Tropical Northern Australia Research Articles

Breeding biology of the White-winged Robin Peneothello sigillata, endemic to montane New Guinea

I studied the breeding biology of the high-elevation montane White-winged Robin Peneothello sigillata (Petroicidae) at Kumul Lodge, southern slope of Mt Hagen, Papua New Guinea, from 2 October to 17 November 2011. Three nests were watched for 18 hours during incubation; each had one egg. Female daylight incubation constancy (percentage of daylight hours spent in incubation) was 73%, the mean duration of incubation sessions was 3.35 minutes and mean absence from the nest was 1.33 minutes, the daily number of incubation sessions was 150 (12.5 per hour), feeding of the incubating female by the male at the nest was 4.6 times/hour, and the total number of nest visits by both male and female during incubation was 17.1/hour. The 100% egg-success in this species suggests that these incubation reproductive traits evolved more in response to food availability and perhaps food limitation than a high risk of nest predation. Nestling care by the female included brooding and the feeding of the young. Nestling care by males included delivering food to the female and feeding the nestling. Both the male and the female removed faecal sacs. Mean brooding constancy (percentage of time spent brooding) was 43.3% for the entire 17–21-day nestling period. Mean number of food-delivery trips by both the male and the female over the entire nestling period was 7.2/hour/nestling and the total number of nest visits by both the male and the female was 17.6/hour. Two nests were attended solely by the breeding pairs, and cooperative breeding, with one immature helper, occurred at one other nest. During the nestling stage, the high nest success, coupled with high nest visitation, suggests that the risk of nest predation is low. Peak nestling growth (to >90% of adult weight) occurred at 10–13 days of age. To elucidate the breeding strategy of the White-winged Robin, its reproductive traits are discussed and compared with those of other robins of New Guinea, tropical northern Australia, and temperate Australia and New Zealand.

Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

AbstractRiparian trees located in seasonally dry environments may be reliant on groundwater supplies, but the prevalence and magnitude of groundwater uptake is often unclear. Using soil water matric potential and water stable isotopes, we examined the relative contributions of soil water and groundwater to the dry season water uptake of five riparian tree species along an intermittent river of tropical northern Australia. Because xylem water was depleted in deuterium relative to source water (average offset −14.0‰), we numerically removed this offset and assessed the effect of the correction on mixing model results. We also estimated the isotopic composition of unbound soil water (i.e., the portion of soil water not tightly bound to soil particles) from bulk soil water data by using an empirical formulation from the literature and tested whether considering unbound soil water as a source would affect our results. Despite the hot and dry surface environment, we found that soil moisture was available for trees at relatively shallow (~0.7–1.5 m) depths. When unbound soil water and corrected xylem water data were considered, most tree species used a combination of this soil moisture source and groundwater from the capillary fringe. However, not correcting for isotopic effects resulted in large underestimations of the groundwater contributions to tree water uptake. Our findings suggest that ignoring soil isotopic effects and deuterium depletion in xylem water may reduce the validity of source water partitioning assessments. Further research is needed on the likely causes for deuterium depletion in xylem water.

Stream Suspended Mud as an Indicator of Post-Mining Landform Stability in Tropical Northern Australia

