Climate change impacts on mesophotic regions of the Great Barrier Reef.

Climate change projections for coral reefs are founded exclusively on sea surface temperatures (SST). While SST projections are relevant for the shallowest reefs, neglecting ocean stratification overlooks the striking differences in temperature experienced by deeper reefs for all or part of the year. Density stratification creates a buoyancy barrier partitioning the upper and lower parts of the water column. Here, we mechanistically downscale climate models and quantify patterns of thermal stratification above mesophotic corals (depth 30 to 50 m) of the Great Barrier Reef (GBR). Stratification insulates many offshore regions of the GBR from heatwaves at the surface. However, this protection is lost once global average temperatures exceed ~3 °C above preindustrial, after which mesophotic temperatures surpass a recognized threshold of 30 °C for coral mortality. Bottom temperatures on the GBR (30 to 50 m) from 2050 to 2060 are estimated to increase by ~0.5 to 1 °C under lower climate emissions (SSP1-1.9) and ~1.2 to 1.7 °C under higher climate emissions (SSP5-8.5). In short, mesophotic coral reefs are also threatened by climate change and research might prioritize the sensitivity of such corals to stress.

eDNA metabarcoding reveals biodiversity and depth stratification patterns of dinoflagellate assemblages within the epipelagic zone of the western Coral Sea.

BACKGROUND: Dinoflagellates play critical roles in the functioning of marine ecosystems but also may pose a hazard to human and ecosystem health by causing harmful algal blooms (HABs). The Coral Sea is a biodiversity hotspot, but its dinoflagellate assemblages in pelagic waters have not been studied by modern sequencing methods. We used metabarcoding of the 18 S rRNA V4 amplicon to assess the diversity and structure of dinoflagellate assemblages throughout the water column to a depth of 150 m at three stations in the Western Coral Sea. Additionally, at one station we compared metabarcoding with morphological methods to optimise identification and detection of dinoflagellates. RESULTS: Stratification of dinoflagellate assemblages was evident in depth-specific relative abundances of taxonomic groups; the greatest difference was between the 5-30 m assemblages and the 130-150 m assemblages. The relative abundance of Dinophyceae (photosynthetic and heterotrophic) decreased with increasing depth, whereas that of Syndiniales (parasitic) increased with increasing depth. The composition of major taxonomic groups was similar among stations. Taxonomic richness and diversity of amplicon sequence variants (ASVs) were similar among depths and stations; however, the abundance of dominant taxa was highest within 0-30 m, and the abundance of rare taxa was highest within 130-150 m, indicating adaptations to specific depth strata. The number of unclassified ASVs at the family and species levels was very high, particularly for Syndinian representatives. CONCLUSIONS: Dinoflagellate assemblages in open water of the Coral Sea are highly diverse and taxonomically stratified by depth; patterns of relative abundance along the depth gradient reflect environmental factors and ecological processes. Metabarcoding detects more species richness than does traditional microscopical methods of sample analysis, yet the methods are complementary, with morphological analysis revealing additional richness. The large number of unclassified dinoflagellate-ASVs indicates a need for improved taxonomic reference databases and suggests presence of dinoflagellate-crypto and-morphospecies.

Extract or conserve? The Hartwick-rule and sustainable environmental governance.

The legal safeguards for sustainable environmental governance are often inadequate, inefficient, and amenable to political maneuverings. Australia recently approved the Carmichael coal mine, rail, and expansion of the Abbot Port projects. These projects, along with many others in the region, have dire consequences for the groundwater system (Currell et al., 2017) [5], the Great Barrier Reef (Kroon et al., 2016; Grech et al., 2016) [6,7], and climate change (Taylor and Meinshausen, 2014) [8]. Here we show that incorporating the Hartwick-rule in economic analysis renders many of these projects unviable with or without the opportunity and externality costs. The Hartwick-rule dictates that exhaustible resource extraction can ensure weak sustainability if resource rents can be invested in such a way that the produced capital outweighs the consumed natural capital (Hartwick, 1977) [9]. We put forward two main arguments; one, resource rents belong to the society and many projects are only viable when these rents are invested with a certain growth rate; second, economic analysis shall incorporate the Hartwick-rule and shall be applied prior to recourse to the legal safeguards. Our analysis can be applied to any non-renewable natural resource extraction decision making.

