Il Nino Phytoplankton

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Continental Shelf Research 27 (2007) 958980

Intra-seasonal and inter-annual variations in phytoplankton

biomass, primary production and bacterial production at

North West Cape, Western Australia: Links to the 19971998

El Nin o event

Miles Furnas

Australian Institute of Marine Science, PMB No. 3, Townsville MC, Queensland 4810, Australia

Received 20 April 2006; received in revised form 4 December 2006; accepted 3 January 2007

Available online 16 January 2007

Abstract

Phytoplankton biomass, community and size structure, primary production and bacterial production were measured at

shelf and continental slope sites near North West Cape, Western Australia (20.51S22.51S) over two summers

(OctoberFebruary 19971998 and 19981999), and in April 2002. The North West Cape region is characterized by

upwelling-favorable, southwesterly winds throughout the summer. Surface outcropping of upwelled water is suppressed by

the geostrophic pressure gradients and warm low-density surface waters of the southward flowing Leeuwin Current. Strong

El Nin o (ENSO) conditions (SOIo0) prevailed through the summer of 19971998 which resulted in lower sea levels along

the northwestern Australian coast and a weaker Leeuwin Current. La Nin a conditions prevailed during the 19981999summer and in April 2002. During the summer of 19971998, the North West Cape region was characterized by a

shallower thermocline (nutricline), resulting in larger euphotic zone stocks of inorganic nitrogen and silicate over the

continental slope. There was evidence for episodic intrusions of upper thermocline waters and the sub-surface chlorophyll

maximum onto the outer continental shelf in 19971998, but not in 19981999. Pronounced differences in phytoplankton

biomass, community size structure and productivity were observed between the summers of 19971998 and 19981999

despite general similarities in irradiance, temperature and wind stress. Phytoplankton primary production and bacterial

production were 2- to 4-fold higher during the summer of 19971998 than in 19981999, while total phytoplankton

standing crop increased byo2-fold. Larger phytoplankton (chiefly diatoms in the410mm size fraction) made significant

contributions to phytoplankton standing crop and primary production during the summer of 19971998, but not

19981999. Although there were no surface signs of upwelling, primary production rates near North West Cape

episodically reached levels (38 g C m2 day1) characteristic of eastern boundary Ekman upwelling zones elsewhere in the

world. Bacterial production (0.0061.2 g C m2

day1

) ranged between 0.6 and 145 percent (median 19 percent) ofconcurrent primary production. The observed differences between years and within individual summers suggest that

variations in the Leeuwin Current driven by seasonal or ENSO-related changes in the Indonesian throughflow region may

have episodic, but significant influences on pelagic productivity along the western margin of Australia.

Published by Elsevier Ltd.

Keywords:Upwelling; Phytoplankton; Primary production; Leeuwin current; ENSO; Indian Ocean; Ningaloo reef

ARTICLE IN PRESS

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1. Introduction

The eastern margin of the southern Indian Ocean

bordering the Australian continent exhibits a

number of geographical and meteorological simila-

rities to the eastern boundary upwelling zones ofthe Atlantic (NW Africa, Namibia) and Pacific

(California Current, Peru) Oceans. In all these

systems, the dominant wind flow is parallel to the

coast and toward the equator. The equatorward

winds drive an Ekman divergence at the continental

boundary that forces surface waters offshore, which

are replaced from below. In the Atlantic and Pacific

Oceans, the upwelled waters are cool and nutrient-

rich, supporting highly productive pelagic ecosys-

tems (Cushing, 1971). In contrast, high intensity

upwelling does not occur along the western coast of

Australia. This is due to the Leeuwin Current,which transports warm, low salinity (low density)

and low-nutrient waters southward from the equa-

torial Indian Ocean along the western margin of

Australia (Cresswell and Golding, 1980; Church

et al., 1989). Instead of cold water, high phyto-

plankton productivity and large tonnage pelagic

fisheries, the western margin of Australia supports

corals (Veron, 1995) and small-tonnage fisheries of

coastal finfish, rock lobsters, tropical prawns and

pearl oysters (Caputi et al., 1996;Penn et al., 2005).

The Leeuwin Current defines the oceanographyof the western Australian continental margin

(Cresswell et al., 1989; Godfrey and Ridgway,

1985). Organized flow begins near the southern

end of the Australian North West Shelf and

proceeds southward along the continental margin,

driven by the steric height gradient between the

Timor Sea and the Southern Ocean (Godfrey and

Ridgway, 1985). Unlike the major eastern boundary

currents of the Atlantic and Pacific Oceans, the

Leeuwin Current flows away from the equator,

directly into the dominant wind field. These winds

are generated by the pressure difference between the

anticyclone in the southern Indian Ocean and the

equatorial zone of low pressure (Hastenrath, 1991).

Currents and sea level in the Timor Sea are strongly

influenced by equatorial flow regimes in the Indian

Ocean (Springtall et al., 2002), the Indonesian

Throughflow (Wyrtki, 1987; Vranes et al., 2002)

and climate drivers of ENSO events (Chambers and

Tapley, 1999). The strength of the Leeuwin Current

varies seasonally, with an early summer (November

January) minimum (Godfrey and Ridgway, 1985).

Over shorter time scales, southerly wind stress also

influences coastal sea level height and exerts a braking

effect on southward current speeds (Cresswell et al.,

1989).

The Leeuwin Current and its variability affect the

marine ecology of Western Australia in a number of

ways. The warm waters of the Leeuwin Currentallow scleractinian corals to grow to the southern

tip (351S) of the West Australian coastline (Veron,

1995). Year-to-year variations in the strength and

main current axis of the Leeuwin Current influence

the recruitment dynamics of a number of commer-

cial fish and invertebrate species, particularly the

Western Rock Lobster (Caputi et al., 1996, 2001;

Griffin et al., 2001; Pearce and Phillips, 1988).

Biological and physical processes underlying these

variations are still not well resolved (Caputi et al.,

2001;Griffin et al., 2001).

At North West Cape (221S), the Leeuwin Currentforms a relatively narrow (o50 km) stream (Taylor

and Pearce, 1999). Satellite imagery of sea surface

temperature indicates that the main axis of the

current moves laterally relative to the coast. When

the Leeuwin Current is displaced offshore, a narrow

coastal boundary current (the Ningaloo Current)

can transport cooler surface water northward along

the seaward margin of the Ningaloo fringing reef

(Taylor and Pearce, 1999;Hanson et al., 2005a).

Relatively little is known regarding the pelagic

productivity and ecology of the southern NorthWest Shelf and adjoining continental margin.

Waters of the southern North West Shelf are

oligotrophic, with rapid (hoursdays) turnover of

dissolved nutrient species, particularly of nitrogen

(Furnas and Mitchell, 1999). Primary production

rates ranging between 0.45 and 2.5 g C m2 day1

have been measured in the vicinity of the Mon-

tebello and Lowendal Islands (201 250S) under

winter (AugustSeptember) conditions (Furnas

and Mitchell, 1999). Concurrent water column

bacterial production rates ranged between 0.1 and

0 . 6 g C m2 day1. Near-surface winter (August)

primary production rates measured in Exmouth

Gulf ranged between 15 and 40mg C m3 day1

(Ayukai and Miller, 1998).Hanson et al. (2005a,b)

measured primary production in continental

slope waters between North West Cape (221S) and

Cape Leeuwin (341S) under non-El Nin o condi-

tions. The highest production rates (0.51.3 g C m2

day1) were measured in the boundary zones

bordering Ningaloo Reef and Shark Bay (261S).

