The Roebuck Bay Intertidal Benthic Mapping Programme (ROEBIM-97)

Objectives
ROEBIM-97 was envisaged as a pilot study which would provide the groundwork for future foodweb studies, mapping studies of foraging waders, environmental impact assessments, and sedimentological and geological investigations in the Roebuck Bay area. This would be achieved by producing:

• Detailed comparative maps of sediment characteristics and the distribution of all macrobenthic invertebrate species (i.e. those retained on a 1-mm mesh sieve) living in the upper 40 cm of the sediments of Roebuck Bay exposed at low tide.
• Preliminary data on the sedimentology of Roebuck Bay based on a small number of longer cores at selected sites.
• Maps showing the other sediment features, vegetation cover and densities of crab- and mudskipper holes over the same area.
• Preliminary data on nutrient loads, organic matter and carbonate content of the sediments.
• An interpretation of any association between sediment characteristics and the diversity of the intertidal benthic community, and also comparison with intertidal wetlands elsewhere in the world.
• A detailed interpretation of the population variation of different benthic taxa on a tropical intertidal flat, something that has never before been achieved at the envisaged scale.

Methodology

• Benthic and sedimentological sampling of as much of the intertidal area of Roebuck Bay as possible based on a predetermined grid with intersections every 200 m north of Crab Creek and every 500 m south of Crab Creek. The latter area to be sampled mainly by hovercraft.
  
• At every sampling station three cores were taken with a diameter of 10.2 cm covering three times 1/120 m² (1/40 m²) to a maximum depth of 40 cm (or less if a shell layer was encountered). This sediment was sieved over a 1-mm mesh, with sieved samples carried ashore to BBO's new wet laboratory (the Pearson Laboratory) for sorting and processing. In addition, a sediment sample was cored to a depth of 10 cm with diameter 5 cm for grain-size analyses. Crab- and mudskipper-holes were counted and notes of the remaining visible epifauna were made, as were notes on sediment grain size, surface colour, etc.
• Sampling points were located according to their geographical coordinates by hand-held GPS.
• Longer geological cores to be taken at selected sites after exploratory ground surveys of the geology and sedimentology of the area.

Participation
CALM: Grant Pearson, Tanya Butler, Jim Lane; Curtin University: Ron Watkins, Lee Coshell (Coshell & Associates), Ed Oldmeadow, Bob Hickey, Markus Pepping (on exchange from the University of Kiel, Germany), Ross Carew; NIOZ: Marc Lavaleije, Theunis Piersma, Pieter Honkoop, Petra de Goeij; BBO: Mavis Russell, Danny Rogers (ANCA/University of Melbourne), Ali Pentelow, Helen MacArthur, Ian Snadden, Olivier Vachez, Sheila Foster-Nixon, Kathy Fletcher, Chris Hassell, Janet Sparrow; miscellaneous volunteers: Ted Costello, Betty Gilbert, Colma Keating, Grecian Sandwell, Carmen Benck, Rob from Broome, Matt from somewhere. It should be pointed out that the great majority of participants in ROEBIM-97 were self-funded, with limited logistical and financial support from the participating institutions.

