Chemotaxonomic responses of autotrophic periphyton communities to nutrient additions in pools of an intermittent stream

1. The algal groups present in periphyton communities form an important base of autochthonous food webs in freshwater streams. Nitrogen (N) and phosphorus (P) are key macronutrients in aquatic systems. Excess nutrients benefit some algal groups over others. 2. We paired a nutrient-diffusing substrata limitation experiment with high performance liquid chromatography to (a) identify which nutrient(s) limit periphyton production, and (b) how the periphyton biomass and community structure changes between isolated pools of differing hydrological characteristics along an intermittent dryland stream. 3. Unique peaks for 21 pigments were identified and matched with published values. We then produced a PERMANOVA model using pigment ratios and CHEMTAX analysis to explore changes in community structure resulting from nutrient addition. 4. Periphyton communities in these pools were co-limited by N and P. Nitrogen additions caused the periphyton to shift from diatom- to chlorophyte-dominated community structure and benefited cyanophyta growth. Phosphorus additions reduced the relative proportion of diatoms and also resulted in an increase in pheophoribide-a, a pigment indicative of cell lysis, demonstrating a detrimental impact of P additions. 5. Outcomes of this study show that when adding nutrient to a system there may be subtle shifts in community composition which can be telescoped up the food web regardless of the system's nutrient status

Phosphorus sorption characteristics and interactions with leaf litter‑derived dissolved organic matter leachate in iron‑rich sediments of a sub‑tropical ephemeral stream

This study investigated the infuence of dissolved organic matter (DOM) additions on phosphate sorption kinetics of iron-rich sediments (39–50% hematite and goethite) from an ephemeral stream in the arid Pilbara region of sub-tropical northwest Australia. While phosphate sorption in stream sediments is known to be strongly infuenced by sediment mineralogy as well as interactions with DOM, the mechanisms and signifcance of DOM on P-release from sediments with high sorption capacities, are largely undescribed. We assessed phosphorus (P) sorption behaviours by adding a range of solutions of known inorganic P concentrations that were amended with variable loadings of DOM derived from leachates of leaf litter to sediments from stream pools during the non-fowing phase. We compared the sorption capacity of the sediments and concurrent changes in DOM composition measured using fuorescence spectroscopy. We show that the low-dose DOM addition (~ 4 mg L−1 DOC) had the efect of reducing sediment P adsorption capacity, while for the high-dose DOM addition (~ 45 mg L−1 DOC), it was increased. The high-dose DOM was similar to pore water DOC and likely saturated sediment surface adsorption sites and produced P–OM–Fe complexes. This resulted in increased removal of P from solution. Sediment P sorption characteristics were well ftted to both Freundlich and Langmuir isotherm models regardless of DOC concentration. Langmuir P sorption maxima ranged from 0.106 to 0.152 mg g−1. General P sorption characteristics of these iron-rich sediments did not difer among pools of contrasting hydrological connectivity. Our results show how humic-rich DOM can modulate the sediment P availability in dryland streams. Unravelling the complexities of P availability is of particular significance to further our understanding of biogeochemical processes in aquatic ecosystems where P often acts as a limiting nutrient

Potential for tree rings to reveal spatial patterns of past drought variability across western Australia

Proxy records have provided major insights into the variability of past climates over long timescales. However, for much of the Southern Hemisphere, the ability to identify spatial patterns of past climatic variability is constrained by the sparse distribution of proxy records. This is particularly true for mainland Australia, where relatively few proxy records are located. Here, we (1) assess the potential to use existing proxy records in the Australasian region—starting with the only two multi-century tree-ring proxies from mainland Australia—to reveal spatial patterns of past hydroclimatic variability across the western third of the continent, and (2) identify strategic locations to target for the development of new proxy records. We show that the two existing tree-ring records allow robust reconstructions of past hydroclimatic variability over spatially broad areas (i.e. > 3° × 3°) in inland north- and south-western Australia. Our results reveal synchronous periods of drought and wet conditions between the inland northern and southern regions of western Australia as well as a generally anti-phase relationship with hydroclimate in eastern Australia over the last two centuries. The inclusion of 174 tree-ring proxy records from Tasmania, New Zealand and Indonesia and a coral record from Queensland did not improve the reconstruction potential over western Australia. However, our findings suggest that the addition of relatively few new proxy records from key locations in western Australia that currently have low reconstruction skill will enable the development of a comprehensive drought atlas for the region, and provide a critical link to the drought atlases of monsoonal Asia and eastern Australia and New Zealand

