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Echosounders are high-frequency sonar systems used to sense fish and zooplankton underwater. Their deployment on a variety of ocean observing platforms is generating vast amounts of data at an unprecedented speed from the oceans. Efficient and integrative analysis of these data, whether across different echosounder instruments or in combination with other oceanographic datasets, is crucial for understanding marine ecosystem response to the rapidly changing climate. Here we present Echopype, an open-source Python software library designed to address this need. By standardizing data as labeled, multi-dimensional arrays encoded in the widely embraced netCDF data model following a community convention, Echopype enhances the interoperability of echosounder data, making it easier to explore and use. By leveraging scientific Python libraries optimized for distributed computing, Echopype achieves computational scalability, enabling efficient processing in both local and cloud computing environments. Echopype's modularized package structure further provides a unified framework for expanding support for additional instrument raw data formats and incorporating new analysis functionalities. We plan to continue developing Echopype by supporting and collaborating with the echosounder user community, and envision that the growth of this package will catalyze the integration of echosounder data into broader regional and global ocean observation strategies. [ABSTRACT FROM AUTHOR]

Wildlife tourism can have adverse effects on the behaviours and movements of animals, with implications for the health and fitness of individuals and populations. We used acoustic-tracking to show that food-based attractants used in shark-tourism increases activity (15%) and burst behaviours (60%) in yellowtail kingfish (Seriola lalandi, n  = 18). Increased activity was restricted to periods when kingfish were on the same side of the island group as berleying tourism vessels, but decreased after operators left the site. Despite the raised activity and frequency of burst swimming events, the physiological condition of kingfish (n  = 39, 6 tagged, and 33 untagged) measured using bioelectrical impedance analysis remained consistent with kingfish from control sites not exposed to tourism. This suggests that kingfish were able to compensate raised energy expenditure by feeding on bait and berley used by operators or through natural foraging. We highlight that the effects of provisioning from wildlife tourism can extend beyond changes in behaviours and movements and can additionally influence the energetic condition of non-focal animals through increased activity. However, supplemental food-sources provided through wildlife tourism may be sufficient to compensate for the increased energy expenditure and lessen the effects of tourism on individual fitness and health. [ABSTRACT FROM AUTHOR]

Growth of structural mass and energy reserves influences individual survival, reproductive success, population and species life history. Metrics of structural growth and energy storage of individuals are often used to assess population health and reproductive potential, which can inform conservation. However, the energetic costs of tissue deposition for structural growth and energy stores and their prioritization within bioenergetic budgets are poorly documented. This is particularly true across marine mammal species as resources are accumulated at sea, limiting the ability to measure energy allocation and prioritization. We reviewed the literature on marine mammal growth to summarize growth patterns, explore their tissue compositions, assess the energetic costs of depositing these tissues and explore the tradeoffs associated with growth. Generally, marine mammals exhibit logarithmic growth. This means that the energetic costs related to growth and tissue deposition are high for early postnatal animals, but small compared to the total energy budget as animals get older. Growth patterns can also change in response to resource availability, habitat and other energy demands, such that they can serve as an indicator of individual and population health. Composition of tissues remained consistent with respect to protein and water content across species; however, there was a high degree of variability in the lipid content of both muscle (0.1–74.3%) and blubber (0.4–97.9%) due to the use of lipids as energy storage. We found that relatively few well-studied species dominate the literature, leaving data gaps for entire taxa, such as beaked whales. The purpose of this review was to identify such gaps, to inform future research priorities and to improve our understanding of how marine mammals grow and the associated energetic costs. [ABSTRACT FROM AUTHOR]

Following the passage of a tropical cyclone (TC) the changes in temperature, salinity, nutrient concentration, water clarity, pigments and phytoplankton taxa were assessed at 42 stations from eight sites ranging from the open ocean, through the coastal zone and into estuaries. The impacts of the TC were estimated relative to the long-term average (LTA) conditions as well as before and after the TC. Over all sites the most consistent environmental impacts associated with TCs were an average 41% increase in turbidity, a 13% decline in salinity and a 2% decline in temperature relative to the LTA. In the open ocean, the nutrient concentrations, cyanobacteria and picoeukaryote abundances increased at depths between 100 and 150 m for up to 3 months following a TC. While at the riverine end of coastal estuaries, the predominate short-term response was a strong decline in salinity and phytoplankton suggesting these impacts were initially dominated by advection. The more intermediate coastal water-bodies generally experienced declines in salinity, significant reductions in water clarity, plus significant increases in nutrient concentrations and phytoplankton abundance. These intermediate waters typically developed dinoflagellate, diatom or cryptophyte blooms that elevated phytoplankton biomass for 1–3 months following a TC. [ABSTRACT FROM AUTHOR]

