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A review of the oceanography of the Dampier Archipelago,

Western Australia

Alan Pearce1, Steve Buchan2, Tony Chiffings3, Nick DAdamo4, Chris Fandry5, Peter

Fearns6, Des Mills5, Rob Phillips7 and Chris Simpson4

1 CSIRO Marine Research, Private Bag 5, Wembley, Western Australia 69132 WNI Oceanographers & Meteorologists, 31 Bishop St, Jolimont, Western Australia 6014

3

24 Victoria St, Guildford, Western Australia 60554 Marine Conservation Branch, Department of Conservation and

Land Management, 47 Henry St, Fremantle, Western Australia 61605 Department of Environmental Protection, Westralia Square,

141 St Georges Terrace, Perth, Western Australia 60006 School of Physical Sciences, Curtin University of Technology,

GPO Box U1987, Perth, Western Australia 68457 International Risk Consultants, 26 Colin St, West Perth, Western Australia 6005

Abstract This review provides an oceanographic background for the biological field surveys ofthe Dampier Archipelago undertaken in July/August 2000. It attempts to summarise the main

oceanographic processes affecting the waters of the Archipelago in the context of the wider North

West Shelf, using both the published literature and previously unpublished data.

The shallow Archipelago waters are strongly influenced by climate- and seasonal-scale processes,

meteorological events and diurnal forcing. El Nino/Southern Oscillation (ENSO) events are

associated with cooler shelf water and lower sea levels (partly linked with changes in the strength of

the Leeuwin Current), while there is correspondingly warmer water and higher sea levels during La

Nina periods.

Winds vary seasonally, with a westerly tendency in summer, a southeasterly trend during the winter,

and more variable winds in the intervening seasons. Relatively strong land/sea breezes of up to 10

m/s can be superimposed on the synoptic pattern in both summer and winter. The most extreme

winds occur during the passage of tropical cyclones through the area between November and April,generating wind gusts of up to 70 m/s. Although an average of 3 to 5 tropical cyclones approach the

North West Shelf each year, typically only 1 or 2 pass within 200 km of Dampier.

Tides in the Dampier Archipelago are semidiurnal with a well-defined spring-neap lunar cycle; the

mean neap and spring tidal ranges are 1.0 and 3.6 m, respectively. The highest astronomical tides

can reach 5 m in height, but storm surges (especially during cyclones) can appreciably raise sea

levels above the predicted tidal height. Tsunamis are rare off Western Australia but can also cause

appreciable sea level surges.

Seas and swell on the open shelf off Dampier are generally heaviest in winter (from the northeast)

and lightest in late summer (typically from the west); only 10% of significant wave heights exceed

1.2 m and the median height is about 0.7 m. Tropical cyclones may generate swell with heights of

up to 10 m at the entrance to Mermaid Sound. Appreciable attenuation takes place within the

Dampier Archipelago itself, with wave heights typically reducing by 50% during propagation down

Mermaid Sound.

Seasonal water temperatures in Mermaid Sound range from about 22C in winter to 30C in summer,

F.E. Wells, D.I. Walker and D.S. Jones (eds) 2003. The Marine Flora and Fauna of Dampier, Western Australia.Western Australian Museum, Perth.


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14 A. PEARCE et al.

with measured extremes of about 20 to 33C. Monthly mean air temperatures vary between around

20C in July and 32C in January/February; monthly mean maximum and minimum temperatures

are 36.1C (in February) and 13.6C (in July) respectively.

Currents in the Dampier Archipelago are largely caused by tides, local winds, large scale ocean

circulation and continental shelf waves, and are strongly influenced by the local topography. Tidalcurrent speeds at the offshore seaward entrances of the Archipelago and in some of the inter-island

water passages can reach 50 cm/s but are generally much weaker in Mermaid Sound (of order 20

cm/s during spring tides and half this during neaps). During winter, the dominantly southerly to

easterly winds drive water westwards between the islands of the Archipelago, while in summer the

prevailing westerlies force the water eastwards through Mermaid Strait. Currents generated by

cyclonic winds in summer can reach 40 cm/s. The Leeuwin Current flows southwestward along the

outer continental shelf, most strongly from February to July with speeds of order 20 cm/s, but has

little effect on the waters inside the Archipelago. The propagation of continental shelf waves forced

by the alongshore wind stress (particularly during the passage of tropical cyclones) can generate

alongshore current surges exceeding 40 cm/s.

The waters of the Archipelago are oligotrophic, and water quality characteristics are highly variable

through the Archipelago. Wind-driven waves are primarily responsible for resuspending sediment inthe water column, and on occasion during spring/summer plankton blooms contribute locally to

turbidity. High spatial and seasonal variability are evident in nutrient and chlorophyll- a distributions,

and the few measurements available do not show any consistent seasonal or cross-shelf pattern.

Chlorophyll-a levels are generally below 0.3 mg/m3, with the highest concentrations exceeding 1 mg/

m3 near the coast in autumn/winter.

Finally, environmental conditions (tides, winds, sea and air temperatures) in the Archipelago during

the 3-week period of the AMSA Workshop in July/August 2000 are briefly described.

INTRODUCTION

The Dampier Archipelago is a complex of over 40 islands stretching from Cape Preston

(11612'E) eastwards to Cape Lambert (11711'E) off the Pilbara coast of the North West Shelf

(NWS). It lies in the biogeographical overlap region between the Tropical Indo-Pacific and

Warm Temperate oceanic zones (Osborne et al., 2000), and the diversity and distribution of the

flora and fauna reflect this overlap. Conditions in this part of the NWS are generally arid-

tropical, with warm water, a relatively high tidal range (compared with the micro-tidal regime

further south), seasonally-varying cross-shelf gradients in water properties and sporadic

disturbance by tropical cyclones during the summer/autumn months. There is a broad and

diverse range of ecological habitats, with a consequent variety of marine communities including

mangroves, algal meadows, sandy beaches, soft sediment fauna, coral reefs and rocky shores

(Osborne et al., 2000). Furthermore, the region is ecologically notable for turtle, marine

mammal and seabird populations. The Archipelago is entirely contained within the 30 m isobathwhich is roughly 25 km from the coast (Figure 1); the bathymetry is generally flat but there are

some deeper channels between the islands.

The Archipelago is an industrially important region with rapid growth over the past 3 decades in

oil/gas processing and other industries, and the port of Dampier is Australias largest tonnage port

(IRC Environment, 2002). There are important pearl oyster and prawn fisheries. In response to the

areas juxtaposition of high conservation values and high levels of current and projected human

usage, it was highlighted as worthy of consideration as a future marine conservation reserve by the

Marine Parks and Reserves Selection Working Group (CALM, 1994). To that end, and consistent

with State Government policy for marine conservation through reserves, the region from Cape

Preston to Nickol Bay has been proposed as a marine conservation reserve under the Conservationand Land Management Act 1984 (Osborne et al., 2000).


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OCEANOGRAPHY OF THE DAMPIER ARCHIPELAGO 15

Figure 1 Location chart of the main islands in the Dampier Archipelago, adapted from Forde (1985). This paper

uses temperature/salinity/nutrient sections along transect AB, wind data from Conzinc Island, weatherinformation from the Withnell Bay area and tidal measurements at King Bay.


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The need to ensure that current and future human usage of the area is ecologically sustainable

has spawned many studies of the oceanography and ecology through tertiary institutions and

both State and Commonwealth research organisations. The North West Shelf Joint

Environmental Management Study (NWSJEMS) is a current joint venture study between theWestern Australian Department of Environmental Protection and CSIRO Marine Research to

develop techniques for regional planning and management of the marine ecosystems on the

western NWS, including the Dampier Archipelago. Much research has also been conducted by

biological, engineering and oceanographic consultants some of the oceanographic work has

been published but much remains in the form of unpublished contract reports.