Results

• For 14 successive days at almost every low tide, day and night, between two to four, three-person 'walking' teams were sent out to sample parts of the intertidal flats. In addition there was a two-person hovercraft team working the least accessible areas and a further team working from a boat sampling sites distant from the shore close to the Low Water Mark. When not involved in the sampling, participants spent time sorting and processing the samples, as well as maintaining/ repairing equipment.
• 800 stations were visited covering the entire northern shore of Roebuck Bay (i.e. the intertidal flats) north of Crab Creek. In addition, we covered approximately half of the eastern shoreline, the strip of soft mud that stretches towards Bush Point along the mangroves of Thangoo Station. A few exploratory cores were taken on the extensive sandy intertidal flat at Bush Point.
• Sediment and benthos cores were taken at 550 suitable stations representing about 45 km² of intertidal area: the remaining 250 sites visited were occupied by rocks and high beach and thus not habitable by benthic invertebrates. All 550 samples were quantitatively analysed for macrobenthic animals. The sediment samples were processed with regard to colour and texture and made ready for laboratory analyses at Curtin University.
• Each station has yielded a list of invertebrate species, along with their number and sizes. Most molluscs and many crustaceans were sorted to species level (even though many of these species may be presently unknown or even undescribed), but all worms (including the polychaetes, but also groups such as phoronids, nemertines and sipunculids) were sorted and counted to family level at best (polychaetes) or phylum level at worst (phoronids and nemertines, for example).
• We processed a total of about 17,000 individual animals, about 10,000 of which were tubeworms. Some of the tubeworms and all other specimens were conserved and stored for future study.
• This yielded a total of more than 200 different taxa (either species, families, or larger taxonomic groupings), suggesting a total diversity of at least 300, and possibly no fewer than 500, intertidal invertebratespecies in Roebuck Bay.
• As we extended the surveyed area, the number of taxa grew steadily. Most taxa appear quite limited in their distribution, each segment of the Roebuck Bay intertidal flats thus offering a unique assemblage and range of species!
• Five 'long' geological cores to a depth of up to 3 m, and 15 geological cores to a depth of 50 cm were taken along traverses out from the shoreline at five localities in the northern part of the bay in order to enable preliminary description of the sedimentological characteristics of the Roebuck Bay intertidal region.
• 16 sets of duplicate sediment cores (32 samples) to a depth of 20 cm were taken at equivalent stations to those of the geological cores, to enable a preliminary analysis of total organic carbon, carbonate, and nutrient elements in the surface layer of sediment from the bay's northern area.

Forward plan

• Laboratory work at the home institutions will include finalization of the identification of selected invertebrate specimens that were too difficult to bring to name with the present means, analysis of sediment samples and geological cores, analysis of nutrient levels in the intertidal surface sediments, and production of maps summarizing the distribution of the different sediments and life forms.
• All data of diverse types will be entered into a multilayer GIS developed at Curtin University (School of Surveying and Land Information). These data will then be analysed and output (maps) using the GIS software. The big advantage of the GIS being the ability to analyse and view the relationships among the disparate datasets. ArcView 3.0 has been chosen as the GIS software because of its relative ease of use and combined raster/vector capabilities.
• All invertebrates need close taxonomic scrutiny to ascertain certainty about species identification, and to get to grips with species identity or composition for all remaining taxa (over 80% of the total!). It is predicted that, given appropriate attention and support, many new and unique species will be described based on the material collected during ROEBIM-97.
• It is planned that a report bringing together the basic biological and geological data from the study will be assembled over the next year and published early in the second half of 1998.
• The completed fieldwork and material collected will provide the basis for (1) a MSc thesis by E. Oldmeadow, School of Applied Geology, Curtin University; (2) a Diploma thesis by M. Pepping at the University of Kiel; and (3) will provide the macrozoobenthic baseline data for a PhD project on wader foraging by D. Rogers at Melbourne University; and (4) give material for several other types of investigation in which other expedition members will be involved.
• Information regarding the project will be placed on a web page for viewing by any interested party (http://www.cage.curtin.edu.au/~rhickey/roebuck/).

Conclusions

• Even at the present state of assessment it is clear that the intertidal flats of Roebuck Bay are the richest known, i.e. the most biodiverse, in the world.
• This high biodiversity may be due to the intriguing sedimentology of the area that has generated a large number of different microhabitats, to its tropical location (tropical habitats generally harbouring greater biodiversity than temperate ones) and/or to its relatively pristine state.
• We believe that ROEBIM-97:
  • will generate baseline data that will be useful in a managerial context for many years to come;
  • has provided an excellent model for future large scale intertidal assessments; and
  • has yielded a wealth of intriguing data that await analysis over the forthcoming months and years.

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Dr. Robert Hickey