Characterizing the Hydrodynamics of Jurien Bay, Western Australia

Kellie Holloway

June 2006

Professor Charitha Pattiaratchi ------This dissertation is submitted in partial fulfillment of the requirements for the Bachelor of Engineering (Applied Ocean Science) ------

Abstract

Abstract

Jurien Bay is a small town on the Central West Coast of Western Australia. With an expanding economy based on fishing, aquaculture and tourism and proximity to Perth, Jurien Bay is expected to grow rapidly over the coming years. Increasing anthropogenic pressure has the potential to compromise the currently pristine natural environment. The livelihood of the town and the economy relies on the quality of the marine environment in particular; hence it is important to understand the dynamics of the system.

This study investigates the characteristics of circulation in Essex Lagoon, a deep basin to the south of the main Jurien Bay settlement. This area is a particularly important for investigation due to the future impact of the adjacent Ardross Estates development and the aquaculture zone located within Essex Lagoon.

An Acoustic Doppler Current Profiler (ADCP) was deployed in summer 2006 to obtain profiles of current velocity and magnitude over three weeks in Essex Lagoon. This data complemented similar data collected during winter 2002 in Essex Lagoon and was used to carry out a seasonal comparison of currents, to characterize circulation patterns and to examine the potential for outside forcing such as atmospheric pressure systems and the Leeuwin Current to influence circulation.

A distinct seasonality was found in the circulation characteristics between summer and winter and this was primarily influenced by seasonality in the wind field. Circulation in summer is dominated by the effect of the diurnal sea breeze, while circulation in winter is influenced by the passing of winter storms. Other mechanisms found to affect currents and circulation were seiching, tides to a small extent and meteorology. It is thought that under extended periods of calm winds, flushing and mixing may become restricted.

The influence of the Leeuwin Current was also detected in the nearshore waters by advection. The incursion of Leeuwin Current waters into Jurien Bay is thought to be linked to the wind field and to characteristic meanders and eddies to the north, near the Abrolhos Islands.

Future directions for this work include linking the known circulation characteristics with density characteristics (salinity, temperature) in order to better understand the stratification and mixing and the influence of the Leeuwin Current. Also, determining the annual frequency and duration of events that may lead to periods of limited flushing would be a useful exercise.

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Acknowledgements

The support and assistance of a number of people in contributing to the completion of this project is gratefully acknowledged.

I would firstly like to thank my supervisor, Professor Charitha Pattiaratchi. Your support, guidance and knowledge throughout the course of this project and throughout my degree are greatly appreciated.

I would also like to thank those who assisted me in Jurien Bay. Melissa Robins helped with deployment; Andrew Tennyson, Peter Quintana and Michael Mulligan (Edith Cowan University) provided assistance with deployment and retrieval of the ADCP, as well as providing the use of their boat. David Fairclough, Steven Moore and Elaine Lek (Murdoch University) provided accommodation in Jurien Bay as well as photos of the moored ADCP and local marine life.

Additional data was supplied by the Bureau of Meteorology and the Department for Planning and Infrastructure’s Coastal Data Centre. Thank you to Philip Kindleysides (Oceanica) and Ray Lawrie (CALM) for supplying aerial imagery of Jurien Bay. My gratitude is also extended to Alan Pearce (formerly CSIRO), who provided several key references and expert advice when sought.

To all my friends and family who have been supportive and encouraging over the past 12 months, thank you so much! I would especially like to thank the special crew from ‘Team Ocean’ with whom I have shared the past four and a half years of learning.

Finally, I would like to dedicate my achievement to my parents. You have provided me with every opportunity for success, and unconditional support and guidance along the way. Thank you!

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iv Table of Contents

ABSTRACT...... I ACKNOWLEDGEMENTS ...... III LIST OF TABLES...... VII LIST OF FIGURES...... VII 1 INTRODUCTION ...... 1 1.1 RATIONALE...... 1 1.2 AIMS ...... 2 1.3 BACKGROUND (PREVIOUS STUDIES)...... 4 2 LITERATURE REVIEW ...... 5 2.1 ENVIRONMENTAL SETTING ...... 5 2.1.1 Climate and Meteorology...... 5 2.1.2 Geology and Geomorphology...... 8 2.1.3 Hydrology and Groundwater...... 9 2.1.4 Biological Setting ...... 10 2.2 CULTURAL SETTING...... 12 2.2.1 History...... 12 2.2.2 Administration ...... 14 2.2.3 Land Use and Future Development...... 14 2.2.4 Commercial Activities...... 14 2.2.5 Recreational Activities...... 18 2.2.6 Research...... 19 2.2.7 Conservation...... 21 2.3 OCEANOGRAPHY ...... 26 2.3.1 Regional Circulation...... 26 2.3.2 Hydrodynamic Processes ...... 29 2.3.3 Physical Properties ...... 37 3 METHODS ...... 40 3.1 DATA COLLECTION ...... 40 3.1.1 Currents ...... 40 3.1.2 Sea Temperature...... 42 3.1.3 Navigation/Position Fixing ...... 42 3.1.4 Winds ...... 43 3.1.5 Aerial Imagery...... 43 3.1.6 Bathymetry...... 43 3.1.7 Satellite Imagery ...... 43 3.1.8 Wave and Sea Level Observations ...... 44 3.1.9 Synoptic Charts...... 44 3.2 DATA ANALYSIS...... 44 3.2.1 Currents ...... 44 3.2.2 Temperature ...... 45

v Table of Contents

3.2.3 Analysis of Wind ...... 45 3.2.4 Spectral Analysis ...... 46 4 RESULTS ...... 47 4.1 CURRENTS ...... 47 4.2 SEA TEMPERATURE ...... 53 4.3 WINDS ...... 55 4.4 SATELLITE IMAGERY ...... 59 4.5 WAVES AND SEA LEVEL OBSERVATIONS...... 61 4.6 SYNOPTIC CHARTS ...... 67 4.7 TROPICAL CYCLONE ‘EMMA’...... 68 4.8 SPECTRAL ANALYSIS ...... 68 5 DISCUSSION...... 75 5.1 CURRENTS ...... 75 5.1.1 Currents and Wind...... 76 5.1.2 Currents and Weather...... 77 5.2 TEMPERATURE ...... 78 5.3 SPECTRAL ANALYSIS ...... 80 6 CONCLUSION...... 83 7 RECOMMENDATIONS ...... 85 8 REFERENCES ...... 86 9 APPENDICES...... 89 9.1 CURRENTS AND WIND...... 89 9.2 CURRENT MAGNITUDE ...... 91 9.3 CURRENT ROSES ...... 93 9.3.1 Summer ...... 93 9.3.2 Winter...... 95 9.4 CURRENT COMPONENTS...... 97 9.4.1 Summer ...... 97 9.4.2 Winter...... 98 9.5 SYNOPTIC CHARTS ...... 99 9.5.1 Summer ...... 99 9.5.2 Winter...... 101

vi List of Tables and Figures

List of Tables

TABLE 2-1 ZONING CATEGORY ALLOCATIONS...... 25 TABLE 2-2 FUNDAMENTAL SEICHE PERIODS FOR ESSEX LAGOON DERIVED FROM MERIAN'S FORMULA FOR SEMI-ENCLOSED LAGOONS...... 31 TABLE 2-3 DOMINANT TIDAL CONSTITUENTS (PATTIARATCHI 2005A; BEER 1997)...... 33 List of Figures