This review of oceanographic conditions in the Dampier Archipelago will provide an

environmental background to the Woodside Dampier Marine Biological Workshop which

was run by the Western Australian Branch of the Australian Marine Sciences Association

(AMSA) between 25th July and 11th August 2000. The Workshop aimed to improve the

understanding of the marine biodiversity of the area, covering most of the Archipelago from

Enderby Island in the west to Delambre Island in the east (i.e. longitudes 11630 to

1175'E). Because of the large amount of oceanographic and biological work that has been

undertaken in the Dampier region over the past 2 decades, this review can at best only

summarize the main features of the physical environment and point to the greater detail

contained in the extensive published and unpublished literature. Other recent reviews of the

area are in Church and Craig (1998), Sherwood et al. (1999), Osborne et al. (2000) and

Heyward et al. (2000), while Jernakoffet al. (1999) have prepared a bibliography relating

to marine research on the NWS.

The climate of the region and local wind regime are dealt with in Section 2 of the paper. Tidal

characteristics are discussed in Section 3, and waves (including both distant swells and locally-

generated seas) are described in Section 4. The temperature/salinity/density distributions acrossthe Archipelago are summarised in Section 5; Section 6 describes the regional and local ocean

current regime, and water quality properties (nutrients and chlorophyll) are discussed in Section

7. Section 8 focusses on the meteorological and oceanographic conditions over the period of

the AMSA Workshop, and some conclusions relevant to the ecology of the area are presented in

Section 9.

CLIMATE AND METEOROLOGY

The climate of the Dampier Archipelago is both arid and tropical, being controlled seasonally

by the meridional position of the large high pressure cells which pass from west to east acrossthe Australian continent (Osborne et al., 2000). These pressure systems with their anticlockwise

wind circulation migrate from a latitude of about 25 30S in winter to 35 40S in summer,

resulting in two broadly defined climatic seasons over northwestern Australia: a warm winter

season from May to September and a hot summer season from October to April. General

information on the climate of the northwest coast and the Dampier Archipelago may be obtained

from Forde (1985), Division of National Mapping (1986), Simpson (1988), Australian Bureau

of Statistics (1989) and Buchan and Stroud (1993).

Winds

While the Bureau of Meteorology has operated a number of meteorological stations in theArchipelago region over recent decades, the only two stations currently providing wind readings


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are automatic weather stations at Karratha Airport, which reports half-hourly, and Legendre

Island, reporting hourly (John Cramb, pers.comm.).

Wind measurements were made by the Western Australian Department of Conservation and

Environment at Conzinc Island (Figure 1) between September 1981 and July 1984 (Pitt andMills, 1985; Forde, 1985) to complement current measurements being undertaken in the

Archipelago. Sample wind roses from Pitt and Mills (1985) are used here to illustrate typical

seasonal wind regimes (Figure 2), while the monthly mean wind vectors in Figure 3 clearly show

the seasonal patterns in wind strength and direction, as well as giving some indication of

interannual variability.

In summer, the more southward position of the high pressure cells results in a preponderance

of warmer winds from the northwest to southwest sector (Figures 2 and 3). There is a pattern of

daytime seabreezes and nighttime land breezes; however wind speeds are typically less than 10

m/s (20 knots).

During winter, on the other hand, the more northerly position of the high pressure belt resultsin a prevailing easterly to southeasterly offshore flow of relatively cool air (the South East

Figure 2 Seasonal wind roses derived from anemometer data at Conzinc Island for January 1983 (representing

summer), April 1982 (autumn), July 1982 (winter) and October 1982 (spring), showing the proportion of

the time that winds were blowing in different speed ranges (on the right) from each compass sector. The

frequency rings are in 5% intervals, the central circle containing the percentage of calms. Reproduced from

Osborne et al. (2000) and Pitt and Mills (1985).


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18 A. PEARCE et al.

Trades) over the northwest, which may be modified by local land/sea breezes. The offshore

winds are enhanced by late night to early morning southeasterly land breezes as the land cools

and are moderated by afternoon northwesterly sea breezes as the land heats. Winds reach speeds

of 10 15 m/s inshore and can occasionally peak over 20 m/s (40 knots) further offshore.

Wind patterns are at their weakest and most variable during the seasonal changeovers between

summer and winter, which occur around April and August (Osborne et al., 2000). While these

wind patterns from Conzinc Island should be representative of the wind field in the Archipelago

generally, Holloway and Nye (1985) caution that coastal wind observations are not always a

good indication of the wind field further out across the continental shelf.

Tropical cyclones

The most extreme winds occur during the passage of tropical cyclones through the area the

coast between Broome and Exmouth is the most cyclone-prone region in Australia (Crowder,

2000). They tend to form between November and April over the Timor Sea or off the northwest

coast, and can result in wind gusts exceeding 70 m/s (140 knots).

Forty-nine tropical cyclones passed within 200 km of Dampier between 1965 and 2000 (an

average of 1.4 per year Bureau of Meteorology pers.comm.), with 75% occurring between

January and March. Of these, the closest passed only 24 km from Dampier. About half of the

cyclones had central winds exceeding 50 m/s (100 knots), although these speeds were notencountered in the Archipelago itself.

Figure 3 Monthly mean wind vectors from Conzinc Island between September 1981 and July 1984, prepared from

data in Pitt and Mills (1985). There were no readings in July and December 1983 and the first 5 months of

1984. The wind speed scale (0 to 5 m/s) is indicated lower left, and the vectors are in the direction to

which the wind is blowing.


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Rainfall and evaporation

Although rainfall patterns are highly variable (Division of National Mapping, 1986), there is

a pronounced seasonal pattern in precipitation and evaporation, both of which have an important

influence on the properties of the coastal waters. Most rainfall occurs over the period January toMay (Figure 4a) when the southern position of high-pressure cells allows the entry of the

tropical low-pressure rain-bearing depressions and cyclonic systems into the northwest of the

State. The mean annual rainfall at the Dampier Salt site near Karratha is 249 mm (Bureau of

Meteorology, 1988), with over half of this amount occurring between January and March.

Figure 4 (a) Monthly mean rainfall for Dampier from Bureau of Meteorology (1988 solid line with circles) and

monthly evaporation (dashed line with diamonds) from Port Hedland (CALM unpublished data). (b)

Monthly mean air temperatures at Dampier, with the monthly minima and maxima (diamonds; fromBureau of Meteorology, 1988).


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Rainfall is highly variable from year to year as a result of occasional heavy downpours the

highest daily rainfalls at Port Hedland in January and February can be four times the long term

monthly averages (CALM unpublished data).

The major river in the Dampier region is the Fortescue River which enters the sea west of theArchipelago. It flows most strongly during summer/autumn (Public Works Department of

Western Australia, 1984), mainly in response to heavy bursts of cyclone-related rainfall, with

monthly flow rates exceeding 50 m3/s about once in every two/three years on average

(unpublished data, Water and Rivers Commission). During flood times, large volumes of

sediment-laden water enter the coastal environment (Osborne et al., 2000).

Monthly evaporation for the Dampier Archipelago far exceeds the rainfall (Figure 4a),

approaching 400 mm during the summer months and dropping to half this amount in winter.

The net annual evaporation is almost 3500 mm, an order of magnitude higher than the net

rainfall for the region.

Air temperature and humidity

The monthly mean air temperature at Karratha (Dampier Salt) peaks at about 32C between

January and March and is at its lowest around 20C in July (Figure 4b data from Bureau of

Meteorology, 1988). The maximum and minimum mean temperatures in individual months are

36.1C (February) and 13.6C (July) respectively, while the extreme recorded temperatures are

47.1C and 4.6C respectively (Osborne et al., 2000).

