Benthic dinoflagellates are fundamental to marine ecosystems, contributing to marine food webs, species interactions, and community diversity. They are also known to produce a range of biotoxins that are harmful to humans and wildlife. This study provides one of the most comprehensive regional surveys of benthic dinoflagellates across Australia's Queensland coast, characterising their diversity, distribution, and ecological associations in three contrasting regions: Townsville, Gladstone, and Hervey Bay. Using high-throughput sequencing of the 18S rRNA gene (V8-V9 region), 44 toxic or potentially toxic taxa were detected, spanning both epiphytic and planktonic genera, including Gambierdiscus, Amphidinium, Ostreopsis, Coolia, Prorocentrum, Karenia, and Alexandrium. Benthic toxic dinoflagellate (BTD) assemblages showed strong spatial structuring driven by region, habitat type, and macroalgal host identity. Several taxa exhibited pronounced host specificity, for instance, Ostreopsis siamensis and Coolia tropicalis were associated exclusively with Sargassum, while Amphidinium klebsii occurred only on Lobophora. Riverine influence and benthic habitat type further shaped community composition, with some toxigenic taxa confined to estuarine environments. Co-occurrence analyses revealed both positive and negative associations among taxa, suggesting that interspecies interactions, alongside environmental filtering, contribute to local community dynamics. These findings underscore the ecological complexity of BTD assemblages and highlight the importance of integrating molecular tools with habitat-based monitoring to assess spatial risk and support early-warning systems for harmful algal bloom (HAB) events in tropical and subtropical marine ecosystems.

Seagrass meadows are critical coastal ecosystems that are increasingly recognised for their potential to form microplastic sinks. This study investigated the impacts of microplastic contamination on Eelgrass (Zostera muelleri), a dominant seagrass in the Indo-Pacific. Through a 90-day mesocosm experiment, Z. muelleri was exposed to three escalating microplastic concentrations (885, 3,540, and 8,850 particles.kg-1 sediment) to evaluate morphological, photosynthetic (pulse amplitude modulation fluorometry), bacterial microbiome (16S rRNA gene amplicon sequencing), and leachate (LC-QToF-MS/MS) responses. Relative to controls (0 particles.kg-1), no significant morphological effects were detected at current environmental concentrations (885 particles.kg-1 sediment), but significant declines in biomass (72-86%), rhizome length (110-190%), and leaf count (8-10 fewer leaves) under elevated microplastic loads. Photosynthetic (Y-Yield) efficiency was not affected, and sediment microbiomes exhibited resilience, with no significant shifts in diversity. Chemical analyses identified 82 leachates in sediments, including ten unique to treatments spiked with microplastics, suggesting leachate toxicity may be related to observed biological impacts. These findings suggest that Z. muelleri may be resilient to microplastic pollution at current observed concentrations but are likely to be negatively affected if microplastic pollution continues to rise. Research on leachate-specific effects is required for targeted mitigation strategies to help conserve these vital ecosystems.

Seagrass meadows are highly-valued coastal ecosystems that have the potential to act as sinks for microplastic pollution through their particle-trapping capabilities. Using wave tank simulations, this study tested how microplastic capture can be affected by particle size, sediment composition, and the presence of seagrass (cores collected from two Zostera muelleri meadows). Two synthetic crumb rubber polymers of differing densities were chosen as model contaminants due to their widespread use in road and sporting infrastructure. Sediment composition and seagrass presence influenced microplastic capture, with site-specific differences between cores from the two meadows. The meadow with greater silt and clay content captured more microplastics in the upper 0-2 cm layer. In contrast, the sand-dominated site captured fewer microplastics in the upper layers, but the lower packing fraction of the sediment allowed for penetration to 6 cm depth. Microplastic size also influenced trapping efficiency, with smaller microplastic particles (250-499 μm) being captured at a rate 177 % higher than larger particles (1.18-2 mm). Polymer density (high vs. low) had no significant effect on trapping capability. These findings demonstrate the importance of sediment composition in the ability of seagrass meadows to act as microplastic sinks. Fine-grained sediments trap microplastics in the upper sediment layers (0-2 cm) where interactions with benthic organisms are more likely to occur, while sandy sediments allow for greater penetration, potentially creating deeper reservoirs. This knowledge highlights the need for policymakers and environmental managers to consider sediment composition and site-specific strategies when addressing microplastic pollution in coastal ecosystems.

