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This project has been funded by the Department of Parks and Wildlife partnership with BHP Billiton “Eighty Mile Beach and Walyarta Conservation Program”, with in-kind support from NIOZ and Wetland Research & Management This report was produced at the Broome Bird Observatory in late October 2016. Abstract 1. This is a report on repeat surveys on the state of the benthic invertebrates at two internationally important areas of intertidal mudflats in northwest Australia (Roebuck Bay and Eighty Mile Beach) during October 2016. In the period 6-19 October 2016, we mapped the invertebrate macrobenthic animals (those retained by a 1 mm sieve) at the main intertidal sites of West Kimberley, WA: Eighty Mile Beach and Roebuck Bay. We revisited almost the entire intertidal area along Eighty Mile Beach that was ‘benthically’ mapped in October 1999. The benthic animals of the northern mudflats of Roebuck Bay had been mapped in 1997, 2000, 2002, and 2006; we revisited as many as possible of these previously established sampling stations along the northern shore. 2. Our team comprised close to 100 participants with greatly varying levels of experience, though similarly high motivation and enthusiasm. At Eighty Mile Beach we visited 816 sampling stations laid out in a grid of 200 m intersections over 7 separate areas along ca. 75 km of beach (from 10 km north of the Anna Plains Station beach access to 65 km south). In the northern part of Roebuck Bay, we visited 534 sampling stations also laid out in a grid with 200 m intersections (but with distance of 400 m in the southeast). We made notes on the surface features on the mud, including the presence or absence of seagrass and various macrofauna. In the course of digging up, sieving, and sorting the mud samples from all stations, we identified and measured 32,500 individual invertebrates. We tried to identify all animals groups up to the level of species if possible, all on the basis of morphological differences. These species were often given field names, as time and means (literature or access to internet) did not allow us to always attach a proper scientific name. In addition, it is very likely that some of the species are still undescribed. Animals were preserved on ethanol for a more thorough scientific identification at a later date. 3. This time we surveyed two very distinct sections of the West Kimberley coast. Roebuck Bay represents a true embayment that is semi-enclosed by mangroves along the eastern, and some of the western shores, and by cliff and pindan woodlands in the north. Eighty Mile Beach stretches over 200 km along the open Indian Ocean facing northwest. In this environment, the intertidal mud- and sandflat area stretches from 1-5 km wide from shore to sea and is enclosed by sand dunes and a few mangroves. Despite the two systems being very important as nonbreeding areas for the same species of long-distance migrant shorebirds, their geomorphology and ecology are very distinct. 4. At both areas the biodiversity of benthic animals was very high compared with other intertidal soft sediment areas across in the world. In Roebuck Bay, 368 species were found, and at Eighty Mile Beach 156 species, providing a total of 433 species/taxa. The most diverse group were the Polychaeta with 167 species, followed by Crustacea (74), Bivalvia (59), Gastropoda (59), and Echinodermata (35). All other groups total less than 12 species. 5. The two areas have 92 species in common, which in the case of Eighty Mile Beach means that 60% of the species also occur in Roebuck Bay. Major groups not found at Eighty Mile Beach were Asteroidea, Brachiopoda, Hirudinea, Oligochaeta, Platyhelminthes, Polyplacophora, and Pycnogonida. Most of these groups were also rare in Roebuck Bay, but the absence of Brachiopoda (lamp-shells) and Pycnogonida (seaspiders) at Eighty Mile Beach came as a surprise. Some species, including two species of Spionidae (Polychaeta) common at Eighty Mile Beach were either absent or extremely rare at Roebuck Bay. Furthermore, a small seacucumber with dark coloured spots all over its body, another larger seacucumber Paracaudina chilensis, two bivalve species of the genus Tellina, and two species of anemones were not found in Roebuck Bay. Yet, Roebuck Bay had many more species (277) not found at Eighty Mile Beach, the most common of these was the relatively large bivalve Tellina piratica, followed by the smooth tusk shell (Laevidentalium lubricatum), and the polychaete family Sternaspidae. 6. The large difference in biodiversity between Roebuck Bay and Eighty Mile Beach may be caused by different factors. Roebuck Bay has a greater variety of sedimentary habitats than Eighty Mile Beach. Eighty Mile Beach is completely exposed to the waves of Indian Ocean, while Roebuck Bay is protected by the peninsula on which Broome is situated. Therefore, notorious ‘ecosystem engineers’ such as the seagrasses occur quite extensively on the intertidal area of Roebuck Bay, but are not found at Eighty Mile Beach. These seagrass mats of Halodula uninervis and Halophila ovalis form special habitat for e.g. the little snail Smaragdia souverbiana. The influence of Broome city by episodic sewage and fertilizer releases, of which blooms of the cyanobacteria Lyngbya are an indication, can may well have a negative influence; opportunistic widespread species other than Lyngbya may of course benefit from the additional nutrient inputs. 7. Local communities and the land-owners actively participated in both expeditions. Several DPAW ranger groups (Yawuru, Karajarri and Nyangumarta) joined the sampling and also aided in the identification of species. Angela Rossen (WAMSI) spearheaded a biodiversity project that involved pupils from Cable Beach Primary School. We believe that we have raised wide awareness and generated considerable enthusiasm for the ecology of a unique contribution of northwest Australia to the world. 8. Based on their outstanding universal values, we recommend that the WA government consider an application of the joint marine reserves of Eighty Mile Beach and Roebuck Bay for World Heritage Status, thus joining China and South Korea in acknowledging and protecting this shared heritage.