Further offshore, primary production rates

were o0 . 2 g C m2 day1. Direct estimates of

ARTICLE IN PRESS

M. Furnas / Continental Shelf Research 27 (2007) 958980 959


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microzooplankton grazing in North West Shelf

waters indicate that picoplankton (o2mm) produc-

tion is almost wholly consumed within the microbial

food web (Moritz et al., 2006; McKinnon and

Duggan, in prep.). Within the North West Cape

region, growth and egg production by macro-zooplankton (chiefly small copepods) are almost

always food-limited (McKinnon and Duggan, 2001,

2003a). The southern North West Shelf produces an

annual catch of benthic-feeding prawns exceeding

1000 tonnes, chiefly from Exmouth Gulf (Sporer

and Kangan, 2005). There are no estimates of

benthic production that might support this fishery.

Diving surveys indicate that there are no substantial

beds of benthic macro-algae or seagrasses within

Exmouth Gulf (McCook et al., 1995).

Large plankton feeders (whale sharks, manta

rays) annually appear along Ningaloo Reef duringthe late summer (MarchMay) to feed on baitfish

schools and euphausiid aggregations (Taylor, 1994,

1996). Whale shark numbers vary from year to year

with some evidence for a link to ENSO processes,

but mechanisms behind these variations are not

known (Wilson et al., 2001). Smaller pelagic

predators such as larval fish are also transported

southward to Ningaloo Reef and the many island

fringing reefs of the southern North West Shelf by

the Leeuwin Current (Sampey et al., 2004). Food

chain processes which support these small pelagicpredators are not well constrained. Both food

availability and temperature appear to contribute

to interannual variability in the development

rate of larval fish off Ningaloo Reef (Meekan

et al., 2003). During the summer, tropical cy-

clones can episodically alter the local composi-

tion of plankton assemblages by along-shelf

displacements of surface water (McKinnon et al.,

2003).

As part of a broader program to define biological

oceanographic processes operating on and adjacent

to the southern North West Shelf and Ningaloo

Reef ecosystem, repeated measurements of primary

and bacterial production, with associated measure-

ments of hydrographic structure, nutrient stocks,

plankton biomass and community structure were

carried out over two summers (19971998 and

19981999). Additional production experiments

were carried out in April 2002. The objectives were

to define summer upwelling processes operating in

the North West Cape region and quantify levels of

primary and secondary production which support

regional pelagic food webs.

2. Methods

2.1. Study location and sampling sites

The productivity and biomass measurements

described herein were largely made near NorthWest Cape, Western Australia (221S) during the

summers of 19971998, 19981999 and in April

2002. Five cruises were carried out in each of the

first two summers (Table 1). For convenience

herein, particular cruises and associated data are

identified by the month in which the cruise began

(e.g. October 1997).

North West Cape lies at the northern end of the

Cape Range Peninsula (Fig. 1). The Cape Range

Peninsula encloses Exmouth Gulf, a large shallow

(most o20 m) embayment which forms the south-

ern end of the Australian North West Shelf. The260 km Ningaloo Reef fringing reef system extends

south from North West Cape. The width of the

continental shelf varies from ca. 42 km, immediately

to the north of North West Cape to ca. 130 km at

the Montebello Islands (201 250S). Much of the shelf

is less than 20 m in depth, with numerous islands

and shoals, many of which support corals or coral

reefs. Along the front of Ningaloo Reef, the

continental shelf is very narrow, with seabed depths

exceeding 500 m within 27 km of the coastline.

Repeated biomass and primary production measure-ments were made at or near two stations (B and E)

located on a cross-shelf transect which was repeatedly

ARTICLE IN PRESS

Table 1

Sampling cruises, dates and sampling stations in the North West

Cape Region

Month Date start Date end Stations

19971998

October 26 Oct 3 Nov NWC001NWC039

November 27 Nov 5 Dec NWC040NWC062

December 28 Dec 4 Jan NWC063NWC096January 24 Jan 1 Feb NWC097NWC125

February 22 Feb 2 Mar NWC126NWC162

199899

October 17 Oct 25 Oct NWC163NWC194

November 17 Nov 24 Nov NWC195NWC242

December 17 Dec 23 Dec NWC243NWC272

January 14 Jan 22 Jan NWC273NWC330

February 11 Feb 19 Feb NWC331NWC373

2002

April 3 Apr 12 Apr NWC374NWC402

For convenience, cruises are identified by the month in which the

sampling started.

M. Furnas / Continental Shelf Research 27 (2007) 958980960


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sampled for oceanographic and biological para-

meters. Station B (211 49.50S1141 20.31E, water

depth 20 m) is representative of the shallow shelf

region at the entrance of Exmouth Gulf. The water

column at station B is generally well mixed and

variably turbid as a result of local sediment

resuspension driven by wind waves. Dissolved

nutrient concentrations are persistently low. Station

E (211 37.30S1141 9.50E; water depth 90 m) is located

on the adjacent continental slope and is more

oceanic in character. The water column at station

E is stratified, with a surface mixed layer that varied

between 20 and 60 m in thickness. Additional shelf

production stations were occupied near Thevenard

ARTICLE IN PRESS

Nin

galooR

eef2215'S

2230'S

2245'S

2145'S

2130'S

2100'S

2115'S

11415'E11400'E11345'E11330'E 11430'E 11445'E 11500'E

Exmouth

Learmonth

0 5 10

N

15 20km

2200'S

Point Cloates

Thevenard

Island

Milyering RS

E

North

West Cape

Exmouth Gulf

Onslow

Indian Ocean

NWC126NWC155

NWC291NWC232

NWC207

NWC219

NWC302

CapeR

ange

TB

B

A

C

D

F

G

H

NWC125

Australia

Fig. 1. Locations of sampling stations in the North West Cape region. Most shelf and slope productivity measurements were made at

stations B and E, respectively. Production station sites other than B, E or TB are identified by the station number. Winds, irradiance and

barometric pressure were measured by a tower-mounted weather station located at Milyering Ranger Station, Cape Range National Park.



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Island (Station TB: 211 23.20S1151 6.70E: water

depth 18 m) and during transits between North

West Cape and Dampier (20.51S). Six production

stations were occupied in slope waters away from

the transect line. Three of these stations (NWC219,

NWC191, NWC302) were in close proximity toNingaloo Reef. Hydrographic profiles, nutrient

characteristics and phytoplankton populations at

these slope sites are similar to those at Station E.

Meteorological data (wind speed and direction,

barometric pressure, solar radiation) in the study

area was measured at half-hourly intervals by an

automatic weather station located at the Milyering

Ranger Station, 36 km southwest of North West

Cape. The station is located on a flat plain approx.

1 km inland from the coast, and is directly exposed

to the dominant southwesterly winds of the region,

though some local wind steering may have beeninduced by the low-lying Cape Range (200300 m

high) which runs along the length of North West

Cape. Records of sea level at Exmouth (December

1997 onward) were obtained from the Western

Australian Department of Planning and Infrastruc-

ture (WADPI). Monthly average values for sea level

at Broome, WA (181S) were obtained from the

Australian National Tidal Facility, Flinders Uni-

versity, South Australia.

2.2. Oceanographic sampling

Water samples for phytoplankton biomass and

productivity measurements were collected between

0830 and 0930 local time. Initially, a surface-

to-bottom CTD cast was made at the sampling site.

