Exmouth Gulf Research Articles

The use of Airborne EM to investigate coastal carbonate aquifer, seawater intrusions and sustainable borefield yield at Exmouth, Western Australia

Exmouth, a regional centre located 1260km north of Perth, Western Australia relies entirely on groundwater for its water supply. Its borefield extracts groundwater from an unconfined limestone aquifer within the Cape Range Group. Groundwater flows easterly from Cape Range into Exmouth Gulf where it discharges above a saline wedge at the base of the aquifer. The current borefield extraction has insufficient capacity to meet increased water demand from population growth and tourists during holiday periods. In 2016-2017, Water Corporation decided to investigate options for improving borefield production from the existing infrastructure. This study comprised an airborne electromagnetic (AEM) survey, a desktop review, 3D hydrogeological modelling and pumping tests. The AEM survey and new hydrogeological modelling have established a clear relationship between the extent of the saltwater interface and the location of karstic features. The AEM survey effectively mapped the saline water distribution. It identified existing bores in thicker lower salinity areas and away from the saline wedge. Twenty-four hour pumping tests of these bores were undertaken, producing flow rates much higher than the established normal production rates. Hydrogeological modelling indicated that these bores could accommodate substantial additional sustainable production. The AEM survey and study also identified bores in areas of higher conductivity and salinity where extraction rates should not be increased, or should be reduced.

Drone photogrammetry and KMeans point cloud filtering to create high resolution topographic and inundation models of coastal sediment archives

AbstractCoastal areas are vulnerable to the impacts of tropical cyclones (TC), tsunamis and other water super‐elevation events, but the frequency of these events is often poorly represented by conventional records. Coastal overwash deposits (including washover fans) can provide a longer‐term archive of event frequency. Because of their low‐gradient geomorphic form, washover fans require high accuracy (centimetre‐resolution) topographic models to understand patterns of connectivity and dynamics that control archive formation. Using images collected by a remotely piloted aircraft system (RPAS, or ‘drone’) and Structure‐from‐Motion (SfM) photogrammetry techniques, we apply a novel point‐cloud filtering technique based on KMeans classification of the R‐G‐B colour of each X‐Y‐Z point to remove vegetation and create a centimetre‐resolution and accuracy bare‐earth digital terrain model (DTM) of a washover fan in Exmouth Gulf (Western Australia).Using the RPAS‐SfM orphophoto and DEM data, supported by ground‐penetrating radar (GPR) and field stratigraphic analysis, we show how this approach can be applied to understand dynamics controlling low‐gradient geomorphic landforms, using an example of a washover fan sedimentary archive in northwestern Australia created by extreme overwash events. Our approach reveals the likely role of backflooding and terrestrial runoff in creating backwater environment for sub‐aqueous deposition and good sediment preservation and identifies key areas to target for detailed dating and stratigraphic analysis of a potentially decadal to sub‐millennial resolution sediment archive of TC activity. Copyright © 2018 John Wiley & Sons, Ltd.

Chronostratigraphy and geomorphology of washover fans in the Exmouth Gulf (NW Australia) – A record of tropical cyclone activity during the late Holocene

Washover fans typically form due to barrier overwash or breaching and coastal inundation and generally represent geomorphological and depositional evidence of intense storms. Few studies have investigated the chronostratigraphy of washover fans in order to infer magnitude/frequency patterns of extreme-wave events over longer time scales. Here we present new data on the chronostratigraphy of late Holocene washover fans in the Exmouth Gulf (Western Australia) by using ground penetrating radar and unmanned aerial vehicle (UAV) survey techniques, as well as geomorphological, sedimentological and chronological investigations. This study aims to (i) provide a detailed characterization of the washover fans' geomorphology and stratigraphical architecture; (ii) document depositional processes involved in their formation; (iii) establish a chronostratigraphy based on optically stimulated luminescence (OSL); and (iv) understand the significance of the washover fans for recording past tropical cyclone (TC) activity. The fans consist of multiple sequences of sand, shell debris and coral rubble comprising depositional units related to TC-induced inundation. The units are separated by palaeosurfaces with incipient soil formation, formed during periods of reduced depositional activity. In combination with the interpretation of a UAV-based high-resolution digital surface model, multiple phases of reactivation are inferred. OSL results allow the establishment of a local long-term TC record and suggest storm-induced deposition at ∼170, ∼360, ∼850 and ∼1300 years ago. Further units were dated to ∼1950, ∼2300, and ∼2850 years ago. The chronology of TC events is consistent with other work relating TC activity with El Niño Southern Oscillation (ENSO) and sea surface temperature (SST) patterns, corroborating the regional palaeotempestological relevance of this unique geomorphological record.

