ALBERT

Description: We examined the effect of artificial light on the near shore trajectories of turtle hatchlings dispersing from natal beaches. Green turtle ( Chelonia mydas ) hatchlings were tagged with miniature acoustic transmitters and their movements tracked within an underwater array of 36 acoustic receivers placed in the near shore zone. A total of 40 hatchlings were tracked, 20 of which were subjected to artificial light during their transit of the array. At the same time, we measured current speed and direction, which were highly variable within and between experimental nights and treatments. Artificial lighting affected hatchling behaviour, with 88% of individual trajectories oriented towards the light and spending, on average, 23% more time in the 2.25 ha tracking array (19.5 ± 5 min) than under ambient light conditions (15.8 ± 5 min). Current speed had little to no effect on the bearing (angular direction) of the hatchling tracks when artificial light was present, but under ambient conditions it influenced the bearing of the tracks when current direction was offshore and above speeds of approximately 32.5 cm s –1 . This is the first experimental evidence that wild turtle hatchlings are attracted to artificial light after entering the ocean, a behaviour that is likely to subject them to greater risk of predation. The experimental protocol described in this study can be used to assess the effect of anthropogenic (light pollution, noise, etc.) and natural (wave action, current, wind, moonlight) influences on the in-water movements of sea turtle hatchlings during the early phase of dispersal.

Description: Mesoscale eddies are common physical features in the ocean that are important for the transport of heat and momentum, and for the distribution of biochemical materials in the ocean. The existence of an anomalous, poleward flowing surface current, the Leeuwin Current (LC), drives the highest eddy kinetic energy environment among all the global eastern boundary currents off Western Australia (WA). There is a limitation in comprehensive understanding of spatial and temporal variability, in particular, the formation and vertical structure of mesoscale eddies. Therefore, this study aimed to define the climatology and vertical structure of mesoscale eddies off WA. Eddy characteristics were derived off WA (10‒40ºS, 105‒130ºE) using a vector geometry-based eddy detection algorithm applied to daily satellite altimetry data over the period 1993-2020. The vertical structure of cyclonic eddies (CE) and anti-cyclonic eddies (AE) in the Perth canyon were examined using ocean glider data over the period 2010-2017 obtained from the Australian Integrated Marine Observing System (IMOS). The seasonal and inter-annual variability and the lifecycles of mesoscale eddies were documented off WA. Shorter-lived eddy formations (〈30 days) were concentrated along the boundaries of the LC with the CE (AE) along the offshore (inshore) boundary and were related to the generation of vorticity. In contrast to the general phenomenon, the concentrations of Chlorophyll were higher in AE compared to CE whilst both eddy types were hotspots of biological activity in the oligotrophic eastern Indian Ocean.

Description: In the eastern margin of the Indian Ocean, anomalous poleward-flowing Leeuwin Current (LC) interacts with the local circulation and topographic features along its path promoting instabilities and a highly energetic eddy field. Eddies play a major role on transferring physical and biogeochemical properties over a wide range of spatial and temporal scales. In particular, sub-mesoscale eddies (SME) are believed to intensify vertical fluxes and are crucial for energy transport. However, as continuous high spatial and temporal resolution data are essential for SME characterization due to their short lifespans and length scale, they are still poorly understood. Thus, this research aims to analyse the spatiotemporal variability of sub-mesoscale eddies distribution and characteristics as a response of LC dynamics along the Rottnest Continental Shelf (ROT). We applied an eddy detection and tracking algorithm to long-term (2010-2018) surface current observations obtained using High-Frequency Radar. LC interactions with the Capes Current and offshore eddies promoted zones with high horizontal shear that were linked to SME generation regions. Counter-clockwise (AC) and clockwise (C) SME were prevalent at the eastern and western boundaries of the LC, respectively, remaining close to their generation spots with 25-50h lifespans. Most of AC (C) SME were formed in August (September). AC eddies generation hotspots migrated meridionally with season, whilst C eddies were clustered in a preferential location, but migrated counter-clockwise over the seasons. The analysis of the long-time series provided detailed information of the generation and lifespans of sub-meso scale eddies and their variability in time in the study region.

Description: Sea-level oscillations and associated current variability, responding to meteorological forcing with periods in the range of 3-15 days (“weather-band”), are ubiquitous along continental shelves globally. However, the investigation of these weather-band sea-level (WBSL) variations over a long period (~10 years) and the understanding their forcing are limited, particularly along North-West Australia. The aims of this research were to: (1) investigate WBSL along North-West Australia from 2009 to 2018; (2) assess the different types of meteorological forcing that contribute to WBSL; and (3) evaluate different meteorological drivers that contribute to the generation of continental shelf waves (CSWs). These aims were achieved through the analysis of long-term sea level records from 7 tide gauge stations, between Port Hedland and Geraldton, separated by 1400 km, together with concurrent meteorological data. The most energetic weather events occurred during the cyclone season (Nov-Apr) and other strong weather events were present during austral winter, particularly along the southern stations. WBSL was linked to five different meteorological forcing conditions: tropical storms (cyclones and depressions); frontal systems, west coast trough and sub-tropical high pressure systems. Over the period of analysis, ~50% of WBSL were attributed to propagating signals and were identified as continental shelf waves (CSW). These waves had mean phase speeds of 4.92±0.6m/s within the study region. CSW’s occurred throughout the year with the most energetic being generated by tropical cyclones. Other WBSL signals were generated by large scale weather systems that resulted in simultaneous sea level changes over the study region.