example the Spring tides), time is of the essence. The team uses a small hoverraft to cross the flats in order to reach the less accessible areas of deep mud.
Photo from Petra de Goeij


The research concentrates on the top layer of the sediments, even using a hovercraft to cross the vast stretches of deep mud. A grid system is superimposed over the research area and a hand held GPS system locates the 600 grid points that are spread out over 40 square kilometres on the north end of the bay.

At every grid point the mud is sampled by pushing a plastic tube to the depth of a metre or so into the sediments and extracting a core (left).
The contents are then washed over a fine sieve to separate out the living creatures from the debris.

Photo from Jan Drent.

These are then taken back to the laboratory set up beside the Broome Bird Observatory and teams of researchers work frantically to identify the creatures from each sample (below) . In this way from the hundreds of samples collected, gradually a picture can be built up of the life in different parts of the mudflats. Photo from Petra de Goeij.

While these are very small samples they can look at large scale patterns and the 40 square kilometres they are mapping. The research has now mapped the northern end of the Bay four times over a ten year period, making it possible to look at changes in the benthos over this period.
What is emerging is that the ecosystems in Roebuck Bay are varied and dynamic and are influenced by many things.


When the first survey was taken, the northern shores of the bay were covered in seagrass, but in 2004 (after Cyclone Rosita hit in 2000) there was none. However, in 2006, the seagrass beds had recovered, suggesting that cyclones have a far greater impact on biodiversity in the sediments of the bay than previously thought. Similarly the animals themselves are not constant. In 1990, for instance there was a big settlement of a particular small mussel but they have since died out. Recently there has been a surge in tunicates, small invertebrates that live in sand pockets (and look like a grain of sand) although these were previously very uncommon.

A major interest is what has been the influence of humans on the dynamics of the bay. Several have been identified so far including the seepage of waste water from Broome town. This has been shown to affect the bay’s algal composition which in turn forms the food sources for many of the invertebrates in the sediments. Clearly there has already been a large influence on the birds of the bay, evidenced, for example, by the 60% decline in numbers of birds using the northern shore of the bay (the area closest to Broome and the only accessible part of the bay via Crab Creek Road) over the past 15 years. 


Above: 'Mudbashers' returning from sampling session. Photo from Grant Pearson.
There are several hundred square kilometres of intertidal flats in the bay and Theunis and colleagues have only mapped ten percent in the northern corner. Especially in the Crab Creek corner at the time of the Spring tides the view is of mud flats stretching out almost endlessly.
Below: They are home to a myriad mudskippers - just one of the countless number of intertidal species in the bay about which scientists know, at best, very little. Photo from Chris Hassell


Why is Roebuck Bay of such scientific interest? Basically it stems from its outstanding biodiversity.    
There are four main reasons that help explain the bay's rich biodiversity:
* it is in a very biogeographically rich region: overall, marine biodiversity in the indo-pacific region is high and this is probably because over eons there have been very few changes in the environment so there has been time for many species to evolve and adapt to specialised habitats. * it has a wide range of different habitats because it has a great diversity of very different sediments.
* it is a buffered environment and so the habitats are retained and not scowered or altered by heavy wave or river actions.
* so far the touch of human hand has been light.

However Theunis is disturbed that over his ten years of expeditions to the bay, nothing has happened to give the bay higher protection. It is vulnerable to the impacts from increase in human use for example, disturbance of the roosts, fishing and agriculture could all have serious effects.
Worryingly, the northern shore is ‘a no man’s land’ - it is not a national park or protected from development.
The only protection for the area is a Ramsar listing. With Broome and the Kimberley generally suffering increased population pressures as a result of the WA mining boom there is no guarantee that these uniquely diverse mudflats are off limits from their direct or indirect consequences.

Text: V.B. December 2008

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