Chenier-type ridges in Giralia Bay (Exmouth Gulf, Western Australia) - Processes, chronostratigraphy, and significance for recording past tropical cyclones

Past coastal flooding events may be inferred from geomorphic and sedimentary archives, including particular landforms (e.g., beach ridges, washover fans), deposits (e.g., washover sediments in lagoons) or erosional features (e.g., erosional scarps within strandplains). In Giralia Bay, southern Exmouth Gulf (Western Australia), sandy ridge sequences in supratidal elevations form the landward margin of extensive mudflats. The formation of these ridges is assumed to be mainly driven by tropical cyclones (TCs), although their depositional processes need to be clarified. We investigated the supratidal sandy ridge sequence in Giralia Bay by carrying out process monitoring, geomorphological mapping by means of an unmanned aerial vehicle survey, as well as sedimentological and geochronological investigations and multivariate statistics. Based on the resulting data, this study aims at (i) identifying the most important driving processes to form the sandy ridges; (ii) establishing their chronostratigraphy; and (iii) understanding their significance for recording past TC activity. Trench excavations revealed sandy units that are interbedded with mud layers at the base, similar to the present distal mudflat sediments. On top, mud intercalations recede, and sand layers of varying grain size distribution dominate. In the upper part of the trenches, younger sediment layers onlap older ones documenting the stepwise seaward accretion of the ridges onto the mudflat. While our data suggests that tidal processes have only limited effects on ridge activity, sediment transport, erosion and deposition seems to be driven by both TC-induced storm surges and high magnitude precipitation events causing surface discharge. Most accretionary sand units are thus assumed to represent events of morphodynamic activity during TC-induced flooding since the mid-Holocene. Ridge activity is recorded in a roughly decadal resolution and over historical as well as prehistorical/Holocene time scales. While the ridges do not represent beach or chenier ridges sensu stricto, they may be described as chenier-type ridges due to their stratigraphical architecture. Ridge evolution, however, over a millennial time scale seems to be indicated by the landward rise of the sequence possibly corresponding to the mid-Holocene sea-level highstand of Western Australia of at least 1–2m above present mean sea level.

Genomic heterogeneity and prevalence of hepandensovirus in Penaeus esculentus from Western Australia, and P. merguiensis from the Gulf of Carpentaria, Australia

Decapod Hepandensovirus 1 (HDV), formerly known as hepatopancreatic parvovirus, has been associated with stunting, lowered production and outright mortalities in prawns in aquaculture. Despite the fact that broodstock are sourced and aquaculture farms are planned in the regions of northern and Western Australia, data on these parvoviruses from this region are limited. The prevalence of HDV in Penaeus esculentus and Penaeus merguiensis is moderate (34–51%) in southern Western Australia, Exmouth Gulf and the Gulf of Carpentaria but statistically higher (P<0.05) in Shark Bay (82%). We speculate this is due to the topography of Shark Bay combined with the currents of the Indian Ocean gyre (IOG). Despite an on average 8–12% genomic heterogeneity, the nucleotide sequences of HDV in WA most closely align with HDV in regions associated with the IOG; Thailand, India, Tanzania, Madagascar; eastern Asia, Korea and less commonly, with sequences from the eastern coast of Australia. This potentially changes the paradigm of a single strain of HDV being ubiquitous in Australia and there was little risk in moving broodstock from WA to the eastern states, so there was no testing of broodstock for HDV. There is no strong evidence to clarify whether the strain of HDV in WA P. esculentus came from either its' nearest genetic relatives, P. monodon or P. semisulcatus or from P. merguiensis from the Solanderian province of Australia. P. esculentus HDV appears to be most related to strains within the IOG. The HDV nucleotide heterogeneity of wild prawns contrasts strongly to studies undertaken with prawns from aquaculture where genetic selection may have occurred.