Relative humidity in the Dampier/Karratha area is appreciably higher during summer than in

winter and spring, with summer morning humidities exceeding 50% (dropping in the afternoons)

and winter/spring values generally between 30% and 40%. The least humid month is September.

TIDES AND SEA LEVEL CHANGES

The tidal regime for the NWS is described in some detail by Easton (1970), who pointed out

the large increase in tidal range (low to high water) between Exmouth and Broome, while

Holloway (1983) has discussed the main features of the tides near Dampier using current

measurements taken offshore of the Archipelago. The main tide gauge in the Dampier region

has operated at King Bay since 1982.

The tides are semidiurnal (two highs and lows each day) with a slight diurnal inequality

(difference in heights between successive highs and lows). There is a well-defined spring-neap

lunar cycle, with spring tides occurring approximately 2 days after the new and full moon. The

largest tides of the year occur in April. A typical tidal chart from Dampier (Figure 5, from

Osborne et al., 2000) shows the spring tides peaking at about 4.5 m around the 4th and 19th July,

and neaps of about 1.5 m near the 11th and 26th July.

The standard tide levels for King Bay, Dampier are (Australian Hydrographic Service, 2002):

Highest Astronomical Tide (HAT) 5.1 m

Mean High Water Springs (MHWS) 4.5 m

Mean High Water Neaps (MHWN) 3.2 m

Mean Sea Level (MSL) 2.7 m

Mean Low Water Neaps (MLWN) 2.2 m

Mean Low Water Springs (MLWS) 0.8 mLowest Astronomical Tide (LAT) 0.1 m


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Computer modelling has shown that the tidal wave propagates in from the open ocean

in the northwest. Tidal ranges increase from 2 m at the entrance to the Archipelago to

2.2 m at the head of Mermaid Sound and reach 2.8 m in Nickol Bay. Relatively large

sea level gradients between the Sound and Nickol Bay give rise to high current flows in

the channels connecting the two basins (tidal currents are discussed in more detail in

Section 6).

Superimposed on the astronomical tides are meteorological tides resulting from changes in

atmospheric pressure and strong onshore or offshore winds. Storm surges during cyclones, in

particular, can appreciably raise sea levels above the predicted astronomical tidal height and

inundate low-lying areas.Tsunamis are rare off Western Australia their likelihood at any specific location on the NWS

is less than 1 in 100 years (WNI, 1998). They are generated by submarine earthquakes or

volcanic activity and can result in long waves of very high energy propagating across the ocean

before devastating coastal communities in their path.

While there is some evidence of early tsunami activity along the northwest coast (Easton

1970), only three tsunamis have actually been recorded in this region: Krakatoa in August 1883,

one originating in the Sunda Arc in August 1977, and the well-documented event of June 1994.

During the last-named, an earthquake south of Java resulted in a 4 m surge causing some

damage near Northwest Cape (Foley, 1994). The tide gauge at King Bay near Dampier recorded

a brief 50 cm surge, although there is anecdotal evidence (Foley, 1994) that the actual wave

amplitude may have been much higher, indicating that there is some degree of susceptibility to

tsunami activity in the Archipelago.

WAVES

Wave climatology

The seastate of the NWS comprises contributions from four main sources: swell generated in

the Southern Ocean, winter easterly swell, tropical cyclone swell and local wind-generated sea.

Note that all discussion relates to the significant wave height, which is a characteristic wave

height representing the average height of the highest one third of the waves; individual waveheights may be up to twice as high as the significant wave.

Figure 5 Typical tidal record from King Bay (Figure 1) for the month of July 1996, from Osborne et al. (2000).


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Figure 6 Typical summer and winter significant wave roses for (upper) an exposed location near Legendre Island at

the entrance to Mermaid Sound and (lower) a sheltered location within Mermaid Sound, showing the

proportion of time the waves were coming from each compass sector. The wave height scale is in the lower

centre of the diagram, and the frequency rings are in 15% intervals. Data courtesy of WNI Science andEngineering (now WNI Oceanographers & Meteorologists) and Woodside Energy Ltd.


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Wave measurements have been made at several locations (west of Legendre Island, east of

Malus Island and west of Withnell Bay) for many years in the early 1980s, while real-time

measurements adjacent the Woodside Shipping Channel have been ongoing since 1991. All

wave data collected within the Dampier Archipelago are proprietary to the industry clients forwhom the data were collected Hamilton (1997) provides a listing of wave data sources. The

only publicly available samples of the Dampier wave measurements are those presented in

Buchan & Stroud (1993).

Exposed North West Shelf waters

Other than the unpublished data sources detailed in Hamilton (1997), the only general

descriptions of seastates in exposed waters off the Dampier Archipelago are those presented in

the regional perspective of Osborne et al. (2000) and the marine data review of Heywood et al.

(2000). Buchan & Stroud (1993) provide additional samples of wave data in water depths

ranging from 30 to 125m across the continental shelf off Dampier.Wave conditions affecting the oceanic or northwest-facing coastlines of the Dampier

Archipelago are represented by wave measurements conducted off the northwestern tip of

Legendre Island (upper roses in Figure 6 WNI unpublished data). Apart from cyclone activity,

seastates tend to be heaviest from the northeast in winter (June and July) and lightest in late

summer (April) when they come typically from the west and northwest, though some swell

persists from the northwest all year. Only 10% of seastates exceed 1.2 m, with the median height

being 0.7 m.

Southern Ocean swell typically arrives at the outer edge of the continental shelf from the south

and southwest, before refracting (changing direction) around the Montebello Islands (which are

to the west of Dampier) to reach Mermaid Sound from the northwest after significant

attenuation. It tends to be higher (typically 2 m) during winter than in summer (about 1 m)

because the generating storms move further north in winter. Swell periods are generally of the

order of 12 to 16 seconds.

Where sufficient fetch is available (at least 200 km), the synoptic winter easterlies which

prevail over the NWS may generate an easterly or northeasterly swell of 6 to 8 seconds period.

Such swell will have some influence in the outer shelf portions of the NWS region (1 to 2 m

height) but should only affect northern shores of the offshore islands of the Dampier

Archipelago.

Tropical cyclones, which occur between the months of November and April (see Section 2),

will generate waves which tend to propagate radially out from the storm centre. Depending upon

such parameters as storm size, intensity, relative location and forward speed, tropical cyclonesmay generate swell of 6 to 18 seconds period from any direction, with heights ranging up to 10

m at the entrance to Mermaid Sound.

Local wind-generated sea typically ranges in period from 2 to 6 seconds, but may attain 7

seconds under very persistent forcing. Wave heights are extremely variable, ranging up to 6 m

under non-tropical cyclone conditions.

Seastates within the Dampier Archipelago

Within the Archipelago itself, sea conditions are strongly influenced by sheltering although

other factors such as refraction and bottom friction also act to reduce wave height and influence

wave direction. Typically, wave heights reduce by at least 50% during propagation downMermaid Sound.


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24 A. PEARCE et al.

The Archipelago is well sheltered from Southern Ocean swell due to the shallow waters

between Barrow Island to the west and the mainland. Sheltering also has a profound effect on

tropical cyclone wave penetration into the Archipelago, and cyclone-generated wave heights

inside Mermaid Sound are unlikely to exceed 5 m and will often be reduced to less than 2.5 m.Wave conditions near Withnell Bay in Mermaid Sound are illustrated in the wave roses of

Figure 6 (WNI unpublished data). At this site, swell is constrained to arrive from due north and

local wind waves are usually fetch-limited and conform to the direction of the forcing wind

(typically southwesterly in summer, but with increasing incidence of easterlies in winter). In

contrast to the offshore conditions described above, 90% of wave heights in southern Mermaid

Sound are less than 0.6 m with a median height of about 0.35 m.