Fuelling the climate and science 'denial machine' on social media: A case study of the Great Barrier Reef's 2021 'in danger' recommendation on Twitter, YouTube and Facebook.

Australian climate policy has been stifled by a network of free-market and extractive industry-advocating actors, yet there is little empirical evidence to show how these actors and information flows behave in online communication spaces during Australian environmental conflicts. Focusing on the UNESCO 2021 'in danger' recommendation for the Great Barrier Reef for 6 weeks, this mixed-methods study of Twitter, Facebook and YouTube uses social network analysis, including cluster analysis and in-depth close reading. We find that a small, yet significant, mix of ideologically aligned partisan actors are fuelling the 'denial machine' in Australia by co-opting a scientific report's findings to argue that the Great Barrier Reef has recovered, and to contest the need for climate action. This article offers insights into the central actors and tactics that could erode public support for Australian climate policy, with similarities to strategies already established in the United States. It also contributes to furthering multi-platform analyses.

Natural recovery of corals after severe disturbance.

Ecosystem recovery from human-induced disturbances, whether through natural processes or restoration, is occurring worldwide. Yet, recovery dynamics, and their implications for broader ecosystem management, remain unclear. We explored recovery dynamics using coral reefs as a case study. We tracked the fate of 809 individual coral recruits that settled after a severe bleaching event at Lizard Island, Great Barrier Reef. Recruited Acropora corals, first detected in 2020, grew to coral cover levels that were equivalent to global average coral cover within just 2 years. Furthermore, we found that just 11.5 Acropora recruits per square meter were sufficient to reach this cover within 2 years. However, wave exposure, growth form and colony density had a marked effect on recovery rates. Our results underscore the importance of considering natural recovery in management and restoration and highlight how lessons learnt from reef recovery can inform our understanding of recovery dynamics in high-diversity climate-disturbed ecosystems.

Hot spots of bleaching in massive Porites coral colonies.

Coral bleaching events have become more frequent and severe due to ocean warming. While the large-scale impacts of bleaching events are well-known, there is growing recognition of the importance of small-scale spatial variation in bleaching and survival probability of individual coral colonies. By quantifying bleaching in 108 massive Porites colonies spread across Lizard Island, Great Barrier Reef, during the 2016 bleaching event, we investigated how hydrodynamic exposure levels and colony size contribute to local variability in bleaching prevalence and extent. Our results revealed that exposed locations were the least impacted by bleaching, while lagoonal areas exhibited the highest prevalence of bleaching and colony-level bleaching extents. Such patterns of bleaching could be due to prolonged exposure to warm water in the lagoon. These findings highlight the importance of considering location-specific factors when assessing coral health and emphasize the vulnerability of corals in lagoonal habitats to rapid and/or prolonged elevated temperatures.

Physical, biological and anthropogenic drivers of spatial patterns of coral reef fish assemblages at regional and local scales.

