NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHORS

International Council for the Exploration of the Sea Annual Science Conference 2003

Theme Session P: Physical-Biological Interactions in Marginal and Shelf Seas

CM 2003/P:16

THE UNIQUE CONTINENTAL SHELF DYNAMICS OFF WESTERN : PHYSICAL CONTROLS ON PHYTOPLANKTON PRODUCTIVITY

C.E. Hanson*, C.B. Pattiaratchi and A.M. Waite

Centre for Water Research, The University of , 35 Stirling Hwy, Crawley, Western Australia 6009 Australia

*Corresponding author Email: [email protected] Phone: +618 9380 1698 Fax: +618 9380 1015

Abstract

Continental shelf waters off Western Australia are dominated by the Leeuwin Current, an anomalous eastern boundary current that transports tropical water poleward and generates large- scale downwelling. However, in summer the inner shelf dynamics are influenced by wind- generated countercurrents (the Ningaloo and Capes Currents) that flow towards the equator and have the potential to create localized upwelling. Prior to the current study, there had been no investigations into the links between physical dynamics and phytoplankton productivity within or between any of these currents. In a series of cross-shelf transects encompassing the Leeuwin, Ningaloo and Capes Currents, we found strong associations between the nitracline and phytoplankton biomass that lead to persistent deep chlorophyll maxima throughout the study area. The countercurrents were highly productive (~ 800-1300 mg C m-2 d-1), in notable contrast to the low productivity signature (< 200 mg C m-2 d-1) within the boundary current. Seaward offshoots and coastal upwelling associated with the Ningaloo and Capes Currents provided a mechanism to access high nutrient concentrations normally confined to the base of the Leeuwin. This may be an important process leading to seasonal peaks in shelf productivity on this otherwise oligotrophic coast.

Keywords: primary production; coastal upwelling; oligotrophic; deep chlorophyll maximum; eastern ; Leeuwin Current; Ningaloo Current; Western Australia, Gascoyne, 21°S to 30°S and 111°E to 115°E

1 1.0 Introduction

Eastern boundary currents are present in all of the major ocean basins, and generally consist of an equatorward surface flow accompanied by large-scale upwelling, high rates of primary production and abundant fisheries (Wooster and Reid, 1963; Mann and Lazier, 1996). Off the coast of Western Australia (WA), the unusual poleward-flowing Leeuwin Current (LC) restricts the eastern arm of the Indian Ocean gyre to offshore , generating large-scale downwelling as it travels along the continental shelf break (Pearce, 1991). The LC is known to reduce coastal nutrient levels (Johannes et al., 1994), influence biological species distributions (Morgan and Wells, 1991) and limit levels of secondary productivity (Lenanton et al., 1991; Caputi et al., 1996). As opposed to the dominance of pelagic finfish stocks in other eastern boundary regions, the major fishery off Western Australia (WA) is benthic rock lobster, with its life cycle and recruitment strongly tied to the dynamics of the LC (Phillips et al., 1991). Inshore of the Leeuwin Current, a system of equatorward coastal countercurrents are driven by southerly winds which prevail during the austral summer months (December to March). Part of that system, the Capes Current (CC) extends along the southwest coast of WA (Pearce and Pattiaratchi, 1999) and the Ningaloo Current (NC) extends along the northwest coast (Taylor and Pearce, 1999). The combination of these wind-forced shelf currents and localized Ekman-driven upwelling (Gersbach et al., 1999; Woo and Pattiaratchi, 2003) seasonally restricts the inshore extent of the LC (Pearce and Pattiaratchi, 1999). The Gascoyne continental shelf extends from (21.3°S) to (26.5°S), Western Australia (WA), and encompasses the northern portion of LC waters (Pearce, 1991). Control of phytoplankton production in coastal regions such as the Gascoyne is often closely tied to ambient nutrient levels, which in turn are strongly influenced by the local oceanography (Mann and Lazier, 1996). Along the west coast of WA, oligotrophic conditions in the nutrient-poor LC are thought to support only a limited amount of phytoplankton biomass and productivity. Phytoplankton biomass data in the Gascoyne is quite sparse, however a review of field studies throughout WA characterized coastal waters as low chlorophyll (< 1 mg chl a m-3) environments, with higher chlorophyll a values considered representative only of shelf waters or estuaries subjected to anthropogenic nutrient inputs (Pearce et al., 2000). The only available data indicate that primary productivity off Western Australia is 150-250 mg C m-2 d-1 (Koblentz-Mishke et al., 1970; FAO, 1981), but these data were sourced from the International Indian Ocean Expedition (Kabanova, 1968), the Australian portion of which only sampled open ocean waters along 110°E (~ 400 km offshore; Jitts, 1969). To date, we are lacking both regional estimates of phytoplankton biomass and productivity in the nearshore and coastal Gascoyne region, and an understanding of the intermediate links between the physical environment and secondary productivity. The aim of the present paper is to understand how the Leeuwin, Ningaloo and Capes Currents impact water column production along the Gascoyne shelf. The oligotrophic LC is known to dominate the region, and the commonly accepted paradigm is that conditions remain oligotrophic along this coast through suppression of upwelling-driven production. We tested this hypothesis using field data from the northern portion of the LC and associated coastal countercurrents.