Tree Rings Show Recent High Summer-Autumn Precipitation in Northwest Australia Is Unprecedented within the Last Two Centuries

An understanding of past hydroclimatic variability is critical to resolving the significance of recent recorded trends in Australian precipitation and informing climate models. Our aim was to reconstruct past hydroclimatic variability in semi-arid northwest Australia to provide a longer context within which to examine a recent period of unusually high summer-autumn precipitation. We developed a 210-year ring-width chronology from Callitris columellaris, which was highly correlated with summer-autumn (Dec–May) precipitation (r = 0.81; 1910–2011; p < 0.0001) and autumn (Mar–May) self-calibrating Palmer drought severity index (scPDSI, r = 0.73; 1910–2011; p < 0.0001) across semi-arid northwest Australia. A linear regression model was used to reconstruct precipitation and explained 66% of the variance in observed summer-autumn precipitation. Our reconstruction reveals inter-annual to multi-decadal scale variation in hydroclimate of the region during the last 210 years, typically showing periods of below average precipitation extending from one to three decades and periods of above average precipitation, which were often less than a decade. Our results demonstrate that the last two decades (1995–2012) have been unusually wet (average summer-autumn precipitation of 310 mm) compared to the previous two centuries (average summer-autumn precipitation of 229 mm), coinciding with both an anomalously high frequency and intensity of tropical cyclones in northwest Australia and the dominance of the positive phase of the Southern Annular Mode

Multi-decadal variations in Southern Hemisphere atmospheric ¹⁴C: Evidence against a Southern Ocean sink at the end of the Little Ice Age CO₂ anomaly.

Northern Hemisphere-wide cooling during the Little Ice Age (LIA; CE 1650-1775) is associated with a ~5 ppmv decrease in atmospheric carbon dioxide. Changes in terrestrial and ocean carbon reservoirs have been postulated as possible drivers of this relatively large shift in atmospheric CO₂, potentially providing insights into the mechanisms and sensitivity of the global carbon cycle. Here we report decadally-resolved radiocarbon (¹⁴C) levels in a network of tree rings series spanning CE 1700-1950 located along the northern boundary of, and within, the Southern Ocean. We observe regional dilutions in atmospheric radiocarbon (relative to the Northern Hemisphere) associated with upwelling of ¹⁴CO₂–depleted abyssal waters. We find the inter-hemispheric ¹⁴C offset approaches zero during increasing global atmospheric CO₂ at the end of the LIA, with reduced ventilation in the Southern Ocean and a Northern Hemisphere source of old carbon (most probably originating from deep Arctic peat layers). The coincidence of the atmospheric CO₂ increase and reduction in the inter-hemispheric ¹⁴C offset imply a common climate control. Possible mechanisms of synchronous change in the high latitudes of both hemispheres are discussed

Tracking down carbon inputs underground from an arid zone Australian calcrete.