As a fundamental component for marine ecosystem functioning, phytoplankton require an appraisal of the extent of their adaptive potential to interpret the effects of natural or human-induced changes in marine habitats. To this aim, the study of the present ecological characteristics of phytoplankton species represents the first effort to assess their adaptive potential in a wide spectrum of environmental variables. Using a set of nine time series, this study aimed at describing the ecological niche of 10 worldwide-distributed species and testing whether individual species occupy the same niche across different environments. The species showed wide variations in frequency, average abundance and seasonal pattern among sites, while their annual maxima occurred over ample ranges of physical–chemical variables, with marked differences from site to site depicting a considerably wide global niche for each species. Although in some of the species this result can be attributed to the existence of multiple, morphologically similar taxa hardly identifiable in light microscopy, other cases could be explained by intraspecific diversity and/or enhanced adaptation potential in phytoplankton populations. Although probably not applicable to species with more restricted distribution ranges, these results should be considered when projecting present species distributions into a future scenario of climate change. [ABSTRACT FROM AUTHOR]

Heterotrophic bacterial diazotrophs (HBDs) are ubiquitous in the pelagic ocean, where they have been predicted to carry out the anaerobic process of nitrogen fixation within low-oxygen microenvironments associated with marine pelagic particles. However, the mechanisms enabling particle colonization by HBDs are unknown. We hypothesized that HBDs use chemotaxis to locate and colonize suitable microenvironments, and showed that a cultivated marine HBD is chemotactic toward amino acids and phytoplankton-derived DOM. Using an in situ chemotaxis assay, we also discovered that diverse HBDs at a coastal site are motile and chemotactic toward DOM from various phytoplankton taxa and, indeed, that the proportion of diazotrophs was up to seven times higher among the motile fraction of the bacterial community compared to the bulk seawater community. Finally, three of four HBD isolates and 16 of 17 HBD metagenome assembled genomes, recovered from major ocean basins and locations along the Australian coast, each encoded >85% of proteins affiliated with the bacterial chemotaxis pathway. These results document the widespread capacity for chemotaxis in diverse and globally relevant marine HBDs. We suggest that HBDs could use chemotaxis to seek out and colonize low-oxygen microenvironments suitable for nitrogen fixation, such as those formed on marine particles. Chemotaxis in HBDs could therefore affect marine nitrogen and carbon biogeochemistry by facilitating nitrogen fixation within otherwise oxic waters, while also altering particle degradation and the efficiency of the biological pump. [ABSTRACT FROM AUTHOR]

Fisheries are currently under pressure to provide increasing amounts of seafood, causing a growing number of marine stocks to be harvested at unsustainable levels. To ensure marine resources remain sustainable, careful management of biological stocks and their genetic integrity is required. The Eastern Rock Lobster, Sagmariasus verreauxi , is commercially harvested along the New South Wales (NSW) coast of eastern Australia and is managed as a single unit. Due to overfishing, the NSW S. verreauxi stock was severely depleted in the mid-1990s but has since been rebuilding. This study evaluates the population genetic structure, putative local adaptation, and potential of a population bottleneck for NSW S. verreauxi. Using neutral single nucleotide polymorphisms (SNPs), we determined NSW S. verreauxi consist of a single genetic stock, with outlier SNPs detecting weak genetic divergence among offshore locations, and evidence of population bottlenecks at all locations. Our findings (i) confirm a single management unit is appropriate; (ii) can be used as a baseline for future genetic monitoring of NSW S. verreauxi ; and (iii) highlights the importance of implementing routine genetic monitoring and collecting temporal samples to understand the full impact of overfishing on a species resilience. [ABSTRACT FROM AUTHOR]