Quantifying shark depredation in a recreational fishery in the Ningaloo Marine Park and Exmouth Gulf, Western Australia

The authors thank all of the recreational fishers interviewed throughout the boat ramp surveys, without whom the study would not have been possible. Gratitude is also extended to the Department of Biodiversity Conservation and Attractions, Government of Western Australia, for supplying boat ramp visitation data, and to the US National Oceanic and Atmospheric Administration for providing SST data for this study. Peter Barnes from the Department of Biodiversity Conservation and Attractions provided local support and information relating to the fishery and the Ningaloo Marine Park. Thanks also go to Claire Smallwood and Karina Ryan from The Department of Primary Industries and Regional Development, Government of Western Australia, for their advice on boat ramp survey design. Brett Molony and Stephen Newman from the Department of Primary Industries and Regional Development also provided important background information on the recreational and commercial fisheries in this region, as well as useful feedback on the manuscript. Matias Braccini is thanked for offering guidance on the statistical analysis conducted in this study. Matt Gillett and Leyland Campbell from Recfishwest provided information on recreational fishing in Western Australia. Lastly, the authors thank Rory McAuley, Alastair Harry and Hannah Zilles from The Department of Primary Industries and Regional Development, Government of Western Australia, as well as 3 other anonymous reviewers, for providing feedback on the manuscript. This study was funded by the Australian Federal Government and The University of Western Australia, through an International Postgraduate Research Scholarship and Australian Postgraduate Award for J.D.M, with additional funding also received from the Jock Clough Marine Foundation.

Pseudotrichonotus belos new species, first record of the fish family Pseudotrichonotidae from Australia (Teleostei: Aulopiformes).

Pseudotrichonotus belos new species, described from three specimens trawled in 100-120 m offshore between Exmouth Gulf and Shark Bay, Western Australia, represents the first record of the sand-diving fish family Pseudotrichonotidae from Australian waters. It differs from its two congeners in having a more posteriorly positioned dorsal fin (predorsal length 39.6-41.2% SL) and fewer dorsal- and anal-fin rays (31-33 and 12, respectively).

Distribution, abundance, diversity and habitat associations of fishes across a bioregion experiencing rapid coastal development

Knowledge of the factors that influence spatial patterns in fish abundance, distribution and diversity are essential for informing fisheries and conservation management. The present study was conducted in the nearshore Pilbara bioregion of north-western Australia where the dynamic marine environment is characterised by large embayments, numerous islands and islets, coexisting with globally significant petrochemical and mineral industries. Within Western Australia, this nearshore bioregion has high biodiversity and is considered to play an essential role in the recruitment of species of commercial importance. To better inform future investigations into both ecological processes and planning scenarios for management, a rapid assessment of the distribution, abundance and associations with nearshore habitats of fishes across the region was conducted. Baited remote underwater stereo-video systems (stereo-BRUVs) were used to simultaneously sample the fish assemblage and habitat composition. Generalised additive mixed models (GAMMs) were used to determine whether the abundance of fishes were related to habitat and a range of environmental variables (visibility, depth, distance to 30 m and 200 m depth isobars, boat ramps and the nearest large embayment (Exmouth Gulf). A diverse fish assemblage comprising 343 species from 58 families was recorded. The abundance and distribution patterns of fishery-target species and of the five most common and abundant species and families were linked positively with areas of high relief, hard coral cover, reef and macroalgae and negatively with the distance to the nearest oceanic waters (200 m depth isobar). This study provides information that can contribute to future marine spatial planning scenarios for management of the Pilbara using a unique, analytical approach that has broad application in biogeography.

Rapid Reactivation of Cyanobacterial Photosynthesis and Migration upon Rehydration of Desiccated Marine Microbial Mats.

Desiccated cyanobacterial mats are the dominant biological feature in the Earth’s arid zones. While the response of desiccated cyanobacteria to rehydration is well-documented for terrestrial systems, information about the response in marine systems is lacking. We used high temporal resolution hyperspectral imaging, liquid chromatography, pulse-amplitude fluorometry, oxygen microsensors, and confocal laser microscopy to study this response in a desiccated microbial mat from Exmouth Gulf, Australia. During the initial 15 min after rehydration chlorophyll a concentrations increased 2–5 fold and cyanobacterial photosynthesis was re-established. Although the mechanism behind this rapid increase of chlorophyll a remains unknown, we hypothesize that it involves resynthesis from a precursor stored in desiccated cyanobacteria. The subsequent phase (15 min–48 h) involved migration of the reactivated cyanobacteria toward the mat surface, which led, together with a gradual increase in chlorophyll a, to a further increase in photosynthesis. We conclude that the response involving an increase in chlorophyll a and recovery of photosynthetic activity within minutes after rehydration is common for cyanobacteria from desiccated mats of both terrestrial and marine origin. However, the response of upward migration and its triggering factor appear to be mat-specific and likely linked to other factors.