2.0 Materials and Methods

2.1 Study area

An oceanographic cruise was undertaken off Western Australia from 13 to 27 November 2000 (early austral summer) aboard the R.V. Franklin (FR10/00), incorporating eleven onshore/offshore transects and a total of 118 stations (Fig. 1). The study region was located between the Abrolhos

2 Islands and North West Cape (21°S to 30 °S; Fig. 1), and encompassed the entirety of the Gascoyne continental shelf (0 – 200 m isobath), shelf break (200 – 300 m isobath) and offshore (300 – 4000 m isobath) waters.

2.2 Oceanographic sampling and laboratory analyses

Water samples were obtained using 24 General Oceanics 5 L Niskin bottles mounted on a rosette equipped with Seabird CTD, dissolved oxygen sensor, fluorometer and Li-Cor LI-192SA underwater quantum sensor. Between three and thirteen discrete depths were sampled at each station (dependent on bottom depth), including surface (roughly 2 m), and above, below and within the fluorescence maximum (as determined by the downcast fluorometer trace). Dissolved inorganic nutrients (nitrate + nitrite, phosphate and silicate) were analyzed for all depths using a shipboard Autoanalyzer. Detection limits were 0.1 µM for nitrate + nitrite (hereafter nitrate) and 0.01 µM for phosphate. Two litre water samples were filtered for chlorophyll (chl) a and pheopigments onto Whatman GF/F filters, stored at -20°C and returned to the laboratory for analysis. Pigments were extracted in 90% acetone with grinding, and measured using a Turner Designs Fluorometer following the acidification technique of Parsons et al. (1989). At 18 ‘production stations’, primary productivity versus irradiance (P vs. I) experiments were performed using the small-volume, short-incubation-time 14C incorporation technique (Lewis and Smith, 1983), with photosynthetron equipment and modifications as per Mackey et al. (1995; 1997). Due to the opportunistic nature of the biological sampling program, water from some stations was collected at night and held in the dark at ambient seawater temperature until processing at dawn the following morning. For the first six experiments, irradiance levels within the photosynthetron (maximum of 400 µE m-2 s-1) were insufficient to reach the maximum photosynthetic rate for shallow-water (< 50 m) samples (UNSAT). From the seventh production station onwards, additional incubations of the surface and next-deepest sample were conducted in natural sunlight (SAT) at two light levels (100% and 30% of incident irradiance). All samples were counted on-board the ship using a LKB Rackbeta liquid scintillation counter. Additional water samples were collected at the surface and DCM of production stations for particulate organic carbon (POC) and particulate nitrogen (PN) analysis (4 L filtered on pre- combusted Whatman GF/F filters and stored at -20°C until analysis by mass spectrometer, following the preparation techniques of Knap et al. (1996).