The symbiosis between bacteria and sponges has arguably the longest evolutionary history for any extant metazoan lineage, yet little is known about bacterial evolution or adaptation in this process. An example of often dominant and widespread bacterial symbionts of sponges is a clade of uncultured and uncharacterised Proteobacteria. Here we set out to characterise this group using metagenomics, in-depth phylogenetic analyses, metatranscriptomics, and fluorescence in situ hybridisation microscopy. We obtained five metagenome-assembled-genomes (MAGs) from different sponge species that, together with a previously published MAG (AqS2), comprise two families within a new gammaproteobacterial order that we named UTethybacterales. Members of this order share a heterotrophic lifestyle but vary in their predicted ability to use various carbon, nitrogen and sulfur sources, including taurine, spermidine and dimethylsulfoniopropionate. The deep branching of the UTethybacterales within the Gammaproteobacteria and their almost exclusive presence in sponges suggests they have entered a symbiosis with their host relatively early in evolutionary time and have subsequently functionally radiated. This is reflected in quite distinct lifestyles of various species of UTethybacterales, most notably their diverse morphologies, predicted substrate preferences, and localisation within the sponge tissue. This study provides new insight into the evolution of metazoan–bacteria symbiosis. [ABSTRACT FROM AUTHOR]

Knowledge about reproductive movements can be of important conservation value for over-exploited species that are vulnerable when moving between and within key reproductive habitats. Lack of knowledge persists around such movements in the overfished school shark Galeorhinus galeus in Australia. Management assumes all pregnant females migrate between adult aggregations in the Great Australian Bight, South Australia, and nursery areas around Bass Strait and Tasmania. We tracked 14 late-term pregnant females tagged in South Australia using satellite-linked pop-up archival tags to investigate extent, timing, and routes of migrations. We found partial migration, with some females (n  = 7) remaining near aggregating areas throughout the pupping season, some migrating to known nursery areas (n  = 3), and one migrating ∼3 000 km to New Zealand. We conclude female movements and pupping habitats are less spatially constrained than assumed and propose females use cool-water routes along the shelf break to reduce energy costs of migration. Migrating females using these routes faced greater fishing pressure than sharks in inshore areas and were not protected by inshore shark fishing closures designed to protect them. This study demonstrates the complexity of reproductive movements that can occur in wide-ranging species and highlights the value of explicit movement data. [ABSTRACT FROM AUTHOR]

Marine Group II (MGII) archaea represent the most abundant planktonic archaeal group in ocean surface waters, but our understanding of the group has been limited by a lack of cultured representatives and few sequenced genomes. Here, we conducted a comparative phylogenomic analysis of 270 recently available MGII metagenome-assembled genomes (MAGs) to investigate their evolution and ecology. Based on a rank-normalised genome phylogeny, we propose that MGII is an order-level lineage for which we propose the name Candidatus Poseidoniales (after Gr. n. Poseidon, God of the sea), comprising the families Candidatus Poseidonaceae fam. nov. (formerly subgroup MGIIa) and Candidatus Thalassarchaeaceae fam. nov. (formerly subgroup MGIIb). Within these families, 21 genera could be resolved, many of which had distinct biogeographic ranges and inferred nutrient preferences. Phylogenetic analyses of key metabolic functions suggest that the ancestor of Ca. Poseidoniales was a surface water-dwelling photoheterotroph that evolved to occupy multiple related ecological niches based primarily on spectral tuning of proteorhodopsin genes. Interestingly, this adaptation appears to involve an overwrite mechanism whereby an existing single copy of the proteorhodopsin gene is replaced by a horizontally transferred copy, which in many instances should allow an abrupt change in light absorption capacity. Phototrophy was lost entirely from five Ca. Poseidoniales genera coinciding with their adaptation to deeper aphotic waters. We also report the first instances of nitrate reductase in two genera acquired via horizontal gene transfer (HGT), which was a potential adaptation to oxygen limitation. Additional metabolic traits differentiating families and genera include flagellar-based adhesion, transporters, and sugar, amino acid, and peptide degradation. Our results suggest that HGT has shaped the evolution of Ca. Poseidoniales to occupy a variety of ecological niches and to become the most successful archaeal lineage in ocean surface waters. [ABSTRACT FROM AUTHOR]

Understanding the processes driving species distributions is a key question in ecology, yet obtaining pertinent data for many species can be challenging. Tag-recapture data from voluntary recreational fisheries programmes is an undervalued citizen science data resource and can provide valuable observation records for data-poor species. Here, we highlight the utility of such data by compiling four tag-recapture datasets (n = 20 878 tags) to describe the movements and distribution of a piscivorous predator, tailor Pomatomus saltatrix, with respect to their environment. We compared species distribution models that explored a trade-off between temporal duration and spatial extent. Both models revealed sea surface temperature (SST) was an important factor in describing the distribution and density of tailor. However, the model with broader spatial extent indicated a non-linear response to SST, peaking at 21.5°C, while the model with temporal longevity showed a linear SST response. This disjunct showcases the importance of building models across an adequate geographic extent to ensure covariate responses are not truncated. This is the first study to comprehensively describe the distribution and migrations of adult tailor across their eastern Australian distribution. The results are discussed in light of ocean warming along eastern Australia, and the subsequent implications for using tag-recapture data in fisheries management. [ABSTRACT FROM AUTHOR]