FIGURE 1-1 LOCATION OF JURIEN BAY, ADAPTED FROM JURIEN BAY REGIONAL PERSPECTIVE (MARINE PARKS AND RESERVES AUTHORITY 2000)...... 2 FIGURE 1-2 JURIEN BAY AERIAL IMAGE, COURTESY DEPARTMENT OF LAND INFORMATION (CAPTURE) AND THE DEPARTMENT FOR CONSERVATION AND LAND MANAGEMENT (SUPPLY)...... 3 TH FIGURE 2-1 TYPICAL WINTER ANTICYCLONE 6 AUGUST 2002 (BOM 2006A)...... 6 TH FIGURE 2-2 TYPICAL SUMMER ANTICYCLONE 13 MARCH 2006 (BOM 2006A)...... 7 FIGURE 2-3 A TYPICAL WIND FIELD GENERATED BY A TROPICAL CYCLONE (PATTIARATCHI 2005B).... 7 FIGURE 2-4 BATHYMETRY OF JURIEN BAY, VIEW FROM THE NORTH (LEFT) AND FROM THE SOUTH (RIGHT) (CHUA 2002)...... 9 FIGURE 2-5 PROPOSED AQUACULTURE ZONES (EVERALL 1998)...... 17 FIGURE 2-6 ZONING OF THE JURIEN BAY MARINE PARK (CALM & MPRA 2005) ...... 23 FIGURE 2-7 SCHEMATIC FIGURE SHOWING MAJOR CURRENTS OF THE EASTERN INDIAN OCEAN, PARTICULARLY THE LEEUWIN CURRENT AND THE INDONESIAN THROUGHFLOW (SRFME 2005) ...... 28 FIGURE 2-8 SCHEMATIC OF THE PROPAGATION OF A CONTINENTAL SHELF WAVE (PATTIARATCHI 2005A)...... 30 FIGURE 2-9 IDEALISED EKMAN SPIRAL IN THE NORTHERN HEMISPHERE (PATTIARATCHI 2005B). ... 36 FIGURE 3-1 (A) ADCP MOORED IN ESSEX LAGOON FEBRUARY/MARCH 2006; (B) ADCP BEFORE DEPLOYMENT...... 42 TH FIGURE 4-1 CURRENT VELOCITY IN N-S, E-W COMPONENTS FOR JURIEN BAY – 27 FEBRUARY – TH 20 MARCH 2006...... 48 TH TH FIGURE 4-2 CURRENT VELOCITY IN N-S, E-W COMPONENTS FOR JURIEN BAY – 30 JULY – 12 AUGUST 2002...... 49 TH TH FIGURE 4-3 SUMMER SURFACE CURRENT HISTOGRAM FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006...... 50 TH TH FIGURE 4-4 SUMMER SEABED CURRENT HISTOGRAM FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006...... 50 TH TH FIGURE 4-5 SUMMER SURFACE CURRENT HISTOGRAM FOR JURIEN BAY – 30 JULY - 12 AUGUST 2006...... 51 TH TH FIGURE 4-6 SUMMER SEABED CURRENT HISTOGRAM FOR JURIEN BAY – 30 JULY - 12 AUGUST 2006...... 52 TH TH FIGURE 4-7 SEABED WATER TEMPERATURE FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006 ...... 54 TH TH FIGURE 4-8 SEABED WATER TEMPERATURE FOR JURIEN BAY 30 JULY – 12 AUGUST 2002...... 55 TH TH FIGURE 4-9 WIND VELOCITY AND DIRECTION FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006 (BOM 2006A)...... 56 TH TH FIGURE 4-10 WIND VELOCITY AND DIRECTION FOR JURIEN BAY – 30 JULY – 12 AUGUST 2006 (BOM 2002)...... 57 TH TH FIGURE 4-11 WIND VELOCITY AND DIRECTION HISTOGRAM FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006 (BOM 2006A)...... 58 TH TH FIGURE 4-12 WIND VELOCITY AND DIRECTION HISTOGRAM FOR JURIEN BAY – 30 JULY – 12 AUGUST 2002 (BOM 2002) ...... 59 FIGURE 4-13 SATELLITE SEA SURFACE TEMPERATURE IMAGE OF THE WESTERN AUSTRALIAN COAST TH – 5 MARCH 2006 (CSIRO 2006) ...... 60 FIGURE 4-14 SATELLITE SEA SURFACE TEMPERATURE IMAGE OF THE WESTERN AUSTRALIAN COAST TH – 16 MARCH 2006 (CSIRO 2006) ...... 61 TH TH FIGURE 4-15 WAVE HEIGHTS FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006 (DPI 2006)62 TH TH FIGURE 4-16 WAVE HEIGHTS FOR JURIEN BAY – 30 JULY – 12 AUGUST 2002 (DPI 2006) ...... 63

