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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]

Abstract: The results presented in this study provide the first broad-scale regional characterisation of the phytoplankton-nitrogen dynamics across the continental shelf between the Abrolhos Islands and Cape Leeuwin, Western Australia. We demonstrate key features of N-uptake, including nitrate and ammonium uptake and N fixation, across the dominant current systems of the west coast of Australia during a 2-week period in spring–summer. The surface waters of the region were N impoverished with low phytoplankton biomass. Nitrate and ammonium uptake rates were among the lowest on record, and f-ratios were high compared to other oligotrophic systems (f-ratio ∼0.5 throughout), indicating an equal preference for nitrate and ammonium uptake. Areas of high phytoplankton biomass were most likely fuelled by nitrate transported into the euphotic zone during temporally short bursts of upwelling on ∼weekly, rather than monthly time scales. When upwelling had relaxed, phytoplankton productivity was largely maintained by microbial regeneration in the euphotic zone. A large proportion (20–60%) of the phytoplankton biomass was comprised of picoplankton and nanoplankton cyanobacteria. However, rates of N2 fixation were 2 orders of magnitude lower than dissolved inorganic N uptake, indicating that N2-fixation did not significantly contribute to phytoplankton productivity during the study. [Copyright &y& Elsevier]

Abstract: The Leeuwin Current (LC) is an anomalous poleward-flowing eastern boundary current that carries warm, low-salinity water southward along the coast of Western Australia. We present an introduction to a new body of work on the physical and biological dynamics of the LC and its eddies, collected in this Special Issue of Deep-Sea Research II, including (1) several modelling efforts aimed at understanding LC dynamics and eddy generation, (2) papers from regional surveys of primary productivity and nitrogen uptake patterns in the LC, and (3) the first detailed field investigations of the biological oceanography of LC mesoscale eddies. Key results in papers collected here include insight into the source regions of the LC and the Leeuwin Undercurrent (LUC), the energetic interactions of the LC and LUC, and their roles in the generation of warm-core (WC) and cold-core (CC) eddies, respectively. In near-shore waters, the dynamics of upwelling were found to control the spatio-temporal variability of primary production, and important latitudinal differences were found in the fraction of production driven by nitrate (the f-ratio). The ubiquitous deep chlorophyll maximum within LC was found to be a significant contributor to total water column production within the region. WC eddies including a single large eddy studied in 2000 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. During the Eddies 2003 voyage, a more detailed study comparing the WC and CC eddies illuminated more mechanistic details of the unusual dynamics and ecology of the eddies. Food web analysis suggested that the WC eddy had an enhanced “classic” food web, with more concentrated mesozooplankton and larger diatom populations than in the CC eddy. Finally, implications for fisheries management are addressed. [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 question of why anticyclonic eddies appearing along the coast of Western Australia tend to be more productive, and have higher phytoplankton biomass than the cyclonic eddies, remains open. Here we present results from a one-dimensional physical–biological model that explores several possible explanations. In particular, we examine the influence of nutrient conditions during eddy formation and subsequent vertical mixing on the biological production of two counter-rotating mesoscale eddies sampled off Western Australia during October 2003. Observed differences in primary production between the two eddies cannot be adequately explained in the model by differences in mixed-layer depth and vertical nutrient flux. Instead, entrainment of productive shelf waters during the formation of the anticyclonic eddy is suggested to account for up to three quarters of the primary production within the eddy as it moves off-shore. At the same time, the longer residence time of sinking detritus within the deep mixed layer of the anticyclonic eddy is shown to increase the importance of regenerated nitrogen and contribute to its enhanced production. A general under-estimation of production is discussed in the context of biological and physical processes not resolved by the model. [Copyright &y& Elsevier]