Plectorhinchus caeruleonothus, a new species of sweetlips (Perciformes: Haemulidae) from northern Australia and the resurrection of P. unicolor (Macleay, 1883), species previously confused with P. schotaf (Forsskål, 1775).

Two distinct haemulid fishes from Australia and the Indo-Australian Archipelago respectively have long been confused with Plectorhinchus schotaf (Forsskål, 1775). Plectorhinchus caeruleonothus sp. nov. is described from 17 specimens collected off western and far northern Australia, between the Monte Bello Islands, Western Australia and Torres Strait, Queensland. It has also been confirmed outside this range by photographs taken at Ningaloo Reef and Exmouth Gulf, Western Australia, and at Claremont Isles and Lizard Island, Queensland. The new species is unique among the genus in having a combination of dorsal-fin rays XII, 18-20, lateral-line scales 56-61, gill rakers 7-9 on the upper limb and 18-20 on the lower limb of the first arch, nostrils minute, and fresh colouration in adults including body uniformly grey, cheek, opercles and posterior margin of the opercular membrane uniformly blue-grey, and rim of orbit and upper edge of maxilla dusky yellow. In contrast to its closest congeners, the juveniles have a distinctive pattern of narrow creamish-white to pale grey stripes on a dark grey to chocolate brown background on the head and body, and oblique dark stripes progressing with growth to spots on the caudal fin. Plectorhinchus unicolor (Macleay, 1883) from Japan to northern Australia is resurrected from the synonomy of P. schotaf and redescribed on the basis of the holotype and 24 non-type specimens. Plectorhinchus unicolor is most similar to P. schotaf, but can be distinguished by fresh colouration, modal dorsal and pectoral-fin ray counts and DNA barcoding. Plectorhinchus schotaf appears to be restricted to the region from southeast Africa to the Arabian Sea, including the Red Sea and Persian Gulf. Plectorhinchus griseus (Cuvier in Cuvier & Valenciennes, 1830) from Indian and Sri Lankan Seas has previously been treated as a junior synonym of P. schotaf, but in accordance with Smith (1962), is here confirmed as a valid species, readily distinguished from the latter by a concavity in the lateral profile of the snout in adults, deep body and high soft dorsal-fin ray count. Comparison of the CO1 genetic marker utilised in DNA barcoding also resulted in significant genetic divergences between the new species, P. unicolor and their closest sampled congeners. Some behavioural observations are also presented for the species treated, including aggressive interactions between individuals of the new species, the likes of which have not previously been recorded among species of Plectorhinchus.

Echolocation parameters of Australian humpback dolphins (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the wild.

Echolocation is a key sensory modality for toothed whale orientation, navigation, and foraging. However, a more comparative understanding of the biosonar properties of toothed whales is necessary to understand behavioral and evolutionary adaptions. To address this, two free-ranging sympatric delphinid species, Australian humpback dolphins (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus), were studied. Biosonar clicks from both species were recorded within the same stretch of coastal habitat in Exmouth Gulf, Western Australia, using a vertical seven element hydrophone array. S. sahulensis used biosonar clicks with a mean source level of 199 ± 3 dB re 1 μPa peak-peak (pp), mean centroid frequency of 106 ± 11 kHz, and emitted at interclick intervals (ICIs) of 79 ± 33 ms. These parameters were similar to click parameters of sympatric T. aduncus, characterized by mean source levels of 204 ± 4 dB re 1 μPa pp, centroid frequency of 112 ± 9 kHz, and ICIs of 73 ± 29 ms. These properties are comparable to those of other similar sized delphinids and suggest that biosonar parameters are independent of sympatric delphinids and possibly driven by body size. The dynamic biosonar behavior of these delphinids may have, consequently, allowed for adaptations to local environments through high levels of control over sonar beam properties.