Species abundance, diversity and community assemblage structure are determined by multiple physical, habitat and management drivers that operate across multiple spatial scales. Here we used a multi-scale coral reef monitoring dataset to examine regional and local differences in the abundance, species richness and composition of fish assemblages in no-take marine reserve (NTMR) and fished zones at four island groups in the Great Barrier Reef Marine Park, Australia. We applied boosted regression trees to quantify the influence of 20 potential drivers on the coral reef fish assemblages. Reefs in two locations, Magnetic Island and the Keppel Islands, had distinctive fish assemblages and low species richness, while the Palm and Whitsunday Islands had similar species composition and higher species richness. Overall, our analyses identified several important physical (temperature, wave exposure) and biological (coral, turf, macroalgal and unconsolidated substratum cover) drivers of inshore reef fish communities, some of which are being altered by human activities. Of these, sea surface temperature (SST) was more influential at large scales, while wave exposure was important both within and between island groups. Species richness declined with increasing macroalgal cover and exposure to cyclones, and increased with SST. Species composition was most strongly influenced by mean SST and percent cover of macroalgae. There was substantial regional variation in the local drivers of spatial patterns. Although NTMR zoning influenced total fish density in some regions, it had negligible effects on fish species richness, composition and trophic structure because of the relatively small number of species targeted by the fishery. These findings show that inshore reef fishes are directly influenced by disturbances typical of the nearshore Great Barrier Reef, highlighting the need to complement global action on climate change with more targeted localised efforts to maintain or improve the condition of coral reef habitats.

The effect of catchment load reductions on water quality in the crown-of-thorn starfish outbreak initiation zone.

Crown-of-Thorns Starfish (CoTS) population outbreaks contribute to coral cover decline on Indo-Pacific reefs. On the Great Barrier Reef (GBR), enhanced catchment nutrient loads are hypothesised to increase phytoplankton food for CoTS larvae in the outbreak initiation zone. This study examines whether catchment load reductions will improve water quality in this zone during the larval period. We defined the i) initiation zone's spatial extent; ii) larval stage's temporal extent; and iii) water quality thresholds related to larval food, from published information. We applied these to model simulations, developed to quantify the effect of catchment load reductions on GBR water quality (Baird et al., 2021), and found a consistently weak response of chlorophyll-a, total organic nitrogen and large zooplankton concentrations in the initiation zone. Model results indicate marine and atmospheric forcing are more likely to control the planktonic biomass in this zone, even during major flooding events purported to precede CoTS outbreaks.

Spatial and temporal variability in tropical off-reef zooplankton across broad spatial and temporal scales.

Productivity of oligotrophic coral reefs is largely dependent on the constant influx of zooplankton. However, our understanding of how zooplankton communities in tropical reef-associated regions vary over large spatial and temporal scales is limited. Using the Australian continuous plankton recorder dataset, we explored if, and to what extent, the off-reef zooplankton community along the Queensland shelf (including most of the Great Barrier Reef lagoon) varied with latitude, month, and diel time. The zooplankton community was consistently dominated by copepods (∼60%) which, with appendicularians, chaetognaths, non-copepod crustaceans, and thaliaceans, comprised ∼98% of the zooplankton. However, the abundance of these taxonomic groups did not vary predictably across latitude, month, or diel time, with these gradients only explaining 5% of community variation. At the scales sampled herein the composition of zooplankton was highly predictable in terms of broad taxonomic groups but variation in the relative abundance of these groups was not predictable.

Vegetative fragment production as a means of propagule dispersal for tropical seagrass meadows.

BACKGROUND AND AIMS: Long distance dispersal (LDD) contributes to the replenishment and recovery of tropical seagrass habitats exposed to disturbance, such as cyclones and infrastructure development. However, our current knowledge regarding the physical attributes of seagrass fragments that influence LDD predominantly stems from temperate species and regions. The goal of this paper is to measure seagrass fragment density and viability in two tropical species, assessing various factors influencing their distribution. METHODS: We measured the density and viability of floating seagrass fragments for two tropical seagrass species (Zostera muelleri and Halodule uninervis) in two coastal seagrass meadows in the central Great Barrier Reef World Heritage Area, Australia. We assessed the effect of wind speed, wind direction, seagrass growing/senescent season, seagrass meadow density, meadow location and dugong foraging intensity on fragment density. We also measured seagrass fragment structure and fragment viability; i.e., potential to establish into a new plant. KEY RESULTS: We found that seagrass meadow density, season, wind direction and wind speed influenced total fragment density, while season and wind speed influenced the density of viable fragments. Dugong foraging intensity did not influence fragment density. Our results indicate that wave action from winds combined with high seagrass meadow density increases seagrass fragment creation, and that more fragments are produced during the growing than the senescent season. Seagrass fragments classified as viable for Z. muelleri and H. uninervis had significantly more shoots and leaves than non-viable fragments. We collected 0.63 (±0.08 SE) floating viable fragments 100 m-2 in the growing season, and 0.13 (±0.03 SE) viable fragments 100 m-2 in the senescent season. Over a third (38%) of all fragments collected were viable. CONCLUSION: There is likely to be a large number of viable seagrass fragments available for long distance dispersal. This study's outputs can inform dispersal and connectivity models that are used to direct seagrass ecosystem management and conservation strategies.