TEMPERATURE, SALINITY AND DENSITY

Temperature measurements have been undertaken in Mermaid Sound for the petroleumindustry since the early 1980s but the data are largely unpublished. Temperatures were recorded

between the water surface and seabed in 14 m water depth at the Dolphin Berth between

December 1983 and December 1984, and mid-depth temperature records continued at that

location until September 1987 (WNI unpublished data). Surface water temperatures have also

been monitored at the Navaid 9 Directional Waverider buoy near Withnell Bay (Figure 1) since

November 1992 (WNI unpublished data).

A series of 6-weekly surveys within the Dampier Archipelago was conducted in 1982/83 by

the then-named Western Australian Department of Conservation and Environment (DCE)

(Forde, 1985). At each station, vertical profiles of temperature and salinity were made between

the surface and the seabed at times of neap tides, enabling both the cross-shelf and vertical

temperature and salinity distributions to be assessed. On the larger scale, open-shelf surface

temperatures have been obtained from satellite measurements (Reynolds and Smith, 1994).

Seasonal temperature cycle

Continuous temperature measurements from the Navaid Waverider buoy near Withnell Bay

showed the following monthly mean, minimum and maximum temperatures at the site (WNI

unpublished data the extremes of the annual cycle are bolded):

Month Mean C Min C Max C

January 29.9 27.5 32.4

February 30.4 27.8 33.1March 30.1 28.5 32.4

April 28.6 25.5 30.5

May 25.9 23.4 29.6

June 23.9 20.4 27.7

July 22.6 20.7 25.0

August 22.3 20.8 25.1

September 24.2 22.5 26.4

October 25.5 23.5 28.2

November 26.6 24.5 30.0December 28.3 25.8 31.7


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The annual temperature range was almost 8C, dropping from the peak of 30.4C in February

to a trough of about 22.5C in July/August (Figure 7).

For comparison, surface temperatures on the open continental shelf have been extracted from

monthly satellite-derived sea surface temperatures (Reynolds and Smith, 1994), covering the 1-

degree latitude/longitude block off Dampier (116 to 117E, 20S to the coast it effectively

extends midway across the continental shelf). The annual temperature range for these open shelf

waters was from 24.0C in August up to 29.3C in March, i.e. about 5C, with the offshore cycle

lagging the inshore pattern by about a month.

This seasonally-reversing cross-shelf temperature gradient is graphically illustrated by thermal

satellite imagery. During summer (upper image in Figure 8), the shallow coastal waters were

warmer than those offshore because of a net input of heat from the sun and the atmosphere. Thecool Ningaloo Current (Taylor and Pearce, 1999) was pushing northwards past Northwest Cape

and Barrow Island, with just a hint that it may have skirted the northern Monte Bello Islands

and then continued eastward towards the Dampier Archipelago. In winter, on the other hand,

strong heat loss to the atmosphere and differential cooling occurred so that the nearshore waters

were much cooler than the offshore region (Figure 8, lower). There was little evidence of a net

alongshore drift, but the tongues of warmer water intruding towards the coast may indicate

active cross-shelf mixing which would have important implications for the transport of

planktonic larvae between inshore and offshore waters.

Spatial and seasonal variability within the Dampier Archipelago can be assessed from the

DCE 6-weekly surveys conducted in 1982/83. The time-series of measurements at the offshoreend of transect AB (Figure 1) illustrates the seasonal changes in the vertical hydrological

Figure 7 Monthly mean water temperatures from the Navaid site near Withnell Bay (dashed line with circles, data

courtesy of WNI Science and Engineering (now WNI Oceanographers & Meteorologists) and Woodside

Energy Ltd) and from the open shelf (Reynolds dataset, 20 to 21S and 116 to 117E, solid line with

diamonds).


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26 A. PEARCE et al.

Figure 8 NOAA Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature images of the

Dampier region in (upper panel) January, representing summer and (lower panel) July 2000 for winter. The

colour scales, ranging from red (warmest) to blue (coolest), have been selected specifically to reveal the

thermal structure in most detail and so are different for each image. The black line marks the 200m

isobath, which represents the edge of the continental shelf. Mottled white/blue bands at the top of the

January image are clouds. Data courtesy of the Western Australian Satellite Technology and ApplicationsConsortium (WASTAC).


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structure of the inner continental shelf. The water column was only weakly stratified from April

until October 1982 (Figure 9), but between November 1982 and March 1983 the surface waters

were warmer by up to 3C and more saline than the near-bottom waters. Surface temperature

approached 29C in mid-summer (matching the Reynolds open-shelf temperatures in Figure 7),

dropping to about 22C during the winter months. The seasonal salinity cycle was less clearlydefined; surface salinities varied between 35.2 and 35.7 psu for most of the year, rising to 36.1

psu between December and February. As would be anticipated, the water density (sigma-t) was

greatest in winter when the water was coldest.

Cross-shelf hydrological structure

There were also distinctive seasonal changes in the cross-shelf gradients along transect AB.

Surface waters within Mermaid Sound at the coastal site B warmed and cooled more rapidly

than those offshore (Figure 10). As discussed above, the inshore water was cooler than that

offshore from April to mid-September and then warmer until mid-March 1983, with a

temperature range some 4C greater than at the shelf station. Throughout the year, the surfacesalinity (and density) decreased from inshore to offshore, with summer salinities at the coastal

Figure 9 Seasonal variation of the near-surface (solid line) and near-bottom (dashed line) water properties of the

inner continental shelf off the Dampier Archipelago in 1982/83 the station is at the outer end A of the

transect shown in Figure 1. The upper panel shows the density (sigma-t), the centre panel the water

temperature and the lower panel the salinity.


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28 A. PEARCE et al.

site B within the Sound rising to over 36.5 between November 1982 and April 1983 as a result

of coastal evaporation.

Mermaid Sound possesses a characteristic winter hydrographic regime, typified by the

survey in June 1982 (Figure 11, left, from Forde, 1985). The structure shows denser (cooler and

more saline) water formed within the Archipelago and wedging seaward beneath open-shelfwater. Both the temperature and salinity gradients contributed positively to the observed

inshore-offshore horizontal density difference. Horizontal and vertical gradients were strongest

at the seaward edge of the Archipelago. Similar structures were found from the surveys of May,

August and September 1982 with transitional structures occurring in the April and October 1982

surveys. Temperature/salinity diagrams from the winter surveys suggested downslope advection

of near-bottom waters and vertical mixing across the pycnocline this vertical mixing appeared

to intensify in areas of steeper slope and higher tidal velocity at the seaward edge of the

Archipelago.

The observed lower layer thickness was about 5 m with a vertical density change across the

layer interface of about 0.3 kg/m3 for the June 1982 cruise at the segment of steepest bottomslope (Figure 11). This is consistent with a density current speed of order 0.1 m/s, assuming

Figure 10 Seasonal variation of the near-surface (3 m) water properties at three locations in the Dampier

Archipelago in 1982/83. The solid line is for the offshore station (near A in Figure 1), the dashed line is

for the innermost station (near B), and the dotted line mid-way along the transect. The upper panel shows

the density (sigma-t), the centre panel the water temperature and the lower panel the salinity.


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near-critical flow, so density outflow of coastal water may be a significant flushing mechanism

during winter. Transverse vertical sections suggest that the denser water tends to flow in the

direction of the steepest slope and hence converges towards areas of greater depth to the

northwest of Mermaid Sound. Other dissolved materials associated with waters of the inner

Archipelago would be transported and mixed in a similar manner to heat and salt.