vii List of Tables and Figures

TH TH FIGURE 4-17 SEA LEVEL OBSERVATIONS FOR JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006 (DPI 2006)...... 64 TH TH FIGURE 4-18 WATER LEVEL OBSERVATIONS OBTAINED FROM THE ADCP – 27 FEBRUARY – 20 MARCH 2006...... 65 TH TH FIGURE 4-19 SEA LEVEL OBSERVATIONS FOR JURIEN BAY – 30 JULY – 12 AUGUST 2002 (DPI 2006)...... 66 TH TH FIGURE 4-20 WATER LEVEL OBSERVATIONS OBTAINED FROM THE ADCP – 30 JULY – 12 AUGUST 2002...... 67 FIGURE 4-21 SYNOPTIC IMAGE OF TROPICAL CYCLONE EMMA (BOM 2006A)...... 68 FIGURE 4-22 SPECTRAL ANALYSIS OF NORTH-SOUTH SURFACE AND SEABED CURRENTS FOR JURIEN TH TH BAY – 28 FEBRUARY – 20 MARCH 2006...... 69 FIGURE 4-23 SPECTRAL ANALYSIS OF EAST-WEST SURFACE AND SEABED CURRENTS FOR JURIEN TH TH BAY – 28 FEBRUARY – 20 MARCH 2006...... 70 TH FIGURE 4-24 SPECTRAL ANALYSIS OF ADCP WATER LEVELS FOR JURIEN BAY – 28 FEBRUARY – TH 20 MARCH 2006...... 71 FIGURE 4-25 SPECTRAL ANALYSIS OF NORTH-SOUTH SURFACE AND SEABED CURRENTS FOR JURIEN TH TH BAY – 30 JULY – 12 AUGUST 2002...... 72 FIGURE 4-26 SPECTRAL ANALYSIS OF EAST-WEST SURFACE AND SEABED CURRENTS FOR JURIEN TH TH BAY – 30 JULY – 12 AUGUST 2002...... 73 TH TH FIGURE 4-27 SPECTRAL ANALYSIS OF ADCP WATER LEVELS FOR JURIEN BAY 30 JULY – 12 AUGUST 2002...... 74 FIGURE 5-1 WINTER SEA TEMPERATURES – JURIEN BAY, SHOWING THE INFLUENCE OF THE LEEUWIN CURRENT (D'ADAMO & MONTY 1997)...... 79 TH FIGURE 9-1 CURRENT VELOCITY AND DIRECTION WITH DEPTH AND TIME AT JURIEN BAY – 27 TH FEBRUARY – 20 MARCH 2006...... 89 TH FIGURE 9-2 CURRENT VELOCITY AND DIRECTION WITH DEPTH AND TIME AT JURIEN BAY – 30 JULY – TH 12 AUGUST 2006 ...... 90 TH FIGURE 9-3 NON-DIRECTIONAL CURRENT VELOCITY WITH DEPTH AND TIME AT JURIEN BAY – 27 TH FEBRUARY – 20 MARCH 2006...... 91 TH FIGURE 9-4 NON-DIRECTIONAL CURRENT VELOCITY WITH DEPTH AND TIME AT JURIEN BAY – 30 TH JULY – 12 AUGUST 2002...... 92 FIGURE 9-5 CURRENT ROSES FOR FEBRUARY/MARCH 2006 FOR (A) 1.73M, (B) 2.23M, (C) 2.73M, (D) 3.23M, (E) 3.73M, (F) 4.23M, (G) 4.73M, (H) 5.23M, (I) 5.73M, (J) 6.23M, (K) 6.73M, (L) 7.23M, (M) 7.73M ABOVE THE SEABED...... 94 FIGURE 9-6 CURRENT ROSES FOR JULY/AUGUST 2002 FOR (A) 1.23M, (B) 2.03M, (C) 3.03M, (D) 4.03M, (E) 5.03M, (F) 6.03M, (G) 7.03M, (H) 8.03M, (I) 9.03M ABOVE THE SEABED...... 96 TH TH FIGURE 9-7 CURRENT VELOCITY IN N-S, E-W COMPONENTS AT JURIEN BAY – 27 FEBRUARY – 20 MARCH 2006 ...... 97 TH TH FIGURE 9-8 CURRENT VELOCITY IN N-S, E-W COMPONENTS AT JURIEN BAY – 30 JULY – 12 AUGUST 2002...... 98

viii Introduction 1 Introduction

1.1 Rationale

Jurien Bay is a small town located 270 km north of Perth on the Central West Coast of Western Australia. As the designated regional centre of the Central West Coast, Jurien Bay is predicted to grow rapidly in the forthcoming years as a commercial entity and popular tourist destination. This rapid growth will inevitably place increased anthropogenic pressure on the natural environment.

In Jurien Bay, the marine environment is integral to all aspects of the community and the economy and it is important that development impacting the quality of this valuable resource is appropriately managed. Currently in pristine condition and with unique habitats and biodiversity, it has been identified as representative of the Central West Coast and included within the Jurien Bay Marine Park, which extends from Wedge Island to Green Head (Figure 2-6). Factors potentially affecting the quality include the addition of excess nutrients to marine waters, disturbance of habitats due to greater population pressure and the inherent problem of human waste disposal. The problem of excess nutrients is particularly important, as Western Australian waters are naturally oligotrophic. Thus any small change in nutrients can be of great consequence.

There is a unique opportunity in Jurien Bay for sustainable growth of the human population by managing anthropogenic stress as it occurs. A good understanding and predictive capacity of the mixing and circulation of nearshore marine waters is crucial. The benefits of detailed knowledge of the characteristics of the local oceanography are many and varied. Information such as flushing characteristics, the fate of excess nutrients and contaminants and larvae dispersal characteristics assist with zoning boundaries for the marine reserve and appropriate sites for aquaculture, coastal structures and wastewater outlets. Benefits extend to activities such as commercial and recreational fishing effort, maritime transport, mineral resource development, ocean based energy sources and naval defence.

The focus area of this study is Essex Lagoon, south of the main township of Jurien Bay. This area is currently a lesser used area of Jurien Bay, however, has potential to be greatly affected by new growth in the future. The current Ardross Estates Development is adjacent to Essex Lagoon and the designated aquaculture development area encompasses Essex Lagoon. The purpose of this study therefore,

1 Introduction is to characterize the circulation characteristics in Essex Lagoon in order to assist in management in the future.

1.2 Aims

To address the rationale presented above, the aims of this project are to:

1. Obtain data to examine the seasonality of currents in Jurien Bay;

2. Characterize the water circulation patterns in Essex Lagoon, Jurien Bay;

3. Examine the potential for outside influences such as weather systems and the Leeuwin Current to affect circulation in Jurien Bay.

Figure 1-1 Location of Jurien Bay, adapted from Jurien Bay Regional Perspective (Marine Parks and Reserves Authority 2000)

2 Introduction

Figure 1-2 Jurien Bay aerial image, courtesy Department of Land Information (capture) and the Department for Conservation and Land Management (supply).

3 Introduction 1.3 Background (Previous Studies)

Three major previous studies have investigated the hydrodynamics of Jurien Bay. These have involved monitoring and modeling of short winter and summer periods and inferred scenarios for spring and autumn.

In 1997, the Department for Conservation and Land Management (CALM) conducted model simulations and collected field data with the intent of characterizing the wind driven circulation and salinity-temperature fields in Jurien Bay. Data was collected from 28 January to 6 February 1997 in order to obtain direct measurements of currents in Jurien Bay and to provide validation for current predictions from the numerical hydrodynamic Hamburg Shelf Ocean Model (HAMSOM). A secondary aim was to observe vertical and horizontal temperature, salinity and density stratification during summer. Results from the study indicated that salinity and temperature were significantly higher in the nearshore zone than the shelf zone. Mixing analysis revealed that typical summer afternoon sea breezes would fully mix the water column eliminating vertical temperature, salinity or density stratification that may have developed. Currents were found to be primarily wind driven and that near bottom flows in deep basins were very weak and at times, in the opposite direction to the wind (D'Adamo & Monty 1997).