2.3 Data processing and production calculations

Attenuation coefficients (Kd) were calculated from a linear regression of the natural log of PAR vs. depth, according to the relation:

ln Ed(0) = -Kdz + ln Ed(z) where Ed(0) and Ed(z) are the values of downwelling PAR at the surface and at z m, respectively (Kirk, 1994). A smoothed irradiance profile was then plotted and used to determine the depth of the euphotic zone, as defined by the depth of the 0.1% light level. In-situ fluorescence was calibrated with extracted chl a data using linear regression, and used as a proxy for phytoplankton biomass. Where data permitted (minimum of 5 data points), a separate regression was performed for each station; otherwise, stations were calibrated using pooled data for that transect (r2 = 0.76-0.92). The deep chlorophyll maximum (DCM) was taken as the depth of maximum subsurface chl a concentration. The top of the nitracline was defined as the depth where the nitrate concentration equaled 0.2 µM, as linearly interpolated between Niskin sampling depths. In these extremely oligotrophic waters, this value was considered more

3 representative of the nitracline than the criteria of 1.0 µM commonly used for other regions (e.g. Maranon and Holligan, 1999; Moran and Estrada, 2001). For the purposes of estimating integrated production, Pm for the UNSAT samples was taken as the maximum photosynthetic rate achieved in the photosynthetron (at ~ 400 µE m-2 s-1), providing a conservative estimate of this parameter. UNSAT samples were also necessarily without a measure of the photoinhibition parameter (β). For SAT experiments where both Pm and β were characterized for irradiance levels of up to 2000 µE m-2 s-1, surface samples did not display photoinhibition, as is common for phytoplankton adapted to high light conditions (Kana and Glibert, 1987; Maranon and Holligan, 1999; Basterretxea and Aristegui, 2000; Moran and Estrada, 2001). Samples associated with the DCM did exhibit photoinhibition, but for the purposes of calculating in-situ production this was of little importance, as light levels at these depths were below the inhibition threshold. Thus, for UNSAT samples we have assumed that β = 0. The implications of these assumptions, for volumetric (per m-3) and areal (per m-2) production estimates and the relative contribution of the DCM, will be addressed in a future paper. Depth and time integrations were computed as in Mackey et al. (1995) and Walsby (1997). Final values were calculated both for actual irradiance data (recorded at five minute intervals by a deck-board Li-Cor LI-192SB Quantum Sensor) and theoretical sine curves of irradiance (based on latitude and date). Calculation of photon flux density just under the water surface was corrected for reflectance by incorporating solar elevation, effective zenith angle and surface wind roughening * * (from five minute wind averages). Chlorophyll-normalized photosynthetic parameters (Pm or Ps , α* and β*) were linearly interpolated between sample depths, and calibrated fluorescence was used to scale photosynthetic parameters at 2 m depth intervals. Attenuation coefficients were those measured at each station, although if the data did not exist the mean value of Kd for the study area was used. Trapezoidal integration was then used to calculate the double integral of photosynthesis through depth and time (mg C m-2 d-1). These calculations provide an estimate of daily gross production, as no attempt was made to correct for losses of carbon via respiration or grazing. All integrations were to the 0.1% light level or bottom.

3.0 Results

To facilitate interpretation of geographical patterns, CTD and production stations were sorted into groups based on water mass temperature-salinity (TS) characteristics, Acoustic Doppler Current Profiler (ADCP) data and sea-surface temperature (SST) images as analyzed by Woo and Pattiaratchi (2003). The four surface water masses considered herein are: Leeuwin Current (LC), Ningaloo Current (NC), Shark Bay outflow (SB) and Capes Current (CC).