The CTD (Seabird SBE25) was fitted with a 4p

underwater light sensor (Biospherical) and chlor-

ophyll fluorometer (Chelsea). Immediately there-

after, discrete water samples were collected using

Niskin bottles from 5 (shelf stations) or 67 (slope

stations) depths spread throughout the water

column or upper 75 m. Underwater light profiles

made at mid-day indicated that the euphotic zone

extended through the full water column at both sites

(I18m at B510% of I0, I75m at E2% of I0). At

stationB, water samples were collected from depths

where calculated in situ light levels nominally

matched levels in shaded tanks of a deck incubator

(100%, 50%, 30%, 20% and 8% of full sunlight).

At station E, water samples were collected from

depths with nominal in situ light levels of 100%,

50%, 30%, 20%, 8%, 4% and 2% of full sunlight.

At other shallow shelf production stations, the

sampling profile was truncated or compressed

depending on local depth and turbidity conditions.

Additional stations were occupied at sites B and E

throughout the study period for other plankton

biomass and rate measurements. Descriptions of

general water column properties at B and E includedata from these stations.

At the production stations, sub-samples of water

were taken from the sampling bottles for determina-

tion of dissolved nutrients (NH4+, NO2

, NO3,

PO43, Si, DON, DOP), chlorophyll a in phyto-

plankton size fractions, phytoplankton production

in size fractions, bacterial production and phyto-

plankton community structure. On a number of

cruises when suitable deionized water was available,

NH4+ concentrations were determined at sea using a

modified (low-contamination) version of the phe-

nol-hypochlorite method (Solorzano, 1969; Dudeket al., 1996). Normally, the sub-samples of water

taken for nutrient determinations were frozen for

later analysis ashore, using standard methods

implemented on a segmented flow analyzer. Dis-

solved organic N and P concentrations were

estimated by difference from total inorganic N

and P in the samples after 16 h of UV oxidation

(Armstrong and Tibbitts, 1968).

Chlorophyll a standing stocks and primary

production rates were determined or estimated in

four size classes (total population, 410mm,102 mm, o2mm). Chlorophyll standing stocks

and primary production rates in the size fractions

not directly measured (e.g. 102 mm, o2mm) were

calculated by difference. Procedures for phyto-

plankton community size fractionation, primary

production measurements and bacterial production

measurements are given in detail elsewhere

(Fuhrman and Azam, 1980, 1982; Furnas and

Mitchell, 1996, 1999). Chlorophyll concentrations

(mg Chl a m3) and hourly primary production

rates (mg C m3 h1) were summed by trapezoidal

integration through the euphotic zone to give areal

standing crop (mg Chla m2) and primary produc-

tion (mg C m2 h1) estimates. Daily production

rates (g C m2 day1) were estimated by dividing

the areal production measured during the 4-h mid-

day incubation period by the fraction of total daily

irradiance in that interval. Surface irradiance (PAR)

was measured at 1-min intervals by a logging

radiometer with the sensor (Biospherical QSR-240)

in the ships superstructure.

Water sub-samples were taken from surface, mid-

depth and near-bottom sampling bottles (60 or 75 m

ARTICLE IN PRESS



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at Sta E) and preserved for later counts

of microphytoplankton and microzooplankton

(Moritz et al., 2006; Furnas, unpubl. data). Sub-

samples (10 ml) were also taken for at-sea (live)

enumeration of phototrophic picoplankton popula-

tions (Synechococcus,Prochlorococcus, small eukar-yotes) by flow cytometry (Furnas, unpublished

data).

3. Results

Surface winds in the vicinity of North West Cape

over the summers of 19971998 and 19981999 had

a dominant northeastward (equatorward) flow,

which roughly parallels the coast (Fig. 2). The

median wind directions (meteorological convention)

and speeds calculated for the two summers from the

Milyering data are virtually the same (2141 at

5.4ms1 vs. 2131 at 5.5 m s1). Coastal winds

exhibited a strong diel variation in mean strength

(Fig. 3) and direction due to intense solar heating ofthe adjacent continental land mass. Most com-

monly, the strongest winds came from the southwest

and prevailed from late afternoon to the early

morning hours. The lightest winds occurred during

the early morning hours; however, the timing and

intensity of diel wind changes varied from day

to day.

Computed offshore Ekman transport arising

from the low-pass filtered shore-parallel wind stress

ARTICLE IN PRESS

-5

0

5

-5

0

5

10

-5

0

5

October November December January February March

-5

0

5

10

-10

-5

0

5

10

1520


1998-99 Smoothed Wind (m sec-1)

1997-98 Smoothed Wind (m sec-1)

-10-5

0

5

10

15

20Off shore Ekman Flow (cm sec-1)LongshoreWind (m sec-1)

Longshore Wind (m sec-1) Off shore Ekman Flow (cm sec-1)

Fig. 2. Time series of smoothed winds measured at Milyering Ranger Station, computed longshore wind velocity and estimated shore-

normal surface Ekman flows at North West Cape over the 19971998 and 19981999 summer periods. Wind vectors are plotted using the

oceanographic convention. Smoothed wind speed and direction were calculated as a 48-h running vector average. Longshore wind was

calculated assuming a coastline orientation of 0301in the study area. Data was recorded at 30 min intervals and used for processing derived

values. Time series were then sub-sampled at a 3-h interval for plotting. Shaded vertical bars indicate the timing of cruises in the North

West Cape region.



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ranged from 5 to 15 cm s1 during periods of

sustained southwesterly winds (Fig. 2). The wind

records in both summers are characterised by

episodic reversals at intervals of one to several

weeks. During these interludes, computed offshore

Ekman transport was either greatly reduced, orchanged to onshore flow. Two tropical cyclones

occurred during the period shown in the wind

records. Tropical cyclone Tiffany passed approxi-

mately 150 km north of North West Cape in

January 1998. The cyclone had no influence on

barometric pressure at Milyering (Fig. 4). At

Tiffanys closest approach, the North West Cape

region experienced ca. 48 h of strong northerly and

north-easterly winds that forced shelf waters south-

ward into the Exmouth Gulf region (McKinnon

et al., 2003). Tropical Cyclone Vance (central

pressure 920 hPa, gust winds in excess of 260kmh1) produced the pronounced variation in

wind vectors as it passed over North West Cape in

March 1999.

Other meteorological variables differed little

between the summers of 19971998 and 19981999

(Fig. 4), although barometric pressure at Milyering

was slightly higher in the summer of 19971998.

Daily irradiance in both years increased from

ca. 47 E m2 in October to 55 E m2 in late

December and declined again to 41 E m2 in March.

There was a pronounced difference between mean

sea levels recorded in the summers of 19971998 and19981999 (Fig. 5). Mean sea levels were consis-

tently lower at both Broome and Exmouth during

the summer of 19971998. Sea levels recorded over

the summer of 20012002 (not shown) were similar

to those in 19981999. El Nin o events are generally

characterized by lower sea level along the Western

Australian coast and a weaker Leeuwin Current

(Caputi et al., 2001). During both years, there was

an overall increase in sea level over the course of the

summer. The mean between-year difference in

summer water levels at Broome (21.5 cm) wassimilar to that recorded at Exmouth (25 cm). Based

on sea levels measured at Fremantle, W.A.,

estimated transport in the Leeuwin current during

1999 was the strongest since 1930 (Caputi et al.,

2001). The lower sea levels during the summer of

19971998 also coincide with dramatically altered

oceanographic conditions in parts of the Indo-West

ARTICLE IN PRESS

0

2

4

6

8

0 4 8 12 16 20 24

Time of Day

MeanWindSpeed(ms-1)

1 Std. dev.

1997-98

1998-99

Fig. 3. Diel changes in mean wind speed within half-hour bins at

Milyering Ranger Station over the summer periods 1 October to

1 April 19971998 and 19981999. The error bar shows a

representative 1 standard deviation range for a time bin.