In contrast to the winter situation, the summer hydrographic regime (illustrated by the

December 1982 survey on the right of Figure 11) is characterised by vertical stratification on

the open continental shelf and elevated salinity in the shallower coastal waters due to the

localised effects of evaporation. Strong vertical gradients of temperature and salinity were foundat the seaward end of the transect (water depth ~ 30 m) with warm saline water overlaying

cooler less saline water from below the shelf thermocline. The outer zone of Mermaid Sound

(depths ~2025 m) exhibited strong horizontal gradients of temperature and salinity with little

vertical stratification; these gradients were opposed, however, in terms of their contribution to

the resultant density gradients which were accordingly small.

WATER CIRCULATION IN THE DAMPIER ARCHIPELAGO

There is a wealth of information (largely unpublished) on the circulation of the Dampier

Archipelago and associated regional waters, including extensive current measurements offshoreof the Archipelago for the oil industry. Analysis of current data across the continental shelf

Figure 11 Vertical density/temperature/salinity structure along Transect AB (Figure 1) for (left panel) the winter

survey in June and (right panel) the summer survey in December 1982, reproduced from Forde (1985).


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30 A. PEARCE et al.

northwest of the Archipelago by Holloway and Nye (1985) and Holloway (1995) have shown a

high degree of current variability associated with the semidiurnal tides and the Leeuwin Current.

A series of current measurements undertaken by the DCE between September 1981 and July

1984 between the coast and the 40 m isobath (Figure 12) revealed the dominant currentcharacteristics of the region (Mills et al. 1986) and were used by Mills (1985) to validate a

numerical depth-averaged circulation model of the Archipelago waters. All these measurements

have yielded some insight into the dynamics of the currents in the area. Phillips and Luettich

(2001) have presented a detailed hydrodynamic model for the region.

The major forces driving circulation in the Dampier Archipelago are large scale ocean

circulation, tides, local winds (including tropical cyclones) and non-tidal long period waves

(continental shelf waves and meteorological effects). Tides contribute most to the

instantaneous water movement, giving rise to oscillatory but regular ebb and flood currents.

The contributions from the other forces are seasonal, less regular in nature and generally

weaker than the tidal flows, but their effects are nevertheless important as they generate a net

residual drift that is responsible for transporting dissolved and particulate matter throughout

the system. Once in motion, circulation is influenced by the local topography water is

topographically steered between the islands and mainland and can become chaotically stirred

and trapped by secondary circulations which are set up in the lees of islands and coastal

headlands.

Large-scale ocean circulation

The Dampier Archipelago is influenced by the larger-scale ocean circulation on the NWS,

which is in turn linked with major Southeast Indian Ocean and Indo-Pacific current regimes

such as the throughflow of Pacific Ocean equatorial waters into the Timor Sea via the

Indonesian Archipelago (Godfrey and Ridgway, 1985; Sherwood et al., 1999). This transport of

warm, low-salinity water sets up a strong poleward sea level gradient along the Western

Australian coastline which drives the Leeuwin Current (Cresswell and Golding, 1980; Godfrey

and Ridgway, 1985).

The Leeuwin Current tends to flow southwestwards along the outer continental shelf off the

Archipelago and is strongest from February to July, with peak speeds of 25 to 30 cm/s

(Holloway and Nye, 1985; Holloway, 1995), although periods of strong southwesterly winds

can weaken or force short-term reversals of the current which are also associated with weak

upwelling events on the outer shelf. These events tend to occur in summer when the Leeuwin

Current is weak and most variable, and may provide some of the nitrate required for primary

production on the NWS (Holloway et al., 1985). Recent observations using buoys and satellitealtimeter estimates of the surface currents show that eddies and meanders are associated with

these shelf processes (as seen, for example, in Figure 8), although their influence on the

hydrodynamics of the Dampier Archipelago is yet to be clarified.

It is likely that the Leeuwin Current has only a minor direct influence on the circulation within

the islands of the Dampier Archipelago. The DCE studies indicated that it could distort the

regular tidal current profile at the outer edge of the Archipelago with a significant residual drift

of up to 20 cm/s towards the west (DAMS-27, Figure 12a). There may also have been some

response at the seaward end of Mermaid Sound (DAMS-30) although the residual drift there

was towards the south. Contemporaneous measurements much nearer the coast (DAMS-28,

Figure 12a) showed smaller drifts, however, indicating that the waters inside the Archipelagowere less exposed to the Leeuwin Current.


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Figure 12 Chart showing the current meter sites during the Department of Conservation and Environment studies

for (a) summer (November to April) and (b) winter (May to October) this split is more to spread the

observations over two diagrams and reduce overlap rather than to suggest any real seasonal differences.

The lines depict the relative frequencies (%) of the dominant current directions at each site (derived from

Mills et al., 1986), and the number next to each position is the DAMS site reference (see text). In cases

where there would be interference between the frequency lines at adjacent sites, one site has been slightlyoffset to avoid the overlap. The current meter deployments were typically a few weeks.


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32 A. PEARCE et al.

Tidal currents

As described in Section 3, the tides in the Dampier Archipelago have a semidiurnal nature,

giving rise to four current reversals per day, and there is a well-defined spring-neap lunar cycle

resulting in considerable variation in the speed of the tidal currents over a 14-day period. Tidalcurrents are usually 90 out of phase with tide height, with maximum speeds occurring at mid-

tide and slack water coinciding with high and low tides.

The DCE current measurements showed that the tidally-reversing currents dominated the flow

at all sites within the Archipelago, so the current directions were essentially bi-modal (Figure

12). The currents between the islands in both summer and winter tended to flow along the

topography, as would be anticipated, but further offshore the tidal currents were generally

directed onshore-offshore (also shown by Holloway, 1983).

Sample time-series plots of typical offshore and nearshore current regimes (Figure 13)

illustrate the dominant tidally-reversing current system. On occasion, current speeds at the

offshore site (DAMS-27) reached 50 cm/s while currents of similar peak speeds wereencountered in some of the inter-island water passages. Within a tidal cycle, the current speed

could vary between almost zero and 40 cm/s over a period of 6 hours. Closer inshore (DAMS-

1, see Figure 12a), the currents were only about half the speed observed at the offshore site,

with a small proportion of current speeds exceeding 20 cm/s. The current directions show the

180 tidal reversals, particularly at the coastal site. Further details of the DCE measurements

are described in the original reports (Mills, 1985; Mills et al., 1986).

Current vectors computed from a numerical model of the area (details in Phillips and Luettich,

2001) show more clearly the regional variability of the flow. At mid-flood on a spring tide, for

example (Figure 14), tidally-driven waters generally flow in a southeasterly direction across the

inner continental shelf towards the coast and are channelled through the islands and along

Mermaid Sound and Mermaid Strait, converging near the Intercourse Islands off Dampier. The

flow ebbs in the opposite direction with a comparable speed.

The areas of strongest tidal currents are generally found towards the outer extremities of the

Archipelago and occur when the water is falling or rising most rapidly. During spring tides,

currents achieve speeds of around 40 50 cm/s through the seaward reaches of Mermaid Sound

(matching the DCE current meter measurements) and about 30 40 cm/s in the channel between

Eaglehawk and Enderby Islands (see Figure 1). Similar speeds are seen in the channel south of

Rosemary Island. Channels connecting Mermaid Sound and Nickol Bayare subject to strong

flows due to the difference in water levels between the two basins, with currents exceeding

2 m/s (about 4 knots) being observed between Angel and Dolphin Islands (Forde, 1985).

Local wind forcing

Due to the relative shallowness in the Archipelago, winds are an important driving force.

Local winds firstly impart motion to the surface waters and then, after several hours, to the

whole water column. Elevation or depression of water adjacent to coastlines gives rise to water

surface slopes and pressure gradient forces, and (in conjunction with the applied wind stress

and bottom friction stress) determines the wind-induced transport.