Most studies investigating the effects of anthropogenic environmental stressors do so in conditions that are often optimal for their test subjects, ignoring natural stressors such as competition or predation. As such, the quantitative results from such studies may often underestimate the lethality of certain toxic compounds. A well-known example of this concept is illustrated by the marked increase in the lethality of pesticides when larval amphibians are concurrently exposed to the odor of potential predators. Here, we investigated the interaction between background levels of environmental predation risk (high vs. low) and ocean acidification (ambient vs. elevated CO2) in 2 × 2 design. Wild-caught juvenile damselfish, Pomacentrus amboinensis, were exposed in the laboratory to the different risk and CO2 conditions for 4 days and released onto coral reef patches. Using a well-established field assay, we monitored the in situ behavior and mortality of the damselfish for 2 days. We predicted that juvenile fish exposed to elevated CO2 and high-risk conditions would display more severe behavioral impairments and increased mortality compared to fish exposed to elevated CO2 maintained under low-risk conditions. As expected, elevated CO2 exposure led to impaired antipredator responses and increased mortality in low-risk fish compared to ambient CO2 controls. However, we failed to find an effect of elevated CO2 on the behavior and survival of the high-risk fish. We hypothesized that the results may stem from either a behavioral compensation or a physiological response to high risk. Our results provide insights into the interactive nature of environmental and natural stressors and advance our understanding of the predicted effect of ocean acidification on aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Oceanographic fronts are strong discontinuities in water mass properties and can result in differences in biota over short horizontal distances. The vertical structuring of epipelagic euphausiids was investigated across an ephemeral thermohaline front between Leeuwin Current Water (LCW) and Sub Tropical Surface Water (STSW) in the south-east Indian Ocean. Vertical ontogenetic layering was evident and euphausiid larvae were most abundant in the 0–50 m stratum in both water masses. Differences in assemblages between water masses were mostly driven by the numerical abundance of shared species; most species were in greater abundance in LCW. LCW was characterized by a uniform water column and no significant structuring of assemblages with depth. Structuring was, however, evident in STSW and analyses indicated this was a result of stratification of temperature, salinity and dissolved oxygen. Higher chlorophyll a concentrations and zooplankton settled volumes were evident in LCW, as well as the upper 100 m of STSW. Decreases in temperature and salinity were correlated with the variation in assemblages at >100 m depth in STSW. This is the first study to examine vertical structuring and environmental correlations of euphausiid life stages and assemblages across a thermohaline front in the south-east Indian Ocean. [ABSTRACT FROM AUTHOR]

Estimating the biomass of gas-bladdered organisms in the mesopelagic ocean is a simple first step to understanding ecosystem structure. An existing two-frequency (38 and 120 kHz) acoustic and optical probe was lowered to 950 m to estimate the number and size of gas-bladders. In situ target strengths from 38 and 120 kHz and their difference were compared with those of a gas-bladder resonance-scattering model. Predicted mean equivalent spherical radius gas-bladder size varied with depth, ranging from 2.1 mm (shallow) to 0.6 mm (deep). Density of night-time organisms varied throughout the water column and were highest (0.019 m−3) in the 200–300 m depth range. Predictions of 38 kHz volume-backscattering strength (Sv) from the density of gas-bladdered organisms could explain 88% of the vessel's 38 kHz Sv at this location (S 40.9, E 166.7). Catch retained by trawls highlighted the presence of gas-bladdered fish of a similar size range but different densities while optical measurements highlighted the depth distribution and biomass of gas-inclusion siphonophores. Organism behaviour and gear selectivity limits the validation of acoustic estimates. Simultaneous optical verification of multifrequency or broadband acoustic targets at depth are required to verify the species, their size and biomass. [ABSTRACT FROM AUTHOR]