Ciguatera poisoning is the most prevalent non-bacterial seafood illness globally, with an estimated 10,000 to 50,000 cases reported annually. It is caused by consumption of seafood (mainly fish) that have accumulated ciguatoxins produced primarily by benthic dinoflagellates of the genus Gambierdiscus. Ciguatera poses a significant public health and fisheries challenge in tropical and subtropical regions. Although these dinoflagellates are widespread across tropical and subtropical marine ecosystems, their ecology, taxonomy, and environmental drivers remain poorly understood in many regions, limiting the capacity to monitor and predict ciguatera risk. This study uses DNA metabarcoding (targeting the V8-V9 regions of the 18S rRNA gene) to investigate the diversity, spatial distribution, and ecological associations of Gambierdiscus species across multiple coastal sites in Queensland, Australia. Results revealed the distribution of known species (G. carpenteri, G. holmesii) and several potential new phylotypes, notably a genetically distinct group (Clade II GBR) that may represent an undescribed lineage. Importantly, this study provides the first confirmed detection of G. holmesii, a known ciguatoxin producer, in Hervey Bay and Gladstone, extending its known geographic range from offshore to inshore reefs and coastal habitats of the Great Barrier Reef. This study is also the first documented record of any Gambierdiscus taxa in the Gladstone coastal region. Taxa composition was influenced by macroalgal host, and shallow coral reef habitats supported the highest species diversity and detection frequency, suggesting their role as ecological hotspots. Canonical correspondence analysis (CCA) identified associations between species and macroalgal hosts, including a strong link between G. holmesii and Cladophora sp., while G. carpenteri showed no specific host preference. Overall, this study provides ecological characterisation of Gambierdiscus communities across diverse tropical and subtropical habitats in Queensland, contributing to regional ciguatera risk assessment and global efforts to understand the ecology and distribution of benthic toxic dinoflagellates.

Crustaceans account for similar to 25 % of seafood consumption worldwide, however global concerns related to seafood safety have been raised, especially in terms of metal contaminants. Edible tissue concentrations of metals were examined in commercially important velvet crab Necora puber sampled around the Orkney islands, Scotland. Tissue concentrations of non-essential metals (cadmium and lead) were compared to permissible levels (< 0.50 mg.kg(-1) wet wt., European Commission). Cadmium concentrations were below this in all crabs sampled, whilst lead was above this limit in two N. puber individuals from one location. No correlation was observed between metal concentrations and other biometric factors, including the presence of black spot shell disease and claw deformities. Sex differences in tissue levels were observed with female crabs accumulating more cadmium, copper, manganese, and cobalt. This study provides relevant baseline data regarding metal accumulation in N. puber to inform future, recommended monitoring studies.

The capacity of many macroalgae to rapidly absorb soluble inorganic nutrients and convert them into primary biomass provides opportunities for their use in the bioremediation of nutrient-enriched waters. Marine-based macroalgal cultivation has the potential to target diffuse source nutrient discharges and produce valuable bioproducts such as fertiliser, soil conditioning agents and agricultural feed additives and phycocolloids. Whilst macroalgal diversity offers benefits for improving and maintaining healthy marine ecosystems, it also presents a challenge for determining the best candidate species for cultivation as a nutrient biofilter and, ultimately, a source of bioproducts. We developed and applied a multi-criteria selection model to identify likely candidate macroalgal species for cultivation in Queensland’s coastal waters for biofiltration and bioproduct development, focusing on native species and product development for the agronomical and horticultural sectors. From a species database of 1380 macroalgal species, candidates were initially selected based on their regional abundance, distribution and morphological characteristics considered amenable to in-situ cultivation and harvesting. From a resulting shortlist of 17 species, five systematic literature searches were applied to identify biofilter potential, bioproduct potential and current cultivation status. Of the shortlisted species, Ulva lactuca, Gracilaria edulis and Hypnea cervicornis ranked most highly in the multi-criteria analysis, with several other species showing good potential, but requiring further investment in understanding key biological processes and the development of cultivation protocols.