0102030405060

990

1000

1010

1020

r

Milyering Barometric Pressure (hPa)

Milyering Daily Irradiance (E m-2

)

97-98

98-99


Tiffany

Vance

Fig. 4. Barometric pressure (hPatop) and daily irradiance (E m2ottom) recorded at Milyering Ranger Station over the summers of

19971998 and 19981999. Names identify the timing of tropical cyclones which affected the North West Cape region.



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Pacific during the 19971998 El Nin o event (e.g.

Abram et al., 2003;Chambers and Tapley, 1999).

Hydrographic conditions near North West Cape

varied to some degree on a cruise-to-cruise basis

during both the 19971998 and 19981999 sum-

mers; however, there were persistent differences

between periods characterized by higher and lower

primary production. Fig. 6 shows cross-slopesections of chlorophyll and water temperature,

which typify distributions of these variables during

the periods of higher primary production in the

summer of 19971998 (October, January, February)

and generally throughout the less productive

summer of 19981999. During the low productivity

interval of November and December 1997 and April

2002, hydrographic sections and profiles were more

like those observed over the summer of 19981999.

During the El Nin o conditions of the summer of

19971998, there was a general shoaling of iso-

therms against the continental slope. On a number

of occasions, water masses contiguous with the

offshore thermocline and sub-surface chlorophyll

maximum or isolated boluses of water cooler than

24 1C were observed on the outer continental shelf

just inshore of North West Cape. Where apparently

unconnected boluses of intruded water were ob-

served on the shelf, their temperature/salinity

characteristics were similar to the offshore thermo-

cline layer. The intruded water, whether connected

to the thermocline or not, was characterized by

elevated chlorophyll concentrations.

Throughout the summer of 19981999, in con-

trast, the 241C isotherm and associated deep

chlorophyll maximum layer consistently remained

below the shelfbreak. No obvious upwelling, sig-

nificant on-shelf extensions of the sub-surface

chlorophyll maxima or intruded masses of thermo-

cline-derived water were observed. In six of nine

cross-shelf sections sampled over the continentalslope during the 19981999 summer period, iso-

therms were either horizontal, or exhibited a

deepening toward the coast, as would be expected

from the geostrophic pressure field associated with

the Leeuwin Current.

Because of the differing degrees of southward

transport by the Leeuwin Current in 19971998 and

19981999, there were pronounced between-year

differences in the temperature/salinity/nutrient

characteristics of surface waters at stations B and

E. Fig. 7 shows monthly averages of temperature,

salinity, oxidized nitrogen and silicate in the mixed

layer at these stations. These values are typical of

conditions over the outer continental shelf and

continental slope. Regional water temperatures

increased steadily over both summers at both shelf

and slope stations. Over the summer of 19981999,

average conditions in the mixed layer at both

stations B (520 m depth stratum) and E (535 m

depth stratum) were consistently warmer, and from

November onward, less saline when the Leeuwin

Current was flowing more strongly and transporting

tropical waters southward from the western Timor

ARTICLE IN PRESS

4.0

4.2

4.4

4.6

4.8

5.0

Sep Oct Dec Jan Feb Mar Apr

Mean Monthly Sea Level (m) at Broome (18S)

97-98

98-99

1.0

1.2

1.4

1.6

1.8

2.0Smoothed Sea Level (m) at Exmouth (21S)

97-98

98-99

mean difference = 21.5 cm

mean difference = 25 cm

Nov

Fig. 5. Mean monthly sea levels recorded at Broome (181S) and 30-min sea levels recorded at Exmouth over the summers of 19971998

and 19981999.



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Sea. In summer, the western Timor Sea is char-

acterised by warm (2631 1C), low-salinity (34.5

35.1%), low-density (sigmatE21.822.6) water

masses (M. Furnas, unpubl. data).

Because of the reduced thickness of the oligo-

trophic surface mixed layer over the continental

slope in the summer of 19971998, depth-weighted

average concentrations of dissolved oxidized nitro-

gen (NO2+NO3) and silicate in the surface layer

(035 m) were significantly greater than in the

summer of 19981999 (Fig. 7; Table 2). With the

exception of the November 1997 cruise, concentra-

tions of dissolved oxidized nitrogen and silicate in

shelf waters (B) did not differ greatly between years,

most likely due to active uptake by phytoplankton.

Because of analytical problems, there was insuffi-

cient PO43 data to make a similar month-by-month

comparison. Where high quality NH4+ data is

ARTICLE IN PRESS

ABCDEFGH

Jan 17-18 1999

Temperature (C)

Jan 26-27 1998

Chlorophyll (g L-1)

Jan 26-27 1998Temperature (C)

Jan 17-18 1999Chlorophyll (g L-1)

0

-50

-100

-150

-200

-250

-50

-100

-150

-200

-250

0

-50

-100

-150

-200

-250

0

-50

-100

-150

-200

-250

0

Dep

th(m)

Dep

th(m)

Dep

th(m)

Dep

th(m)

Fig. 6. Cross shelf sections of temperature and chlorophyll fluorescence representative of conditions observed during high productivity

periods in the summer of 19971998 (top two panels) and low-productivity periods over the summer of 19981999 (bottom two panels).



10/23

available, surface concentrations were very low

(o0.05 mM) at both stations B and E, and without

a clear between-year difference. Particulate nitrogen

(PN) concentrations in shelf waters (station B) were

significantly (po0.05) higher than in slope waters

on a month by month basis over both the

19971998 and 19981999 summers. In all three

sampling campaigns, average surface layer chlor-

ophyll concentrations at B were approximately

twice those measured at E. Mean surface layer

chlorophyll concentrates at both stations B and E

were significantly higher during the summer of

19971998 than in the summer of 19981999

(po0.05). The small number of 2002 stations

precluded a rigorous comparison with previous

years.

Representative vertical profiles of temperature,

salinity, underwater light, size-fractionated chloro-

phyll a and size-fractionated primary production

are shown in Fig. 8. During the summer of

19971998, larger phytoplankton (chiefly diatoms)

retained by a filter with 10 mm pores were respon-

sible for a significant proportion of both biomass

and primary production at stations B and E. In

contrast, phytoplankton larger than 10 mm were a

relatively minor contributor to community structure

ARTICLE IN PRESS

20

22

24

26

28

30

9798 Sta. B

9899 Sta. B

9798 Sta. E

9899 Sta. E

Oct Nov Dec Jan Feb

Cruise

35.1

35.2

35.3

35.4

35.5

35.6

0

2

4

6

8

10

0.0

0.2

0.4

0.6

0.8

1.0

Oct Nov Dec Jan Feb

Cruise

Tempera

ture

(C)

NO

2+NO

3(M)

Silica

te(M)

Sa

linity

()

Fig. 7. Temporal changes in the cruise-averaged depth-weighted mean mixed layer water temperature, salinity, oxidized nitrogen

concentration and silicate concentration at stations B (520 m) and E (535 m) over the summers of 19971998 and 19981999.