3.1 Phytoplankton biomass and nutrients

A common feature throughout much of the study area was higher chl a concentration at depth, either near the sea bottom or as distinct peaks within the water column. Along Cape Range Peninsula (Transects B and C; Fig. 1), surface chl a averaged 0.3 mg chl a m-3 while the DCM averaged 0.6 mg chl a m-3 (n = 16). Deep chlorophyll maxima generally increased in depth from onshore to offshore, with highest chl a concentrations found in shelf waters (Fig. 2). The maximum chl a concentration for the study region was found along the inshore edge (55 to 75 m isobath) of Transect F (Fig. 1), where a near-seabed plume of hypersaline Shark Bay outflow contained up to 2.4 mg chl a m-3. Excluding this unusual feature, Gascoyne shelf waters were generally characterized by a maximum concentration of 1.0 mg chl a m-3. DCMs were quite distinct in offshore (300 – 1000 m isobath) waters south of Point Cloates (Fig. 1), where near surface biomass

4 was extremely low (< 0.1 mg chl a m-3; Fig. 2b-d). Nitrate, depth-integrated to the sea bottom or mean 0.1 % light level of 105 m) also exhibited distinctive cross-shelf patterns, with local minima often found both inshore and offshore of the shelf break (Fig. 3). A strong positive correlation between DCM depth and the nitracline was found throughout the study area (Fig. 4), with the deepest maxima and nitraclines found in LC waters. This DCM-nitracline coupling is further apparent in a plot of chlorophyll distribution along the 500 m isobath from north to south through the study area (Fig. 5). While the LC generally flowed most strongly along the 200 m isobath (Woo and Pattiaratchi, 2003), current shear and mixing with shelf waters modified the LC’s structure along this landward edge of the flow; the LC’s chlorophyll signature is thus best typified along this 500 m isobath. The three northern transects (A – C) were strongly influenced by upwelling and coastal NC offshoots, with the LC found as a distinct flow from Transect D south (Fig. 5). A shallow (~ 50 m) DCM at North West Cape (Transect A) and Cape Range Peninsula (Transect B) coincided with a shallow (≤ 50 m) nitracline, while at the majority of transects to the south (C to F, H, I) the DCM was located between 90 and 100 m and the nitracline between 70 and 100 m (Fig. 5). Exceptions in the southern transects included a shallow (~ 65 m) DCM and nitracline (~ 50 m) at Transect G, and an extremely deep (185 m) nitracline at Transect J, with chlorophyll maxima at 90 and 160 m. Overlaid on the chlorophyll contours of Figure 5 is the maximum alongshore surface velocity for offshore waters (as measured by ADCP and reported in Woo and Pattiaratchi, 2003), which is here used as a proxy for the strength of the Leeuwin Current. Highest velocities (-0.64 to -0.68 m s-1) were measured in the south, at Transects I and J respectively, while the weakest flow (-0.26 to -0.38 m s-1) was found in the central portion of the study area (Transects F to H; Fig. 5). The euphotic zone (0.1 % light level) in both Leeuwin Current and inshore waters averaged 105 m (± 15 SD; calculated from a mean attenuation coefficient of 0.066 ± 0.010 m-1, n = 27), and is overlaid on the chlorophyll contours of Figure 5. The majority of the DCMs along the 500 m isobath were located between 0.1 and 0.3 % irradiance, with shallow extremes along Cape Range Peninsula (3.7 %) and at Transect G (1.5 %). Particulate organic carbon collected on a GF/F filter can be composed of not only phytoplankton, but also bacteria, microzooplankton and detritus. To determine the fraction attributable to autotrophs, a linear regression of POC on chl a was performed, with the intercept taken as that portion of POC that was not associated with live phytoplankton (Eppley et al., 1977). Separate regressions for surface and DCM samples yielded:

Surface: y = 67.5x + 43.6, r2 = 0.47, p < 0.005 DCM: y = 65.1x + 23.6, r2 = 0.32, p < 0.05

The intercepts were subtracted from total POC to yield phytoplankton POC, which was found to be significantly higher (t-test, p < 0.01) at the DCM than surface (and matched by higher chl a; Fig. 6). The regression coefficients are a measure of phytoplankton-specific C:chl a (Townsend and Thomas, 2002), and were not significantly different between surface and DCM datasets (Student’s t, p < 0.001). This allowed calculation of the weighted regression coefficient underlying the two slopes, which equalled 65.4 and will be used as the common C:chl a for this study.

