Structure of mesoscale eddies in the vicinity of Perth Submarine Canyon

Mesoscale eddies represent discrete, rotating fluid particles that are different compared to their ambient aquatic environment. Understanding the dynamics of mesoscale eddies requires observations, not only of their horizontal structure, such as is available through satellite data, but also of their vertical structure. This study investigates the surface and subsurface characteristics of mesoscale eddies in the vicinity of Perth submarine canyon (30.5‒33.5º S, 112‒116º E) off the southwest coast of Western Australia. Satellite remote sensing (altimetry, temperature, and ocean color) observations were used to understand the surface characteristics while the vertical structure was investigated using ocean glider data collected between 2010 and 2017 through the Integrated Marine Observing Systems (IMOS). Eight Seaglider missions that intersected eddies revealed nine distinct vertical structures, comprising four cyclonic and five anti-cyclonic eddies. Isotherms and isohalines exhibited upwelling in cyclonic eddies, corresponding to mixed layer depth shoaling, and downwelling in anti-cyclonic eddies, aligning with mixed layer depth deepening. Anti-cyclonic eddies exhibited higher surface chlorophyll concentrations than cyclonic eddies, with coastal eddies, regardless of their sense of rotation, displaying elevated surface chlorophyll levels attributed to the entrainment of coastal waters. Offshore eddies featured lower surface chlorophyll concentrations and a distinct subsurface chlorophyll maximum

The influence of turbulent bursting on sediment resuspension under fluvial unidirectional currents

Laboratory experiments were undertaken in a unidirectional current flume in order to examine the role of turbulence on incipient sediment motion. An acoustic Doppler velocimeter was used to measure the instantaneous three-dimensional velocity components and acoustic backscatter (related to suspended sediment concentration). The relationship between wall turbulence (in particular, the "bursting" phenomenon) and resuspension of a non-cohesive sediment bed was examined. The results within a range above and below the measured critical velocity suggested that: 1) the contribution of turbulent bursting events remained identical in both experimental conditions; 2) ejection and sweep events contributed more to the total sediment flux than up-acceleration and down-deceleration events; and 3) wavelet transform revealed a correlation between the momentum and sediment flux in both test conditions. Such similarities in conditions above and below the measured critical velocity highlighted the need to re-evaluate the accuracy of a single time-averaged critical velocity for the initiation of sediment entrainment

A Systematic Review of How Multiple Stressors from an Extreme Event Drove Ecosystem-Wide Loss of Resilience in an Iconic Seagrass Community

A central question in contemporary ecology is how climate change will alter ecosystem structure and function across scales of space and time. Climate change has been shown to alter ecological patterns from individuals to ecosystems, often with negative implications for ecosystem functions and services. Furthermore, as climate change fuels more frequent and severe extreme climate events (ECEs) like marine heatwaves (MHWs), such acute events become increasingly important drivers of rapid ecosystem change. However, our understanding of ECE impacts is hampered by limited collection of broad scale in situ data where such events occur. In 2011, a MHW known as the Ningaloo Niño bathed the west coast of Australia in waters up to 4°C warmer than normal summer temperatures for almost 2 months over 1000s of kilometres of coastline. We revisit published and unpublished data on the effects of the Ningaloo Niño in the seagrass ecosystem of Shark Bay, Western Australia (24.6 – 26.6o S), at the transition zone between temperate and tropical seagrasses. Therein we focus on resilience, including resistance to and recovery from disturbance across local, regional and ecosystem-wide spatial scales and over the past 8 yearsThermal effects on temperate seagrass health were severe and exacerbated by simultaneous reduced light conditions associated with sediment inputs from record floods in the south-eastern embayment and from increased detrital loads and sediment destabilisation. Initial extensive defoliation of Amphibolis antarctica, the dominant seagrass, was followed by rhizome death that occurred in 60-80% of the bay’s meadows, equating to decline of over 1000 km2 of meadows. This loss, driven by direct abiotic forcing, has persisted, while indirect biotic effects (e.g. dominant seagrass loss) have allowed colonisation of some areas by small fast-growing tropical species (e.g. Halodule uninervis). Those biotic effects also impacted multiple consumer populations including turtles and dugongs, with implications for species dynamics, food web structure, and ecosystem recovery. We show multiple stressors can combine to evoke extreme ecological responses by pushing ecosystems beyond their tolerance. Finally, both direct abiotic and indirect biotic effects need to be explicitly considered when attempting to understand and predict how ECEs will alter marine ecosystem dynamics