Sensitivity of the Indo-Pacific coral Acropora millepora to aromatic hydrocarbons.

The risks posed by petroleum spills to coral reefs are poorly understood and quantifying acute toxicity thresholds for aromatic hydrocarbons to reef-building corals is required to assess their sensitivity relative to other taxa. In this study, we exposed Acropora millepora to toluene, naphthalene and 1-methylnaphthalene (1-MN) in a flow-through system and assessed survivorship and sublethal responses including growth, colour and the photosynthetic performance of symbionts. Median 50% lethal concentrations (LC50s) decreased over the 7-d exposure period, reaching asymptotic values of 22,921, 5,268, 1167 µg L-1 for toluene, naphthalene and 1-MN, respectively. Corresponding toxicokinetic parameters (εLC50) defining the time progression of toxicity were 0.830, 0.692, and 0.256 d-1, respectively. Latent effects after an additional 7-d recovery in uncontaminated seawater were not observed. Effect concentrations (EC50s) for 50% growth inhibition were 1.9- to 3.6-fold lower than the LC50s for each aromatic hydrocarbon. There were no observed effects of aromatic hydrocarbon exposure on colour score (a proxy for bleaching) or photosynthetic efficiency. Acute and chronic critical target lipid body burdens (CTLBBs) of 70.3 ± 16.3 and 13.6 ± 18.4 µmol g-1 octanol (± standard error) were calculated for survival and growth inhibition based on 7-d LC50 and EC10 values, respectively. These species-specific constants indicate adult A. millepora is more sensitive than other corals reported so far but is of average sensitivity in comparison with other aquatic taxa in the target lipid model database. These results advance our understanding of acute hazards of petroleum contaminants to key habitat-building tropical coral reef species.

Estimating the aquatic risk from exposure to up to twenty-two pesticide active ingredients in waterways discharging to the Great Barrier Reef.

Pesticides decrease the quality of water reaching the Great Barrier Reef (GBR), Australia. Up to 86 pesticide active ingredients (PAIs) were monitored between July 2015 to end of June 2018 at 28 sites in waterways that discharge to the GBR. Twenty-two frequently detected PAIs were selected to calculate their combined risk when they co-occur in water samples. Species sensitivity distributions (SSDs) for the 22 PAIs to fresh and marine species were developed. The SSDs, the multi-substance potentially affected fraction (msPAF) method, Independent Action model of joint toxicity and a Multiple Imputation method were combined to convert measured PAI concentration data to estimates of the Total Pesticide Risk for the 22 PAIs (TPR22) expressed as the average percentage of species affected during the wet season (i.e., 182 days). The TPR22 and percent contribution of active ingredients of Photosystem II inhibiting herbicides, Other Herbicides, and Insecticides to the TPR22 were estimated. The TPR22 ranged from <1 % to 42 % of aquatic species being affected. Approximately 85 % of the TPR22 estimates were >1 % - meaning they did not meet the Reef 2050 Water Quality Improvement Plan's pesticide target for waters entering the GBR. There were marked spatial differences in TPR22 estimates - regions dominated by grazing had lower estimates while those with sugar cane tended to have higher estimates. On average, active ingredients of PSII herbicides contributed 39 % of the TPR22, the active ingredients of Other Herbicides contributed ~36 % and of Insecticides contributed ~24 %. Nine PAIs (diuron, imidacloprid, metolachlor, atrazine, MCPA, imazapic, metsulfuron, triclopyr and ametryn) were responsible for >97 % of TPR22 across all the monitored waterways.