Global plastic production is estimated to be 400 million tonnes per annum, with 5.25 trillion fragments floating in our oceans. Microplastics (< 5 mm) have the potential to disproportionately accumulate and become trapped in mangroves and seagrass meadows, creating plastic ‘sinks’. This is concerning as these ecosystems are of great ecological and economic importance, with microplastics causing harm to inhabiting flora and fauna. However, accurately measuring microplastic abundance, comparing findings, and determining potential impacts are difficult due to a lack of standardised sampling protocols. Therefore, a systematic literature review was completed to review currently adopted microplastic sampling methods in surface water and sediment in seagrass and mangrove ecosystems. These were compared with recommendations from existing governmental and institutional groups as a first step to standardising methods for future sampling procedures in seagrasses and mangroves.

Land-based aquaculture provides dietary protein to the world's population in a sustainable way, but issues related to release of nitrogen rich wastewater limits its expansion. Sedimentation of naturally occurring microalgae that assimilate excess nitrogen, is slow and land intensive. Electro-flocculation, used in wastewater treatment processes, is a potential alternative for aquaculture. Trials of different electro-flocculation configu-rations applied to three prawn farm pondwater samples containing varying microalgal assemblages are reported. In 64 % of trials, electro-flocculation reduced total nitrogen (TN) and dissolved inorganic nitrogen (DIN) loads within regulatory limits. TN was reduced up to 83.2 % (10.93 to 1.83 mg.L-1) within 20 mins in stationary water, and DIN to 90.6 % (3.19 to 0.30 mg.L-1) in 102 mins trials in flowing water. Bellerochea and Gloeocapsa spp. were dominant in wastewater. The role of microalgal community composition on flocculation is discussed, including evidence Bellerochea promotes flocculation. This study confirmed electro-flocculation quickly reduces TN and DIN.

Occurrence of microplastics (MPs) in freshwater environments, particularly reservoir and lakes, is an emerging concern. There are limited studies in Pakistan on microplastic pollution in the lacustrine environments and those that exist do not provide sufficient information on the spatial distribution of MPs in offshore surface water. The aims of this study were to determine microplastic abundance in Rawal Lake, Pakistan and to ascertain if sampling methodology influences microplastic counts. Surface water samples were collected from 10 sites; 5 tributaries, 2 human settlement and 3 fishing and boating areas using two different sampling techniques: 100 mu m mesh trawl and 20 L sample through a 45 mu m mesh sieve. A significant difference was observed in the abundance of MPs across two methods with the sieve method yielding 2.8 & PLUSMN; 1.44 particles/L and trawl yielding 0.025 & PLUSMN; 0.024 particles/L. Tributaries and boating/fishing area had higher microplastic abundance than the residential area regardless of sampling method. Filaments were the dominant shape of MPs in both type of samples followed by fragments in trawl samples and films in sieved samples. Microbeads were only detected in trawl samples. MPs within size range 0.1-0.9 mm were mostly fragments (82%). MPs were diverse in colors with white/transparent and black MPs common. Polypropylene was the main type of microplastic in Rawal Lake (40-74%). Scanning Electron Microscopy (SEM) of MPs showed cracks, roughness and striations on the particles. Energy Dispersive Spectroscopy (EDS) detected heavy metals (Fe, Cu, Ni, Pb, Zn, Co and Cr) in MPs. Findings suggest that microplastic pollution in Rawal Lake may pose great risk to aquatic and human life through leaching of inherent/ adsorbed heavy metals and therefore requires future investigation.