11/23

and biomass during the summer of 19981999.Numerically, phytoplankton populations were

dominated by unicellular cyanobacteria (Synecho-

coccus, Prochlorococcus) in the o2mm size fraction

(104105 ml1: Furnas, unpubl. data). At most

stations, the picoplankton size fraction was a

significant, if not the dominant proportion of the

chlorophyll standing crop. Diatom assemblages

(410mm fraction) were highly diverse, especially

during the summer of 19971998 (Furnas, unpubl.

data). They were most often dominated by multi-

species assemblages of Chaetoceros and Pseudo-

nitzschia. Mid-sized phytoplankton (210 mm) were

relatively small contributors to biomass in both

shelf and slope waters during both summers. In

individual station profiles, peak chlorophyll con-

centrations typically occurred at mid- to lower

depths in the water column or euphotic zone.

Peak primary production and chlorophyll-specific

photosynthesis rates were usually found at

depths between the 50% and 20% mid-day

isolumes. Despite considerable differences in

cell size and taxonomic composition, ranges of

assimilation numbers (mgC mgchl1 h1) for

the experimental size fractions were of similarorder.

Pronounced day-to-day, intra-seasonal and

inter-annual differences in chlorophyll standing

crop (Fig. 9), primary production (Fig. 10) and

the size distribution of biomass and produc-

tion were observed. At station B, the highest

standing crops and areal productivities were

measured at the beginning (October) and end

(FebruaryMarch) of the summer sampling period.

Fluctuations in both standing crop and produc-

tion at station B were largely due to variations

in the biomass of and production by the 410mm

size fraction, particularly in the summer of

19971998. Chain-forming diatoms (e.g. Chaeto-

ceros,Rhizosolenia,Pseudonitzschia) were the major

component of this size fraction (Furnas, unpubl.

data). With one exception (January 1998), chlor-

ophyll standing crop at station E was considerably

less variable on a day-to-day and between-cruise

basis than that observed at station B. Absolute

standing crops at station E (780 mg m2;

median 24mgm2) were approximately three

times those observed at B (328 m g m2;

ARTICLE IN PRESS

Table 2

Seasonal averages of depth-weighted mean mixed-layer nutrient and chlorophyll concentrations at stations B (521 m) and E (535 m) for

the summers of 19971998, 19981999 and April 2002

Year Sta. Nox NH4+ PN PO4

3 PP Si Chl a

mM mg L1

19971998 B Mean 0.10* 0.05 2.0* 0.07* 0.15* 3.0* 0.61*

1 S.D. 0.03 0.09 0.3 0.05 0.03 1.7 0.34

n 21 21 10 18 11 21 21

E Mean 0.44* 0.10 1.2 0.11 0.06 4.5* 0.34*

1 S.D. 0.35 0.21 0.3 0.04 0.02 2.3 0.21

n 16 14 5 12 3 16 16

19981999 B Mean 0.02 0.02 1.5 0.12 0.09 2.15 0.48

1 S.D. 0.02 0.02 0.5 0.04 0.02 0.8 0.42

n 20 20 10 12 8 20 16

E Mean 0.14 0.01 1.0 0.15 0.05 2.38 0.24

1 S.D. 0.15 0.01 0.2 0.08 0.03 1.1 0.14

n 20 6 11 11 20 20

April 2002 B Mean 0.01 0.02 2.3 0.08 3.2 0.70

1 S.D. 0.01 0.02 0.1 0.01 0.4 0.13

n 3 3 2 3 3 4

E Mean 0.03 0.03 1.3 0.07 2.5 0.23

1 S.D. 0.04 0.01 0.01 0.3 0.04

n 2 2 1 2 2 2

n, number of stations where the variable was measured. *Statistically significant difference between 19971998 and 19981999 (t test,

po0.05). Nox NO2+NO3

.



12/23

median 7 m g m2), largely due to the greater

depth of the water column rather than absolute

differences in mean water column chlorophyll

concentration. Chlorophyll stocks at station E were,

in most cases, predominantly in the o2 mm size

fraction (mean o2mm chlorophyll 62 percent of

total chlorophyll). In April 2002, chlorophyll

standing crops at station B were intermediate

between standing crops measured in the summers

of 19971998 and 19981999 while at station E,

chlorophyll standing crops were similar to those

found in 19981999 (Fig. 9). The median standing

crop of chlorophyll at station E is close to that

(21mgm2) reported by Hanson et al. (2005a) for

ARTICLE IN PRESS

24

Temperature (C)

NWC039

Slope

3-Nov-97

NWC161

Shelf

2-Mar-98

NWC207

Slope19-Nov-98

NWC273

Shelf

14-Jan-99

Sal C

% IoChl (fluor) < 2 m


13/23

the northern Leeuwin Current region in November

2000.

During the high productivity periods of the

19971998 summer, estimated daily production rates

at station B ranged between 1.1 and 4.6 g C m2

(average 2.770.9 (S.D.) g C m2 day1, n 10).

Daily production rates of 1.1 and 2.5 g C m2 were

measured at two inner shelf sites (NWA097,

NWA125) during transit legs. During the mid-

summer low-production period of 19971998, daily

production at station B averaged 1.070.1gCm2

day1 (n 5). In contrast, daily primary production

at station B over the early and late portions of the

19981999 summer averaged only 0.770.4gCm2

(n 10), declining to an average rate of 0.37

0.1gCm2 day1 during the mid-summer period.

Estimated daily production rates only exceeded

1 g C m2 on two occasions at shelf stations during

the summer of 19981999, compared to 12 times in

19971998. Daily primary production rates measured

at station TB over the 19981999 summer (n 5)

averaged 0.570.2gCm2). In April 2002, estimated

daily primary production at station B averaged

0.670 . 2 g C m2.

Day-to-day variations in primary production at

station E were often quite pronounced, particularly

during the summer of 19971998. The presence and

movement of eddies (e.g. McKinnon et al., 2003) is

ARTICLE IN PRESS

0

5

10

15

20

25

0

5

10

15

20

25

0

20

40

60

80

0

20

60

40

October November December January February April 02

Chlorophyll standing crop (mg m-2)

Shelf Stations 1997-98

Slope Stations 1997-98



NN

N

> 10 m fraction

2-10 m fraction

< 2 m fraction

Unfractionated

Fig. 9. Standing crops of phytoplankton chlorophyll in the 410mm, 102mm and o2mm size fractions at shelf and slope stations over the

19971998 and 19981999 summers, and April 2002. Standing crop data from shelf station other than B are identified by an asterix.

Standing crop at slope stations bordering Ningaloo Reef are identified by N.



14/23

presumed to be the primary cause of this variability.