Limited cross-species virus transmission in a spatially restricted coral reef fish community.

The Great Barrier Reef (GBR)-the largest coral reef ecosystem in the world-supports over 1,200 fish species with some of the highest population densities and diversities observed in vertebrates, offering a high potential for virus transmission among species. As such, the GBR represents an exceptional natural ecosystem to determine the impact of host community diversity on virus evolution and emergence. In recent decades, the GBR has also experienced significant threats of extinction, making it one of the most vulnerable ecosystems on the planet. Despite the global importance of the GBR, our understanding of virus diversity and connectivity in tropical reef fishes remains poor. Here, we employed metatranscriptomic sequencing to reveal the viromes of sixty-one reef fish species. This identified transcripts representing 132 putative viral sequences, 38 of which exhibited strong phylogenetic relationships with known vertebrate-associated viral genera, including a novel Santee-Cooper ranavirus (Iridoviridae). We found little evidence for virus transmission between fish species living within a very restricted geographical space-a 100-m2 coral reef ecosystem-suggesting that there might be important host barriers to successful cross-species transmission despite regular exposure. We also identified differences in virome composition among reef fish families, such that cryptobenthic reef fishes-characterized by small body sizes and short life spans-exhibited greater virome richness compared to large reef fishes. This study suggests that there are important barriers to cross-species virus transmission and that successful emergence in a reef fish community likely requires active host adaptation, even among closely related host species.

Adult Corals Are Uniquely More Sensitive to Manganese Than Coral Early-Life Stages.

Manganese (Mn) is an essential element and is generally considered to be one of the least toxic metals to aquatic organisms, with chronic effects rarely seen at concentrations below 1000 µg/L. Anthropogenic activities lead to elevated concentrations of Mn in tropical marine waters. Limited data suggest that Mn is more acutely toxic to adults than to early life stages of scleractinian corals in static renewal tests. However, to enable the inclusion of sufficient sensitive coral data in species sensitivity distributions to derive water quality guideline values for Mn, we determined the acute toxicity of Mn to the adult scleractinian coral, Acropora muricata, in flow-through exposures. The 48-h median effective concentration was 824 µg Mn/L (based on time-weighted average, measured, dissolved Mn). The endpoint was tissue sloughing, a lethal process by which coral tissue detaches from the coral skeleton. Tissue sloughing was unrelated to superoxidase dismutase activity in coral tissue, and occurred in the absence of bleaching, that is, toxic effects were observed for the coral host, but not for algal symbionts. We confirm that adult scleractinian corals are uniquely sensitive to Mn in acute exposures at concentrations 10-340 times lower than those reported to cause acute or chronic toxicity to coral early life stages, challenging the traditional notion that early life stages are more sensitive than mature organisms. Environ Toxicol Chem 2023;42:1359-1370. © 2023 Commonwealth Scientific and Industrial Research Organisation. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Model of the Origin of a Ciguatoxic Grouper (Plectropomus leopardus).