Microplastic (MP) ingestion has been widely recorded in aquatic organisms, but few studies focus on cnidarians and ctenophores, which form a significant contribution to marine trophic interactions. Scyphozoans (Cyanea capillata, C. lamarckii and Aurelia aurita), hydrozoan (Cosmetira pilosella) and ctenophores (Beroe cucumis and Pleurobrachia bachei) collected opportunistically from Orkney, Shetland and the North Sea were thermally disintegrated, with a subsample of ingested plastics analysed using FTIR. A total of 1,986 MPs were counted (94% fibres), the majority (84.4%) in the four cnidarian species. Highest MP concentrations were recorded in B. cucumis (0.956 ml-1), whilst C. pilosella yielded the lowest (0.014 ml-1). The main polymers in digestate were PET and PP, with 27% discounted as non-plastics. In feeding trials, A. aurita ingested a greater quantity of PET fibres (60-80%), compared to nylon (0%) and HDPE fibres (0%). This study demonstrates cnidarians and ctenophores, a largely overlooked group, are a potential route for MPs entry into food webs.

A multidisciplinary approach was used to assess chemical ecological dietary interactions between marine organisms as a tool to isolate novel ecologically relevant compounds with biotechnological potential. First, laboratory-based feeding preference assays of the sea hare Dolabrifera nicaraguana (previously known as D. dolabrifera), an anaspidean mollusc, were conducted by simultaneously offering six food options collected from nearby tidal pools in the Coiba National Park in the Tropical Eastern Pacific of Panama. An evaluation of preferred dietary repertoire revealed D. nicaraguana significantly preferred cf. Lyngbya sp. over the cyanobacterium Symploca sp., green alga Chaetomorpha sp., and red alga Spyridia sp. A no-choice feeding assay using cf. Lyngbya sp. or green alga Cladophora sp. supported this finding. Secondly, we conducted bioactivity-guided fractionation using the preferred food source of D. nicaraguana, the ‘hair-like” cf. Lyngbya sp. from which we also isolated and elucidated two new depsipeptide compounds, veraguamide M (1) and veraguamide N (2). Veraguamides M (1) and N (2) showed in vitro activity toward the malaria-causing parasite Plasmodium falciparum with GI50 values of 4.2 and 4.3 μM, respectively, and therapeutic windows of 7.0–8.0 (based on moderate cytotoxicities to mammalian Vero cells with GI50 values of 29.3 and 34.1 μM, respectively). Veraguamide N (2) was also active against Leishmania donovani, the causative agent of visceral leishmaniasis, with a GI50 value of 6.9 μM. We then evaluated sequestration of these new compounds by D. nicaraguana used in the feeding assays and found trace amounts of the dietary sequestered compounds. Finally, we evaluated sequestration of these new compounds by the sea hare Stylocheilus rickettsi (previously known as S. striatus) that were grazing on the cf. Lyngbya sp. used in the feeding assays and found both to be sequestered. This study is the first example whereby compounds with significant activity against tropical parasites have been found in both the sea hare S. rickettsi and its cyanobacterial food source. These results suggest that chemical ecological studies involving sea hares and cyanobacteria continue to provide a diverse source of bioactive compounds with biotechnological potential.