Also in 1997, a PhD thesis was conducted by Dr Peta Sanderson on the coastal geomorphology of Jurien Bay. As part of this study, the waves and wind driven currents were studied in summer and winter to investigate the influence of hydrodynamics on sediment resuspension and transport. Results indicated that under prevailing summer south-west to south-east winds, currents were approximately northward, and influenced by topography. Spectral analysis revealed a dominance in diurnal summer wind patterns, as well as evidence of the effect of passing anticyclonic high pressure systems, continental shelf waves and seiching within the lagoon. Sanderson also concluded that currents in Jurien Bay were primarily wind driven with low variability (Sanderson 1997).

In 2002, James Chua conducted a hydrodynamic modeling study validated using winter current data collected with an Acoustic Doppler Current Profiler (ADCP). Flushing times were modeled for spring/summer, autumn and winter. Particle tracks highlighted Favourite Lagoon as a potential area of recirculation, indicating potential for poor flushing. As in other studies, the coastal circulation patterns were concluded to be predominantly wind driven (Chua 2002).

4 Literature Review

2 Literature Review

This section provides an overview of the known physical, biological, oceanographic, social and cultural attributes of Jurien Bay.

2.1 Environmental Setting

The environmental setting of Jurien Bay includes the climate and meteorology, the geological and geomorphological characteristics, the hydrology and groundwater and the biological characteristics.

2.1.1 Climate and Meteorology

Jurien Bay experiences a typical Mediterranean climate of hot, dry summers and cool wet winters. During winter, mild weather and light winds are interspersed with occasional north-westerly gales and storms due to periodic low pressure cyclonic systems from the south. Fine summer conditions are occasionally interrupted by strong winds and heavy rain resulting from periodic low pressure tropical cyclones from the north (D'Adamo & Monty 1997; Marine Parks and Reserves Authority 2000). Air temperatures range from an average maximum temperature of 30.7 °C in February to an average minimum temperature of 9.4 °C in August (BOM 2006a). Maximum average monthly rainfall is 115.3 mm in July and minimum 6 mm in December (BOM 2006a).

Wind conditions along the coast of Western Australia display a distinct seasonality. Typical summer wind conditions in Jurien Bay comprise a moderate to strong afternoon seabreeze cycle, often exceeding 8 m/s following moderate offshore morning easterlies. An analysis of Jurien Bay winds in 1997 indicated south to south- westerly winds prevailing for the past 25 years (Sanderson 1997). The predominant summer easterlies and southerlies result in a net northward wind stress, further amplified by the afternoon sea breeze. In winter, winds and therefore wind stress is far more variable, with the passing of winter storms, and have been found to be northerly to westerly for almost half the observations. Strong wind events exceeding 8 m/s are predominantly from the southwest (Sanderson 1997).

The diurnal sea breeze system is a significant meteorological phenomenon along the Western Australian coast. It is generated due to the differing thermal properties of land and sea. While the land mass both absorbs and releases heat very quickly, the water mass gains and releases heat at a much slower rate. The diurnal heating and

5 Literature Review cooling of the land mass creates pressure differences which result in the sea breeze during heating and the opposing land breeze during cooling. In Western Australia, the sea breeze is unique, in that it blows shore parallel, as opposed to the idealized cross-shore sea breeze due to a complex relationship between the sea breeze and synoptic weather patterns. Sea breezes in Western Australia have also been found to be some of the strongest winds worldwide, often reaching storm intensity (Pattiaratchi et al. 1997). In Jurien Bay, low tidal ranges and the shelter of the offshore reef system result in low wave energy within the nearshore zone. Hence, local wind waves and currents generated by the sea breeze are influential on the circulation patterns. While their influence on coastal hydrodynamics has not been extensively investigated, studies in Perth coastal waters found that sea breezes affect currents almost instantaneously, significantly deepening the mixing layer (Pattiaratchi et al. 1997). It is generally accepted that due to high wind intensities and low tidal ranges in Western Australia, the sea breeze is an important driving force of coastal hydrodynamics (Pattiaratchi et al. 1997).

Western Australian coastal weather is influenced by the seasonal movement of a belt of anticyclonic high pressure systems (HPS) traveling eastward with a period of 7 – 10 days (Breckling 1989; Marine Parks and Reserves Authority 2000; Sanderson 1997). In winter, the belt is centred around 30° S resulting in cool, moist westerly winds (Figure 2-1). Cold fronts occur amid successive passing high pressure cells result in episodic north-west to south-west gales (Mills et al. 1996). During summer, the belt is centred around 40° S, resulting in dry easterlies and fine, warm weather (Figure 2-2) (Marine Parks and Reserves Authority 2000).

Figure 2-1 Typical winter anticyclone 6th August 2002 (BOM 2006a)

6 Literature Review

Figure 2-2 Typical summer anticyclone 13th March 2006 (BOM 2006a)

Tropical cyclones are small, intense low pressure cells which periodically cross the north-west coast of Western Australia during the summer months. They develop over the ocean and propagate fairly unpredictably until they meet the coastline (Pugh 2004). At the coastline, they generate high winds and heavy rains often resulting in flooding and property damage. The passage of a tropical cyclone over a coastal boundary can result in storm surges and the generation of continental shelf waves (see section 2.3.2.1) which then have implications for circulation patterns in nearby coastal waters.

Figure 2-3 A typical wind field generated by a tropical cyclone (Pattiaratchi 2005b).

7 Literature Review

2.1.2 Geology and Geomorphology

The Jurien Bay shoreline is aligned generally north-south. Coastal landforms include curved sandy beaches, low dunes, sand promontories, foredune plains, rocky bluffs and headlands and low limestone cliffs. Tamala limestone deposited during the Quaternary period, around 240,000 years ago, underlies yellow quartz sand and surface sand layers of calcium carbonate, derived from marine sediments. Sediments are nutrient poor, porous and water repellent (Marine Parks and Reserves Authority 2000).

The nearshore topography of Jurien Bay (Figure 2-4) is a complex system of shore- parallel Tamala limestone reefs, emergent rocky outcrops, islands and intertidal rock platforms found inside the 20 m depth contour (Marine Parks and Reserves Authority 2000; Sanderson 1997). These form part of the longest temperate limestone reef system in Australia, which stretches from Trigg Island in the south, to Dongara in the north (CALM & MPRA 2005). Offshore wave energy is largely dissipated on these reefs by wave breaking, refraction, diffraction and reflection, creating a sheltered, shallow (<10 m depth) lagoonal environment landward of the reef (Sanderson 1997; CALM 2005; Marine Parks and Reserves Authority 2000). Inshore, shore perpendicular sand spits and sand banks have developed in the lee of the offshore islands and reef, the largest of which occurs at Island Point in the lee of Boullanger Island. Deep basins (>10 m depth) occur immediately north and south of Island Point in Favourite Lagoon and Essex Lagoon (CALM & MPRA 2005; Marine Parks and Reserves Authority 2000). Essex Lagoon is located 3 km to the south of Island Point with an average depth of 10 m and is 4 km wide and 4 km in length (Chua 2002).