Published data were used to model the transfer of ciguatoxins (CTX) across three trophic levels of a marine food chain on the Great Barrier Reef (GBR), Australia, to produce a mildly toxic common coral trout (Plectropomus leopardus), one of the most targeted food fishes on the GBR. Our model generated a 1.6 kg grouper with a flesh concentration of 0.1 µg/kg of Pacific-ciguatoxin-1 (P-CTX-1 = CTX1B) from 1.1 to 4.3 µg of P-CTX-1 equivalents (eq.) entering the food chain from 0.7 to 2.7 million benthic dinoflagellates (Gambierdiscus sp.) producing 1.6 pg/cell of the P-CTX-1 precursor, P-CTX-4B (CTX4B). We simulated the food chain transfer of ciguatoxins via surgeonfishes by modelling Ctenochaetus striatus feeding on turf algae. A C. striatus feeding on ≥1000 Gambierdiscus/cm2 of turf algae accumulates sufficient toxin in <2 days that when preyed on, produces a 1.6 kg common coral trout with a flesh concentration of 0.1 µg/kg P-CTX-1. Our model shows that even transient blooms of highly ciguatoxic Gambierdiscus can generate ciguateric fishes. In contrast, sparse cell densities of ≤10 Gambierdiscus/cm2 are unlikely to pose a significant risk, at least in areas where the P-CTX-1 family of ciguatoxins predominate. The ciguatera risk from intermediate Gambierdiscus densities (~100 cells/cm2) is more difficult to assess, as it requires feeding times for surgeonfish (~4-14 days) that overlap with turnover rates of turf algae that are grazed by herbivorous fishes, at least in regions such as the GBR, where stocks of herbivorous fishes are not impacted by fishing. We use our model to explore how the duration of ciguatoxic Gambierdiscus blooms, the type of ciguatoxins they produce, and fish feeding behaviours can produce differences in relative toxicities between trophic levels. Our simple model indicates thresholds for the design of risk and mitigation strategies for ciguatera and the variables that can be manipulated to explore alternate scenarios for the accumulation and transfer of P-CTX-1 analogues through marine food chains and, potentially, for other ciguatoxins in other regions, as more data become available.

Dispersal of the pesticide diuron in the Great Barrier Reef.

Pesticides from urban and agricultural runoff have been detected at concentrations above current water quality guidelines in the Great Barrier Reef (GBR) marine environment. We quantify the load of the pesticide diuron entering GBR waters using the GBR-Dynamic SedNet catchment model. After comparison of simulated distributions with observations at 11 monitoring sites we determined a half-life of diuron in GBR marine waters of 40 days. We followed diuron dispersal in the GBR (2016-2018) using the 1 km resolution eReefs marine model. The highest diuron concentrations in GBR waters occurred in the Mackay-Whitsunday region with a spike in January and March 2017, associated with 126 and 118 kg d-1 diuron loads from Plane Creek and the O'Connell River respectively. We quantify areas of GBR waters exposed to potentially ecotoxic concentrations of diuron. Between 2016 and 2018, 400 km2 and 1400 km2 of the GBR were exposed to concentrations exceeding ecosystem threshold values of 0.43 and 0.075 µg L-1 respectively. Using observed mapped coral and seagrass habitat, 175 km2 of seagrass beds and 50 km2 of coral habitats had peak diuron concentrations above 0.075 µg L-1 during this period. While the highest concentrations are localised to river plumes and inshore environments, non-zero diuron concentrations extend along the Queensland coast. These simulations provide new knowledge for the understanding of pesticide dispersal and management-use in GBR catchments and the design of in-water monitoring systems.

Large-scale impact of the 2016 Marine Heatwave on the plankton-associated microbial communities of the Great Barrier Reef (Australia).

The Great Barrier Reef (GBR) is the world's largest coral ecosystem and is threatened by climate change. This study investigated the impact of the 2016 Marine Heatwave (MHW) on plankton associated microbial communities along a ∼800 km transect in the GBR. 16S rRNA gene metabarcoding of archived plankton samples collected from November 2014 to August 2016 in this region showed a significant increase in Planctomycetes and bacteria belonging to the genus Vibrio and Synechococcus during and after the heatwave. Notably, Droplet Digital PCR and targeted metagenomic analysis applied on samples collected four months after the MHW event revealed the presence of several potential pathogenic Vibrio species previously associated with diseases in aquatic animals. Overall, the 2016 MHW significantly impacted the surface picoplankton community and fostered the spread of potentially pathogenic bacteria across the GBR providing an additional threat for marine biodiversity in this area.

