Roebuck Bay Invertebrate and bird Mapping 2006 (ROEBIM-06) : Research Report

1. This is a report on a survey of the benthic ecology of the intertidal flats along the northern shores of Roebuck Bay in June 2006. In the period 11-20 June we mapped both the invertebrate macrobenthic animals (those retained by a 1 mm sieve) over the whole of the northern intertidal area of Roebuck Bay and the shorebirds that depend on this food resource. The northern mudflats previously had been benthically mapped in 1997, 2000 and 2002. In addition to the mapping efforts, as a reach-out to the Broome community, the project incorporated the ‘Celebrate the Bay Forum’ on 17 June on the CALM grounds in Broome. This one-day event was visited by about 150 people and was widely considered successful in generating enthusiasm for the ecology of the bay and concerns about its future well-being.2. Our team comprised 38 participants with greatly varying levels of experience, but all with similarly high motivation and enthusiasm. We visited 532 sampling stations laid out in a grid with 200 m intersections. We made notes on the surface features of the mud, including the presence or absence of seagrasses. In the course of digging up, sieving and sorting the mudsamples from all the stations, we identified and measured more than 12,000 individual invertebrates. These animals represented 185 taxa at taxonomic levels ranging from species (bivalves, gastropods, brachiopods, some of the echinoderms and sipunculids), families (polychaete worms, crustaceans and sea anemones) to orders and phyla (Phoronida, Echiura, Nemertini and Tunicata).3. Linear seagrass Halodula uninervis and oval seagrass Halophila ovalis were quite widespread again. They showed a level of recovery to the coverage earlier reported in June 1997, after their disappearance during the passage of cyclone Rosita in June 2002.4. Of the 185 different taxa encountered in the mudcores, most had been found during earlier surveys. Nevertheless, about 26 taxa had apparently not been encountered before, including several small bivalves belonging to the Galeomnatidae. The relatively strong presence of the very small Galeomnatidae in the samples, and the relative abundance of minuscule transparent organisms such as skeleton shrimps Caprellidae retrieved, also compared with previous years, may indicate that the sorters, who routinely checked each other’s trays at the end of each sorting, did a particularly thorough job. The 12,000 individual invertebrates found in the 532 samples is similar to the number retrieved from the 1000 sampling stations visited during the mapping of all intertidal flats in Roebuck Bay in June 2002.5. At a considerable number of sampling stations across the intertidal flats we noted the presence of a new kind of large snail, the ‘ornate’ Ingrid-eating snail Nassarius bicallosus, occurring alongside the very similar scavenger Nassarius dorsatus in Roebuck Bay. Only a few individuals of Nassarius bicallosus had been found in Roebuck Bay before.6. For all six suspension-feeding (Siliqua and Anomalocardia) and deposit-feeding (Tellina) bivalves, the distributions in 2006 were remarkably similar to those recorded in the surveys of 1997 and 2002. Given the stark and repeatable gradients in sediment type and tidal height this is perhaps not surprising, but given their wide distributions across these gradients and their variable recruitment patterns, perhaps it is.7. Two 1-5 cm long species of tuskshell, or Scaphopoda, have previously been found on the intertidal flats of Roebuck Bay. The two species are pretty similar, but one has a smooth and the other a ribbed surface. In 2006 the smooth tuskshell Laevidentalium occurred widespread over all parts of the intertidal flats, living in very muddy as well as quitesandy place, but the ribbed tuskshell Dentalium only occurred at the muddier sites in the Crab Creek corner and in the muds near Dampier Creek and the nearby mangal edge.8. The long-armed brittle stars Amphiura sp. were among the most widespread species of the bay. Despite, or due to, their similarity, Amphiura tenuis and Amphiura catephes usually occurred together, A. catephes being the less numerous species, occurring much in the soft muddy areas of Crab Creek Corner where Amphiura tenuis did live.9. All polychaete worm families were very widely distributed, occurring over much broader ranges of sediment types and tidal heights than the bivalve species. These widespread distributions could perhaps be explained as a result of the summation of much more limited species-specific distributions. ‘Pickled’ specimens were collected to make a start with polychaete species assignments.10. During the previous surveys Tunicates were always at a few places in the intertidal, but in June 2006 they occurred in remarkable densities over remarkable extends of intertidal habitat along the northern shores. Probably four species were common: two or three solitary living species that were buried close to the sediment surface, sometimes occurring in carpet-like densities and always occurring in colonies, and a rooted, colonial, form that also occurred in colonies but not over the same extent as the solitary species.11. Grey-tailed Tattlers were widespread on the western flats of the bay, just as during previous surveys. In contrast, the feeding distribution of Great Knots and Red Knots which feed on bivalves and show a preference for feeding sites near the sea-edge has varied over the years. In mid June 2006, Great Knots were found over a wide area of mudflats, albeit with the highest concentrations occurring in the east of the bay. In contrast, we could only find a single feeding concentration of Red Knots – in the far east of the bay, just south of Crab Creek. This distribution of Red Knots came as a surprise to us, as the species tends to prefer slightly sandier sediments than Great Knot, e.g. the Dampier Flats. Indeed, we found rather few shorebirds on these western flats. It is possible that the cause of the discrepancy lies on high tide roosts rather than on the intertidal flats. The closest available roost sites to the Dampier Flats, Quarry Beach and Simpson’s Beach, are both heavily disturbed in the dry season. For shorebirds that cannot tolerate the disturbance levels at these roost sites and therefore roost elsewhere, the costs of commuting to the Dampier Flats to feed may be too high.12. A biodiversity hot spot analysis revealed that overall macrozoobenthic invertebrate diversity was highest in parts of the Dampier Flats and in the narrow intertidal zone just south of the Broome Bird Observatory. Overall biodiversity was negatively correlated with penetrability, a measure of the silt content of the sediments. However, when bivalves alone were considered, biodiversity peaked in areas adjacent to where overall biodiversity was highest and the relationship with siltiness was reversed: the highest diversity of bivalves was found in the muddiest parts of the intertidal flats of Roebuck Bay.