8 Literature Review

Figure 2-4 Bathymetry of Jurien Bay, view from the north (left) and from the south (right) (Chua 2002).

2.1.3 Hydrology and Groundwater

Jurien Bay is influenced by two distinct drainage basins. The Hill River estuary is halfway between the Cervantes and Jurien Bay townsites and originates in the Dandaragan Plateau. The catchment area is around 692 km2 and flow is erratic and seasonal. The estuary has a barred entrance which usually restricts exchange with the ocean, however, occasionally the river will break through the barrier, resulting in significant freshwater discharge (Marine Parks and Reserves Authority 2000).

Small creeks make up the second drainage basin in Jurien Bay. These creeks do not reach the ocean directly, but accumulate in coastal wetlands. Some of this water drains into the sea by percolating through the underlying porous limestone (Marine Parks and Reserves Authority 2000).

Both deep and shallow aquifers occur in the Jurien Bay region. These provide a valuable potable water source for the region (Marine Parks and Reserves Authority 2000). Flow from these aquifers is generally seaward and results in ground water discharge at the shoreline. A corresponding salt water wedge intrudes inland beneath the fresh water layer (Marine Parks and Reserves Authority 2000). Saline groundwater is a potentially valuable resource for the aquaculture industry (Everall 1998).

In general, coastal waters in Jurien Bay are of very high quality. Water quality surveys conducted in February 2004 detected low metal concentration and no

9 Literature Review organic chemicals. The main sources of contaminants originate from agricultural run off, stormwater drainage and harbours and marinas (McAlpine et al. 2005). Western Australian waters are considered to be oligotrophic and therefore particularly susceptible to environmental nutrient increases from agriculture, industrial and urban land use, wastewater discharge, contaminated groundwater and riverine discharge (Lourey et al. 2006). The circulation, mixing and stratification of the water column is an important control on the local impact of elevated nutrient concentrations (Thompson & Waite 2003).

2.1.4 Biological Setting

Western Australia consists of 18 distinct marine biogeographical regions. Jurien Bay falls within the Central West Coast marine bioregion, spanning approximately 600 km from Kalbarri in the north, to Rottnest Island in the south. The Central West Coast region includes the greatest diversity of seagrasses in Australia and one of the largest temperate limestone reef systems. A diverse mix of both tropical and temperate species exist due to the influence of the Leeuwin Current from the north and the Capes Current from the south (Marine Parks and Reserves Authority 2000). Jurien Bay is considered to be broadly representative of species and habitats of the Central West Coast and this fact provided part of the rationale for the location of the Jurien Bay Marine Park.

A wide variety of marine habitats in Jurien Bay support an abundance of tropical and temperate flora and fauna. Landward of the limestone reef the dominant habitats are the subtidal and intertidal limestone reefs, bare sand and seagrass meadows, but these also include rocky shores with wide wave cut platforms, limestone pavements and submerged sandbanks (Marine Parks and Reserves Authority 2000; Sanderson 1997). In Jurien Bay, an important source of nutrients to the nearshore zone is the breakdown of wrack material which naturally strands itself on the shoreline.

A biological survey conducted by CALM in April/May 1997 recorded a total of 400 species, of which around 35% were of tropical origin. Nine species of seagrass were identified, 134 species of large algae, 205 invertebrate species and 63 species of fish. Among these, unique species not previously observed, endemic species and species at the extent of their geographical range were discovered (Marine Parks and Reserves Authority 2000).

There are a number of faunal species of special significance in and around Jurien Bay. These include several species of cetacean (whales, dolphins and porpoises),

10 Literature Review seals and sea lions, marine seabirds, waders and shore birds, leafy sea dragon, Great White Sharks and the Western Rock Lobster (Marine Parks and Reserves Authority 2000).

Several species of whales and dolphins frequent the Jurien Bay region, including the Killer Whale (Orcinus orca), Bottle-nosed Dolphin (Tursiops truncates) and Common Dolphin (Delphinus delphis). Baleen whales are known to be present off Jurien Bay in deep waters, including the Great Blue Whale (Balaenoptera musculus), Humpback Whale (Megaptera novaeangliae) and Southern Right Whale (Eubalaena australis). Many species are listed as rare or endangered and are protected under Western Australian legislation. Their presence provides the potential for nature based tourism ventures such as whale and dolphin watching, especially during annual southern and northern migration periods (Marine Parks and Reserves Authority 2000).

The Australian Sea Lion (Neophoca cinerea) resides in the Jurien Bay area, using nearby Buller, North Fisherman and Beagle Islands as breeding sites. Jurien Bay is the main breeding area of the Central West Coast, with a small population at the Abrolhos Islands. The Australian Sea Lion is endemic to Australia and is specially protected under state wildlife legislation (Marine Parks and Reserves Authority 2000). There are currently two commercial tourist operators offering visitors to Jurien Bay the opportunity to swim with sea lions.

The small islands and rocky outcrops off Jurien Bay provide undisturbed breeding grounds for a number of sea birds, free from introduced ground predators and human disturbance. Many species of birds, including the Fairy Tern (Sterna nereis), Crested Tern (Sterna bergii) and Pied Cormorant (Phalacrocorax varius), have been found nesting on the offshore islands. There are also several species of shore birds on the mainland due to a wide variety of suitable habitats. Beaches, local wetlands (including the Hill River estuary), salt lakes and blackwater wetlands provide nesting sites for species such as the Red Caped Plover (Charadrius ruficapillus) and the Hooded Plover (Esacus magnirostris), a species considered vulnerable by Birds Australia.

The Leafy Sea Dragon (Pycodurus eques) and Great White Shark (Carcharodon carcharias) are protected species under the Fish Resources Management Act 1994. Unconfirmed sightings of the Leafy Sea Dragon have been reported in the Jurien Bay region. Similarly, the Great White Shark is occasionally sighted in the area (Marine Parks and Reserves Authority 2000).