Molecules and morphology reveal overlooked populations of two presumed extinct Australian sea snakes (Aipysurus: Hydrophiinae).

The critically endangered leaf-scaled (Aipysurus foliosquamaI) and short-nosed (A. apraefrontalis) sea snakes are currently recognised only from Ashmore and Hibernia reefs ~600km off the northwest Australian coast. Steep population declines in both species were documented over 15 years and neither has been sighted on dedicated surveys of Ashmore and Hibernia since 2001. We examine specimens of these species that were collected from coastal northwest Australian habitats up until 2010 (A.foliosquama) and 2012 (A. apraefrontalis) and were either overlooked or treated as vagrants in conservation assessments. Morphological variation and mitochondrial sequence data confirm the assignment of these coastal specimens to A. foliosquama (Barrow Island, and offshore from Port Hedland) and A.apraefrontalis (Exmouth Gulf, and offshore from Roebourne and Broome). Collection dates, and molecular and morphological variation between coastal and offshore specimens, suggest that the coastal specimens are not vagrants as previously suspected, but instead represent separate breeding populations. The newly recognised populations present another chance for leaf-scaled and short-nosed sea snakes, but coastal habitats in northwest Australia are widely threatened by infrastructure developments and sea snakes are presently omitted from environmental impact assessments for industry. Further studies are urgently needed to assess these species’ remaining distributions, population structure, and extent of occurrence in protected areas.

Radiocarbon dating and wood density chronologies of mangrove trees in arid Western Australia.

Mangrove trees tend to be larger and mangrove communities more diverse in tropical latitudes, particularly where there is high rainfall. Variation in the structure, growth and productivity of mangrove forests over climatic gradients suggests they are sensitive to variations in climate, but evidence of changes in the structure and growth of mangrove trees in response to climatic variation is scarce. Bomb-pulse radiocarbon dating provides accurate dates of recent wood formation and tree age of tropical and subtropical tree species. Here, we used radiocarbon techniques combined with X-ray densitometry to develop a wood density chronology for the mangrove Avicennia marina in the Exmouth Gulf, Western Australia (WA). We tested whether wood density chronologies of A. marina were sensitive to variation in the Pacific Decadal Oscillation Index, which reflects temperature fluctuations in the Pacific Ocean and is linked to the instrumental rainfall record in north WA. We also determined growth rates in mangrove trees from the Exmouth Gulf, WA. We found that seaward fringing A. marina trees (∼10 cm diameter) were 48±1 to 89±23 years old (mean ± 1σ) and that their growth rates ranged from 4.08±2.36 to 5.30±3.33 mm/yr (mean ±1σ). The wood density of our studied mangrove trees decreased with increases in the Pacific Decadal Oscillation Index. Future predicted drying of the region will likely lead to further reductions in wood density and their associated growth rates in mangrove forests in the region.

The Yannarie Solar Project: Design of a Solar Saltfield in Western Australia to Safeguard the Natural Environment

The Northwest Coast of Western Australia is the location for a number of large solar saltfields. More than 10 million tonnes of high grade solar salt is exported annually from these saltfields; predominantly servicing the chloralkali industries of Northern and Southeast Asia. Straits Resources Limited is a mining company with operations in Australia and Indonesia. It has identified the solar salt industry as an opportunity to diversify its resource portfolio and build a longer-term position within the resource sector. Access has been approved by the Government of Western Australia to a large area in the eastern Exmouth Gulf region of Western Australia suitable for a solar saltfield with an ultimate capacity as high as 10 million tonnes per annum. All new resources projects in Australia must proceed through a rigorous environmental approval process at both the Federal (Commonwealth) and State Government levels. Straits commissioned a team of saltfield design, environmental and engineering consultants to design an economically viable saltfield that minimises impacts to the environment. There has been a series of iterative changes in its design based on feedback from environmental and cultural heritage studies. This has enabled the saltfield to be specifically located within a defined footprint to avoid sensitive areas such as mangroves, tidal creeks and algal mats. Comprehensive studies have been undertaken on the local marine and terrestrial flora and fauna (including migratory bird and marine fauna), together with surveys for cultural heritage, soils, hydrology and a sweep of other parameters including hydrodynamic modelling of the marine environment. A commercial trawling fishing industry operates in the waters of Exmouth Gulf that is also the permanent home or on the migratory path of a number of significant marine fauna, including whales, turtles, and dugongs. The project, known as the Yannarie Solar Project, is progressing through the environmental approval processes of the Australian Commonwealth and Western Australian Governments. The conclusion is that the technical findings of the suite of studies that examined the environmental aspects of the engineering requirements of the saltfield provide a sound basis for project approval. Assuming that approval is given, and the current schedule maintained, construction would commence in 2008 and shipments of salt in 2011.