3.2 Production stations

Production stations sampled each of the four main surface water masses (Fig. 7), and are displayed in Figure 8 with values of depth-integrated primary production. Areal production over the entire region was variable, ranging from 110 to 1310 mg C m-2 d-1 with a mean ± SD of 560 ± 420 mg C m-2 d-1 (n = 18). The values presented are for clear-sky sine curve irradiance, which allowed a spatial comparison regardless of cloud conditions. Integrated productivity calculated from

5 measured irradiance data averaged 16 ± 6 (SD) % lower than that calculated from theoretical irradiance. The most productive waters (≥ 840 mg C m-2 d-1) were found: 1) in a coherent grouping along Cape Range Peninsula and just south of Point Cloates (NC), and 2) in a single highly productive (990 mg C m-2 d-1) inner shelf station at the southernmost extent of the study area (CC; Fig. 8). The majority of Leeuwin Current waters had productivity of 200 mg C m-2 d-1 or less (range 110 – 530 mg C m-2 d-1; Fig. 8). The single production station at the higher end of the range (530 mg C m-2 d-1) was found at the shelf break (250 m isobath) on Transect I, the only measurement of productivity rates in the LC’s core (all other stations were ≥ 1000 m depth). Shark Bay outflow was characterized by integrated productivity between 550 and 560 mg C m-2 d-1 (Fig. 8). Grouped together, the shelf currents (NC, SB, CC) were significantly more productive (Mann- Whitney test, p < 0.001) than the Leeuwin Current. As seen in the vertical profiles of chl a and volumetric production (Fig. 9), well-defined deep chlorophyll maxima at the base of the LC were associated with deep peaks in productivity, with production at DCM depths accounting for 5 to 40% of water column productivity (mean = 20 ± 12 %, n = 10). Shelf currents were typified by shallower and generally more productive DCMs (accounting for 25 to 80% of integrated productivity) than the LC, although distinct subsurface production peaks were not common in the NC or CC.

4.0 Discussion

In this early summer field study, the Gascoyne continental shelf was found to be a region of dynamic physical oceanographic processes, a detailed description of which is presented in Woo and Pattiaratchi (2003) and summarized here in Figure 10. The key features included: 1) the southward-flowing Leeuwin Current, centred along the shelf break (~ 200 m isobath) and associated with downwelling south of Point Cloates; 2) the Ningaloo Current, located inshore of the LC and associated with coastal upwelling along Cape Range Peninsula (with a contribution from colder water below the LC); 3) entrainment of NC waters into the LC via coastal offshoots at Transects B and C; 4) hypersaline Shark Bay outflow, which mixed with LC water and formed a distinctive water mass that flowed poleward from Shark Bay; and 5) the northern extension of the Capes Current, encountered on the inner shelf at the southern limit of the study region (Fig. 5). This discussion investigates how the processes summarized above directly affect phytoplankton biomass distributions and rates of primary production. First, we elucidate regional patterns in the coupling of physical and biological processes along the Gascoyne shelf, and second we look at mechanisms important in DCM formation. We then consider the implications of our findings for higher trophic levels.

4.1 Regional patterns in coupled physical-biological processes

The Leeuwin Current is a well-documented feature of the west coast of WA (Pearce, 1991; Woo and Pattiaratchi, 2003), and nitrate and phosphate concentrations in the mixed layer of the LC are known to be < 0.2 µM (Pearce et al., 1992), similar to offshore surface waters of the Indian Ocean (Rochford, 1980). The nutricline at the base of the LC can be between 100 to 200 m deep (Pearce et al., 1992; Pearce, 1997), and the absence of large-scale upwelling along the coast of WA is thought to prevent deep nutrient concentrations from reaching surface waters (Pearce, 1991). In the early summer of 2000, we found the majority of Leeuwin Current stations had productivity levels typical of oligotrophic waters such as the Coral Sea (off eastern Australia; Furnas and Mitchell, 1996), the eastern Mediterranean (Ignatiades et al., 2002) and the North Pacific gyre (Hayward et al., 1983); this eastern boundary current was therefore strongly oligotrophic (defined as < 270 mg C m-2 d-1; (Nixon, 1995). However, the physical dominance of the LC was offset by