Hawksbill turtle (Eretmochelys imbricata) populations have experienced global decline because of a history of intense commercial exploitation for shell and stuffed taxidermied whole animals, and harvest for eggs and meat. Improved understanding of genetic diversity and phylogeography is needed to aid conservation. In this study, we analyzed the most geographically comprehensive sample of hawksbill turtles from the Indo-Pacific Ocean, sequencing 766 bp of the mitochondrial control region from 13 locations (plus Aldabra, n = 4) spanning over 13 500 km. Our analysis of 492 samples revealed 52 haplotypes distributed in 5 divergent clades. Diversification times differed between the Indo-Pacific and Atlantic lineages and appear to be related to the sea-level changes that occurred during the Last Glacial Maximum. We found signals of demographic expansion only for turtles from the Persian Gulf region, which can be tied to a more recent colonization event. Our analyses revealed evidence of transoceanic migration, including connections between feeding grounds from the Atlantic Ocean and Indo-Pacific rookeries. Hawksbill turtles appear to have a complex pattern of phylogeography, showing a weak isolation by distance and evidence of multiple colonization events. Our novel dataset will allow mixed-stock analyses of hawksbill turtle feeding grounds in the Indo-Pacific by providing baseline data needed for conservation efforts in the region. Eight management units are proposed in our study for the Indo-Pacific region that can be incorporated in conservation plans of this critically endangered species. [ABSTRACT FROM AUTHOR]

Measurements of mean volume backscattering strength (Sv, dB re 1 m-1) at ocean-basin scale were made using 38-kHz hull-mounted echosounders on ships of opportunity as part of Australia's Integrated Marine Observing System. The data were collected on vessels of various designs, none of which were purposely built for collecting high-quality acoustic data. A full range of weather extremes affected the quality of the data and could cause large biases in Sv. To remove first-order biases and improve processing efficiency, a sequence of new and existing data processing filters were applied in a semi-automated procedure. These filters were designed to mitigate the effects of three types of noise: impulsive (less than one ping), transient (multiple pings), and background (hours or longer). A filter was also applied to identify signals that were attenuated by air bubbles beneath the transducer. These filters were applied to data from transits across the Southwest Pacific, Indian, and Southern Oceans to produce quality-controlled Sv datasets that are now available from a publicly accessible repository. These filters may be relevant to other open-ocean acoustic observing endeavours, and one or more could be used to mitigate bias in data from a range of acoustic applications. [ABSTRACT FROM AUTHOR]

The small salp, Thalia democratica, is known globally to form dense swarms, yet the physiological and oceanographic factors that influence the magnitude and occurrence of these swarms are poorly understood. In this study, two numerical models were used to explore T. democratica population dynamics. A Lefkovitch matrix model identified that the survival of juvenile oozoids was the most important population metric promoting population growth and a size-structured population model was developed to further explore the dynamics of T. democratica. The size-structured model tracks cohorts of four life stages and incorporates size-dependent reproduction and mortality. Model outputs of average generation time and mean abundances of each life stage corresponded well to previously reported values. Factors that promote juvenile oozoid abundance were the most sensitive parameters influencing salp abundance. A 10-year time-series simulation using this population model identified that salp abundances in the Tasman Sea are proportionally higher in winter and spring, consistent with previous studies. This model shows that temperature and phytoplankton concentration are sufficient drivers of large-scale patterns of salp abundance across season and latitude. These results demonstrate the importance of future research focusing on identifying the environmental conditions that influence the size at which females reproduce. [ABSTRACT FROM AUTHOR]

The strong La Niña of 2010-2011 provided an opportunity to investigate the ecological impacts of El Niño-Southern Oscillation on coastal plankton communities using the nine national reference stations around Australia. Based on remote sensing and across the entire Australian region 2011 (La Niña) was only modestly different from 2010 (El Niño) with the average temperature declining 0.2%, surface chlorophyll a up 3% and modelled primary production down 14%. Other changes included a poleward shift in Prochlorococcus and Synechococcus. Along the east coast, there was a reduction in salinity, increase in nutrients, Chlorophytes and Prasinophytes (taxa with chlorophyll b, neoxanthin and prasinoxanthin). The southwest region had a rise in the proportion of 19-hexoyloxyfucoxanthin; possibly coccolithophorids in eddies of the Leeuwin Current and along the sub-tropical front. Pennate diatoms increased, Ceratium spp. decreased and Scrippsiella spp. increased in 2011. Zooplankton biomass declined significantly in 2011. There was a reduction in the abundance of Calocalanus pavo and Temora turbinata and increases in Clausocalanus farrani, Oncaea scottodicarloi and Macrosetella gracilis in 2011. The changes in the plankton community during the strong La Niña of 2011 suggest that this climatic oscillation exacerbates the tropicalization of Australia. [ABSTRACT FROM AUTHOR]

