Note:

Submarine canyons are important to continental shelf ecosystems. They have a strong influence on shelf circulation and the distribution of biota. The Perth Canyon is a long, deep canyon on the Western Australian coastline that has attracted attention as a feeding area for pygmy blue whales (Balaenoptera musculus brevicauda). Despite existing on a highly oligotrophic coast, the Perth Canyon has the ability to support sufficient krill to feed these massive mammals. The aim of this study was to examine the physical processes within the Perth Canyon, and consider how these could affect productivity. Research areas included the interaction of the Leeuwin Current and Leeuwin Undercurrent with the canyon, the circulation within the canyon, the effect of wind forcing and the occurrence of upwelling. The oceanography of the Western Australian coast including seasonal productivity changes was also examined. This study utilised numerical modelling and collection of field data to develop a thorough understanding of the Perth Canyon. The numerical model ROMS (Regional Ocean Modelling System) was used to simulate a long stretch of coastline in which the Perth Canyon was centrally located. The model forced the Leeuwin Current and Undercurrent using density gradients, and the seasonal Capes Current was then generated by applying a surface wind stress. The simulations showed that primarily the Leeuwin Undercurrent interacted with the canyon. Eddies continually formed within the canyon, which enhanced vertical transport and could contribute to entrapment of passive drifters. The addition of wind had no discernible effect on canyon circulation although vertical velocities increased everywhere and shallow upwelling occurred along the shelf. The field data comprised moored temperature loggers, field cruises, and sundry data from satellite imagery, weather stations and whale observations. / The temperature loggers, located on the canyon rim, indicated the range of processes that affect the canyon region. These processes included seasonal changes in the wind, the seasonal changes and meanders of the Leeuwin Current, storms, the near-diurnal sea breeze and inertial period changes, and other internal waves. The temperature loggers also indicated sporadic upwelling at the canyon rims, although this upwelling rarely extended into the Leeuwin Current. The field cruises gathered CTD, ADCP, nutrients and acoustic backscatter data. The water masses near the canyon were identified from their temperature, salinity and oxygen signatures. The deep chlorophyll maximum exhibited high spatial variability around the canyon. The circulation, in conjunction with the simulated circulation from ROMS, reiterated that eddies filled the canyon below its rims, and suggested that passive drifters would aggregate within the head. The acoustic backscatter reinforced this, showing that biota concentrated near the head of the canyon, which is where the whales were most often sighted feeding. The conclusions of this study were that the canyon is a region of enhanced productivity where upwelling is enhanced and aggregation of passive drifters is likely. Upwelling occurs more strongly when the Leeuwin Current is weakened or has meandered out of a region. Seasonal changes in productivity in the canyon conform to seasonal productivity arguments for the entire coastline, which accounts for the seasonal aggregation of blue whales. The physical processes in the Perth Canyon are variable and all are important to this marine ecosystem.

The circulation along the south coast of Western Australia was examined using field data and numerical modelling. Physical processes in this region, particularly along the continental shelf and slope regions, were poorly understood due to a paucity of field measurements. Data were collected during a research cruise on RV Southern Surveyor (04/2006) during April 2006 consisting of 18 CTD transects from Twilight Cove (126oE) to Cape Leeuwin (115oE) and was augmented by shipborne ADCP data. The field data set provided a detailed understanding of three major current systems: Leeuwin Current (LC), Leeuwin Undercurrent (LU) and Flinders Current (FC). The LC along the south coast exhibits different characteristics when compared to that along the west coast. The LC flows into the colder and lower salinity subantarctic environment of the south coast. This is evident in a strong geopotential gradient off the south-west corner of Australia (Cape Leeuwin) resulting in rapid acceleration of the LC as it reaches a maximum velocity in this region. Numerical modelling studies, using the Regional Ocean Modelling System (ROMS) indicated that wind stress is an important component of the dynamics in this region. This was identified when comparing summer and winter conditions when the winds act in opposite directions, from north-westerly to southeasterly respectively. Along the shelf break and slope, the Flinders Current (FC) interacts with LC. As the dominant current, the FC serves both as a surface and as an undercurrent, transporting sub Antarctic mode water (SAMW). This interconnection the FC and LU can be seen clearly from the salinity, temperature and velocities within the depth range 200-700m postulating a connection between subsurface waters off Tasmania (origin of the Flinders Current) and the tropical Indian Ocean through the Flinders and Leeuwin Undercurrents.