6 dynamics that occurred near the margins of LC flow, most notably in the highly productive shelf countercurrents. Localized upwelling and seaward offshoots along Cape Range Peninsula resulted in high nitrate concentrations in the euphotic zone, probably sourced from the base of the Leeuwin Current given the close proximity of NC and LC water masses along the narrow (6 – 17 km) shelf (Woo and Pattiaratchi, 2003). The substantial uptake rates (~ 1050 to 1300 mg C m-2 d-1) associated with the NC north of Point Cloates (Fig. 8) were linked to this upwelling and nutrient enrichment. In NC production stations south of Point Cloates, water column nitrate was strongly depleted, yet productivity ranged up to 1050 mg C m-2 d-1. Similar conditions were seen at the CC production station south of Shark Bay, where the northward flowing water mass at the inshore stations was postulated to originate from upwelled water beyond the study region (Woo and Pattiaratchi, 2003). The low nitrate/high productivity signature associated with both of these flows is consistent with an aging upwelled water mass (Dugdale et al., 1990). Carbon uptake rates in upwelling zones are known to peak a number of days after nitrate enrichment occurs (Kudela et al., 1997), given the physiological time lag between nitrate uptake and utilization by phytoplankton (Collos and Slawyk, 1980). Intracellular storage of nitrate can occur in marine microalgae and may under some conditions provide a buffer during low-nitrate conditions (Dortch et al., 1985; Bode et al., 1997). However, of much greater importance during the late stages of upwelling events is the shift from ‘new’ to ‘regenerated’ forms of nitrogen (Dugdale and Goering, 1967), as seen in the upwelling regions off Spain (Bode and Varela, 1994) and California (Kudela et al., 1997). Accordingly, the unmeasured ammonium and/or urea concentrations in the study area would have provided an important connection between nutrient dynamics and production in these shelf currents.

4.2 Controls on vertical distributions of biomass and productivity

Deep chlorophyll maxima have the capacity to fuel substantial fractions of total water column production (Probyn et al., 1995; Richardson et al., 2000). Along the Gascoyne continental shelf, the contribution of these subsurface peaks was found to be highly variable. It thus becomes important to understand both when and where the DCM plays an important role in integrated production along the northwest coast of WA. To do this, we must evaluate the mechanisms leading to DCM formation and assess their significance for the Gascoyne region. In a review of vertical profiles of chlorophyll a, Cullen (1982) noted that two of the chief mechanisms responsible for DCM formation are physiological adaptation of the carbon to chlorophyll a ratio (C:Chl a), and chlorophyll and production maxima near the nitracline (typical tropical structure, TTS). In the first instance, adaptation of the C:Chl a ratio produces chlorophyll distributions that are not reflective of biomass. High irradiance in surface waters results in decreased chlorophyll per cell, while shade adaptation in deeper waters generates more chlorophyll per cell for efficient light utilization (Cullen, 1982). Using the particulate organic carbon (POC) data, we found that deep chlorophyll maxima were also biomass maxima, and thus not an artifact of photoadaptation. The question then turns to the cause of biomass accumulation at depth. In most respects, the distinct DCMs at the base of the Leeuwin Current can be characterized as TTS, where a stable, nutrient-depleted surface layer overlies a nutrient-replete but light limited deeper layer (Cullen, 1982), and new production is limited by the vertical diffusion of nitrate (Dugdale and Goering, 1967). In LC waters, chlorophyll peaks and local productivity maxima were strongly associated with the deep nitracline; however, highest photosynthetic rates were almost invariably located at or just below the surface, where nitrate was below detection limits and chlorophyll was at a minimum (similar to observations in the oligotrophic western Mediterranean; Estrada, 1985). This surface pattern is a deviation from TTS, and raises some interesting questions about the ecology of the system. The lack of biomass (chlorophyll) accumulation in surface waters despite locally maximal productivity rates could be explained by concurrently high grazing rates by microzooplankton,