Mining can cause environmental disturbances and thus mined lands must be managed properly to avoid detrimental impacts in the future. They should be rehabilitated in such a way that post mining landforms behave similarly as the surrounding stable undisturbed areas. A challenge for government regulators and mine operators is setting closure criteria for assessment of the stability of the elevated post-mining landforms. Stability of a landform is often measured by the number and incision depth of gullies. This can assess mass stability and bulk movement of coarse material. However, there is a need for a more sensitive approach to assess catchment disturbances using the concept of waves of fine suspended sediment and thus determine the dynamics of recovery of a post mining landform. A more environmentally meaningful approach would be to assess the fine suspended sediment (FSS, silt + clay (0.45 µm < diameter < 63 µm)) leaving the system and entering downstream waterways. We propose assessing stability through relationships between rainfall event loads of FSS and event discharge (Q) in receiving streams. This study used an innovative approach where, instead of using instantaneous FSS concentration, it used total FSS load in waves of sediment driven through the system by rainfall runoff events. High resolution stream monitoring data from 2004 to 2015 in Gulungul and Magela Creeks, Northern Territory, Australia, were used to develop a relationship between sediment wave and event discharge, ∑FSS α f(Q). These creeks are adjacent to and receive runoff from Ranger Mine. In 2008, a 10 ha elevated waste rock landform was constructed and instrumented in the Gulungul Creek catchment. The earthworks required to build the landform created a considerable disturbance in the catchment, making a large volume of disturbed soil and substrate material available for erosion. Between 2008 and 2010, in the first two wet seasons immediately after construction, the downstream monitoring site on Gulungul Creek showed elevated FSS wave loads relative to discharge, compared with the upstream site. From 2010 onwards, the FSS loads relative to Q were no longer elevated. This was due to the establishment of vegetation on the site and loose fine sediment being trapped by vegetation. Large scale disturbance associated with mining and rehabilitation of elevated landforms causes elevated FSS loads in receiving streams. The predicted FSS loads for the stream as per the relationships between FSS and event discharge may not show a 1:1 relation with the observed loads for respective gauging stations. When downstream monitoring shows that FSS wave loads relative to rainfall runoff event discharge reduce back to pre-construction catchment levels, it will indicate that the landform is approaching equilibrium. This approach to assess landform stability will increase the sensitivity of assessing post-mining landform recovery and assist rehabilitation engineers to heal the land and benefit owners of the land to whom it is bestowed after rehabilitation.

Diversity and phylogenetic relationships of haemosporidian and hemogregarine parasites in Australian lizards

Apicomplexa is a large monophyletic phylum of unicellular, parasitic organisms. Reptiles are hosts to both haemosporidian (Haemosporida) and hemogregarine (Eucoccidiorida) apicomplexan blood parasites. Within reptiles our understanding of their diversity remains limited, with a paucity of information from Australia, despite a high diversity of squamates (snakes and lizards). We provide a preliminary assessment of haemosporidian and hemogregarine diversity occurring in lizards across northern tropical Australia, building on existing data with results from a microscopy and genetic assessment. We screened total of 233 blood slides using microscopy and detected hemogregarines in 25 geckos, 2 skinks and 1 agamid, while haemosporidians were detected in 13 geckos. DNA sequencing of 28 samples of the hemogregarine 18S rRNA (∼900 bp) nuclear gene revealed five lineages of Australian lizard hemogregarines within heteroxenous adeleids. We sequenced 10 samples of Haemosporida mtDNA (cytb & coI: ∼1313 bp) and phylogenetic analysis with 30 previously published sequences revealed that the Australian Haemosporida grouped within the Haemoproteidae but were not supported as a monophyletic clade. Our results demonstrate that there is significant undocumented evolutionary diversity in Australian lizard haemosporidian and hemogregarine parasites, with preliminary evidence of significantly higher infection rates in geckos.

Assessing the value of ecosystem services delivered by prescribed fire management in Australian tropical savannas

The savannas of tropical northern Australia, covering 1.9M km2, are relatively unmodified and support a very sparse human population (0.5 person/km2). Largely marginalised and impoverished Indigenous communities are key stakeholders in the region with legal rights to >60% of the land. Colonisation in the late 19th century significantly impacted long-standing Indigenous land management practices, resulting, until recently, in fire regimes dominated by extensive wildfires emitting, on average, >16Mt of greenhouse gases (GHG) per annum. To manage these emissions, the Australian Government in 2013 enacted an incentivised scheme—the Savanna Burning Methodology (SBM) under the Carbon Farming Initiative Act (2011)—to reduce wildfires through strategically applied prescribed burning. This paper assesses the value of ecosystem services (ES) delivered by fine-scale fire management under the SBM that is now applied to 25% of the 1.2 M km2 regulatory eligible savanna area, abating >7 Mt of GHG emissions per annum. While this scheme delivers and maintains a diverse range of ES supporting (i) the well-being of local Indigenous people, estimated at $189 million/yr (using a substitute value of government expenditure on Indigenous welfare), and (ii) many off-site ES for regional and global populations, the realised market value for GHG emissions abatement represents < 1% (i.e. USD 74.6 million since 2013) of the total value of ES. This assessment emphasizes the: (i) need to recognise the many benefits derived from SB; (ii) challenges associated with valuing ES for regional savanna stakeholders; (iii) further development of incentivised mechanisms for maintaining the flow of ES across sparsely settled northern Australian savannas. This assessment has broader implications globally where Indigenous and local communities aspire to sustainably manage their lands