The aim of paleoenvironmental studies is to reconstruct characteristics of the past environment from fossil assemblages preserved in sedimentary strata. Thus, studies of modern surface assemblages, quantitatively correlated to the environmental parameters, are required before reliable interpretations can be made. For this dissertation, two different techniques were applied in two case studies: a reconstruction making use of a benthic foraminiferal transfer function from the intertidal marshes in the eastern Mississippi Sound, Alabama/Mississippi, and a qualitative reconstruction of ocean current activity on the Western Australian shelf. Modern salt-marsh foraminifera were collected from Grand Bay, Pascagoula, Fowl River, and Dauphin Island across several elevation transects and different salinity regimes. Cluster analysis yielded nine dead biofacies and five live assemblages from Open Estuarine to Upland Transition. Canonical correspondence analysis indicated a strong relationship between distributions of dead biofacies and elevation. Both dissolution of calcareous species in the organic marsh sediment and the long-term accumulative nature of the dead assemblage favored the use of non-estuarine dead assemblages. A Weighted Average-Partial Least Squared transfer function was applied to the surface data and yielded a Root Mean Squared Error of Prediction (RMSEP) of 0.14 m, which represents 33% Mean Range of Tide at Grand Bay and 39% at Dauphin Island. The transfer function was applied to two sedimentary cores from Grand Bay and two from Dauphin Island, which revealed disparate developments between the regions during the last 1,900 years. While both Dauphin Island cores indicated relative sea-level trends aligned with other Gulf of Mexico studies, Grand Bay was likely impacted by a river avulsion event disconnecting Grand Bay from fluvial sediment influx, and by the erosion of a protective headland, Grand Batture Island. Sediments spanning the last ~100 years contained increased abundances of low marsh foraminifera likely associated with coastline erosion, which was most prominently displayed by a lithology shift towards grey silt in the Dauphin Island cores. Surface carbonate sediments from Western Australia’s Northwestern Shelf and Carnarvon Ramp were collected from 127–264 m water depth. Foraminiferal assemblages changed between 127 m and 145–264 m due to rapidly decreasing water temperature in the thermocline, and loss of sufficient light for support of “larger” benthic foraminifera. Latitudinal differences were likely caused by three factors: (1) limited influence of the warm Leeuwin Current to support tropical taxa at the sampled depths, (2) reduced habitat diversity on the narrow Carnarvon Ramp compared to surrounding shelves, and (3) differing water-mass characteristics. The gathered information was used to interpret the assemblages from a Carnarvon Ramp core (total depth 300 m), providing insight into the activity of the warm, surficial Leeuwin Current for the last 3.54 My (Pliocene). Abundant infaunal taxa were inferred to indicate low oxygenation, increased supply of organic matter, and high sea-surface productivity during the absence of the Leeuwin Current above the coring site. Dominance of epifaunal species signified higher oxygenation at the sediment-water interface when upwelling of nutrient-rich waters was effectively suppressed by the Leeuwin Current. Around 1.14 Ma, waning of hypoxic conditions was initiated until a more substantial change was marked at 0.91 Ma. Suspension-feeding sponges became common sediment constituents during a Leeuwin Current flow optimum at ~0.6 Ma. The epifaunal taxa dominance persisted on the modern shelf, yet short episodes of infaunal peaks were likely caused by lateral shifts and fluctuating influence of the Leeuwin Current during more intense glacial cycles.