Estimated daily production rates at station E over

the summer of 19971998 ranged between 0.5

and 8.3 g C m2 (mean 3.172 . 2 g C m2 day1)

with no obvious temporal variation over the

course of the summer. The high areal produc-

tion rates at station E were largely due to the

greater depth range over which significant produc-

tion occurred, rather than to very high production

rates within the surface layer or a narrow depth

stratum. Through the summer of 19981999, in

contrast, estimated daily production at station E

was only slightly greater than 1 g C m2 (mean-

1.371 . 6 g C m2 day1). Production rates (but not

standing crop) at station E varied considerably

between experiments on an individual cruise during

19971998, but not in 19981999. In contrast, to the

diatom-driven variations at Station B, fluctuations

in production at station E were largely due to

changes in production by the o2mm size fraction

(primarily unicellular cyanobacteria). Primary pro-

duction rates in MarchApril 2002 at both sites

ARTICLE IN PRESS

1

2

3

4

5

1

2

1

2

3

4

5

1

2

3

4

5


0

0

0

0

Daily Primary Production (g C m-2day-1)

Shelf Stations - 1997-98

Shelf Stations - 1998-99

Slope Stations - 1997-98

Slope Stations - 1998-99

8.3

6.3

> 10 m fraction

2-10 m fraction

< 2 m fraction

Unfractionated

NN

N

4.9

Fig. 10. Daily primary production by phytoplankton in the410mm, 102mm and o2mm size fractions at shelf and slope stations over the

19971998 and 19981999 summers, and April 2002. Production estimates from shelf station other than B are identified by an asterix.

Production estimates at slope stations bordering Ningaloo Reef are identified by N.



15/23

were similar in magnitude to rates measured in the

summer of 19981999 (Fig. 10).

There were pronounced inter-annual (po0.001)

and between-site (po0.001) differences in daily

production to biomass (P/B) ratios (Eassimilation

numbers; Fig. 11) of phytoplankton assemblages.

Overall, averages of daily P/B ratios measured over

the summer of 19971998 (185 g C g chl1 day1 at

station B and 108 g C g chl1 day1 at station E)

were twice those recorded during the summer of

19981999 (79 g C g chl1 day1 at station B and

5 7 g C g c h l1 day1 at station E). With one

high ratio excluded (February 1999), the average

P/B ratio at station E over the summer of

19981999 was 39 g C g chl1 day1. P/B ratios

recorded in April 2002 at both stations were

similar to those measured during the summer of

19981999.

With a small number of outliers (45 times the

local 95% confidence interval) excluded, rates of

bacterial carbon production estimated from 3H-

thymidine uptake were correlated with measured

primary production rates (Fig. 12). Ranges of bacterial

production at station B over the summers of

19971998 and 19981999 were 0.20.6 (med-

ian 0.36gCm2day1) and 0.030.6gCm2 day1

(median 0.07gCm2 day1), respectively. Ranges

of bacterial production measured at slope stations

were 0.31.1 (median 0.62gCm2 day1) and

0.011.2gCm2 day1 (median 0.11gCm2 day1)

for the 19971998 and 19981999 summers, respec-

tively. The higher bacterial biomass production at the

slope stations reflects the greater depth of the euphotic

zone (4590m vs. 1621 m) at these stations. With one

exception (February 1999, slope stations), monthly

averages of bacterial production rates measured during

ARTICLE IN PRESS

0

50

100

150

200

250

300

0

50

100

150

0

50

100

150

0

50

100

150

200

250

300

P/B (g C g Chl-1day-1) - Shelf Stations 1997-98




N NN


Fig. 11. Daily production to biomass (P/B) ratios for the total phytoplankton population at shelf and slope stations over the 19971998

and 19981999 summers, and April 2002. P/B ratios from shelf station other than B are identified by an asterix. P/B ratios at slope stations

bordering Ningaloo Reef are identified by N.



16/23

the summer of 19971998 were consistently higher

than averages for comparable times and locations in

19981999 (po0.01). Estimated daily bacterial produc-

tion at shelf stations (1 outlier excluded) averaged

21710 (1 S.D.) percent of the concurrent phytoplank-

ton rate in 19971998, compared to 18710 percent in19981999. At slope stations (three outliers removed),

19971998 daily bacterial production averaged 1979

percent of concurrent phytoplankton production and

1779 percent in 19981999. A linear regression

between estimated daily bacterial and phytoplankton

production rates (four outliers removed) had a slope of

0.13 with no clear difference between shelf and slope

stations. Apart from experimental artifact or utiliza-

tion of previously excreted carbon, there is no obvious

explanation for the four high and outlying bacterial

production rates.

4. Discussion

Despite the absence of surface manifestations of

upwelling, phytoplankton and bacterial production

rates measured in the vicinity of North West Cape

over two summer periods spanned ranges similar to

those recorded in much larger and more well-known

equatorial and eastern boundary current upwelling

systems (Table 3). There were pronounced differ-

ences between phytoplankton standing crops, com-

munity size structure and primary production

measured during the summer of 19971998 and

similar rates or values measured over the summer of

19981999 and in April 2002. There were concur-

rent differences in the structure and inferred

strength of the Leeuwin Current at North West

Cape. These differences suggest that inter-annualvariations in pelagic production in this system are

caused by climate-associated mechanisms that in

some ways mirror forcing processes operating in the

larger upwelling systems of the eastern Pacific

Ocean. The 1997 El Nino was at its maximum

strength during the late (southern) summer of

19971998 (Fig. 13). While upwelling and associated

productivity decreased in the eastern and equatorial

Pacific during the 19971998 El Nin o(Strutton and

Chavez, 2000;Chavez et al., 2002a, b), phytoplank-

ton biomass and productivity increased at North

West Cape. This increase was associated with a

relaxation of the Leeuwin Current and a thinning of

the low-density surface layer which brought ther-

mocline waters with higher nutrient concentrations

well up into the euphotic zone (see also Hanson et

al., 2005a). The upwelled water did not, however,

breach the low-density layer of Leeuwin Current

water at the surface in either summer. The absence

of surface outcropping of cooler water makes North

West Cape a cryptic upwelling system.

The low-biomass, low-productivity regime domi-

nated by picoplankton that was sampled over the

ARTICLE IN PRESS

Shelf

Slope

Bact Prod = 0.13 (Phyto Prod) + 0.06

r2= 0.67 p


17/23

La Nin a summer of 19981999 and in April 2002 is

most likely the norm for the Leeuwin Current

system at North West Cape. Within this regime,

episodic El Nin o coupled changes in the strength of

the Leeuwin Current produce upwelling conditions

favorable for diatom-dominated communities and

high productivity. Significant changes in community

structure and productivity can occur in these

intervals even when local environmental parameters

that influence upwelling (wind stress) and phyto-

plankton productivity (light intensity, temperature,

near-surface nutrient levels) are similar. Because

high productivity at North West Cape is likely

restricted to El Nin o events and there is no

pronounced surface temperature or chlorophyll

signature of upwelling, it is also likely that high

productivity events will be missed by infrequent

surveys, or may be difficult to detect with satellite

ocean color and thermal imagery.