Large planktivores require high-density prey patches to make feeding energetically viable. This is a major challenge for species living in tropical and subtropical seas, such as whale sharks Rhincodon typus. Here, we characterize zooplankton biomass, size structure and taxonomic composition from whale shark feeding events and background samples at Mafia Island, Tanzania. The majority of whale sharks were feeding (73%, 380 of 524 observations), with the most common behaviour being active surface feeding (87%). We used 20 samples collected from immediately adjacent to feeding sharks and an additional 202 background samples for comparison to show that plankton biomass was ~10 times higher in patches where whale sharks were feeding (25 vs. 2.6 mg m-3). Taxonomic analyses of samples showed that the large sergestid Lucifer hanseni (~10 mm) dominated while sharks were feeding, accounting for ~50% of identified items, while copepods (<2 mm) dominated background samples. The size structure was skewed towards larger animals representative of L.hanseni in feeding samples. Thus, whale sharks at Mafia Island target patches of dense, large, zooplankton dominated by sergestids. Large planktivores, such as whale sharks, which generally inhabit warm oligotrophic waters, aggregate in areas where they can feed on dense prey to obtain sufficient energy. [ABSTRACT FROM AUTHOR]

Smale, D. A., Kendrick, G. A., Harvey, E. S., Langlois, T. J., Hovey, R. K., Van Niel, K. P., Waddington, K. I., Bellchambers, L. M., Pember, M. B., Babcock, R. C., Vanderklift, M. A., Thomson, D. P., Jakuba, M. V., Pizarro, O., and Williams, S. B. 2012. Regional-scale benthic monitoring for ecosystem-based fisheries management (EBFM) using an autonomous underwater vehicle (AUV). – ICES Journal of Marine Science, 69: 1108–1118.Monitoring marine habitats and biodiversity is critical for understanding ecological processes, conserving natural resources, and achieving ecosystem-based fisheries management (EBFM). Here, we describe the application of autonomous underwater vehicle (AUV) technology to conduct ongoing monitoring of benthic habitats at two key locations in Western Australia. Benthic assemblages on rocky reefs were sampled with an AUV, which captured >200 000 geo-referenced images. Surveys were designed to obtain 100% coverage of 25 × 25 m patches of benthic habitat. In 2010, multiple patches were surveyed at 15–40-m depths at three reference sites at the Houtman Abrolhos Islands and at six reference sites at Rottnest Island. The following year, repeat surveys of the same geo-referenced patches were conducted. Benthic assemblages at the Houtman Abrolhos Islands were varied in that one reference site was dominated by hard corals, whereas the other two were macroalgae dominated. Conversely, assemblages at Rottnest Island were dominated by the kelp Ecklonia radiata. The AUV resurveyed each patch with high precision and demonstrated adequate power to detect change. Repeated observations at the reference sites will track natural variability in benthic habitat structure, which in turn will facilitate the detection of ecological change and ultimately feed back into EBFM processes. [ABSTRACT FROM PUBLISHER]

We describe a climate-driven range expansion of the red-tide dinoflagellate Noctiluca scintillans into the Southern Ocean (45°31′S 147°E). Sea surface height data showed that a warm-core eddy moving southwards from Tasmania was the potential vector for the transport of Noctiluca. We provide evidence for active feeding of Noctiluca on Southern Ocean phytoplankton. Possible competition with other grazers may have implications for food web dynamics were Noctiluca to become established in the Southern Ocean. [ABSTRACT FROM PUBLISHER]

We investigated the space–time dynamics of picophytoplankton in South Australian continental shelf waters from February 2008 to January 2009, focusing on localized physical events. We discriminated six picophytoplankton populations by flow cytometry, including Synechococcus (SYN1, SYN2), Prochlorococcus (PROC1, PROC2) and small and large picoeukaryotes (EUKS, EUKL). Local physical events observed included downwelling and dense waters outflowing from a nearby gulf in winter–early spring 2008, upwelling in summer and early spring 2008 and eddy formation in January 2009. Each population responded differently to these events, which resulted in up to four orders of magnitude changes in their abundances. Population-specific hotspots reflected a succession of distinct dominant communities associated with the strength of upwelling events, changes in fluorescence maximum depths and local downwelling and mixing processes. The unexpected high abundances and local dominance of Prochlorococcus in summer reflected the possible influence of eastward and westward current transports and the presence of a High-Light (PROC1)- and Low-Light (PROC2)-adapted ecotypes. This study highlights the role of localized physical events in the dominance of all three picophytoplankton groups that may be critical for the high productivity of the study region, and suggests the importance of hydroclimatic forcing for inter-annual changes in picophytoplankton communities. [ABSTRACT FROM AUTHOR]