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Conventional photosystem II (PSII) herbicides applied in agriculture can pose significant environmental risks to aquatic environments. In response to the frequent detection of these herbicides in the Great Barrier Reef (GBR) catchment area, transitions towards ‘alternative’ herbicides are now widely supported. However, water quality guideline values (WQGVs) for alternative herbicides are lacking and their potential ecological impacts on tropical marine species are generally unknown. To improve our understanding of the risks posed by some of these alternative herbicides on marine species under tropical conditions, we tested the effects of four herbicides on the widely distributed diatom Chaetoceros muelleri. The PSII herbicides diuron, propazine, and tebuthiuron induced substantial reductions in both 24 h effective quantum yields (ΔF/F m ′) and 3-day specific growth rates (SGR). The effect concentrations, which reduced ΔF/F m ′ by 50% (EC 50 ), ranged from 4.25 µg L −1 diuron to 48.6 µg L −1 propazine, while the EC 50 s for SGR were on average threefold higher, ranging from 12.4 µg L −1 diuron to 187 µg L −1 tebuthiuron. Our results clearly demonstrated that inhibition of ΔF/F m ′ in PSII is directly linked to reduced growth (R 2  = 0.95) in this species, further supporting application of ΔF/F m ′ inhibition as a valid bioindicator of ecological relevance for PSII herbicides that could contribute to deriving future WQGVs. In contrast, SGR and ΔF/F m ′ of C. muelleri were nonresponsive to the non-PSII herbicide haloxyfop at the highest concentration tested (4570 µg L −1 ), suggesting haloxyfop does not pose a risk to C. muelleri . The toxicity thresholds (e.g. no effect concentrations; NECs) identified in this study will contribute to the derivation of high-reliability marine WQGVs for some alternative herbicides detected in GBR waters and support future assessments of the cumulative risks of complex herbicide mixtures commonly detected in coastal waters.

Herbicide contamination of nearshore tropical marine ecosystems is widespread and persistent; however, risks posed by most ‘alternative’ herbicides to tropical marine microalgae remain poorly understood. Experimental exposures of the important but understudied microalgae Rhodomonas salina to seven individual Photosystem II (PSII) inhibitor herbicides (diuron, metribuzin, hexazinone, tebuthiuron, bromacil, simazine, propazine) led to inhibition of effective quantum yield (ΔF/F m ′) and subsequent reductions in specific growth rates (SGR). The concentrations which reduced ΔF/F m ′ by 50% (EC 50 ) ranged from 1.71-59.2 µg L −1 , while the EC 50 s for SGR were 4-times higher, ranging from 6.27-188 µg L −1 . Inhibition of ΔF/F m ′ indicated reduced photosynthetic capacity, and this correlated linearly with reduced SGR (R 2  = 0.89), supporting the application of ∆F/F m ’ inhibition as a robust and sensitive indicator of sub-lethal toxicity of PSII inhibitors for this microalga. The three non-PSII inhibitor herbicides (imazapic, haloxyfop and 2,4-Dichlorophenoxyacetic acid (2,4-D)) caused low or no toxic responses to the function of the PSII or growth at the highest concentrations tested suggesting these herbicides pose little risk to R. salina . This study highlights the suitability of including R. salina in future species sensitivity distributions (SSDs) to support water quality guideline development for the management of herbicide contamination in tropical marine ecosystems.

The increasing frequency and severity of coral bleaching underscores the need for effective coral reef restoration programs. These initiatives include deploying coral fragments or early recruits (spat), with large-scale coral seeding success dependent on improving coral survival by minimizing competition from algae and benthic invertebrates. This study presents a proof-of-concept field experiment assessing the effectiveness of two commercial non-biocidal fouling release coatings (FRCs) and an FRC wax coating in reducing fouling on coral seeding devices. Ceramic devices treated with FRCs were deployed with Acropora millepora microfragments and monitored over 46 weeks. Coated devices experienced significantly less fouling than uncoated controls, particularly during the critical early months when coral spat are most vulnerable. The best performing coating maintained over 10 times more clear surface area than uncoated devices, providing sustained protection without affecting coral survival. Corals also successfully overgrow the coatings by trial’s end. These findings suggest that FRCs could protect smaller coral spat from overgrowth, reducing early mortality until they reach a size escape threshold. Such coatings also hold promise for large-scale restoration projects, coral nurseries, and aquaculture. Future research should evaluate their efficacy across diverse habitats, particularly areas with high macroalgal cover, to optimize their application in restoration strategies. 