Larval fish assemblages were investigated during a three-year multidisciplinary project conducted off the coast of south-western Australia. Larvae were sampled using replicated oblique bongo net tows along a five-station transect extending from inshore (18m depth) to offshore waters (1000m depth). A total of 148 taxa from 93 teleost families were identified. Larvae of Gobiidae, and Blenniidae were abundant inshore, while larvae of pelagic and reef-dwelling families, such as Clupeidae, Engraulidae, Carangidae and Labridae were common in continental shelf waters. Larvae of oceanic families, particularly Myctophidae, Phosichthydae and Gonostomatidae, dominated offshore assemblages. Inshore larval fish assemblages were the most seasonal, in terms of species composition and abundance, with offshore assemblages the least so. Multivariate statistical analyses revealed larval fish assemblages to have a strong temporal and spatial structure. Assemblages were closely correlated to water masses, with species distributions reflecting both cross shelf and along-shore oceanographic processes and events. The strength and position of the warm, southward flowing Leeuwin Current, and of the cool, seasonal, northward flowing Capes Current were shown to drive much of the variability in the marine environment, and thus larval fish assemblages. Many of the distinctions between larval fish assemblages on the continental shelf were attributable to patterns of abundance in clupeiform larvae. While larvae of Engraulis australis and Spratelloides robustus showed clear seasonal and spatial distribution patterns, larvae of Sardinops sagax and Etrumeus teres were found throughout the year, with high interannual variability in abundance. Abundances of larvae from all pelagic clupeiform species were negatively correlated to microzooplankton concentrations. Peaks of abundance of S. sagax and E. teres, in particular, appeared to be better aligned with favourable transport and retention conditions. A detailed comparison of the horizontal and vertical distribution of larval fishes highlighted the influence of contrasting oceanographic conditions between summer and winter on larval fish assemblages. Although most fish larvae were found above the thermocline, depth distributions differed between taxa, and were shown to influence their offshore transport. Neustonic fish larvae showed potential for significant dispersion during summer, as a result of offshore Ekman transport. Mesoscale Leeuwin Current eddies were a feature of the oceanography of the region, and their influence on larval fish assemblages was examined in both an anti-cyclonic eddy (warm-core) and a cyclonic eddy (cold-core). The warm-core eddy contained larval fish assemblages that were distinct from those in the cold-core eddy, with lower larval fish concentrations, especially in the eddy centre. Although the eddies originated near the continental shelf, larval fish assemblages within both eddies were largely oceanic, probably a result of the age of the eddies when they were sampled (about 5 months). Overall, larval fish assemblages showed strong temporal and spatial structure, and were well aligned to water masses in the region. The unique oceanography off south western Australia thus has considerable implications for both larval fish transport, and potential recruitment to regional fisheries.

Thesis (Ph.D.)--University of Western Australia, 2007.

The condition at settlement of the western rock lobster pueruli presumably reflects both their energetic condition in reaching the near shore and subsequent recruitment into adult populations. In recruiting to the near shore the pueruli swim across the continental shelf where oceanographic conditions are complex and likely influence the success of recruitment. The results from the biochemical studies are interpreted in light of the oceanography off the coast of Western Australia. I investigated the nutritional condition of larval phyllosomata, post-larval puerulus and first instar post-pueruli juveniles of the spiny lobster, Panulirus cygnus, to determine energy use during the non-feeding transitional puerulus stage. Biochemical analyses of lipid, fatty acid (FA) and protein revealed that lipid, in particular phospholipids, is primarily used for energy during the nonfeeding puerulus stage. Monounsaturated FA showed the greatest decline with development, whereas the polyunsaturated FA showed a high degree of sparing, suggesting these FA are not used as a substrate for energy production. The knowledge gained on the biochemistry of energy use in P. cygnus was then used to investigate the spatial and temporal variability in the nutritional condition, in particular lipid condition, of puerulus collected at three near shore locations (Alkimos, Jurien Bay and Dongara) along the Western Australia coast, and one offshore location (Houtman Abrolhos Islands). The one offshore location was chosen as I hypothesised that arriving pueruli are likely to be in a better state of nutrition than those arriving at more coastal locations where the potential journey from offshore larval feeding grounds to the near shore is considerably greater. This element of my research showed lipid levels to be inversely related, generally, to shelf width but were variable, suggesting pueruli may travel complex trajectories to reach nearshore settlement. The lipid and FA composition of pueruli was also consistent with spatial and seasonal variation in Leeuwin Current and coastal productivity regimes.

[Truncated abstract] The study of eastern ocean boundary currents has been principally restricted to the Pacific and Atlantic ocean regions. The traditional view of the circulation near eastern ocean boundaries is that upwelling-favourable winds force surface waters offshore, leading to upwelling of cold, nutrient-rich subsurface water at the coast, the formation and offshore advection of a coastal front, and the generation of alongshore currents, generally having an equatorward surface flow and a poleward undercurrent. The eastern ocean boundary system of the southern Indian Ocean, off the west coast of Australia, is unique compared with these regions because a warm, poleward surface flow, known as the Leeuwin Current, dominates the dynamics over the continental shelf. Satellite imagery has shown the Leeuwin Current consists of a complex system of meanders, jet-like streams, and eddies, and has a seasonal and interannual variability. The oceanic circulation of the region between Carnarvon (latitude 25°S) and Jurien Bay (latitude 31°S) was examined using observational and remotely sensed data in conjunction with a detailed numerical modelling study. The model was validated using in situ ADCP and CTD data, and the horizontal eddy viscosity parameterization was tested against field observations. ... The resulting offshore meander grew laterally, shallowed, and closed to form an anticlockwise eddy to the original clockwise eddy’s south, forming a characteristic LC eddy pair (dipole). The model demonstrated the LC and Leeuwin Undercurrent (LUC) coupling played an important role in the onset of eddies at both sites. When an energy diagnostic scheme was used, the dominant instability process linked to the anticlockwise eddy’s development at site 1 was a mixed mode barotropic and baroclinic instability. The baroclinic instability’s source was the available potential energy stored within the mean lateral density gradient. The LC’s meandering southerly flow interacting with the LUC’s northerly subsurface flow generated the horizontal shear that sourced the barotropic instability. The dominant instability process at site 2 was baroclinic in origin. Possible links between the eddy field dynamics and the shelfslope region’s alongshore topographic variability were considered. The results of a suite of five model runs, differing only in the specification of bottom topography, were contrasted to investigate the effects. Except for the expected alongshore variability, delay in the onset of instabilities, varying growth rates, and some differences in the dominant wavebands’ mesoscale patterns, the overall impression was the response was similar.