The seasonal variation in wind over the Archipelago was described in Section 2 above.

During winter (June to August), winds are dominantly from southerly to easterly directions

(Figure 2), and the diurnal land/sea breeze reinforces the southerly to easterly winds in the

morning and moderates them in the afternoon and the night. Easterly winds drive waterwestwards between the islands of the Archipelago. During summer on the other hand, winds


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Figure 13 Current speeds, directions and water temperatures from the DCE current meter deployments at (upper

panel) offshore site DAMS-27 near the 40 m isobath northwest of Dampier, and (lower panel) site

DAMS-1 close inshore west of Burrup Peninsula, reproduced from Mills et al. (1986). Note thedifference in the speed scale on the y-axis. The two sites are shown in Figure 12.


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34 A. PEARCE et al.

prevail from the west, driving water eastwards through Mermaid Strait and between Rosemary

and Enderby Islands whilst water in Mermaid Sound is forced northwards towards the seaward

entrance.

As part of a marine conservation reserve planning process for the Department of Conservation

and Land Management, numerical simulations were conducted in which particles were released

at various locations in the Archipelago and tracked over a number of tidal cycles (Figure 15).

Under the influence of tide and an autumnal wind forcing (winds varying between typicalsummer and winter conditions), these trajectories show a general drift towards the southwest

throughout the Archipelago superimposed on the back-and-forth motion of the tide. Also

evident is the enhanced drift during neap tides, which has been observed in measured currents

(Mills et al., 1986) and is attributed to the relative weakness of the neap tidal currents (Forde,

1985). During these periods, the energy imparted by wind to water is more efficiently

transformed into advective kinetic energy as the flow is less turbulent and energy dissipation by

bottom friction occurs at a lesser rate. As a result, moderate to strong persistent winds are

capable of distorting or completely nullifying the small neap tidal current, resulting in a large

net excursion of water.

Around the islands, the flow is complex with particles becoming trapped in secondarycirculations this is particularly evident off Dampier and around Legendre Island. The particles

Figure 14 Simulated spring tide flood currents in the Dampier Archipelago derived from a numerical model. Theinset shows the tidal cycle, and the arrow gives the current speed scale (courtesy of WNI Science and

Engineering; now WNI Oceanographers & Meteorologists).


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released in Nickol Bay tend to remain there, suggesting that this may be a relatively enclosed

system (at least under the wind forcing used in this particular simulation) although some

exchange with Mermaid Sound water is likely to take place via the strong currents around

Legendre and Dolphin Islands.

Tropical cyclones

Extreme winds from tropical cyclones can have a profound affect on the circulation, includingthe generation of continental shelf waves (CSW) along the coast which can lead to enhanced

current speeds superimposed on the tidal currents in the Dampier region. Current meter records

from moorings on the open continental shelf northwest of the Archipelago clearly showed the

response of the currents (in the form of CSWs) to the enhanced alongshore wind stress during

the passage of six tropical cyclones in the summers of 1981 and 1982 (Webster, 1985).

Forde (1985) described the response to two tropical cyclones (Ian and Jane) which passed

through the area during the DCE study. During TC Ian in March 1982, the winds were initially

southerly in excess of 20 m/s before slowly swinging northerly and moderating to 7 m/s over a

few days. Currents recorded 2 m above the seabed in the 18 m deep channel between Malus

Island and West Lewis Island (site DAMS-4 in Figure 12b) flowed continuously to the west for36 hours before turning east for the next 12 hours. Current speeds peaked at 40 cm/s (Webster,

Figure 15 Simulated tracks of particles released at various locations in the Archipelago over a 6-week period under

the influence of tide and an autumnal wind regime (courtesy of the Department of Conservation and

Land Management and WNI Science and Engineering; now WNI Oceanographers & Meteorologists).


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36 A. PEARCE et al.

1985, Mills et al., 1986) generating a net excursion of about 18.2 km to the west. During TC

Jane, winds were initially east-northeast at 10 m/son 7th January 1983 before increasing to more

than 20 m/s two days later. Currents recorded 4 m above the seabed in Mermaid Sound shipping

channel (DAMS-3, Figure 12a) initially flowed south for 46 hours, followed by a 26 hournortherly reversal. Current speeds of about 35 cm/s were recorded, resulting in a net excursion

of about 16.6 km.

More recently, the passage of TC Connie (which crossed the coast to the south of Port

Hedland in January 1997) resulted in a positive storm surge of 2 m at Port Hedland, generating

a continental shelf wave that propagated southwestwards along the coast towards the

Archipelago. Measured currents near Withnell Bay indicated a northward flow for about 18

hours, double-peaking at around 40 cm/s (Figure 9.2 in Steedman, 1988) the tidal contribution

at this time was around 20 cm/s, so the CSW augmented the ebb current towards the north but

suppressed the flood current towards the south, accounting for the double peak.

It should be noted that tropical cyclones are not the only source of CSWs (Provis and Radok,

1979). Current meter records from Mermaid Sound reveal that these events occur typically once

or twice per month throughout the year, bringing strong current surges of a few days duration;

tidal flows in one direction are often doubled while the reversals are suppressed (WNI

unpublished data).

WATER QUALITY

The outer waters of the Archipelago show typical oceanic variations in water quality

characteristics such as water temperature and salinity (Section 5), turbidity and oxygen. Within

the Archipelago, relatively restricted flushing can lead to variations outside normal oceanic

ranges in water quality. For example, turbidity can be up to an order of magnitude greater

nearshore compared with offshore (Simpson, 1988), and throughout the year there may be

pronounced cross-shelf gradients in turbidity levels through the Archipelago (Forde, 1985).

Mud is transported into the marine environment by fluvial processes, or is eroded from older

sedimentary deposits, or is biogenically generated (Semeniuket al., 1982). Wind-driven waves

are primarily responsible for resuspending sediment in the nearshore areas while offshore

sediment resuspension is mainly caused by oceanic swells. Many coral species can tolerate

sediment deposition rates of up to about 50 g/m2/day but periodically these rates may be

exceeded by natural processes throughout the Archipelago during the year (Simpson, 1988).

Gilmour (2001a) found that coral communities within the inner-Archipelago are periodically

exposed to higher levels of acute sediment deposition, following resuspension due to tides andstorm activity. For example, during both 1998 and 2000, mean sediment deposition at inner-

reefs ranged from 81 to 201 g/m2/day, due to the high frequency of storm and cyclone

disturbance (Gilmour, 2001a). Periods of acute sediment deposition are likely to reduce coral

survival, even for species that are resistant to sedimentation (e.g. Gilmour, 2001b). In addition

to elevating levels of sediment deposition, severe storm and cyclone activity can cause

concomitant physical damage to benthic communities and accumulations of calcareous rubble

and sands in Mermaid Sound.

Dampier Archipelago Marine Study

Data from the Dampier Archipelago Marine Study of the mid 1980s showed that the watersof the Archipelago are oligotrophic. Phytoplankton throughout the Archipelago are present in


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generally low densities for much of the year (Semeniuk et al., 1982), except perhaps on

occasions during spring/summer when plankton blooms contribute locally to turbidity; there are

also periodic occurrences of red-tide producing dinoflagellate blooms ofTrichodesmium.

High spatial and seasonal variability were evident in nutrient and chlorophyll-a concentrationsfrom a series of four surveys undertaken by DCE between October 1980 to September 1981.

Surface, mid-depth and near-bottom samples were taken at 11 stations from King Bay out to the

30m contour, and the samples have been combined here into depth-averaged values. Although

the surveys were undertaken during neap tides, each transect took more than 6 hours to complete

and therefore extended across a large proportion of a tidal phase. Because of the significant

water movements that can take place over a tidal period (Section 6) and the low number of

samples, interpretation of these results must be viewed with some caution.