Pesticides are ubiquitous in the catchments of the Great Barrier Reef (GBR) and regularly discharge into the nearshore waters. Effective management of pesticides requires suitable water quality guideline values (WQGVs), and further ecotoxicological data for many pesticides are needed to improve the reliability of environmental risk assessments. To help address this issue, toxicity thresholds were determined to two species of tropical marine microalgae Tisochrysis lutea and Tetraselmis sp. for a suite of herbicides detected in the GBR. Photosystem II (PSII) herbicides significantly reduced growth with no effect concentration (NEC) and 10% effect concentration (EC10) values spanning two orders of magnitude from 0.60 µg L −1 for diuron to 60 µg L −1 for simazine across both species. However, growth was insensitive to the non-PSII herbicides. The NEC/EC10 thresholds for most herbicide-microalgae combinations were greater than recent WQGVs intended to protect 99% of species (PC99); however, metribuzin was toxic to T. lutea at concentrations lower than the current PC99 value, which may have to be revisited. The toxicity thresholds for alternative herbicides derived here further inform the development of national and GBR-specific WQGVs, but more toxicity data is needed to develop WQGVs for the > 50 additional pesticides detected in catchments of the GBR.

Widespread contamination of nearshore marine systems, including the Great Barrier Reef (GBR) lagoon, with agricultural herbicides has long been recognised. The fate of these contaminants in the marine environment is poorly understood but the detection of photosystem II (PSII) herbicides in the GBR year-round suggests very slow degradation rates. Here, we evaluated the persistence of a range of commonly detected herbicides in marine water under field-relevant concentrations and conditions. Twelve-month degradation experiments were conducted in large open tanks, under different light scenarios and in the presence and absence of natural sediments. All PSII herbicides were persistent under control conditions (dark, no sediments) with half-lives of 300 d for atrazine, 499 d diuron, 1994 d hexazinone, 1766 d tebuthiuron, while the non-PSII herbicides were less persistent at 147 d for metolachlor and 59 d for 2,4-D. The degradation of herbicides was 2-10 fold more rapid in the presence of a diurnal light cycle and coastal sediments; apart from 2,4-D which degraded more slowly in the presence of light. Despite the more rapid degradation observed for most herbicides in the presence of light and sediments, the half-lives remained > 100 d for the PS II herbicides. The effects of light and sediments on herbicide persistence were likely due to their influence on microbial community composition and its ability to utilise the herbicides as a carbon source. These results help explain the year-round presence of PSII herbicides in marine systems, including the GBR, but more research on the transport, degradation and toxicity on a wider range of pesticides and their transformation products is needed to improve their regulation in sensitive environments.

Over 30 herbicides have been detected in catchments and waters of the Great Barrier Reef (GBR) and their toxicity to key tropical species, including the coral endosymbiotic algae Symbiodiniaceae, is not generally considered in current water quality guideline values (WQGVs). Mutualistic symbionts of the family Symbiodiniaceae are essential for the survival of scleractinian corals. We tested the effects of nine GBR-relevant herbicides on photosynthetic efficiency (ΔF/F m ′) and specific growth rate (SGR) over 14 days of cultured coral endosymbiont Cladocopium goreaui (formerly Symbiodinium clade C1). All seven Photosystem II (PSII) herbicides tested inhibited ΔF/F m ′ and SGR, with toxicity thresholds for SGR ranging between 2.75 and 320 µg L −1 (no effect concentration) and 2.54–257 µg L −1 (EC 10 ). There was a strong correlation between EC 50 s for ΔF/F m ′ and SGR for all PSII herbicides indicating that inhibition of ΔF/F m ′ can be considered a biologically relevant toxicity endpoint for PSII herbicides to this species. The non-PSII herbicides haloxyfop and imazapic did not affect ΔF/F m ′ or SGR at the highest concentrations tested. The inclusion of this toxicity data for Symbiodiniaceae will contribute to improving WQGVs to adequately inform risk assessments and the management of herbicides in tropical marine ecosystems.

In order to examine the potential interactive pressures of local pollution and global climate change, we exposed corals and crustose coralline algae (CCA) to three agricultural photosystemII (PSII) herbicides at four temperatures (26–32°C). The coral Acropora millepora was 3–to 10-fold more sensitive to the three herbicides than the CCA Neogoniolithon fosliei. While the photosynthesis of CCA was not affected by the herbicide concentrations used (< 1 μg L−1), temperatures of 31°C and 32°C alone significantly inhibited photosynthetic efficiency (ΔF:F′m) and caused chronic photoinhibition (reduced Fv : Fm) and substantial bleaching. Environmentally relevant concentrations of each herbicide increased the negative effects of thermal stress on coral at 31°C and 32°C. Mixed model analyses of variance showed that the effects of elevated sea surface temperatures (SST) and herbicide on photosynthetic efficiency of coral symbionts were additive. Furthermore, the effect of either diuron or atrazine in combination with higher SST (31°C and 32°C) on chronic photoinhibition was distinctly greater than additive (synergistic). Reducing the herbicide concentration by 1 μg L−1 diuron above 30°C would protect photosynthetic efficiency by the equivalent of 1.8°C and reduce chronic photoinhibition by the equivalent of a 1°C reduction. Reduced water quality increases the vulnerability of corals to elevated SSTs, and effective management of local water quality can reduce negative effects of global stressors such as elevated SST.