The complete mitochondrial genome of the Australian Common Rock Rat, Zyzomys argurus

The Common Rock Rat Zyzomys argurus is an abundant small- to medium-sized Murid rodent that is endemic to Australia. It is a nocturnal mammal with a mostly herbivorous diet. This species is native to the wet/dry tropics of Northern Australia and can be identified from other rock rats on the basis of its small size and its tail length (which is at least equivalent to its head-body length). Here, we describe the complete mitochondrial genome of Z. argurus and compare it to other Rodentia. The Z. argurus circular mitogenome was 16,261 bp and contained 13 protein-coding genes, two rRNA genes, 22 tRNAs and a control region (D-loop) of 859 bp. Phylogenetic analysis of selected, published sequenced mitogenomes reveal it is most closely related to the Lakeland Downs mouse Leggadina lakedownensis in the order Rodentia.

Hydrology drives variation in spawning phenologies and diversity of larval assemblages of Australian wet–dry tropical fish

Abstract Setting ecologically based water‐extraction rules requires an understanding of the relationships between hydrology and the completion of aquatic organism life cycles. Successful reproduction of riverine fishes has been linked to hydrological variation across diverse climatic regions. However, the reproductive ecology for many species in wet–dry tropical northern Australia is poorly understood, and consequently there is little knowledge on how temporal (seasonal) and spatial hydrological variation influences fish reproduction in this region. This study aimed to quantify the hydrology‐related reproductive ecologies of freshwater fish across the wet–dry seasonal cycle and between differing hydrological classes in a northern Australian river (Daly River, Northern Territory) subject to increasing interest in water extraction. Sampling of larval and juvenile fish was undertaken over 1 year at eight sites classified into perennial and intermittent flow types. Fish spawning phenologies were developed and spatial–temporal dynamics in larval fish assemblages were investigated using model‐based multivariate and univariate analyses. Spawning occurred in all hydrological seasons, with low‐flow dry season periods identified as important for many fish taxa. For a different fish assemblage, spawning mostly occurred during high‐flow wet season periods. Larval assemblages in the wet season were more diverse than in the dry and wet–dry seasons. Perennial sites maintained higher diversity than intermittent sites year‐round; however, larvae were more abundant in intermittent sites for some taxa. Spatial and temporal differences in larval assemblages were largely explained by variation in the occurrence and relative abundance of three aseasonally spawning taxa (Craterocephalus spp., Melanotaenia spp. and Ambassis spp.) and the largely wet‐period spawning of Terapontidae spp. These results add to growing evidence of variation in spawning phenologies among fish species and the use of multiple hydrological phases for aseasonal reproduction. The diverse range of hydrological conditions used for spawning and rearing, and the differences between hydrological river classes among taxa, suggests that water‐extraction and environmental flow rules should aim to maintain hydrological conditions catchment‐wide during both low and high‐flow periods to reduce the risk of impacting fish reproduction. In particular, the results suggest that if perennially flowing sites began to flow intermittently, such as through over‐extraction of water or due to climate change, less diverse fish assemblages may result. This study expands our understanding of the relationships between hydrology and the reproductive ecology of freshwater fish in wet–dry tropical climates and may inform future ecologically based approaches to setting water‐extraction rules.