Within recent years, the issue of marine litter has become increasingly relevant to the citizens of Scotland. Inputs from shipping and fishing activities with additional contributions from society's throw away lifestyle has inundated healthy seas, endangered wildlife and degraded the health of the oceans. International, European and Scot's law governing the marine environment exists on a legally and non-legally binding basis, with the strictest of regulations concerning marine litter more commonly found on a non-legally binding basis. A global response is necessary to curb the issue but despite commitments to protect the marine environment, many States that have committed to reducing environmental pollution via international treaties are lagging on their promises. The introduction of marine planning to reduce conflict between users and the marine environment has the potential to address marine litter and to uphold commitments for healthy seas under the Marine Strategy Framework Directive. This paper, using a scientific literature-based analysis, evaluates international, European and Scottish environmental law that addresses aspects of marine litter, including inputs, types of marine litter and the outcomes of parties found guilty of contributing to the issue. Integration of these policies within regional marine plans utilising local knowledge has become the first step in tackling this ubiquitous problem, as well as increasing awareness of policies and marine planning amongst the public. A survey was carried out in Scotland to establish awareness levels on these topics: awareness of marine litter (100%), marine planning (34%) and regional marine plans (33%). Raising public awareness of marine planning will aid its effectiveness in reducing the presence and impact of marine litter.

Seagrasses have global distribution and are highly productive and economically valuable habitats. They are sensitive and vulnerable to a range of human-induced pressures, including ongoing exposure to marine litter, such as microplastic particles (<5 mm). In this study, a Zostera marina bed in Deerness Sound, Orkney was selected to determine whether microplastics accumulate in seagrass beds and adhere to seagrass blades. Sediment, seagrass blade, biota and seawater samples were collected. 280 microplastic particles (0.04 to 3.95 mm (mean = 0.95 mm ± 0.05 SE)) were observed in 94% of samples collected (n = 111). These were visually categorised into type (fibre, flake, fragment) and colour, and 50 were successfully identified as plastic using ATR-FTIR. Fibres contributed >50% of the total microplastics observed across all samples. This is the first known study on Z. marina to describe microplastic loading within a seagrass bed and to identify microplastic adherence to seagrass blades.

Around the coastline of the UK, macro-debris has been observed in average densities of over 700 items per metre. Systematic beach-cleans were conducted at 35 sites around the Scottish Orkney Islands, in order to quantify and categorise the level of marine debris found there. Litter was collected from 100 m transects and categorised by its material, broad source (terrestrial or marine) and potential sector source. Variation between sites, and the relative contribution of pre-determined environmental variables in influencing said variation, were analysed using the "capscale" function for a canonical analysis of principle coordinates (CAP). 513 items/m were observed, (77% plastic), with "String/cord (< 1cm diameter)" being the most abundant and widely distributed litter type. 47% of macro-debris was attributed to the fishing sector and < 10% to leisure, living and tourism-associated activities. Conversely, the unique regional hydrodynamics must be examined further, before the source of any given item can be categorically assigned.

Sponges play a vital role in the world's most complex and vulnerable marine ecosystems. Various in situ studies have suggested that sponge morphologies (developed from exposure to a range of biophysical factors) can be considered as ecological indicators to current detrimental environmental changes such as climate change, overfishing, pollution and dredging for coastal development. Regional and long-term taxonomic data on sponges within each geographic range is not always available, especially from the Great Barrier Reef (GBR), due to dearth of sponge research. In this study, to understand large-scale variation and advance sponge research and knowledge, morphological characteristics were adopted as a rapid practical way to identify sponges from phototransect images of a long-term dataset from the GBR. Biennial surveys were carried out in 2008-2014 from 28 pairs of take and no-take zones of the GBR. To evaluate the temporal changes in sponge morphology and correlation between abiotic factors, remote-sensed data such as chlorophyll a, current, wave height and sea surface temperature (SST) during the survey period were analyzed. Results showed sponges were ubiquitous in all six surveyed locations and their distribution was spatially heterogeneous. Encrusting forms were dominant followed by upright, massive, cups and tabular growth forms. Sponges were more prevalent in Innisfail, Pompey and Townsville compared to Cairns, Swain and Capricorn Bunker. Biennial observations showed greater sponge coverage in 2010 and 2014, especially in the central GBR, which may be related to the geomorphology and habitat of reefs along with its influence by wind and wave action. Also, the aftermath of Cyclone Hamish (2009) and Yasi (2011) would have triggered suspended particulate matter that are beneficial to sponge growth. Geostrophic current showed a weak relationship on encrusting, upright and massive forms, whereas, chl-a, wave height and SST appeared to have no effect on sponge morphology, suggesting sponges may be resilient to adverse conditions in the GBR. Whilst selected sponge morphologies can act as environmental proxies to monitor adverse conditions, further in situ research on other environmental parameters such as turbidity, sedimentation, cyclone, tides are required to bring substantial conclusions on sponge morphologies as ecological indicators.