Impact of cyclones and aquatic macrophytes on recruitment and landings of tiger prawns Penaeus esculentus in Exmouth Gulf, Western Australia

The cover of seagrasses and macroalgae, landings and fishery-independent measures of spawning stock and recruitment for brown tiger prawns, were monitored immediately following a major cyclone in Exmouth Gulf, Western Australia in March 1999. Anecdotal evidence on the extent of seagrass from 1990 to 1998 suggests that the cyclone caused a major, immediate disruption and loss of the seagrass/macroalgal beds (to ≤2% cover), the critical prawn nursery habitat, and mangroves in the shallow inshore waters of the system. Prawn landings and recruitment to the fishery were not affected in the year of the cyclone, but were markedly lower in the two years immediately afterwards and then increased as the cover of macrophytes increased to over 40% in 2003. Tiger prawn landings and catch rates were not affected in Shark Bay, a system 500 km south of Exmouth Gulf that did not experience cyclonic disturbance. Seagrasses in Exmouth Gulf showed a succession of species from small colonising species (Halophila ovalis and Halodule uninervis) to larger, broad-leaved species (Cymodocea serrulata, Syringodium isoetifolium) only two years after the cyclone. The recruitment and landings of tiger prawns were correlated significantly with the total cover of macroalgae and seagrass. The large loss of seagrass and macroalgae reduced the settling habitat for postlarvae and the nursery habitat for juvenile tiger prawns, probably leading to the lower recruitment to the fishery. These findings suggest that the extent of seagrass and macroalgae are some of the factors defining the productivity of the tiger prawn fishery in Exmouth Gulf.

Unprecedented mass bleaching and loss of coral across 12° of latitude in Western Australia in 2010-11.

BackgroundGlobally, coral bleaching has been responsible for a significant decline in both coral cover and diversity over the past two decades. During the summer of 2010–11, anomalous large-scale ocean warming induced unprecedented levels of coral bleaching accompanied by substantial storminess across more than 12° of latitude and 1200 kilometers of coastline in Western Australia (WA).Methodology/Principal FindingsExtreme La-Niña conditions caused extensive warming of waters and drove considerable storminess and cyclonic activity across WA from October 2010 to May 2011. Satellite-derived sea surface temperature measurements recorded anomalies of up to 5°C above long-term averages. Benthic surveys quantified the extent of bleaching at 10 locations across four regions from tropical to temperate waters. Bleaching was recorded in all locations across regions and ranged between 17% (±5.5) in the temperate Perth region, to 95% (±3.5) in the Exmouth Gulf of the tropical Ningaloo region. Coincident with high levels of bleaching, three cyclones passed in close proximity to study locations around the time of peak temperatures. Follow-up surveys revealed spatial heterogeneity in coral cover change with four of ten locations recording significant loss of coral cover. Relative decreases ranged between 22%–83.9% of total coral cover, with the greatest losses in the Exmouth Gulf.Conclusions/SignificanceThe anomalous thermal stress of 2010–11 induced mass bleaching of corals along central and southern WA coral reefs. Significant coral bleaching was observed at multiple locations across the tropical-temperate divide spanning more than 1200 km of coastline. Resultant spatially patchy loss of coral cover under widespread and high levels of bleaching and cyclonic activity, suggests a degree of resilience for WA coral communities. However, the spatial extent of bleaching casts some doubt over hypotheses suggesting that future impacts to coral reefs under forecast warming regimes may in part be mitigated by southern thermal refugia.