The Darwin Prospective Melioidosis Study: a 30-year prospective, observational investigation

The global distribution of melioidosis is under considerable scrutiny, with both unmasking of endemic disease in African and Pacific nations and evidence of more recent dispersal in the Americas. Because of the high incidence of disease in tropical northern Australia, The Darwin Prospective Melioidosis Study commenced in October, 1989. We present epidemiology, clinical features, outcomes, and bacterial genomics from this 30-year study, highlighting changes in the past decade. The present study was a prospective analysis of epidemiological, clinical, and laboratory data for all culture-confirmed melioidosis cases from the tropical Northern Territory of Australia from Oct 1, 1989, until Sept 30, 2019. Cases were identified on the basis of culture-confirmed melioidosis, a laboratory-notifiable disease in the Northern Territory of Australia. Patients who were culture-positive were included in the study. Multivariable analysis determined predictors of clinical presentations and outcome. Incidence, survival, and cluster analyses were facilitated by population and rainfall data and genotyping of Burkholderia pseudomallei, including multilocus sequence typing and whole-genome sequencing. There were 1148 individuals with culture-confirmed melioidosis, of whom 133 (12%) died. Median age was 50 years (IQR 38-60), 48 (4%) study participants were children younger than 15 years of age, 721 (63%) were male individuals, and 600 (52%) Indigenous Australians. All but 186 (16%) had clinical risk factors, 513 (45%) had diabetes, and 455 (40%) hazardous alcohol use. Only three (2%) of 133 fatalities had no identified risk. Pneumonia was the most common presentation occurring in 595 (52%) patients. Bacteraemia occurred in 633 (56%) of 1135 patients, septic shock in 240 (21%) patients, and 180 (16%) patients required mechanical ventilation. Cases correlated with rainfall, with 80% of infections occurring during the wet season (November to April). Median annual incidence was 20·5 cases per 100 000 people; the highest annual incidence in Indigenous Australians was 103·6 per 100 000 in 2011-12. Over the 30 years, annual incidences increased, as did the proportion of patients with diabetes, although mortality decreased to 17 (6%) of 278 patients over the past 5 years. Genotyping of B pseudomallei confirmed case clusters linked to environmental sources and defined evolving and new sequence types. Melioidosis is an opportunistic infection with a diverse spectrum of clinical presentations and severity. With early diagnosis, specific antimicrobial therapy, and state-of-the-art intensive care, mortality can be reduced to less than 10%. However, mortality remains much higher in the many endemic regions where health resources remain scarce. Genotyping of B pseudomallei informs evolving local and global epidemiology. The Australian National Health and Medical Research Council.

Occurrence and dynamics of potentially pathogenic vibrios in the wet-dry tropics of northern Australia

Bacteria from the Vibrio genus are a ubiquitous component of coastal and estuarine ecosystems with several pathogenic Vibrio species displaying preferences for warm tropical waters. We studied the spatial and temporal abundance of three key human potential pathogens V. parahaemolyticus, V. cholerae and V. vulnificus in northern tropical Australia, over the wet and dry seasons, to identify environmental parameters influencing their abundance. Quantitative PCR (qPCR) analysis revealed that V. parahaemolyticus occurred more frequently and in higher abundance than V. cholerae and V. vulnificus across all locations examined. All three species were more abundant during the wet season, with V. parahaemolyticus abundance correlated to temperature and conductivity, whereas nutrient concentrations and turbidity best explained V. vulnificus abundance. In addition to these targeted qPCR analyses, we assessed the composition and dynamics of the entire Vibrio community using hsp60 amplicon sequencing. Using this approach, 42 Vibrio species were identified, including a number of other pathogenic species such as V. alginolyticus, V. mimicus and V. fluvialis. The Vibrio community was more diverse in the wet season, with temperature and dissolved oxygen as the key factors governing community composition. Seasonal differences were primarily driven by a greater abundance of V. parahaemolyticus and V. vulnificus during the wet season, while spatial differences were driven by different abundances of V. harveyi, V. campbellii, V. cholerae and V. navarrensis. When we related the abundance of Vibrio to other bacterial taxa, defined using 16S rRNA gene amplicon sequencing, V. parahaemolyticus was negatively correlated to several taxa, including members of the Rickettsiales and Saccharimonadales, while V. vulnificus was negatively correlated to Rhobacteriaceae and Cyanobiaceae. In contrast, V. alginolyticus, V. harveyi and V. mediterranei were all positively correlated to Cyanobacteria. These observations highlight the dynamic nature of Vibrio communities and expands current understanding of the processes governing the occurrence of potentially pathogenic Vibrio spp. in tropical coastal ecosystems.