Ciguatoxins (CTXs) produced by benthic Gambierdiscus dinoflagellates, readily biotransform and bioaccumulate in food chains ultimately bioconcentrating in high-order, carnivorous marine species. Certain shark species, often feeding at, or near the top of the food-chain have the ability to bioaccumulate a suite of toxins, from both anthropogenic and algal sources. As such, these apex predators are likely sinks for CTXs. This assumption, in conjunction with anecdotal knowledge of poisoning incidents, several non-specific feeding trials whereby various terrestrial animals were fed suspect fish flesh, and a single incident in Madagascar in 1994, have resulted in the widespread acceptance that sharks may accumulate CTXs. This prompted a study to investigate original claims within the literature, as well as investigate CTX bioaccumulation in the muscle and liver of 22 individual sharks from nine species, across four locations along the east coast of Australia. Utilizing an updated ciguatoxin extraction method with HPLC-MS/MS, we were unable to detect P-CTX-1, P-CTX-2 or P-CTX-3, the three primary CTX congeners, in muscle or liver samples. We propose four theories to address this finding: (1) to date, methods have been optimized for teleost species and may not be appropriate for elasmobranchs, or the CTXs may be below the limit of detection; (2) CTX may be biotransformed into elasmobranch-specific congeners as a result of unique metabolic properties; (3) 22 individuals may be an inadequate sample size given the rare occurrence of high-order ciguatoxic organisms and potential for CTX depuration; and (4) the ephemeral nature and inconsistent toxin profiles of Gambierdiscus blooms may have undermined our classifications of certain areas as CTX hotspots. These results, in combination with the lack of clarity within the literature, suggest that ciguatoxin bioaccumulation in sharks remains elusive, and warrants further investigation to determine the dynamics of toxin production, accumulation and transformation throughout the entire food-web.

Ciguatera fish poisoning is a debilitating human neuro-intoxication caused by consumption of tropical marine organisms, contaminated with bioaccumulated ciguatoxins (CTXs). The growing number of cases coupled with the high toxicity of CTXs makes their reliable detection and quantification of paramount importance. Three commonly occurring ciguatoxins, P-CTX-1, 2 and 3 from five different ciguatoxic Spanish mackerel (Scomberomorus commerson), were used to assess the effectiveness of different extraction techniques: homogenization (high powered blending vs. ultrasonication); C-18 column sizes (500 mg vs. 900 mg); and a novel HILIC SPE cleanup. Despite minor differences, blending and sonication proved equally effective. Larger 900 mg columns offered a greater extraction efficiency, increasing detected P-CTX-1 by 37% (P < 0.001). The newly adapted cleanup was highly effective at reducing co-eluting phospholipids thereby reducing matrix effects and increasing detectable CTXs by HPLC-MS/MS. Silica cleanup extraction efficiencies were also compared between the highly effective and validated ciguatoxin rapid extraction method (CREM) and current best practice extraction method employed by Queensland Health (QH). Overall, the QH protocol proved more effective, especially when paired with the newly adapted cleanup, as this increased the amount of extracted P-CTX-1 by 46% (P < 0.01), P-CTX-2 by 10% and P-CTX-3 by 71% (P = 0.001). This study suggests the QH protocol utilizing a 900 mg C-18 column and newly adapted HILIC SPE cleanup was most effective at extracting P-CTX-1, -2, -3. Specifically P-CTX-1, the primary ciguatoxin congener of concern due to its extremely high potency and an ability to cause CFP at 0.1 μg/kg following consumption of carnivorous fish flesh. Despite being more time intensive (an additional 85 min per batch of 12 samples), this will be especially effective for assessing lower toxin burdens, which may be near the limit of detection.