Exchange Flow Variability between Hypersaline Shark Bay and the Ocean

In Shark Bay, a large hypersaline bay in Western Australia, longitudinal density gradients force gravitational circulation that is important for Bay-ocean exchange. First-time observations of vertical stratification and velocity are presented, confirming the presence of a steady, near-bed dense water outflow from Shark Bay’s northern Geographe Channel that persisted through all stages of the tide. Outflow velocities were 2–3 times stronger than the outflows recorded previously in Naturaliste Channel (in the west), and were more resistant to breakdown by tidal mixing. Estimates of turbulent kinetic energy production derived from the variance method showed a more complex structure in the Geographe Channel, due to shear between surface and bottom layers. Turbulence varied between flood and ebb tide, with peak levels of turbulence occurring during reversal of tidal flows. For both channels, the main source of turbulence was tidal flow along the seabed, with the bottom current speed cubed, |Ub3|, providing a reasonable proxy for tidal mixing and prediction of dense water outflows from Shark Bay majority of the time. Orientation and deeper water of the Geographe Channel along the main axis of the longitudinal density gradient provided an explanation for the predominant outflow from the Bay’s northern entrance. These density-driven currents could potentially influence recruitment of commercially fished scallops and prawns through the dispersal and flushing of larvae

Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

Along the majority of Australian shallow coastal regions, summer evaporation increases the salinity of shallow waters, and subsequently in autumn/winter, the nearshore waters become cooler due to heat loss. This results in the formation of horizontal density gradients with density increasing toward the coast that generates gravity currents known as dense shelf water cascades (DSWCs) flowing offshore along the sea bed. DSWCs play important role in ecological and biogeochemical processes in Australian waters through the transport of dissolved and suspended materials offshore. In this study a numerical ocean circulation model of Rottnest continental shelf, validated using simultaneous ocean glider and mooring data, indicated that the passage of cold fronts associated with winter storms resulted in rapid heat loss through evaporative cooling. These conditions resulted in enhancement of the DSWCs due to modifications of the cross-shelf density gradient and wind effects. Specifically, onshore (offshore) directed winds resulted in an enhancement (inhibition) of DSWCs due to downwelling (vertical mixing). Consequently, the largest DSWC events occurred during the cold fronts when atmospheric temperatures reinforced density gradients and onshore winds promoted downwelling that enhanced DSWCs. Advection of DSWCs was also strongly influenced by the wind conditions, with significantly more transport occurring along-shelf compared to cross-shelf

Improving Data Quality for the Australian High Frequency Ocean Radar Network through Real-Time and Delayed-Mode Quality-Control Procedures

Quality-control procedures and their impact on data quality are described for the High-Frequency Ocean Radar (HFR) network in Australia, in particular for the commercial phased-array (WERA) HFR type. Threshold-based quality-control procedures were used to obtain radial velocity and signal-to-noise ratio (SNR), however, values were set through quantitative analyses with independent measurements available within the HFR coverage, when available, or from long-term data statistics. An artifact removal procedure was also applied to the spatial distribution of SNR for the first-order Bragg peaks, under the assumption the SNR is a valid proxy for radial velocity quality and that SNR decays with range from the receiver. The proposed iterative procedure was specially designed to remove anomalous observations associated with strong SNR peaks caused by the 50 Hz sources. The procedure iteratively fits a polynomial along the radial beam (1-D case) or a surface (2-D case) to the SNR associated with the radial velocity. Observations that exceed a detection threshold were then identified and flagged. After removing suspect data, new iterations were run with updated detection thresholds until no additional spikes were found or a maximum number of iterations was reached

DataSheet_5_Daily timing of low tide drives seasonality in intertidal emersion mortality risk.pdf

Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to atmospheric stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival and growth in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline. Hourly water levels (1992 – 2016) from a high-resolution sea level hindcast (http://sealevelx.ems.uwa.edu.au), were linked to maximum surface solar radiation data from the Copernicus ERA5 atmospheric model and minimum atmospheric temperature observations from the Australian Bureau of Meteorology to identify seasonal patterns and historical occurrence of coral emersion mortality risk. Local tidal characteristics were found to dictate the time of day when low water, and therefore emersion mortality risk occurs, varying on a seasonal and regional basis. In general, risk was found to be greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels. This paper highlights the importance of considering emersion history when assessing intertidal environments, including shallow coral reef platform habitats, where critical low water events intrinsically influence coral health and cover. The study addresses a fundamental knowledge gap in both the field of water level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.</p

DataSheet_2_Daily timing of low tide drives seasonality in intertidal emersion mortality risk.pdf

Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to atmospheric stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival and growth in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline. Hourly water levels (1992 – 2016) from a high-resolution sea level hindcast (http://sealevelx.ems.uwa.edu.au), were linked to maximum surface solar radiation data from the Copernicus ERA5 atmospheric model and minimum atmospheric temperature observations from the Australian Bureau of Meteorology to identify seasonal patterns and historical occurrence of coral emersion mortality risk. Local tidal characteristics were found to dictate the time of day when low water, and therefore emersion mortality risk occurs, varying on a seasonal and regional basis. In general, risk was found to be greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels. This paper highlights the importance of considering emersion history when assessing intertidal environments, including shallow coral reef platform habitats, where critical low water events intrinsically influence coral health and cover. The study addresses a fundamental knowledge gap in both the field of water level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.</p

DataSheet_4_Daily timing of low tide drives seasonality in intertidal emersion mortality risk.pdf

Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to atmospheric stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival and growth in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline. Hourly water levels (1992 – 2016) from a high-resolution sea level hindcast (http://sealevelx.ems.uwa.edu.au), were linked to maximum surface solar radiation data from the Copernicus ERA5 atmospheric model and minimum atmospheric temperature observations from the Australian Bureau of Meteorology to identify seasonal patterns and historical occurrence of coral emersion mortality risk. Local tidal characteristics were found to dictate the time of day when low water, and therefore emersion mortality risk occurs, varying on a seasonal and regional basis. In general, risk was found to be greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels. This paper highlights the importance of considering emersion history when assessing intertidal environments, including shallow coral reef platform habitats, where critical low water events intrinsically influence coral health and cover. The study addresses a fundamental knowledge gap in both the field of water level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.</p

DataSheet_3_Daily timing of low tide drives seasonality in intertidal emersion mortality risk.pdf

Sea level exerts a fundamental influence on the intertidal zone, where organisms are subject to immersion and emersion at varying timescales and frequencies. While emersed, intertidal organisms are exposed to atmospheric stressors which show marked diurnal and seasonal variability, therefore the daily and seasonal timing of low water is a key determinant of survival and growth in this zone. Using the example of shallow coral reefs, the coincidence of emersion with selected stressors was investigated for eight locations around the Australian coastline. Hourly water levels (1992 – 2016) from a high-resolution sea level hindcast (http://sealevelx.ems.uwa.edu.au), were linked to maximum surface solar radiation data from the Copernicus ERA5 atmospheric model and minimum atmospheric temperature observations from the Australian Bureau of Meteorology to identify seasonal patterns and historical occurrence of coral emersion mortality risk. Local tidal characteristics were found to dictate the time of day when low water, and therefore emersion mortality risk occurs, varying on a seasonal and regional basis. In general, risk was found to be greatest during the Austral spring when mean sea levels are lowest and a phase change in solar tidal constituents occurs. For all Great Barrier Reef sites, low tide occurs close to midday during winter and midnight in the summer, which may be fundamental factor supporting the historical bio-geographical development of the reef. Interannual variability in emersion mortality risk was mostly driven by non-tidal factors, particularly along the West Coast where El Niño events are associated with lower mean sea levels. This paper highlights the importance of considering emersion history when assessing intertidal environments, including shallow coral reef platform habitats, where critical low water events intrinsically influence coral health and cover. The study addresses a fundamental knowledge gap in both the field of water level science and intertidal biology in relation to the daily timing of low tide, which varies predictably on a seasonal and regional basis.</p