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Weller E.,CSIRO | Weller E.,National Research Flagship | Holliday D.,Murdoch University | Feng M.,CSIRO | And 4 more authors.
Continental Shelf Research | Year: 2011

A continental shelf scale survey from 22°S to 34°S along the Western Australia coast provides the first detailed synoptic examination of the structure, circulation and modification of the southward flowing Leeuwin Current (LC) during the late austral autumn-early winter (May-June 2007). At lower latitudes (22°S-25°S), the LC was masked within a broad expanse of warm ambient surface water, which extended across the shelf and offshore before becoming constrained at the shelf break and attaining its maximum velocity of ~1.0ms -1 at 28°S. The temperature and salinity signature of the LC experienced substantial modification as it flowed poleward; surface temperature of the LC decreased by ~5.25°C while surface salinity increased by ~0.72, consistent with climatology estimates and smaller (larger) for temperature (salinity) than those found during summer. Subsequently, LC water was denser by ~2σ T in the south compared to the north, and the surface mixed layer of the LC revealed only a small deepening trend along its poleward trajectory. Modification of the LC resulted from a combination of mixing due to geostrophic inflow and entrainment of cooler, more saline surrounding subtropical waters, and convective mixing driven by large heat loss to the atmosphere. Air-sea heat fluxes accounted for 50% of the heat lost from the LC in the south, whilst only accounting for 25% in the north, where large geostrophic inflow occurred and the LC displayed its maximum flow. The onshore transport was characterised by distinct jet-like structures, enhanced in the upper 200m of the water column, and the presence of eddies in the vicinity of the shelf break generated offshore transport. © 2011 Elsevier Ltd.

Hassler C.S.,CSIRO | Hassler C.S.,National Research Flagship | Hassler C.S.,University of Technology, Sydney | Djajadikarta J.R.,University of Technology, Sydney | And 4 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2011

This study focuses on the comparison of oceanic and coastal cold-core eddies with inner-shelf and East Australian Current (EAC) waters at the time of the spring bloom (October 2008). The surface water was biologically characterised by the phytoplankton biomass, composition, photo-physiology, carbon fixation and by nutrient-enrichment experiments. Marked differences in phytoplankton biomass and composition were observed. Contrasted biomarker composition suggests that biomarkers could be used to track water masses in this area. Divinyl chlorophyll a, a biomarker for tropical Prochlorophytes, was found only in the EAC. Zeaxanthin a biomarker for Cyanophytes, was found only within the oceanic eddy and in the EAC, whereas chlorophyll b (Chlorophytes) was only present in the coastal eddy and at the front between the inner-shelf and EAC waters. This study showed that cold-core eddies can affect phytoplankton, biomass, biodiversity and productivity. Inside the oceanic eddy, greater phytoplankton biomass and a more complex phytoplankton community were observed relative to adjacent water masses (including the EAC). In fact, phytoplankton communities inside the oceanic eddy more closely resembled the community observed in the inner-shelf waters. At a light level close to half-saturation, phytoplankton carbon fixation (gCd -1) in the oceanic eddy was 13-times greater than at the frontal zone between the eddy and the EAC and 3-times greater than in the inner-shelf water. Nutrient-enrichment experiments demonstrated that nitrogen was the major macronutrient limiting phytoplankton growth in water masses associated with the oceanic eddy. Although the effective quantum yield values demonstrate healthy phytoplankton communities, the phytoplankton community bloomed and shifted in response to nitrogen enrichments inside the oceanic eddy and in the frontal zone between this eddy and the EAC. An effect of Si enrichment was only observed at the frontal zone between the eddy and the EAC. No response to nutrient enrichment was observed in the inner-shelf water where ambient NO x, Si and PO 4 concentrations were up to 14, 4 and 3-times greater than in the EAC and oceanic eddy. Although results from the nutrient-enrichment experiments suggest that nutrients can affect biomass and the composition of the phytoplankton community, the comparison of all sites sampled showed no direct relationship between phytoplankton biomass, nutrients and the depth of the mixed layer. This is probably due to the different timeframe between the rapidly changing physical and chemical oceanography in the separation zone of the EAC. © 2010 Elsevier Ltd.