The Miocene Nullarbor Limestone is the most recent formation in the Cenozoic Eucla Group and was deposited in the Eucla Basin, southern Australia, at ~38°S paleolatitude during the early to middle Miocene. The rocks form the modern surface of the vast, karsted Nullarbor Plain. Older Eucla Group marine carbonates (Eocene-earliest Miocene) are cool-water in nature and dominated by bryozoans and echinoderms. The Nullarbor Limestone is subtropical in composition and rich in coralline algae (rhodoliths and articulated types), large and small benthic foraminifera and molluscs. Diverse zooxanthellate corals are also present but not numerous. Deposition is interpreted to have taken place in three main paleoenvironments: rhodolith gravels, seagrass banks, and open seafloors. The Southern Ocean extended inboard ~450 km from the shelf edge during Nullarbor Limestone deposition. Interpreted paleodepths ranged from the top to the base of the photic zone, implying a small slope over a wide shelf. The Miocene Eucla platform is therefore interpreted to have been epeiric in nature. Paleoenvironment distribution is explained using epeiric platform sedimentation patterns and comparisons with modern environments. Open seafloor environments, the deepest settings, are thought to have been below fair-weather wave base. Rhodolith gravels accumulated at intermediate depths, where waves frequently swept the seafloor. Seagrass banks developed in the shallowest waters farthest inboard, where wave energy had been largely dissipated. Diverse corals, large benthic foraminifera and micrite envelopes inboard and in the western part of the basin support the notion of paleotemperatures generally above 20°C, the upper limit of subtropical carbonate accumulation. Although deposition occurred during the Miocene Climatic Optimum, a simple overall temperature increase cannot completely account for the subtropical nature of these sediments at mid-latitudes. Tropical components decrease from west to east, implying a temperature gradient, probably due to the warm proto-Leeuwin Current. Thus, these subtropical carbonates were deposited at mid-latitudes and their presence did not simply reflect a change in global climate. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2010-12-23 16:05:47.981

[Truncated abstract] This work was the first large-scale biological oceanographic study to be undertaken in the coastal eastern Indian Ocean adjacent to Western Australia, and covered both northwest (Exmouth Peninsula to the Abrolhos Islands) and southwest (Cape Naturaliste to Cape Leeuwin) regions. The study area was dominated by the Leeuwin Current (LC), an anomalous eastern boundary current that transports tropical water poleward and prevents deep nutrients from reaching the surface by creating large-scale downwelling. Indeed, LC and offshore waters were consistently associated with low nitrate concentrations and low phytoplankton biomass and production (< 200 mg C m-2 d-1). However, the physical forcing of the LC was offset, during the summer months, by upwelling associated with wind-driven inshore countercurrents (Ningaloo and Capes Currents), which provided a mechanism to access high nutrient concentrations normally confined to the base of the LC. ... Limited seasonal investigations off the Capes region of southwestern Australia showed that the winter production scenario can be very different than summer conditions, with strong Leeuwin Current flow that meanders onto the continental shelf and entrains seasonally nutrient-enriched shelf waters. However, production in the LC was still low (≤450 mg C m-2 d-1) due to light limitation resulting from both increased light attenuation and reduced surface irradiance characteristic of the winter months. This investigation provides fundamental knowledge on physical-biological coupling off Western Australia, with implications for fisheries management in view of seasonal and inter-annual variability in the strength of both the Leeuwin Current and inshore countercurrents.