Nuisance blooms of toxic cyanobacteria are a common occurrence in many tropical and subtropical locations. Benthic marine cyanobacteria of the genera Lyngbya, Okeania, and Moorea are frequently observed in both Florida and throughout the Caribbean, sometimes forming large mats, and are prolific producers of bioactive secondary metabolites that often act as feeding deterrents to generalist herbivores. Little is known regarding the ecological roles of the secondary metabolite chemistry and the palatability of benthic cyanobacteria to grazers. This study examines the palatability of benthic cyanobacterial species from Florida (IRL1, IRL2, IRL3 and Okeania erythroflocculosa) and Belize (BEL1, BEL2) to a range of macro- and mesograzers in Florida and Belize. Pair-wise feeding assays using artificial diets of Gracilaria tikvahiae or fish food coated with cyanobacterial extracts and a control were used to determine palatability of extracts to Floridian and Belizean generalist grazers. The extracts of IRL1, IRL2, IRL3 and O. erythroflocculosa from Florida did not deter feeding by invertebrate grazers. Reef fish, however, were deterred by the non-polar extracts of IRL1, IRL3 and O. erythroflocculosa. Stylocheilus striatus was stimulated to feed on IRL2 extracts and non-polar extracts from IRL3. Non-polar extracts of BEL1 stimulated feeding in S. striatus; however, no significant difference was observed between BEL2 extracts and the control. Most generalist invertebrate grazers, sympatric and non-sympatric, appear indifferent to cyanobacteria extracts whilst reef fish are more likely to be deterred by cyanobacterial extracts, which may affect species interaction within communities with fluctuating or dominating benthic cyanobacterial blooms.

Species of the genus Gambierdiscus are epiphytic dinoflagellates well known from tropical coral reef areas at water temperatures from 24 to 29 degrees C. Gambierdiscus spp. are able to produce ciguatoxins (CTXs) known to bioaccumulate in fish, and the ingestion of tropical fish that accumulated CTXs and possibly also maitotoxins (MTXs) can cause ciguatera fish poisoning (CFP) in humans. In Australia, ciguatera poisonings have been reported in tropical parts of Queensland and the Northern Territory. Here, we report for the first time the seasonal abundance (April-May 2012/13) of Gambierdiscus spp. (up to 65658255 cells g(-1) wet weight algae) from Merimbula and Wagonga Inlets in temperate southern New South Wales, Australia (37 degrees S) at water temperatures of 16.5-17 degrees C. These are popular shellfish aquaculture and recreational fisheries areas with no reports of ciguatera poisoning. Sequencing of a region of the 28S rRNA gene led to the conclusive identification of Gambierdiscus carpenteri. The cells differed however from the Belize type description, including the absence of a thecal groove, dorsal rostrum and variable hatchet- to rectangular-shaped 2' plate, and were morphologically more similar to Gambierdiscus toxicus. To study the dinoflagellate community structure in detail, a pyrosequencing approach based on the 18S rRNA gene was applied, which confirmed the presence of a single Gambierdiscus species only. Neither CTXs nor MTXs were detected in natural bloom material by LC-MS/MS; however, the extracts were found to be toxic via mouse-bioassay, with symptoms suggestive of poisoning by MTX-like compounds. Understanding the abundance of Gambierdiscus populations in areas with no apparent human health impacts is important towards defining the alternate conditions where sparse populations can create ciguatera problems. (C) 2014 Elsevier B.V. All rights reserved.