Unravelling the taxonomy and identification of a problematic group of benthic fishes from tropical rivers (Gobiidae: Glossogobius).

Flathead gobies (genus Glossogobius) include c. 40 small- to medium-sized benthic fishes found primarily in freshwater habitats across the Indo-Pacific, having biodiversity value as well as cultural and economic value as food fishes, especially in developing countries. To help resolve considerable confusion regarding the identification of some of the larger-growing Glossogobius species, a systematic framework was established using nuclear genetic markers, mitochondrial DNA barcoding and phenotypic evidence for a geographically widespread collection of individuals from the waterways of tropical northern Australia. Species boundaries and distribution patterns were discordant with those previously reported, most notably for the tank goby Glossogobius giuris, which included a cryptic species. Genetic divergence was matched with accompanying unique visual characters that aid field identification. Additional taxonomic complexity was also evident, by comparison with DNA barcodes from international locations, suggesting that the specific names applicable for two of the candidate species in Australia remain unresolved due to confusion surrounding type specimens. Although flathead gobies are assumed to be widespread and common, this study demonstrates that unrealised taxonomic and ecological complexity is evident, and this will influence assessments of tropical biodiversity and species conservation. This study supports the need for taxonomic studies of freshwater fishes to underpin management in areas subject to significant environmental change.

Connectivity of fish communities in a tropical floodplain river system and predicted impacts of potential new dams

Longitudinal and lateral connectivity is important for mobile aquatic species in rivers for reproductive migrations, recruitment, gene flow and access to food resources across habitat types. Water resource developments such as dams and levees may disrupt these connections, causing river fragmentation and loss of access to highly productive habitats such as floodplain wetlands. We used sulfur stable isotopes as a tracer to estimate patterns of fish movement in an unregulated river in tropical northern Australia, taking advantage of observed spatial variation in sulfur isotope values of their food resources across the catchment. We also modelled the flow and barrier related impacts of potential dam development scenarios on fish movement. Fish with isotope values significantly different from local prey values were determined to be migrants. In the ‘no dams’ scenario, movement varied among fish species (0–44% migrant fish within species where n > 5) and sites (0–40% migrant fish within sites where n > 5), and immigration was higher in more connected sites. Impacts of water resource development on fish movement varied between dam scenarios, with predictions that a dam on the main channel of the Mitchell River would have the highest impact of the three individual dam scenarios. This study provides critical information on how flow-mediated connectivity supports patterns of fish community movement in an unregulated river system. The generic quantitative approach of combining tracers of fish movement with connectivity modelling provides a powerful predictive tool. While we used sulfur stable isotopes to estimate fish movement, our approach can be used with other tracers of movement such as otoliths and acoustic telemetry, making it widely applicable to guide sustainable development in other river systems.