11 Literature Review

The Western Rock Lobster is the most valuable single species fishery in Australia, accounting for around 20 % of the total value of Australian fisheries (Department of Fisheries 2004). Lobsters spawn and hatch in deep waters (> 40 m) and larvae then spend 9-11 months in the deep ocean (900-1500 km offshore) before being carried back onshore by ocean currents. Larvae then metamorphose into puerulus (smooth, transparent, miniature lobster) and swim, with the assistance of waves and currents, to settle on inshore reefs where they moult into juveniles. Between November and January every year, juveniles migrate to offshore reefs where they mature and spawn at around six – seven years of age (Department of Fisheries 2004). The extensive limestone reef systems and seagrass meadows at Jurien Bay provide ideal conditions for the life cycle of the Western Rock Lobster. Puerulus settlement is affected by environmental conditions, such as the strength of the Leeuwin Current, temperature and storm conditions (Department of Fisheries 2004). For this reason, knowledge of the oceanography of an area is an important consideration for management of the Western Rock Lobster fishery.

2.2 Cultural Setting

2.2.1 History

2.2.1.1 Indigenous Heritage

Jurien Bay is a culturally significant area for the Nyoongar Aboriginals who inhabitated the area for more than 30,000 years. At least 36 Aboriginal sites have been identified between Guilderton and Dongara and there is evidence to suggest that limestone caves in the area were occupied by Aborigines in the past. Midden deposits have been identified mainly comprising shellfish and fish remains, indicating these were an integral food source. Aboriginal burial sites have been discovered in the coastal dunes in the area (Marine Parks and Reserves Authority 2000).

2.2.1.2 Maritime Heritage

The first white human settlement along the Central West Coast is believed to be established by the survivors of the wrecked Dutch East Indies ship the ‘Gilt Dragon’ in 1656, 150 km south of Jurien Bay. Leeman, around 40 km north of Jurien, is named after the Dutch navigator Abraham Leeman whose Dutch East Indian ship ‘Watch Buoy’ was sent to search for survivors of the ‘Gilt Dragon’ in 1658. In 1801, a French scientific and exploration expedition charted the Central West Coast. As a result of this mission, Jurien was named in honour of French naval administrator Charles Marie Jurien. Following the settlement of the Swan River Colony in 1829,

12 Literature Review shipping along the Western Australian coast increased. Poor charts, unpredictable weather and unknown currents made navigation difficult and the islands and reefs off Jurien were a shipping hazard. There are four known historical shipwrecks in the vicinity of Jurien Bay;

• ‘Cervantes’ – wrecked off Cervantes Island in 1844

• ‘Maid of Lincoln’ – wrecked off Jurien Bay in 1891

• ‘Europa’ – wrecked offshore of the Hill River mouth in 1897

• ‘Lubra’ – a steamship wrecked immediately offshore of the Jurien Bay townsite in 1898 (Marine Parks and Reserves Authority 2000).

2.2.1.3 Squatter Settlements

The small coastal townships of the Central West Coast are generally derived originally from squatter settlements on Crown Land (Ecologia Environmental Consultants 1997). Shacks were originally constructed by farmers who spent holidays on the coast. Cervantes and Green Head are original farmers’ squatter sites, upgraded to legal townsites. After World War II, shacks provided temporary accommodation for cray fishermen during the fishing season. Over the last 25 years, increasing numbers of shacks have been erected along the coast by holiday makers and recreational fishers. In 1988, the Western Australian Government launched a Squatters’ Policy for the progressive removal of squatter shacks. Leases with the Shire of Dandaragan for squatter shacks expired in June 2001, after which shacks were to be demolished and removed.

2.2.1.4 Farming and Fishing

The first agricultural activities in the Jurien Bay hinterland began in the 1850s when millionaire Walter Padbury acquired land for agriculture. Around 1885, a cargo jetty was constructed to facilitate loading of wool, cattle and horse hide onto ships. The functionality of the jetty was compromised by siltation in the early 1900s and a bushfire destroyed the jetty in the 1930s. During World War II, it was rumoured that Japanese submarines were using the sheltered waters of Jurien Bay to surface and recharge. The Australian Army constructed a road into Jurien and stationed soldiers in concrete bunkers near North Head, at the northern extremity of the Bay. This period also gave rise to the origins of the Western Rock Lobster fishery. Rock lobster tails were exported to American troops in the South Pacific and post-war exported

13 Literature Review directly to the United States. The growth of the industry continued and formed the basis of the growth of the town of Jurien Bay (Marine Parks and Reserves Authority 2000).

2.2.2 Administration

Jurien Bay lies within the Shire of Dandaragan and is the Shire’s fastest growing centre. While Dandaragan is the administrative centre of the Shire, Jurien Bay has been designated the major regional centre for the Central West Coast (WAPC 1996).

2.2.3 Land Use and Future Development

The ‘Turquoise Coast Development’ is a current development managed by Ardross Estates Pty Ltd. Property owned by Ardross Estates is cited in the Central Coast Regional Strategy (WAPC 1996) as the key expansion area for Jurien Bay. It covers approximately 2006 ha from the existing townsite south to the Hill River. Currently, the majority of this area consists of natural bushland and sheep grazing areas. The development will include urban, recreational and tourism uses, incorporating internal public open space (EPA 2001).

The development has potential to impact the natural values of the marine environment and adjacent terrestrial environment. Increased use will increase population pressure, coastal structures associated with the development have the potential to change the natural landscape, and storm water and effluent disposal require effective management so as not to compromise the conservation values of the adjacent marine park (EPA 2001). Development in Jurien Bay will require careful planning and management to conserve the pristine environmental and natural values of the area.

2.2.4 Commercial Activities

The major commercial activities based in Jurien Bay are fishing, aquaculture, marine nature-based tourism, petroleum drilling and mineral development.

2.2.4.1 Commercial Fishing

A number of commercial fisheries operate out of Jurien Bay, including commercial wetlining, abalone, shark netting, beach seining for mullet and collection of specimen shells and aquarium fish. The basis of the Jurien Bay economy, however, is the Western Rock Lobster (Panulirus cygnus) fishery. The Western Rock Lobster fishery has the highest value of any commercial, single species fishery in Australia

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(Department of Fisheries 2004; Marine Parks and Reserves Authority 2000) and is of significant value to both the local and state economy. Up to 140 commercial fishing boats operate out of Jurien Bay, Green Head, Cervantes and Wedge with a seasonal catch of, on average, 1.6 million kg live weight (CALM & MPRA 2005). The season runs from November through to June and is sustainably managed using larval recruitment studies, stock density predictions and associated management strategies. A rock lobster larval recruitment monitoring area is located off Boullanger Island and this area is closed to rock lobster fishing. Rock lobster processing facilities exist at Jurien and Cervantes to prepare the catch for export (Marine Parks and Reserves Authority 2000). Taiwan, Japan and China comprise the majority of the international market for Western Rock Lobster, however, there is a significant market for frozen lobster in the USA and an increasing European market. There is also a small, local market (Department of Fisheries 2004).