Although surface manifestations of upwelling

were not observed, phytoplankton standing crop

and primary production rates were several times

higher during the summer of 19971998, as com-

pared to 19981999 or April 2002. Areal primary

production rates near North West Cape varied 4

5-fold during the summer of 19971998, from ca. 1

ARTICLE IN PRESS

Table 3

Ranges of primary production, bacterial production and bacterial production as a proportion of concurrent phytoplankton primary

production recorded in a range of upwelling systems

System Primary production

(gCm2 day1)

Bacterial production

(gCm2 day1)

BP/PP (%) Source

Equatorial Pacific

1992 El Nin o 1.22.0 0.190.24 920 Ducklow et al. (1995)

1992 El Nino 0.31.4 Chavez et al. (1996)

1993 La Nin a 1.3 0.24 17 Kirchman et al. (1995)

19971998 El Nin o 0.21.2 Strutton and Chavez (2000)

19981999 La Nin a 0.21.8 Strutton and Chavez (2000)

Humboldt Current

Peru 1.611 Ryther et al. (1971)

Peru 15 Calienes et al. (1985)

Peru 0.74.3 Chavez et al. (1996)

Chile231S 18.1 Iriarte and Gonzalez (2004)

Chile361S 0.38.7 0.060.56 429 Cuevas et al. (2004)

Chile22361S 0.027.7 Daneri et al. (2000)

Chile22361

S 0.115 0.35.0 20145 Troncoso et al. (2003)Cariaco Basin 0.56.9 Muller-Karger et al. (2001)

California Current 0.33.5 Pilskaln et al. (1996)

0.71.9 Shipe and Brzezinski (2003)

Kuroshio Current

Taiwan 1.54.5 Chen et al. (2004)

Taiwan 0.12.1 0.030.24 7125 Shiah et al. (2000)

NW Spain 0.0511 0.010.43 318 Teira et al. (2003)

0.22.9 Tilstone et al. (2003)

Benguela Current 0.54 Brown and Field (1986)

0.153.5 0.050.78 31100 Brown et al. (1991)

Arabian Sea

Somali Basin 0.82.8 0.120.35 930 Veldhuis et al. (1997),Wiebinga et al. (1997)

Gulf of Aden 0.52.2 0.070.34 1034 Veldhuis et al. (1997),

Wiebinga et al. (1997)

Somali Basin 0.13.8 Savidge and Gilpin (1999)

North West Cape

Sta. E (19971998) 0.68.3 0.291.15 830 This study

Sta. E (19981999) 0.45.8 0.011.19 129 This study

Sta. B (19971998) 0.84.6 0.210.63 841 This study

Sta. B (19981999) 0.21.4 0.020.61 261 This study



18/23

to 8.3 g C m2 day1. Regional phytoplankton

populations 19971998 at both shelf and slope

stations were often dominated by assemblages of

diatoms (410 mm size fraction; Furnas unpubl.

data) similar in composition to those normally

encountered in upwelling systems (e.g. Smayda,

1965;Blasco, 1971) or elsewhere in high energy shelf

and coastal regions of NW Australia (Hallegraeffand Jeffrey, 1984). Diatoms were much less

abundant during the summer of 19981999

when picoplankton (o2mm size fraction) predomi-

nated. Similar low production rates (0.3

0 . 7 g C m2 day1) were measured during the sum-

mer of 19981999 at Thevenard Island where local

vertical mixing and nutrient inputs from the benthos

would be similar to those at station B. Daily

primary production rates 42 g C m2 were recorded

13 times at sites on the shallow continental shelf and

the deeper continental slope over the summer of

19971998, but only three times over a similar time

frame in 19981999. Winter (AugustSeptember)

areal production rates on the southern North West

Shelf, determined by the same method, ranged

between 0.5 and 2.5 g C m2 day1 (Furnas and

Mitchell, 1999), though most production rates

were less than 1 g C m2 day1. Daily primary

production rates in the largely co-latitudinal shelf

waters of the Great Barrier Reef (NE Australia)

range between 0.03 and 5.5 g C m2 day1 (med-

ian 0 . 7 g C m2 day1, n 175; Furnas and

Mitchell, 1989; Furnas, unpubl.). As at North West

Cape, the highest areal production rates in the

Great Barrier Reef under non-disturbed conditions

(ca. 5 g C m2 day1) were measured at slope sites

where local topographically forced upwelling deli-

vers nutrients to the surface layer.

The low ambient concentrations of dissolved

nutrients, especially N and P in shelf waters indicate

that near-surface production of phytoplanktonbiomass is strongly constrained by nutrient avail-

ability. Areal production at station B in both

summers generally tracked changes in the chlor-

ophyll standing crop, most obviously during the

summer of 19971998. In contrast, day-to-day

changes in primary production, but not biomass

(as chlorophyll), was more pronounced at station E.

Reasons for this high level of production, but not

biomass variability are not known. Despite the

shoaling of the thermocline into the euphotic zone

during the summer of 19971998, elevated dissolved

nutrient concentrations were never observed at the

surface. Rapid uptake by phytoplankton and

bacteria is the obvious cause. Estimates of water

column ammonium turnover rates near the Mon-

tebello Islands (201S) in August 1995 (water

temperatures 2223 1C) fell between 0.5 and 4 h

(Furnas and Mitchell, 1999). Measured and inferred

turnover times of dissolved inorganic nitrogen and

phosphate in the low-nutrient co-latitudinal shelf

waters of the Great Barrier Reef range from hours

to days under comparable light and temperature

conditions (Furnas et al., 2005).

ARTICLE IN PRESS

-30

-20

-10

0

10

20

30

Jan-1992 Jan-1994 Jan-1996 Jan-1998 Jan-2000 Jan-2002 Jan-2004

SOI

4.3

4.4

4.5

4.6

4.7

4.8

Broome

Sea

Leve

l(m

)

Monthly SOI (lagged 2 months)

Broome Sea Level

Fig. 13. Monthly mean values of the Southern Oscillation Index (SOI: lagged 2 months) and mean sea level at Broome, Western Australiabetween January 1992 and October 2003. The shaded boxes indicate summer time windows when productivity measurements were carried

out at North West Cape.



19/23

Inter-annual and spatial variations in daily

production to biomass ratios (P/Bassimilation

numberg C g chl1 day1) largely followed the

observed inter-annual and cross-shelf variations in

chlorophyll standing crop and primary production

(Fig. 11). The average daily P/B ratios at shelf andslope stations during the summer of 19971998 were

nearly twice the ratios measured over the summer of

19981999. The observed differences in P/B ratios

reflect the difference in phytoplankton community

composition over the two summers and the growth

potential of the two major phytoplankton func-

tional groups involved (diatoms vs. prokaryotic

picoplankters; Furnas, 1990, 1991; Furnas and

Crosbie, 1999). Despite the low ambient nutrient

concentrations, pelagic diatoms in tropical shelf

waters appear to be capable of in situ growth rates

on the order of 0.71.4day1 (12+ doubl-ings day1;Furnas, 1990, 1991). For phytoplankton

communities with a nominal C/chl a ratio of

5 0 g C g c h l1, daily P/B ratios on the order of 100

to 200 would be indicative of biomass growth rates

in this range. In contrast, populations of pelagic

cyanobacteria (Synechococcus, Prochlorococcus)

usually appear to have in situ growth rates of

p0.7 day1 (o1 doubling day1; Furnas and Cros-

bie, 1999). For a cyanobacteria-dominated popula-

tion with a C/chl a ratio of 50, daily P/B ratio

o100gCgchl1

would be expected.Estimates of bacterial production over the sum-

mers of 19971998 and 19981999 ranged from

0.006 to 1.2gCm2 day1. As with phytoplankton

production, the highest areal bacterial production

rates were estimated at station E because of the

greater depth range integrated. In individual experi-

ments, estimated areal bacterial carbon production

ranged from 3 to 145 percent of concurrent

phytoplankton carbon fixation. Excluding four high

outlier values, bacterial carbon production was

generally on the order of 1020 percent of con-

current 14C-based primary production with little

difference between stations B and E. The observed

range of bacterial/phytoplankton production ratios

is similar to that reported for other upwelling

systems (Table 3). Estimates of carbon growth

efficiencies (GCE) in pelagic bacteria indicate most

healthy populations have CGE values on the order

of 1030 percent (del Giorgio and Cole, 2000;

Anderson and Ducklow, 2001). For CGE values in

this range, the bacteria/phytoplankton production

ratios measured at North West Cape suggest that

pelagic bacteria may process carbon fluxes up to

half the level of phytoplankton primary production.