Diverse stone artefacts around Lake Woods, Central Northern Territory, Australia

Lake Woods is a large freshwater body on the northern edge of Australia’s arid zone where research since the late 1960s has suggested late Pleistocene, later Holocene, and recent human occupations. We report on surface collections and test excavations undertaken at four sites on the western side of the lake basin in 2019. Each site produced a distinctive lithic assemblage, characterised by different types of shaped and retouched lithics and grindstones. Some material matches the ethnographic descriptions of W. Murgatroyd and R. Paton of local stone tool technology. Our work also confirms the hypothesis of M. Smith that there was later Holocene occupation in the area; and is not inconsistent with the hypothesis of J. Bowler that there was a late Pleistocene occupation around the lake, but definitive evidence of this remains ambiguous. Grindstones occur at all sites, indicating that the harvesting of grass seeds was consistently an important subsistence component for people living by the lake. Distinctive lithic types are found at Lake Woods that are characteristic either of tropical northern Australia or arid southern Australia, suggesting the lake was an inter-regional locus for human activity and a potential zone of interaction between different groups at various times in the past.

The use of regional and alluvial groundwater by riparian trees in the wet‐dry tropics of northern Australia

AbstractRiparian trees play a critical role in the ecological function of rivers, yet are threatened by anthropogenic change to the hydrological cycle. Identifying the sources of water used by riparian trees can inform sustainable water policy. We used isotopic analysis complemented by measurements of plant water relations to assess water sources for riparian trees at two sites with contrasting hydrogeological processes; one with an alluvial aquifer overlaying an aquitard, and one where fault‐induced preferential pathways in the aquitard allowed the flow of deeper, older groundwater from a regional aquifer to the alluvium. At both sites, plant water potential, stomatal conductance, and plant water isotope composition in the xylem sap of riparian trees were collected from two landscape positions, the riverbank and floodplain. We used a Bayesian mixing model (MixSIAR) to assess differences in the proportion of water sources for sites and landscape positions. We found that xylem water isotope values differed between the two sites in line with their hydrogeological characteristics, with trees at the regional aquifer site using water sourced from the regional groundwater and trees at the site with only an alluvial aquifer present using a mixture of water sources, with no dominant source identified. Higher plant predawn water potential values at the regional site indicated greater water availability and support the inference that plants were using more groundwater at the regional site compared to the alluvial site. Trees closer to the river had higher isotope values, indicative of surficial water sources i.e. shallow soil water and river water. Our findings show that the water sources used by riparian trees reflect local hydrogeology and resource availability. Water managers should identify and protect plant water sources to ensure maintenance of riparian trees.

An Empirical Analysis of Sediment Export Dynamics from a Constructed Landform in the Wet Tropics

Although plot-scale erosion experiments are numerous, there are few studies on constructed landforms. This limits the understanding of their long-term stability, which is especially important for planning mined land rehabilitation. The objective of this study was to gain insight into the erosion processes in a 30 × 30 m trial plot on a mine waste rock dump in tropical northern Australia. The relationships between rainfall, runoff and suspended and bedload sediment export were assessed at annual, seasonal, inter-event and intra-event timescales. During a five-year study period, 231 rainfall–runoff–sediment export events were examined. The measured bedload and suspended sediments (mainly represented in nephelometric turbidity units (NTU)) showed the dominance of the wet season and heavy rainfall events. The bedload dominated the total mass, although the annual bedload diminished by approximately 75% over the five years, with greater flow energy required over time to mobilise the same bedload. The suspended load was more sustained, though it also exhibited an exhaustion process, with equal rainfall and runoff volumes and intensities, leading to lower NTU values over time. Intra-event NTU dynamics, including runoff-NTU time lags and hysteretic behaviours, were somewhat random from one event to the next, indicating the influence of the antecedent distribution of mobilisable sediments. The value of the results for supporting predictive modelling is discussed.