A small scale finfish commercial fishery exists in Jurien Bay, targeting sharks, West Australian Dhufish (Glaucosoma hebraicum), Pink snapper (Pagrus auratus) and Baldchin groper (Choerodon rubescens). These fish primarily serve the Perth domestic market (Marine Parks and Reserves Authority 2000).

Commercial shell collecting takes place on an occasional basis. There are no resident commercial collectors in Jurien Bay, however collectors from other areas visit from time to time (Marine Parks and Reserves Authority 2000).

2.2.4.2 Aquaculture

A small aquaculture industry currently exists in Jurien Bay, and there is enormous potential for expansion. A number of significant advantages mean Jurien Bay is widely considered a particularly suitable location for aquaculture. Shallow, sheltered lagoons with high water quality, low population pressure and warm sea temperatures (and limited cold upwelling) provide ideal conditions for cage management and high growth rates. Existing infrastructure associated with the existing commercial fishing industry, ample space for expansion and a skilled work base, provide excellent opportunities for efficient growth of the industry (Everall 1998).

The existing aquaculture industry was founded by Jurien Fish Farmers in 1993 based on snapper, black bream and other species consisting of a licensed hatchery in the commercial precinct near the Jurien Boat Harbour and sea cages on mooring licence areas to the south of Island Point. The sea cage structures were designed for local conditions and have been found to successfully withstand a 1 in 50 year storm event.

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Mussel farming ventures originally established south of Island Point were relocated and later discontinued due to poor growth rates. Future growth of the industry requires appropriate planning decisions, further technological advances in feeding techniques and detailed environmental monitoring throughout development (Everall 1998).

Low current velocities and potential for periods of stratification can impede flushing around sea cages. Hence, an understanding of the local hydrodynamics is an important consideration for the development of aquaculture in Jurien Bay. Two special purpose aquaculture zones have been designated in the Jurien Bay Marine Park for development of the aquaculture industry. Area 1, shown in Figure 2-5, is designated for the development of submerged cage technology and higher production techniques. Area 2, located in Essex Lagoon, is the main nearshore zone for sea cage farming and aquaculture operations. Area 2 incorporates a 400m shoreline buffer, however, also encompasses the reef and shallows of Essex Rocks and Three Breaks Reef, hence allowing the potential for the development of mollusc, rock lobster and other methods of culture (Everall 1998).

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Figure 2-5 Proposed aquaculture zones (Everall 1998)

2.2.4.3 Tourism

While the origins of Jurien Bay lie in commercial fishing, a booming tourism industry currently boosts the local economy. Proximity to Perth, pleasant climate, clean

17 Literature Review sheltered waters, sandy beaches, extensive recreational activities and increasing accessibility (Lancelin-Cervantes coastal road link) are fostering an ever expanding tourism industry.

Currently, the peak tourism season occurs in spring and autumn when temperatures are higher than in Perth and the south-west winds are relatively moderate. The visual amenity of the Jurien Bay seascapes, coupled with other local tourist attractions such as wildflowers, the Pinnacles at Cervantes and local national parks make Jurien an increasingly popular destination.

Currently, the majority of tourists are Perth residents. However, with an expanding state tourism industry, demand for unique holiday experiences in proximity to Perth is increasing. Nature-based tourism allow visitors to experience the unique natural environment, while fostering an awareness of the importance of careful management. Whale watching tours operate out of Jurien Bay to coincide with the annual Humpback Whale migration in spring, and other tours provide opportunities to interact with dolphins, the endemic Australian Sea Lion and other wildlife (Tourism Western Australia 2006; Marine Parks and Reserves Authority 2000).

Jurien Bay has a range of tourist accommodation from hotels and motels, caravan parks and self contained cottages and chalets. The Ardross Estates ‘Turquoise Coast Development’ includes opportunities for tourist resort development and other commercial ventures (Tourism Western Australia 2006; Marine Parks and Reserves Authority 2000).

2.2.5 Recreational Activities

Popular recreational activities in the Jurien Bay region include scuba diving, snorkeling, swimming, fishing, boating and water sports, including wind surfing and surfing (CALM 2005).

Recreational fishing is a successful and popular activity for locals and tourists in Jurien Bay. Target species are primarily the Western Rock Lobster, West Australian dhufish (Glaucosoma hebraicum), Pink Snapper (Pagrus auratus), Baldchin Groper (Choerodon rubescens), Abalone (Haliotis sp.) and other finfish and mollusc species There are no restrictions placed on beach fishing along the whole coastline and groynes and jetties provide convenient fishing sites. Line fishing from boats is popular in the lee of the offshore islands and on the offshore limestone reefs. Spearfishing is permitted, and rock lobster and abalone may be collected by hand.

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Western Rock Lobster are also collected with pots. All recreational fishing in Jurien Bay is subject to the management policies of the Jurien Bay Marine Park and is subject to some licence conditions, size restrictions and bag limits (CALM & MPRA 2005; Marine Parks and Reserves Authority 2000).

Jurien Bay provides ideal areas for diving, snorkeling and swimming. The shelter of the lagoon results in low wave energy, and while strong sea breezes do occur, offshore islands, coves and headlands provide ample shelter. The shallow lagoon, crystal clear water and variety of marine life provide good opportunities for snorkeling and SCUBA diving. A dive shop, instructor and tour business operate out of Jurien Bay making SCUBA diving a convenient and enjoyable pastime and an asset to the tourism industry (Marine Parks and Reserves Authority 2000).

Recreational boating is a popular activity for locals and tourists at Jurien Bay. Smaller craft can be easily launched from convenient beach access points, while the Jurien small boat harbour has boat launch facilities for larger vessels. Some restrictions apply to boating in the Jurien Bay area, in the interest of public safety. Calm, sheltered waters, offshore islands, good fishing and naturally sheltered anchorages make Jurien an ideal location for recreational boating (Marine Parks and Reserves Authority 2000).

Other surface water sports available in Jurien Bay include sailing, wind surfing, water skiing, surfing, jet-skiing and parasailing (Marine Parks and Reserves Authority 2000). The suitability of the sheltered waters of Jurien Bay for these activities enhances the potential of the tourism industry (Marine Parks and Reserves Authority 2000).

Coastal land based activities also form the basis of enjoyable recreational activities in Jurien Bay. One of the most popular activities is four-wheel driving along beaches and dunes. Some dune systems around Jurien Bay are scarred by this practice and vulnerable to wind erosion and dune blowout. Such activities require careful management into the future (Marine Parks and Reserves Authority 2000).