Concentrations in inorganic nutrients were generally low (as found by Rochford, 1980) and

there was considerable variation in concentrations for all three sampled nutrients at the very

shallow inshore sites. Inshore to offshore gradients were evident in some months but there was

no consistent seasonal or cross-shelf pattern across all nutrients. The orthophosphate

concentrations (PO4P) in June and September 1981 were about twice those in October 1980 and

March 1981 (Figure 16a), but the latter two transects showed a marked increase from inshore to

offshore; the peak concentration (over 0.4 M/l) was at the 8 km station in June. Concentrations

of NH4N were generally between 0.4 and 0.7 M/l (Figure 16b), except for a rise near the coast

in all seasons and an increase at the outer transect stations in June and September. High nitrate

(NO3N) levels were observed out to 10 km offshore in March 1981 but otherwise the

concentrations were mainly below 0.2 M/l with a slight increase at the coast (Figure 16c).

Apart from elevated levels close inshore (Figure 16d), N:P ratios were generally low

throughout and well below the Redfield ratio of 16:1, indicating that under nutrient-limiting

conditions nitrogen availability would be the most limiting to phytoplankton growth. In Octoberand March the N:P ratio greatly exceeded the Redfield ratio at many of the inshore sites, this

being a result of increases in either ammonium or nitrate nitrogen (or both). These values

suggest that phosphorus rather than nitrogen would be the limiting nutrient, which is unusual for

marine systems, but it is possible that at this time light rather than nutrients was limiting to

phytoplankton growth.

With the exception of June 1981, chlorophyll-a levels beyond 10 km from the coast (Figure

17a) were below 0.3 mg/m3, matching the low concentrations measured during the International

Indian Ocean Expedition of the early 1960s (Krey and Babenerd, 1976). The highest

chlorophylls were in winter (June 1981) as reported by Pearce et al. (2000) for elsewhere along

the Western Australian coast, but the concentration also exceeded 1 mg/m3

near the coast inMarch 1981.

Light attenuation was always low (about 0.05 E/m) outside the Archipelago, but rose to at

least double this value near the coast and exceeded 0.3 E/m close inshore in June and

September (Figure 17b). Large changes in water clarity over short periods of time (in the order

of hours) were observed during the surveys, probably indicative of the rapid tidal water

movements through the Archipelago and perhaps explaining the highly variable conditions near

the coast in June and September 1981.

Satellite remote sensing

The recent availability of satellite-derived chlorophyll images of Western Australian coastalwaters is beginning to provide greater detail on the seasonal distribution of phytoplankton in the


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38 A. PEARCE et al.

a

b

Figure 16 Cross-shelf transects from close inshore (King Bay, stations 1 and 4; near position B in Figure 1)

extending up Mermaid Sound out to the 30 m isobath (station 15; near position A in Figure 1), from

Chiffings (1982): (a) PO4P, (b) NH4N, (c) NO3N and (d) the N:P ratio. At each station, depth-averages of(usually) three observations have been used.


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c

d


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40 A. PEARCE et al.

a

b

Figure 17 Cross-shelf transects (matching Figure 16) for (a) chlorophyll-a, and (b) light attenuation (units E/m).

At each station, depth-averages of (usually) three observations have been used.

region although there are some shallow-water limitations. Some indication of the potential of

remotely-sensed information can be gained from summer and winter images obtained from theSeaWiFS sensor. An image for summer (Figure 18a) shows that there was a distinct nearshore


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Figure 18 SeaWiFS chlorophyll images for the Dampier Archipelago in (upper panel) February 2000, representing

summer conditions, and (lower panel) June 2000 for winter. The images were prepared by the Remote

Sensing and Satellite Research Group (Curtin University), courtesy of Orbital Sciences Corporation and

WASTAC. The logarithmic chlorophyll scale is on the right of each image: the highest chlorophyllconcentrations are in yellow/red and the lowest in green/blue.


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42 A. PEARCE et al.

band of relatively high chlorophyll with no coherent offshore structure it must be cautioned,

however, that there may have been some non-chlorophyll-related influences such as suspended

sediment near the coast as well as bottom reflectance in waters less than 30 m depth, possibly

misrepresenting the true nearshore chlorophyll concentration. The highest apparentconcentrations close inshore (shown in yellow, representing chlorophyll levels approaching 10

mg/m3) may not be valid, and further research into appropriate algorithms for deriving shallow-

water chlorophylls is required.

The band of elevated coastal chlorophylls was appreciably narrower in the winter image

(Figure 18b) but there was a coast-parallel series of high chlorophyll patches approximately 100

km offshore these were apparent in images on adjacent days as well and graphically illustrate

the high degree of spatial variability in the phytoplankton distribution. Insufficient images have

as yet been processed to assess how frequently such complex patterns may occur or to explain

their formation and structure; possible mechanisms could include dinoflagellate blooms and

upwelling due to shoaling internal waves and internal tides.

Improvement in monitoring coastal waters such as those of the Dampier Archipelago will

come from locally tuned algorithms, based on a knowledge of specific optical conditions of

the water column, to produce more accurate estimates of constituent concentrations in the water.

The applicability of SeaWiFS is also limited to some degree by the small number of spectral

bands as well as by the 1 km2 pixel size: when bathymetry and/or substrate reflectance is

variable over scales of less than 1 km, higher spatial resolution sensors may provide better

estimates of water column properties and possibly also characteristics of the substrate. The

MODIS sensors onboard the more recent Terra and Aqua satellites can provide near-daily views

of the earth using 36 spectral channels, including 10 visible channels at spatial resolution

ranging from 250 m to 1 km, while the MERIS instrument onboard the Envisat satellite

(launched in March 2002) can also provide data of promise to remote sensing of coastal waters.The Hyperion on board the Earth Observing-1 satellite provides hundreds of spectral channels

with 30 m resolution. The trade-off for higher spatial resolution is a lower temporal frequency

for viewing. Aircraft-borne hyperspectral sensors such as the Compact Airborne Spectrographic

Imager (CASI) can provide spatial resolution from sub-meter to 10 m.

ENVIRONMENTAL CONDITIONS: 25 JULY TO 11 AUGUST 2000

Longer-term context

There is a close relationship between monthly coastal sea levels, inner shelf SSTs (Reynolds

dataset, see Section 5) and El Nino/Southern Oscillation (ENSO) events as indicated by theSouthern Oscillation Index (SOI) (Figure 19). Over the past decade, ENSO periods (indicated

by negative SOI) were associated with generally lower ocean temperatures and lower sea levels

(which have been interpreted by Pearce and Phillips (1994) and Pearce (1997) as a weaker

Leeuwin Current). Conversely, duringLa Nina periods when the SOI was high, sea levels were

relatively high, the water warmer and the Leeuwin Current presumably stronger. ENSO events

clearly have an important influence on interannual variability in the Archipelago.

In the context of these longer-term environmental conditions, the AMSA Workshop was held

during the prolongedLa Nina event commencing in the late 1990s. Although coastal sea levels

were at an almost record peak (Figure 19b), implying a very strong Leeuwin Current, the inner

shelf water temperature was in fact almost 1C below the longterm average for July 2000(Figure 19a) the reason for this is not known.


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Figure 19 Monthly (a) sea surface temperature (SST) anomalies of the Dampier region (filled circles) and (b) sea

level anomalies at King Bay (filled circles) compared with the monthly Southern Oscillation Index (SOI

open diamonds) over the period 1990 to 2000. The SST anomalies have been derived from the

Reynolds dataset for the area 20 to 21S, 116 to 117E by subtracting the monthly SSTs from the long-

term monthly means, and have been smoothed by a 5-month running mean. The SOI values (from the

Darwin Tropical Diagnostic Statement) and Dampier sea level anomalies (from the National Tidal

Facility) have been similarly smoothed. El Nino/Southern Oscillation (ENSO) events are marked by thesolid horizontal bars.