Hospitalisation for lower respiratory tract infection is associated with an increased incidence of acute myocardial infarction and stroke in tropical Northern Australia

Acute respiratory infections appear to precipitate vascular events. Acute myocardial infarction (AMI) and stroke are the leading cause of death and disability globally. This study was based on a cohort of patients admitted to Townsville University Hospital between January 2006 and December 2016. Using a self-controlled case series design, we investigated the risk of AMI or ischaemic stroke after an episode of pneumonia. We defined the ‘risk interval’ as the first 14 days after hospitalisation for pneumonia and the ‘control interval’ as one year before and one year after the risk interval. Among a population (N = 4557) with a median age of over 70, a total of 128 AMI and 27 stroke cases were identified within 1 year of an episode of pneumonia in this study. Ten and two admissions occurred during the risk interval, while 118 and 25 admissions occurred during the control period. The relative incidence ratios (RIR) of AMI increased after an episode of pneumonia (RIR=4.85, 95% confidence interval (CI) 2.44–9.67). The risk for stroke after the exposure period of 14 days was 4.94 (95% CI 1.12–21.78) considering only the first stroke incidence. The RIR results for AMI and stroke were not altered by adjusting for age, sex or Indigenous status. The risk of AMI and stroke were significantly higher two weeks after an episode of pneumonia.

Temperate southern Australian coastal waters are characterised by surprisingly high rates of nitrogen fixation and diversity of diazotrophs.

Biological dinitrogen (N2) fixation is one mechanism by which specific microorganisms (diazotrophs) can ameliorate nitrogen (N) limitation. Historically, rates of N2 fixation were believed to be limited outside of the low nutrient tropical and subtropical open ocean; however, emerging evidence suggests that N2 fixation is also a significant process within temperate coastal waters. Using a combination of amplicon sequencing, targeting the nitrogenase reductase gene (nifH), quantitative nifH PCR, and 15N2 stable isotope tracer experiments, we investigated spatial patterns of diazotroph assemblage structure and N2 fixation rates within the temperate coastal waters of southern Australia during Austral autumn and summer. Relative to previous studies in open ocean environments, including tropical northern Australia, and tropical and temperate estuaries, our results indicate that high rates of N2 fixation (10–64 nmol L−1 d−1) can occur within the large inverse estuary Spencer Gulf, while comparatively low rates of N2 fixation (2 nmol L−1 d−1) were observed in the adjacent continental shelf waters. Across the dataset, low concentrations of NO3/NO2 were significantly correlated with the highest N2 fixation rates, suggesting that N2 fixation could be an important source of new N in the region as dissolved inorganic N concentrations are typically limiting. Overall, the underlying diazotrophic community was dominated by nifH sequences from Cluster 1 unicellular cyanobacteria of the UCYN-A clade, as well as non-cyanobacterial diazotrophs related to Pseudomonas stutzeri, and Cluster 3 sulfate-reducing deltaproteobacteria. Diazotroph community composition was significantly influenced by salinity and SiO4 concentrations, reflecting the transition from UCYN-A-dominated assemblages in the continental shelf waters, to Cluster 3-dominated assemblages in the hypersaline waters of the inverse estuary. Diverse, transitional diazotrophic communities, comprised of a mixture of UCYN-A and putative heterotrophic bacteria, were observed at the mouth and southern edge of Spencer Gulf, where the highest N2 fixation rates were observed. In contrast to observations in other environments, no seasonal patterns in N2 fixation rates and diazotroph community structure were apparent. Collectively, our findings are consistent with the emerging view that N2 fixation within temperate coastal waters is a previously overlooked dynamic and potentially important component of the marine N cycle.

Interpreting Burkholderia pseudomallei disc diffusion susceptibility test results by the EUCAST method

Interpreting Burkholderia pseudomallei disc diffusion susceptibility test results by the EUCAST method