Variation in wood density and anatomy in a widespread mangrove species

Wood density is an important plant trait that influences a range of ecological processes, including resistance to damage and growth rates. Wood density is highly dependent on anatomical characteristics associated with the conductive tissue of trees (xylem and phloem) and the fibre matrix in which they occur. Here, we investigated variation in the wood density of the widespread mangrove species Avicennia marina in the Exmouth Gulf in Western Australia and in the Firth of Thames in New Zealand. We assessed how variation in xylem vessel size, fibre wall thickness and proportion of phloem within the wood contributed to variation in wood density and how these characteristics were linked to growth rates. We found the wood density of A. marina to be higher in Western Australia than in New Zealand and to be higher in taller seaward fringing trees than in scrub trees growing high in the intertidal. At the cellular level, high wood density was associated with large xylem vessels and thick fibre walls. Additionally, wood density increased with decreasing proportions of phloem per growth layer of wood. Tree growth rates were positively correlated with xylem vessel size and wood density. We conclude that A. marina can have large xylem vessel sizes and high growth rates while still maintaining high wood density because of the abundance and thickness of fibres in which vessels are found.

The North West Cape, Western Australia: A Potential Hotspot for Indo-Pacific Humpback Dolphins Sousa chinensis?

Indo-Pacific Humpback Dolphins Sousa chinensis (Humpback Dolphins hereafter) are listed as ‘near threatened’ on an international level and ‘migratory’ in Australian waters. There is limited information on Humpback Dolphins in Western Australian State waters, where the species remains unlisted. This lack of knowledge hinders the management and conservation of the species in a region of rapidly increasing coastal development. We conducted opportunistic boat-based surveys in April 2010 and present data on the location, size and composition of Humpback Dolphin groups encountered in the near-shore waters around the North West Cape, Western Australia. A total of 42 groups were encountered in a variety of habitats during 145 h on the water over ca. 80 km coastline. Group size ranged from 1 to 15, with a mean (± SE) of 5.3 (± 0.48) individuals. A total of 54 Humpback Dolphins were identified from photographs of the unique markings on their dorsal fins. The lack of a plateau in the cumulative discovery curve of identified individuals over the duration of the study suggests that only a subset of dolphins in the area was identified. This region is close to the south-western limit of the species’ Australian distribution and appears to represent an important location for Western Australian Humpback Dolphins. In light of increasing anthropogenic activity around the North West Cape and Exmouth Gulf, these preliminary findings from a limited survey effort indicate that further research into this population is required.

Nutrient exchange of extensive cyanobacterial mats in an arid subtropical wetland

Cyanobacterial mats cover extensive areas of subtropical arid coastal wetlands and are sites of active nutrient exchange. To assess spatial (low v. high in the intertidal zone) and temporal (day v. night) variability in nitrogen (N) exchange in arid Exmouth Gulf, Western Australia, we measured nutrient exchange (NOx–-N, NH4+ and soluble reactive phosphorus) during tidal inundation and N fixation of cyanobacterial mats before and during an unusual period of heavy rainfall. Additionally, we investigated the species composition within the cyanobacterial mat. We hypothesised that nutrients are released to the floodwater during tidal inundation, that N fixation is a significant path of N incorporation, that highest N fixation rates occur in the low intertidal zone at night, and that the cyanobacterial mat community composition varies across the intertidal zone. Our results showed that nutrients were removed from the floodwater during tidal inundation. N fixation accounted for 34% of N incorporation, with highest rates in the lower intertidal zone during the day. The cyanobacterial mat was dominated by Microcoleus chthonoplastes, but composition varied across the intertidal zone. The present study provided evidence of temporal and spatial variability in nutrient exchange and implied an important role of cyanobacterial mats in coastal production.

Histopathology of oedema in pearl oysters Pinctada maxima

In October 2006, severe mortalities (80 to 100%) were reported in pearl oyster Pinctada maxima production farms from Exmouth Gulf, Western Australia. Only P. maxima were affected; other bivalves including black pearl oysters P. margaratifera remained healthy. Initial investigations indicated that the mortality was due to an infectious process, although no disease agent has yet been identified. Gross appearance of affected oysters showed mild oedema, retraction of the mantle, weakness and death. Histology revealed no inflammatory response, but we did observe a subtle lesion involving tissue oedema and oedematous separation of epithelial tissues from underlying stroma. Oedema or a watery appearance is commonly reported in published descriptions of diseased molluscs, yet in many cases the terminology has been poorly characterised. The potential causes of oedema are reviewed; however, the question remains as to what might be the cause of oedema in molluscs that are normally iso-osmotic with seawater and have no power of anisosmotic extracellular osmotic regulation.