7 which are prevalent members of the oligotrophic food web and can play an important part in nutrient recycling (Azam et al., 1983; Cushing, 1989). An active microbial loop (which encompasses bacteria, small phytoplankton, protozoa, ciliates and microzooplankton) could also explain the notable surface productivity despite the absence of measurable nitrate. Preliminary results from grazing rate experiments off (32°S) indicate that microzooplankton in LC surface waters can consume up to 40% of phytoplankton standing stock over a 24 h period (H. Paterson, unpublished data). Consequently, Leeuwin Current chlorophyll profiles may be influenced by both increased production in a stratum (with a lower limit restricted by euphotic zone depth and the upper limit linked to nitrate concentrations) and high grazing rates in surface waters. Physiological modification of sinking rates near the nitracline/pycnocline may also be important mechanisms for DCM formation (Cullen, 1982). The critical balance between light and nutrients, however, is a key factor in the productivity of many DCM systems (Eppley et al., 1988; Basterretxea et al., 2002), and within the Gascoyne region this balance was clearly impacted by the physical dynamics of the LC. In northern waters, elevated nitrate concentrations in the euphotic zone (due to upwelling and seaward offshoots) would allow phytoplankton to exert biological control over the depth of the nitracline. However, as the LC travelled south, intensifying and deepening along the shelf break, the DCM/nitracline layer was physically depressed towards the base of the 0.1 % light level, very likely inducing light limitation in this population. This biological-physical coupling may have some significance when considering seasonal or inter-annual variation in Leeuwin Current dynamics. The LC is known to flow less strongly in the summer months of November to March (due to increased opposing wind stress; Godfrey and Ridgway, 1985), and also during ENSO (El Niño/Southern Oscillation) years when the north-south geopotential anomaly (the driving force for the Leeuwin Current) is reduced (Pearce and Phillips, 1988; Pattiaratchi and Buchan, 1991). Under these conditions, both the nitracline and associated DCM would shallow, allowing phytoplankton at the nitracline to access higher light levels. This could lead to periods of increased production, in addition to a shift from physical to biological controls on the nitracline depth. The opposite scenario would occur during autumn/winter conditions (April to September) and non-ENSO (La Niña) years, when the LC flows strong and deep, further confining nitrate concentrations to depth. Links between the physical dynamics of the LC’s flow (current strength, eddy formation) and coastal fisheries have been established by Lenanton et al. (1991), Phillips et al. (1991) and Caputi et al. (1996), however concurrent biological dynamics have yet to be examined on an inter-annual basis and may provide further insight into yearly recruitment patterns (Griffin et al., 2001).

4.3 Implications of biomass and productivity patterns for community ecology

This study has revealed the Ningaloo Current as a ‘hotspot’ for areal primary production off Western Australia; the same may be true of the Capes Current, although limited data in the southern region does not allow us to draw this conclusion without further investigation. The uniqueness of the Ningaloo area has been known for some time, as it is the site of the only substantial coral reef system found on the west coast of a (Taylor and Pearce, 1999) and attracts a number of charismatic megafauna (including whale sharks and manta rays; Taylor, 1994). Dense schools of zooplankton are seasonally common in this region, and Wilson et al. (2002) theorized that upwelling near North West Cape might drive the production of large euphausiid populations off Ningaloo reef. The high primary productivity and phytoplankton biomass we observed at Point Cloates and along the Cape Range Peninsula seem a likely link between coastal upwelling and zooplankton productivity, with nutrient inputs via upwelling possibly supporting a more efficient food web than found in more oligotrophic waters (Cushing, 1989). Studies that measure these features (nutrient enrichment, primary and secondary productivity) concurrently and examine their seasonality would be the next step in elucidating the ecological processes of this region.