Thompson P.A.,Marine and Atmospheric Research | Thompson P.A.,National Research Flagship | Bonham P.,Marine and Atmospheric Research | Bonham P.,National Research Flagship | And 6 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2011

The composition and dynamics of the phytoplankton communities and hydrographic factors that control them are described for eastern and western Australia with a focus on the Eastern Australian Current (EAC) and Leeuwin Current (LC) between 27.5° and 34.5°S latitude. A total of 1685 samples collected from 1996 to 2010 and analysed for pigments by high performance liquid chromatography (HPLC) showed the average TChla (monovinyl+divinyl chlorophyll a) concentration on the west coast to be 0.28±0.16γgL-1 while it was 0.58±1.4γgL-1 on the east coast. Both coasts showed significant decreases in the proportions of picoplankton and relatively more nanoplankton and microplankton with increasing latitude. On both coasts the phytoplankton biomass (by SeaWiFS) increased with the onset of winter. At higher latitudes (>27.5°S) the southeast coast developed a spring bloom (September) when the mean monthly, surface chlorophyll a (chla) concentration (by SeaWiFS) was 48% greater than on the south west coast. In this southern region (27.5-34.5°S) Synechococcus was the dominant taxon with 60% of the total biomass in the southeast (SE) and 43% in the southwest (SW). Both the SE and SW regions had similar proportions of haptophytes; ~14% of the phytoplankton community. The SW coast had relatively more pelagophytes, prasinophytes, cryptophytes, chlorophytes and less bacillariophytes and dinophytes. These differences in phytoplankton biomass and community composition reflect the differences in seasonality of the 2 major boundary currents, the influence this has on the vertical stability of the water column and the average availability of nutrients in the euphotic zone. Seasonal variation in mixed layer depth and upwelling on the west coast appears to be suppressed by the Leeuwin Current. The long-term depth averaged (0-100m) nitrate concentration on the west coast was only 14% of the average concentration on the east coast. Redfield ratios for NO3:SiO2:PO4 were 6.5:11.9:1 on the east coast and 2.2:16.2:1 on the west coast. Thus new production (nitrate based) on the west coast was likely to be substantially more limited than on the eastcoast. Short term (hourly) rates of vertical mixing were greater on the east coast. The more stable water column on the west coast produced deeper subsurface chlorophyll a maxima with a 25% greater proportion of picoeukaryotes. © 2010 Elsevier Ltd.

Thompson P.A.,Marine and Atmospheric Research | Thompson P.A.,National Research Flagship | Wild-Allen K.,Marine and Atmospheric Research | Wild-Allen K.,National Research Flagship | And 7 more authors.
Progress in Oceanography | Year: 2011

New observations along the continental shelf of Western Australia provide a novel explanation for the established ~60years relationship between Leeuwin Current (LC) strength and greater winter nitrate concentrations at 32°S plus the inter-annual variation in the magnitude of the annual, shelf-scale, phytoplankton bloom. The potential source of dissolved nitrogen to support the annual shelf scale phytoplankton bloom was identified as thin layers of an unprecedented areal extent, nitrate concentration and shallow nature that were observed off the northwest of Australia. We propose that the dissolved inorganic nitrogen (DIN) in these layers enters the LC at depth and then enters the euphotic zone via by three mechanisms: instability that results in a warm core eddy, cooling that deepens the surface mixed layer and shallowing of the thin layer. During the onset of the annual phytoplankton bloom along the west coast of Australia from 22°S to 34°S the poleward flowing LC was clearly evident as a surface intensified ocean boundary current transporting warmer, lower-salinity, greater-silicate waters in a shallow mixed layer rapidly southward. Between 24 and 26°S the core of the LC was present as a 50-100m deep layer over one or more thin layers, 15-50m thick, with high nitrate and low dissolved oxygen (DO). These layers were of lower salinity, cooler water with markedly reduced DO, high nitrate concentrations and distinct nitrate:silicate (NO3:Si(OH)4) nutrient ratios. As the LC flowed south it cooled and deepened thereby entraining the thin layers of high nitrate water into the euphotic zone. The LC also formed large (greater than 100km diameter) warm core eddies with a deep surface mixed layer that also entrained nitrate from these thin layers. In some locations as far south as 32°S the LC was still present with the thin layer of high nitrate intact but now within the euphotic zone. Thus, the available evidence suggests the LC arises under conditions that favour rapid and shallow nitrification. This nitrification fuels a shelf-scale bloom on a downwelling favourable coast. Depending upon the rate of nitrification the source of the particular organic matter may be local or delivered from the tropics via horizontal advection in a subsurface layer of the LC. © 2011 Elsevier Ltd.

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