Detection of marine oil-like features in Sentinel-1 SAR images by supplementary use of deep learning and empirical methods: Performance assessment for the Great Barrier Reef marine park.

Continuous monitoring of oil discharges in coastal and open ocean waters using Earth Observation (EO) has undeniably contributed to diminishing their occurrence wherever a detection system was in place, such as in Europe (EMSA's CleanSeaNet) or in the United States (NOAA's OR&R). This study describes the development and testing of a semi-automated oil slick detection system tailored to the Great Barrier Reef (GBR) marine park solely based on EO data as no such service was routinely available in Australia until recently. In this study, a large, curated, historical global dataset of SAR imagery acquired by Sentinel-1 SAR, now publicly available, is used to assess classification techniques, namely an empirical approach and a deep learning model, to discriminate between oil-like features and look-alikes in the scenes acquired over the marine park. An evaluation of this detection system on 10 Sentinel-1 SAR images of the GBR using two performance metrics - the detection accuracy and the false-positive rate (FPR) - shows that the classifiers perform best when combined (accuracy >98 %; FPR 0.01) rather than when used separately. This study demonstrates the benefit of sequentially combining classifiers to improve the detection and monitoring of unreported oil discharge events in SAR imagery. The workflow has also been tested outside the GBR, demonstrating its robustness when applied to other regions such as Australia's Northwest Shelf, Southeast Asia and the Pacific.

Microeconomic adaptation to severe climate disturbances on Australian coral reefs.

Coral reefs are increasingly affected by climate-induced disturbances that are magnified by increasing ocean temperatures. Loss of coral reefs strongly affects people whose livelihoods and wellbeing depend on the ecosystem services reefs provide. Yet the effects of coral loss and the capacity of people and businesses to adapt to it are poorly understood, particularly in the private sector. To address this gap, we surveyed about half (57 of 109) of Australian reef tourism operators to understand how they were affected by and responded to severe impacts from bleaching and cyclones. Reef restoration and spatial diversification were the primary responses to severe bleaching impacts, while for cyclone-impacts coping measures and product diversification were more important. Restoration responses were strongly linked to the severity of impacts. Our findings provide empirical support for the importance of response diversity, spatial heterogeneity, and learning for social-ecological resilience.

Direct comparison of runoff of residual and knockdown herbicides in sugarcane using a rainfall simulator finds large difference in runoff losses and toxicity relative to diuron.

Runoff losses of herbicides have rarely been compared simultaneously under the same conditions. Our aim was to directly compare herbicide runoff losses, normalised for the amount present (relative runoff loads) and in absolute terms. Toxicity and runoff concentrations were combined to provide a risk ranking relative to diuron. Four rainfall simulation trials were conducted in sugarcane in the Great Barrier Reef catchment. Herbicides studied were older PSII residuals (atrazine, ametryn, diuron, hexazinone), alternative residuals (isoxaflutole, imazapic, metribuzin, metolachlor, pendimethalin) and knockdown herbicides (glyphosate, 2,4-D, fluroxypyr) and the tracer bromide (Br). Simulations were conducted two days after spraying, before differences due to half-lives were apparent. Two trials had bare soil and two had sugarcane trash. Herbicide runoff losses and concentrations were closely related to the amount applied, runoff amounts and partitioning coefficients. Relative runoff losses and absolute losses were similar for most older and alternative residual herbicides, 2,4-D and Br. Glyphosate and pendimethalin relative runoff losses were low, due to greater sorption. Isoxaflutole, imazapic, and fluroxypyr are applied at much lower rates and runoff losses were low. Herbicides were lost in the dissolved phase, except pendimethalin. There was a large range in toxicity relative to diuron. There is a range of herbicide choices posing less offsite risk than diuron and ametryn, which have high application rates and high toxicity. Herbicide choice should consider application rate, runoff losses, sorption, and toxicity.