Photosystem II herbicides are transported to inshore marine waters, including those of the Great Barrier Reef and are usually detected in complex mixtures. These herbicides inhibit photosynthesis, which can deplete energy reserves and reduce growth in seagrass, but the toxicity of some of these herbicides to seagrass is unknown and combined effects of multiple herbicides on seagrass has not been tested. Here we assessed the acute phytotoxicity of 10 PSII herbicides to the seagrass Halophila ovalis over 24 and/or 48 h. Individual herbicides exhibited a broad range of toxicities with inhibition of photosynthetic activity ( ∆F/F m ′) by 50% at concentrations ranging from 3.5 μg l −1 (ametryn) to 132 μg l −1 (fluometuron). We assessed potential additivity using the Concentration Addition model of joint action for binary mixtures of diuron and atrazine as well as complex mixtures of all 10 herbicides. The effects of both mixture types were largely additive, validating the application of additive effects models for calculating the risk posed by multiple PSII herbicides to seagrasses. This study extends seagrass ecotoxicological data to ametryn, metribuzin, bromacil, prometryn and fluometuron and demonstrates that low concentrations of PSII herbicide mixtures have the potential to impact ecologically relevant endpoints in seagrass, including ∆F/F m ′.

Herbicides are detected year-round in marine waters, including those of the World Heritage listed Great Barrier Reef (GBR). The few previous studies that have investigated herbicide persistence in seawater generally reported half-lives in the order of months, and several studies were too short to detect significant degradation. Here we investigated the persistence of eight herbicides commonly detected in the GBR or its catchments in standard OECD simulation flask experiments, but with the aim to mimic natural conditions similar to those found on the GBR (i.e., relatively low herbicide concentrations, typical temperatures, light and microbial communities). Very little degradation was recorded over the standard 60 d period (Experiment 1) so a second experiment was extended to 365 d. Half-lives of PSII herbicides ametryn, atrazine, diuron, hexazinone and tebuthiuron were consistently greater than a year, indicating high persistence. The detection of atrazine and diuron metabolites and longer persistence in mercuric chloride-treated seawater confirmed that biodegradation contributed to the breakdown of herbicides. The shortest half-life recorded was 88 d for growth-regulating herbicide 2,4-D at 31°C in the dark, while the fatty acid-inhibitor metolachlor exhibited a minimum half-life of 281 d. The presence of moderate light and elevated temperatures affected the persistence of most of the herbicides; however, the scale and direction of the differences were not predictable and were likely due to changes in microbial community composition. The persistence estimates here represent some of the first appropriate data for application in risk assessments for herbicide exposure in tropical marine systems. The long persistence of herbicides identified in the present study helps explain detection of herbicides in nearshore waters of the GBR year round. Little degradation of these herbicides would be expected during the wet season with runoff and associated flood plumes transporting a high proportion of the original herbicide from rivers into the GBR lagoon.

Early post-settlement mortality is a major bottleneck in larval-based coral restoration, largely driven by competitive overgrowth from benthic fouling organisms. Non-biocidal fouling-release coatings (FRCs) may reduce fouling pressure and enhance spat survival, but their efficacy in situ remains poorly quantified. We evaluated whether a commercial FRC could reduce benthic fouling and improve survival of Acropora loripes spat on a mid-shelf Great Barrier Reef. Larvae were settled onto ceramic seeding devices containing either FRC-treated or untreated (control) cores. Devices were deployed on the reef and monitored for fouling cover and spat survival over 46 weeks (~12 months). Relationships between spat survival, fouling, and benthic community composition were assessed. Fouling was substantially lower on FRC-treated devices, with only 25% fouling cover, compared to near-total overgrowth on controls. Importantly, spat survival remained consistently higher on FRC devices (68%) compared to controls (59%) at 46 weeks. Spat survival was negatively associated with device fouling, independent of immediate benthic community composition. This study provides the first in situ mechanistic evidence that FRCs indirectly enhance coral spat survival by mitigating competitive fouling pressure during the critical early growth period. Although the greatest benefit occurred in the first six months, fouling protection persisted throughout the deployment, suggesting that FRCs could provide a scalable solution to improve restoration outcomes. Integration of FRCs into seeding device design represents a promising strategy to support large-scale coral reef restoration under ongoing climate stress.