8 Acknowledgements

We thank the Captain, crew and scientific support staff of the RV Franklin for the successful execution of Cruise FR10/00, and the shipboard scientific party (T. Koslow, B. Nahas, W. Schroeder, P. Thompson, M. Woo) for their assistance and constructive discussions. Brian Griffiths (CSIRO Marine Research) is thanked both for the use of the photosynthetron equipment and for the detailed training provided to C. Hanson. David Griffin (CSIRO Marine Research) supplied real- time satellite imagery, and the staff at CMR Data Centre (particularly Terry Byrne, Gary Critchley and Bob Beattie) provided extensive assistance with the CTD and hydrology datasets. Bridget Alexander and Jamie McLaughlin are thanked for pre- and post-cruise technical support. Financial assistance was provided by a UWA Research Grant, a UWA Vice Chancellor’s Discretionary Grant, and an International Postgraduate Research Scholarship/University Postgraduate Award (to C. Hanson). This contribution is Environmental Dynamics No. ED 1885 CH.

References

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11 Figure 1. Eleven cross-shelf transects undertaken along the Gascoyne continental shelf, Western Australia from 13 to 27 November 2000; CTD casts and Niskin sampling were completed at all stations, while 14C uptake experiments were performed at production stations only (marked by filled circles and station numbers).

12 Figure 2. Cross-shelf contours of chl a for representative transects within the study region – a) Transect C, b) Transect D, c) Transect H, and d) Transect J.

13 Figure 3. Cross-shelf profiles of depth-integrated nitrate (mmol N m-2; note the y-axis for Transect A is twice the magnitude of the other transects). Shelf break (200 – 300 m isobath) stations are marked with open circles, and general water mass groupings are indicated above each profile (LC – Leeuwin Current, NC – Ningaloo Current, SB – Shark Bay outflow, CC – Capes Current).

14 Figure 4. Linear regression of DCM depth vs. the nitracline for all data, with symbols indicating stations associated with each of the four surface water masses. Dashed lines indicate the extent of countercurrent (NC, CC) and SB outflow data.

15 Figure 5. Along-isobath (500 m) chl a distribution as viewed in a cross-section from north (left) to south (right) through the study area. Chlorophyll contours are overlaid with the nitracline (solid line), a mean estimate of euphotic zone depth (0.1 % light; dotted line) and the maximum alongshore surface velocity (as a proxy for LC strength; dashed line).

16 Figure 6. Particulate organic carbon vs. chl a, corrected for non-phytoplankton POC by subtracting non-zero intercepts (which represented carbon not associated with live phytoplankton) from the data (note: four surface data points below 0 mg C m-3 after correction were not included in the figure but were used in the regression). The slopes represent phytoplankton-specific C:chl a (which was not significantly different between surface and DCM datasets; see text), and the dashed lines indicate the extent of surface values.

17 Ningaloo Current

Shark Bay Leeuwin Current Outflow

Capes Current

Figure 7. Mean temperature-salinity (TS) values (calculated for the top 40 m of the water column) for production stations; identification of the Ningaloo Current and Shark Bay outflow stations within the Leeuwin Current TS signature was facilitated with the ADCP and SST data analyses of Woo and Pattiaratchi (2003).

18 NINGALOO CURRENT

SHARK BAY OUTFLOW

CAPES CURRENT

LEEUWIN CURRENT

Figure 8. Schematic of the geographical groupings of production stations as derived from TS relationships, ADCP and SST data (Woo and Pattiaratchi, 2003); bars indicate depth-integrated primary production (mg C m-2 d-1).

19 Figure 9. Vertical profiles of temperature (°C), nitrate (µM), chl a (mg m-3) and photosynthetic rate (mg C m-3 d-1) for representative production stations in the Leeuwin Current, Ningaloo Current, Capes Current and Shark Bay outflow.

20 Figure 10. The generalized surface circulation patterns encountered during the field study (from Woo and Pattiaratchi, 2003).

21