Freshwater ecosystems play a key role in shaping the global carbon cycle and maintaining the ecological balance that sustains biodiversity worldwide. Surficial water bodies are often interconnected with groundwater, forming a physical continuum, and their interaction has been reported as a crucial driver for organic matter (OM) inputs in groundwater systems. However, despite the growing concerns related to increasing anthropogenic pressure and effects of global change to groundwater environments, our understanding of the dynamics regulating subterranean carbon flows is still sparse. We traced carbon composition and transformations in an arid zone calcrete aquifer using a novel multidisciplinary approach that combined isotopic analyses of dissolved organic carbon (DOC) and inorganic carbon (DIC) (δ13CDOC, δ13CDIC, 14CDOC and 14CDIC) with fluorescence spectroscopy (Chromophoric Dissolved OM (CDOM) characterisation) and metabarcoding analyses (taxonomic and functional genomics on bacterial 16S rRNA). To compare dynamics linked to potential aquifer recharge processes, water samples were collected from two boreholes under contrasting rainfall: low rainfall ((LR), dry season) and high rainfall ((HR), wet season). Our isotopic results indicate limited changes and dominance of modern terrestrial carbon in the upper part (northeast) of the bore field, but correlation between HR and increased old and 13C-enriched DOC in the lower area (southwest). CDOM results show a shift from terrestrially to microbially derived compounds after rainfall in the same lower field bore, which was also sampled for microbial genetics. Functional genomic results showed increased genes coding for degradative pathways-dominated by those related to aromatic compound metabolisms-during HR. Our results indicate that rainfall leads to different responses in different parts of the bore field, with an increase in old carbon sources and microbial processing in the lower part of the field. We hypothesise that this may be due to increasing salinity, either due to mobilisation of Cl- from the soil, or infiltration from the downstream salt lake during HR. This study is the first to use a multi-technique assessment using stable and radioactive isotopes together with functional genomics to probe the principal organic biogeochemical pathways regulating an arid zone calcrete system. Further investigations involving extensive sampling from diverse groundwater ecosystems will allow better understanding of the microbiological pathways sustaining the ecological functioning of subterranean biota

Megadroughts in the Common Era and the Anthropocene

Exceptional drought events, known as megadroughts, have occurred on every continent outside Antarctica over the past ~2,000 years, causing major ecological and societal disturbances. In this Review, we discuss shared causes and features of Common Era (Year 1–present) and future megadroughts. Decadal variations in sea surface temperatures are the primary driver of megadroughts, with secondary contributions from radiative forcing and land–atmosphere interactions. Anthropogenic climate change has intensified ongoing megadroughts in south-western North America and across Chile and Argentina. Future megadroughts will be substantially warmer than past events, with this warming driving projected increases in megadrought risk and severity across many regions, including western North America, Central America, Europe and the Mediterranean, extratropical South America, and Australia. However, several knowledge gaps currently undermine confidence in understanding past and future megadroughts. These gaps include a paucity of high-resolution palaeoclimate information over Africa, tropical South America and other regions; incomplete representations of internal variability and land surface processes in climate models; and the undetermined capacity of water-resource management systems to mitigate megadrought impacts. Addressing these deficiencies will be crucial for increasing confidence in projections of future megadrought risk and for resiliency planning

Unique stable isotope signatures of large cyclonic events as a tracer of soil moisture dynamics in the semiarid subtropics

Evaporative flux from soils in arid and semi-arid climates can be very high and may substantially reduce soil moisture retained between infrequent rainfall events. Direct measurement of the evaporative losses from soils is technically challenging. However, environmental tracers such as water stable hydrogen and oxygen isotope composition can be used to calculate evaporation rates if the initial signature of the infiltrating rainwater is distinct from the signature of residual soil moisture. Large tropical cyclones typically result in rainfall events of large volume and very negative δ18O signatures that are significantly lower than the signatures of more frequent and smaller rainfall events. These very negative stable isotope signatures are retained in the soil and can be used to understand the depth of water infiltration, retention and subsequent rate of evaporation from the soil. At our study site in dry subtropical northwest Australia, we repeatedly sampled rainwater and soil moisture prior to, during and after tropical Cyclones Heidi and Lua in 2012. Site inundation from Cyclone Heidi (rainfall 213 mm, δ18O −17.6‰) replenished soil moisture in the unsaturated zone for several months, completely replacing soil moisture down to depths of ~3.5 m and contributing to groundwater recharge. The transient momentary evaporative losses from wet soil at the time of sampling varied between 0.21 and 0.60 mm × day−1 (equivalent to 76 to 220 mm × yr−1 recalculated as an annual rate). During the prolonged dry period between cyclones, evaporative losses decreased to between 8 and 30 mm × yr−1. Mean long-term groundwater recharge for the study site was low (−1). Recharge is primarily driven by infrequent but high-volume cyclones that are an important source of soil moisture and an essential water source for vegetation in this semi-arid environment. However, variation in lithology, position in the landscape and time since the last inundation contribute to highly heterogeneous patterns of δ18O in the vadose zone, which complicates upscaling observations from a local to a regional scale model of evaporative demand