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Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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±0.16μgL−1 while it was 0.58±1.4μgL−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. [Copyright &y& Elsevier]

Abstract The continental shelf of the South Kimberley region of Western Australia is wide (~300 km) and interactions with the Indian Ocean culminate in extreme tides. This results in some of the largest tropical tides in the world (reaching 11 m on spring tide) measured in King Sound (KS), a large embayment which opens to the Indian Ocean. This remote area of Australia is very rich in natural resources and under increasing developmental pressures, but despite this the marine environment has been poorly studied. In Austral autumn (April/May) 2010, we examined phytoplankton biomass and production across four onshore-offshore transects from coastal waters (also within KS) to the 1000 m isobath, and their responses to physical and chemical environmental variables associated with tidal forces during a four week voyage aboard the RV Southern Surveyor. Using temperature and salinity relationships we found three distinct water masses; KS, Kimberley Shelf, and Kimberley Shelf break waters. The highest chlorophyll- a biomass in the study area was measured in a band along the 200 m isobath (27–46 mg Chl a m−2), and associated with greater concentrations of nutrients (332–392 mmol NO 3 - m−2, 650–823 mmol Si m−2, 21–39 mmol PO 4 3- m−2). Phytoplankton biomass inside KS was dominated by large diatoms (as determined from accessory pigment analyses), shifting to a community largely comprised of picophytoplankton offshore. Rates of primary production (PP) on the shelf, via measurements of phytoplankton carbon (14C) uptake from photosynthesis versus irradiance incubations, decreased from coastal areas (222–560 mg C m−2 d−1) to offshore waters (45–78 mg C m−2 d−1). In KS, PP was highest (1692 mg C m−2 d−1) despite low standing stock nutrient concentrations (0.02–1.5 mmol NO 3 - m−2) and relatively high concentrations of suspended particulate material reducing light availability (K d = 0.27 m−1). Highlights • Here we present the first large shelf-scale study of primary production (PP) in the Kimberley region of Western Australia. • Phytoplankton production is notably high (1190 - 1636 mg C m-2 d-1) in macro-tidal Kind Sound, Western Australia. • Physical forcing in this region affects productivity and species differently according to distance from the coast. [ABSTRACT FROM AUTHOR]

In order to strengthen environmental application of nitrate N and O isotopes, we measured the N and O isotopic fractionation associated with cellular nitrate uptake and efflux in the nitrate-assimilating marine diatom Thalassiosira weissflogii. We isolated nitrate uptake and efflux from nitrate reduction by growing the cells in the presence of tungsten, which substitutes for molybdenum in assimilatory nitrate reductase, yielding an inactive enzyme. After growth on ammonium and then N starvation, cells were exposed to nitrate. Numerical models fit to the evolution of intracellular nitrate concentration and N and O isotopic composition yielded distinct N isotope effects (15ε) for nitrate uptake and nitrate efflux (2.0±0.3‰ and 1.2±0.4‰, respectively). The O isotope effects (18ε) for nitrate uptake and nitrate efflux were indistinguishable (2.8±0.6‰), yielding a ratio of O to N isotopic fractionation for uptake of 1.4±0.4 and for efflux of 2.3±0.9. The 15ε for nitrate uptake can account for at most 40% of the organism-level N isotope effect (15εorg) measured in laboratory studies of T. weissflogii and in the open ocean (typically 5‰ or greater). This observation supports previous evidence that most isotope fractionation during nitrate assimilation is due to intracellular nitrate reduction, with nitrate efflux allowing the signal to be communicated to the environment. An O to N fractionation ratio (18εorg:15εorg) of ∼1 has been measured for nitrate assimilation in algal cultures and linked to the N and O isotope effects of nitrate reductase. Our results suggest that the ratios of O to N fractionation for both nitrate uptake and efflux may be distinct from a ratio of 1, to a degree that could cause the net 18εorg:15εorg to rise appreciably above 1 when 15εorg is low (e.g., yielding a ratio of 1.1 when 15εorg is 5‰). However, field and culture studies have consistently measured nearly equivalent fractionation of N and O isotopes in association with low isotope effects, calling for isotopic studies of nitrate transport by other phytoplankton strains. [ABSTRACT FROM AUTHOR]