Understanding trends in hydrologic extremes across Australia

Changes in the hydrologic cycle have far reaching impacts on agricultural productivity, water resources availability, riverine ecosystems, and our ability to manage environmental assets, bushfire risk, and flood hazard. For example, declining rainfall in the southeast of Australia has led to a prolonged period of drought, with serious impacts on agriculture, the environment, and water supply to urban and rural towns. Here, using the continental wide Australian Water Resources Assessment Landscape model (AWRA-L), we evaluate historical trends from 1960 to 2017 in rainfall, soil moisture, evapotranspiration, and runoff to explain changing drought and flooding. Northern parts of Australia have experienced increasing annual rainfall totals, resulting in increased water availability in the tropics with increased soil moisture, evapotranspiration, and runoff, particularly during the hot, wet monsoon season. In contrast, the southwest and southeast coast of Australia have experienced declines in rainfall, particularly in the colder months, corresponding with decreasing evapotranspiration, soil moisture, and runoff. Trends in flooding are aligned with runoff trends, and closely follow trends in rainfall, with changes in soil moisture of secondary influence. Streamflow droughts, measured by the standardised runoff index, are increasing across large parts of Australia, with these increases more widespread than changes in rainfall alone. Increases in rainfall in the tropics of northern Australia appear to be related to decreasing drought occurrence and extent, but this trend is not universal, suggesting changes in rainfall alone are not an indicator of changing drought conditions.

Identifying disturbance in archaeological sites in tropical northern Australia: Implications for previously proposed 65,000‐year continental occupation date

AbstractThe stratigraphic integrity of archaeological deposits in the seasonally wet tropics can be disrupted by termite activity and other processes. Significant questions have been raised about the surprisingly old age estimates for artefacts recovered from two sites in northern Australia: Nauwalabila and Madjedbebe. If accurate, a 65,000‐year date for the latter would represent a 30% increase in the currently accepted length of human occupation of the continent. The oldest estimate for Nauwalabila falls just short of that. These dates would have important implications for human dispersal across Eurasia in the Late Pleistocene. No other data from Australia support them. Smith et al. (2020) claim to refute the notion that termite disturbance accounts for these anomalies. Here we show that the criteria on which the assertion is based are invalid. Radiocarbon data from both sites, not cited by Smith et al. (2020), are consistent with the termite‐disturbance hypothesis. Luminescence data claimed to offer strong support for the early Madjedbebe estimate are also consistent with termite disruption. We conclude that the early dates for human presence at Madjedbebe and Nauwalabila must be rejected along with any chronometric inference about human behaviour to the degree it relies on them.

Elevated Magnesium Concentrations Altered Freshwater Assemblage Structures in a Mesocosm Experiment.

Magnesium (Mg) is a mining-related contaminant in the Alligators Rivers Region of tropical northern Australia. A mesocosm experiment was used to assess Mg toxicity to aquatic freshwater assemblages. Twenty-five 2700-L tubs were arranged, stratified randomly, on the bed of Magela Creek, a seasonally flowing, sandy stream channel in the Alligator Rivers Region of northern Australia. The experiment comprised 5 replicates of 4 nominal Mg treatments, 2.5, 7.5, 23, and 68 mg L-1 , and a control. Phytoplankton biomass, and diatom, zooplankton, and macroinvertebrate assemblages present in the treatment tubs were sampled before and after Mg addition. A significant negative relationship between phytoplankton biomass and Mg was observed 4 wk after Mg addition as measured by chlorophyll a concentrations (r2 = 0.97, p = 0.01). This result was supported by reductions in some major phytoplankton groups in response to increasing Mg concentrations, in the same experiment and from independent field studies. There was a significant negative relationship between zooplankton assemblage similarity (to control) and Mg concentrations (r2 = 0.96, p = 0.002). Seven weeks after Mg addition, macroinvertebrate assemblages were dominated by 3 microcrustacean groups (Ostracoda, Cladocera, and Copepoda), each reaching maximum abundance at intermediate Mg concentrations (i.e., unimodal responses). The responses of phytoplankton and zooplankton were used to derive assemblage effect concentrations (Mg concentrations resulting in x% of the assemblage change [ECx]). Magnesium concentrations resulting in assemblage EC01 values were <3 mg L-1 . Together with candidate guideline values from other laboratory- and field-based lines of evidence, the mesocosm EC01 values were incorporated into a weight-of-evidence framework for a robust regulatory approach to environmental protection. Environ Toxicol Chem 2020;39:1973-1987. © 2020 Commonwealth of Australia. Published by Wiley Periodicals LLC on behalf of SETAC.