Previous measurements of bacterial production in

waters to the north of North West Cape under

somewhat cooler (2223 1C), winter conditions

(Furnas and Mitchell, 1999) indicated bacterial

production was approximately 40% of concurrentprimary carbon fixation. Given the high irradiance

levels (4155 E m2 day1) driving summer photo-

synthesis on the southern North West Shelf and low

ambient nutrient concentrations which constrain the

ability of phytoplankton to produce complex

structural biomolecules (e.g. proteins, nucleic acids)

from the carbon fixed, regional phytoplankton

would be expected to excrete excess (supra-Redfield)

fixed carbon as dissolved organic carbon (DOC),

providing the substrate for significant bacterial C

uptake and biomass production. Pelagic bacteria

are efficient competitors for dissolved nutrients inpelagic systems (Wheeler and Kirchman, 1986) and

would have little difficulty competing with phyto-

plankton for the scarce available nutrients (NH4+,

NO3, PO4

3). Unfortunately, no measurements of

DOC concentrations or excretion were made.

The results herein suggest that large-scale climate

events such as El Nin o and the Indian Ocean Dipole

(Chapman and Tately, 1999) influence biological

productivity along the southern North West Shelf

and Ningaloo Reef primarily by changing the

intensity or volume of the poleward flowingLeeuwin Current. During normal or La Nin a

periods, relatively stronger southward flow in the

Leeuwin Current transports a thicker layer of warm,

low-density water southward past North West

Cape. This enhanced surface flow will inhibit

Ekman upwelling of high-nutrient sub-thermocline

waters by the geostrophic depression of the iso-

pycnal and iso-nutrient surfaces along the conti-

nental slope (e.g. Fig. 6 bottom). In this situation,

water upwelled to replace surface water moved

offshore by Ekman transport would come from the

thicker, low-density, low-nutrient surface layer. The

increased thickness of the surface layer transported

southward by a stronger Leeuwin Current will also

deepen the nutricline so that vertical mixing driven

by internal tides and waves (Holloway et al., 1985)

will bring relatively less water from the nutricline

into the euphotic zone. During El Nin o events, in

contrast, reduced southward transport allows the

nutricline to rise closer to the surface (Fig. 6 top)

where mixing processes driven by internal waves

and tides can mix more nutrients into the euphotic

zone. With a shallower thermocline, internal tides

ARTICLE IN PRESS



20/23

and waves will also be able to intrude the raised

upper thermocline onto the shelf.

The Southern Oscillation Index (SOI) is the

primary indicator of the occurrence and intensity

of El Nin o events in the Indo-Pacific region.

Fluctuations in average sea level along the WesternAustralian coast provide an index of the strength of

the Leeuwin Current (Pearce and Phillips, 1988;

Caputi et al., 2001). Fig. 13 shows the relationship

between average monthly SOI values for 19922004

and average monthly sea levels 2 months later at

Broome (181S). Changes in mean sea level at North

West Cape over a shorter record length are similar

to those at Broome. The El Nin o summer of

19971998 was characterized by strongly negative

values of the SOI and low values of sea level,

indicative of reduced southward transport of

tropical waters by the Leeuwin Current. In contrast,positive SOI values and higher sea levels indicate

stronger Leeuwin Current transport during the La

Nin a summer of 19981999 (Caputi et al., 2001). A

similar relationship between the average SOI and

production was seen in April 2002. Additional

periods of low sea level in Broome (and likely low

transport in the Leeuwin Current) were recorded

during the extended El Nin o of 19921994.

Relationships between summer primary produc-

tion at North West Cape and regional climate are

shown in Fig. 14 where the averages of primary

production rates measured during each of the

monthly cruises are presented in relation to the

average monthly value of the SOI 2 months earlier.There is a clear difference between the summers of

19971998 and 19981999. With one exception,

higher areal production rates in both shelf and slope

waters at North West Cape are clearly associated

with negative values of the SOI. In both summers,

average SOI values tended to increase over the

summer. The mean of both shelf and slope

production rates measured in April 2002 and the

positive average SOI value for that period are

similar to the conditions observed during the

summer of 19981999.

The 19971998 El Nin o was one of the strongeston record (McPhaden, 1999) and produced notice-

able effects in both the Indian and Pacific Oceans

(Abram et al., 2003; Chavez et al., 1999; Chavez et

al., 2002a, b; Escribano et al., 2004). The observed

dynamics of the northern Leeuwin Current system

at North West Cape during the 19971999 El Nin o/

La Nin a events is of interest because a number of

the observed physical and biological responses at

North West Cape are a mirror image of what was

observed off western North America. During its

peak in late-1997 and early-1998, the CaliforniaCurrent upwelling system was characterized by an

increase in sea level (Ryan and Noble, 2002; lower

off NW Australia), higher water temperatures

(Collins et al., 2002; lower off North West Cape;

Meekan et al., 2003) and decreased phytoplankton

biomass and primary production (Chavez et al.,

2002a, b; higher off North West Cape) due to a

replacement and deepening of the mixed layer

(Chavez et al., 2002a, b; thinner off North West

Cape). Diatom biomass off California and Peru was

lower than normal during 1997 (Chavez et al.,

2002a, b; Iriarte and Gonzalez, 2004; higher off

North West Cape). These conditions then reversed

during 19981999 when the 1999 La Nin a set in.

The results reported here are primarily focused

upon the northern Leeuwin Current in the small

area close to North West Cape. The Leeuwin

Current exerts a strong effect upon the marine

ecosystems all along Western Australia coast. Inter-

annual variability in the strength and position of the

Leeuwin Current are known to influence the

distribution and recruitment of important commer-

cial species of fish and invertebrates (Pearce and

ARTICLE IN PRESS

5 0 1 2 3

Average Monthly SOI (lagged 2 months)

Oct

Nov

Dec

Jan

Feb

Apr '02

1997-98 1998-99

Daily Primary Production (g Cm-2)

Shelf

Slope

-25 -20 -15 -10 -5 50 10 15

4 3 2 1

Fig. 14. The monthly means of primary production rates

measured at shelf and slope stations during the summers of

19971998 and 19981999 and in April 2002 in relation to the

monthly average value (2-month lagged) for the Southern

Oscillation Index (SOI).



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Phillips, 1988; Griffin et al., 2001; Caputi et al.,

1996, 2001). While the Leeuwin Current suppresses

large-scale upwelling along the Western Australian

coastline, the results here show that intermittent

bursts of high productivity can occur in favorable

locations or circumstances during El Nin o events. Itwould be unsurprising if the 19971998 El Nin o had

a significant influence on a variety of biological

processes all along the Western Australian coast.

Acknowledgements

I thank A.D. McKinnon as a collaborator in

fieldwork and for criticism of early drafts of the

manuscript. M. Dommisse carried out the Novem-

ber 1997 production experiments. D. McKinnon, S.

Duggan, J. Carleton, M. Dommisse, E. Richardson,H. Mermaid and the crew of the R.V. Lady Basten

greatly assisted with the sampling, and made hot

rough cruises much happier events. This work was

funded by the Australian Institute of Marine

Science.

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