Abstract: 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 PO4 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. [Copyright &y& Elsevier]

Abstract: We measured the natural inventories of nitrogen and carbon stable isotopes within various ecosystem fractions of two counter-rotating eddies associated with the poleward Leeuwin Current (LC), off Western Australia. Isotopic signatures (δ 15N and δ 13C) were used as proxies for trophic transformation of inorganic and organic matter and are the basis for our discussion on food web functions in the two eddies. We present the first measurements of dissolved inorganic nitrogen (DIN) isotopic composition for the eastern Indian Ocean. We show that the large autotrophs (sampled within the >5-μm and >20-μm fractions of particulate organic matter (POM)), including a distinctive diatom population in the warm-core (WC) eddy, are likely to have taken up sources of DIN which were primarily nitrate, while the picoplankton are likely to have assimilated a large fraction of recycled ammonium. We show that phytoplankton in the cold-core (CC) eddy had distinctly more enriched δ 15N signatures than in the WC eddy, probably due to the higher vertical fluxes of nitrate into the CC eddy. A clear negative correlation between mixed-layer depth and δ 15N in POM across both eddies also supports the role of vertical nitrate fluxes in determining the primary δ 15N signature of the autotrophs. Within the WC eddy, there was a significant δ 13C-enrichment in comparison to the CC eddy across all size fractions of the mesozooplankton community, which, in combination with a low C:N molar ratio the >200- and >500-μm mesozooplankton size fractions, suggests a healthier mesozooplankton community, with greater lipid storage, in the WC eddy. This is consistent with the greater productivity and biomass of large diatoms in the WC eddy. Larval fish from the WC eddy also had an enriched δ 13C signature compared to those from the CC eddy. The WC eddy had higher production rates than the CC eddy, and harboured a physiologically healthier population of zooplankton. Paradoxically, this seemed to occur despite higher overall nitrate fluxes into the CC eddy, and may have to do with the particularly active diatom population within the WC eddy operating within the unusually deep mixed layer. [Copyright &y& Elsevier]

Abstract: The Leeuwin Current (LC) is an unusual poleward-flowing eastern boundary current that carries warm, low-salinity water southward along the coast of Western Australia. LC dynamics include the formation of a dynamic mesoscale eddy field whose biological dynamics have not been studied. Satellite altimetry indicates that the eddies studied in the 2003 field programme were dynamically typical of LC eddies, but the warm-core (WC) eddy was relatively large and long-lived. The WC eddy contained relatively elevated chlorophyll a concentrations thought to originate, at least in part, from the continental shelf/shelf break region and to have been incorporated during eddy formation. Primary production per unit volume in the WC eddy was ∼2× higher than in the cold-core (CC) eddy due to an historical accumulation of chlorophyll a over the period since eddy formation (5–6 months), though chlorophyll a-specific daily production was volumetrically ∼50% greater in the CC eddy. In the WC eddy, nitrate uptake rates were 4× greater than in the CC eddy, despite the fact that vertical diffusive fluxes of nitrate into the WC eddy were probably only 50% of those in the CC eddy. We therefore hypothesize that other nitrate sources were important, possibly including isopycnal mixing and/or lateral transport into the eddy from surrounding waters. In addition, a deep mixed layer favoured a large (>5μm) diatom population within the centre of the WC eddy while the CC eddy was persistently stratified, with a shallower mean mixed-layer depth (∼100m vs. ∼200m for the WC eddy) and a well developed deep chlorophyll a maximum at ∼100m composed of 30% prochlorophytes (not capable of taking up nitrate). Both factors probably contributed to higher f-ratios in the WC and in the >5μm phytoplankton relative to total phytoplankton. [Copyright &y& Elsevier]