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44 A. PEARCE et al.

Figure 20 Hourly measurements of (a) sea level, (b) water temperature, (c) wind speed (10-minute means, filled

circles; 3-second gusts, open circles), and (d) air temperature over the period of the AMSA Workshop in

July/August 2000. The sea level measurements were in King Bay (courtesy WA Department of

Transport), the water temperatures from a buoy off Withnell Bay (courtesy of WNI Science and

Engineering (now WNI Oceanographers & Meteorologists) and Woodside Energy Ltd), and the

meteorological data were from a site on Burrup Peninsula near Withnell Bay (courtesy of WNI Science

and Engineering (now WNI Oceanographers & Meteorologists) and Woodside Energy Ltd). The verticallines are at 12-hourly intervals.


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46 A. PEARCE et al.

AMSA Workshop

Over the 2-week period of the Workshop, the tidal regime at King Bay went through a

complete neap-spring-neap cycle (Figure 20a), with a peak tidal range of about 4 m early in

August and (assumed) correspondingly strong tidal currents in the Archipelago.No current measurements are available over this period, but because the water movements are

dominated by tide, the modelled and historically observed currents (Section 6, see also Mills et

al. 1986) may give a fair indication of the likely current patterns at the time.

Sea temperatures near Withnell Bay in Mermaid Sound were initially relatively high (Figure

20b), exceeding 23C on 25th July, but dropped rapidly to oscillate between 21 and 22C for

most of the Workshop period. The diurnal temperature cycle was apparent on most days, with

the water being coolest around 8am and warmest mid-afternoon; the diurnal temperature range

was usually about 0.5C but on at least 3 days temperature rises of almost 1 degree were

recorded. By the end of the Workshop, the temperature had fallen to 20.5C. These temperature

changes were most likely due to air-sea heat flux, possibly with some influence of cross-shelfadvection/mixing events.

The synoptic pressure weather charts (not reproduced here) showed a large high-pressure cell

in the southwest of the State at the start of the Workshop, resulting in strong east to southeast

winds at Dampier. Winds gusted to 20 m/s (40 knots) on 26 th and 27th July (Figure 20c) then

moderated to a calm period of a few days with 5 m/s breezes from the north as the High moved

eastwards. This pattern was repeated in early August as new Highs developed in the south, with

mean wind speeds of about 10 m/s gusting to 15 m/s (30 knots) most mornings. Air

temperatures fluctuated between about 17C in the mornings to 25 or 27C by mid-afternoon

with a pronounced diurnal pattern (Figure 20d). The coolest days were towards the end of July

and coincided with the sharp fall in sea temperature (Figure 20b).

SUMMARY AND ECOLOGICAL SIGNIFICANCE

Some of the environmental characteristics of the Dampier Archipelago have a profound

influence on the ecology of the area. These include the relatively high water temperatures and

tidal ranges (compared with conditions south of Exmouth Gulf) and the seasonal (albeit

irregular) occurrence of tropical cyclones.

Water temperatures greater than 30C in conjunction with high light intensities are known to

cause stress to corals, and this stress can manifest itself through the expulsion of the

zooxanthellae from the coral polyp, sometimes resulting in mortality (Hoegh-Guldberg, 1999).

The expulsion of the zooxanthellae can be detected by a whitening of the coral tissue (coralbleaching), which has been observed as recently as April 1998 in the Dampier Archipelago,

particularly for a variety of hard corals, soft corals, anemones and zoanthids in nearshore

communities (Andrew Heyward, pers. comm and James Gilmour, pers. comm.). Low water

temperatures (under 20C), on the other hand, have also been found to cause bleaching of corals

in the inner part of Mermaid Sound (Chris Simpson, pers. comm.).

The stress on corals from factors other than high temperature such as increased

photosynthetically active radiation (PAR), ultra-violet radiation, sediment, physical disturbance

or predation can superpose to aggravate the effect of extreme temperature. Of particular

relevance to the Dampier Archipelago is the combination of bleaching and sedimentation, as

bleached corals are unable to utilise mechanisms of sediment rejection such as ciliary andtentacular activity, and mucous production.


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The 4 to 5 m spring tides at Dampier, while much lower than those further northeast at

Broome, are nevertheless 5 times greater than the tides south of Exmouth. The gently-sloping

seabed in much of the coastal region results in large expanses of sand and rock which are

alternately exposed and inundated twice-daily, influencing the distribution and survival of thebenthic biota in the intertidal and shallow subtidal zones. In extreme conditions, temperature

(both high and low) and heavy rainfall events can result in thermal or haline stress to the marine

organisms inhabiting these regions, although it should be noted that low tides do not occur

during the middle of the day in summer.

Tropical cyclones probably provide the most energetic events on the NWS, with the potential

to devastate coastal structures and communities. The destructive winds associated with cyclones

are generally accompanied by heavy rains, large coastal sea level surges and high waves. Wave-

induced turbulence can lead to substantial movements in coastal sediments, which can then

smother marine organisms and important benthic habitats. The large waves can also directly

dislodge and damage corals and other benthic communities, while the integrity of coastal

infrastructure can be threatened by the combined destructive effects of violent winds and

associated waves (Forde, 1985; Massel and Done, 1993; Sherwood et al., 1999).

The issue of upwelling (and consequently nutrient supply to the Archipelago) is still a matter

of debate, first addressed by Schott (1933) who suggested that there may be some upwelling

along the NWS in autumn but not in summer. It is clear that there is no large-scale, seasonally-

persistent upwelling of nutrient-rich water off Western Australia as occurs in the Benguela

Current system off southwestern Africa and the Humboldt Current off the west coast of South

America (Pearce 1991). Rochford (1977) examined the available information on biological

enrichment on the NWS and concluded that there was no clear evidence for upwelling.

Nevertheless, episodical penetration of nutrient-rich water onto the outer shelf, whatever the

mechanism, clearly contributes to the local phytoplankton production (Tranter and Leech,1987). As mentioned earlier, current measurements on the open shelf (Holloway et al., 1985;

Holloway, 1994) have suggested that wind-induced upwelling processes (on a range of temporal

and spatial scales) may operate along the outer shelf during the summer months but may not

necessarily result in nutrient enrichment there, and the extent to which these processes may

influence the near-coastal waters is presently unknown.

ACKNOWLEDGEMENTS

SeaWiFS (courtesy of the Orbital Sciences Corporation) and AVHRR satellite imagery were

provided by the Western Australian Satellite Technology and Applications Consortium(WASTAC). Andrew Heyward (Australian Institute of Marine Science) and James Gilmour

(University of Western Australia, Zoology Department) provided timely information on the

corals at Dampier.

Sea levels for Dampier were supplied by the National Tidal Facility, The Flinders University

of South Australia, Copyright reserved, courtesy of Paul Davill. Oceanographic and

meteorological data (both historical and for the period of the Workshop) were provided by WNI

Oceanographers & Meteorologists (Steve Buchan) on behalf of Woodside Energy Ltd, while the

Western Australian Department of Transport (Tony Lamberto) supplied tidal data for the

Dampier Workshop. Kevin Smith (Bureau of Meteorology) extracted information about tropical

cyclones in the Dampier region. Circulation and particle track simulations presented in thispaper were conducted for the Department of Conservation and Land Management by WNI


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48 A. PEARCE et al.

Oceanographers & Meteorologists. Ms Muriel Salpetier (Waters and Rivers Commission)

supplied information on the Fortescue river flows.

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