2.2.6 Research

Due to the projected growth of Jurien Bay and its inclusion in the Jurien Bay Marine Park, the waters and wildlife have been the focus of a number of scientific studies. One of the major facilitators of these studies is the Commonwealth Scientific and

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Industrial Research Organization (CSIRO) through the Strategic Research Fund for the Marine Environment (SRFME).

SRFME is a $20 million research program funded jointly by the Western Australian Government and CSIRO. The goals of the program are to build capability and capacity in marine science in Western Australia; to facilitate strong collaboration among the Western Australian marine science community; and to conduct fundamental and strategic research of benefit to Western Australia and Australia (SRFME 2005). The strategic research approach allows a baseline understanding of Western Australia’s marine ecosystems and how they function over time. Key governmental agencies involved in the SRFME program include the Department of Fisheries, the Department of Conservation and Land Management (CALM), the Department of Environment (DOE), the Department of Industry and Resources (DOIR) and the Office of Science and Innovation (SRFME 2005).

The SRFME program encompasses two main components; Core Projects and Collaborative Linkages. Three Core Projects are carried out by CSIRO marine research staff aiming to meet strategic information requirements for the Western Australian Government. Collaborative linkage projects are carried out in conjunction with PhD students from the four publicly funded Western Australian universities, post doctoral scientists and Western Australian government agencies. The collaborative linkage projects aim to extend the range of research outcomes of SRFME, as well as to complement the core projects (SRFME 2005). A number of research components are being carried out in and around Jurien Bay. The Central West Coast, in particular Jurien Bay, provides unique conditions for research given the diversity of species, the pristine environmental conditions and the management strategies of the Marine Park.

The three SRFME Core Projects are:

• Biophysical Oceanography off Western Australia: Dynamics Across the Continental Shelf and Slope – CSIRO WA CSIRO Tasmania, University of Western Australia (UWA), Murdoch University, Curtin University of Technology, DOE;

• Coastal Ecosystems and Biodiversity – CSIRO WA, CSIRO Tasmania, Curtin University of Technology, UWA, Flinders University;

• Integrated Modelling – CSIRO WA.

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Collaborative Linkage Projects with relevance to Jurien Bay include:

• The Development and Validation of Algorithms for Remotely Sensing Case II Waters – Wojciech Klonowski – Curtin University of Technology;

• Ecological Interactions in Coastal Marine Ecosystems: The Fish Communities and Main Fish Populations of the Jurien Bay Marine Park – Professor Ian Potter, Dr David Fairclough – Murdoch University;

• Ecological Interactions in Coastal Marine Ecosystems: Trophodynamics – Dr Glenn Hyndes – ECU;

• Ecological Interactions in Coastal Marine Ecosystems: Rock Lobster – Dr Glenn Hyndes – ECU;

• Biodiversity of Marine Fauna on the Central West Coast – Dr Fred E. Wells – Western Australian Museum;

• Ecophysiology of Benthic Primary Producers – Associate Professor Paul Lavery (ECU), Dr Russ Babcock (CSIRO);

• Establishing Reference and Monitoring Sites to Assess a Key Indicator of Ecosystem Health (seagrass health) on the Central West Coast of Western Australia – Adjunct Professor Ray Masini – Department of Environment;

• Baseline Biodiversity Monitoring in the Proposed Jurien Bay Marine Park – Dr Chris Simpson - CALM

The natural values of Jurien Bay are a valuable educational resource. Nature-based tourism ventures provide opportunities for educational experience for tourists and the variety of natural and cultural resources provide opportunities for hands-on experience for students in a range of disciplines including geology, coastal geomorphology, marine and terrestrial biology, Aboriginal history, minerals and energy, agriculture and aquaculture (Marine Parks and Reserves Authority 2000).

2.2.7 Conservation

The Jurien Bay Marine Park was formally declared on 26 August 2003 as a multiple use marine park which aims to preserve representative ecosystems, while supporting recreational and commercial activities ‘equitably and sustainably’. Management of

21 Literature Review the Park is entrusted to the Marine Parks and Reserves Authority and the Department of Conservation and Land Management, while fishing and aquaculture is managed in cooperation by the Department of Fisheries (CALM 2005). The Park is bounded to the west by the limit of Western Australia’s territorial waters and to the east by the low water mark. Dynamite Bay at Green Head marks the northern boundary of the park, extending south to Wedge Island, covering a total area of 82,375 ha (CALM & MPRA 2005). The management strategies and objectives are detailed in the Jurien Bay Marine Park Management Plan 2005-2015 (CALM & MPRA 2005).

The vision statement for the Jurien Bay Marine Park is

“In the year 2025, the marine flora and fauna, habitats and water quality of the Jurien Bay Marine Park will be in the same or better condition than in the year 2005. The area will support viable and ecologically sustainable fishing, aquaculture, recreation and nature-based tourism and the marine park will be considered an important asset by the local community.”

The Jurien Bay Marine Park is zoned into classified areas, as required by Section 13B(2) of the CALM Act 1984. Areas and number of zones of each category are shown in Table 2-1. The zones of the Jurien Bay Marine Park are shown in Figure 2-6.

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Figure 2-6 Zoning of the Jurien Bay Marine Park (CALM & MPRA 2005)

Sanctuary zones offer the highest level of protection by excluding human activities that may damage the environment. They provide protection for vulnerable and specially protected species as well as conserving representative habitats to maintain relatively undisturbed environments. Passive recreational activities may be permitted,

23 Literature Review however, extractive activities such as fishing, hunting and mineral exploration are not. Sanctuary zones in the Jurien Bay Marine Park are located at Fisherman Islands, North Head, Pumpkin Hollow, Boullanger Island, Booker Rocks, Nambung Bay, Cavanagh Reef, Grey, Target Rock and Wedge Island (CALM & MPRA 2005).

Special purpose zones allow a particular priority use, for example aquaculture. Activities which are incompatible with the particular priority use are not allowed in special purpose zones. There are four categories of special use zones in Jurien Bay Marine Park.

Scientific Reference special purpose zones exist at Fisherman Islands, Hill River and Green Islands. Their purpose is to study natural processes occurring within the Marine Park free of anthropogenic influences and to obtain comparative data to monitor other zones within the Park.

Aquaculture special purpose zones exist at Cervantes Islands, Emu Rocks Seaward Ledge (northwest of Boullanger Island) and Hill River. Their primary purpose is aquaculture; however, other activities are not automatically excluded unless they conflict with the primary use.

Shore Based Activities special purpose zones exist adjacent to the mainland coast in the Boullanger Island Sanctuary Zone, Nambung Bay Sanctuary Zone and two in the North Head Sanctuary Zone. These zones allow shore based activities, including fishing, and exist to recognize the social importance of shore based activities as well as the minimal environmental impact.

One Puerulus Monitoring special purpose zone exists at Boullanger Island. This zone allows monitoring of rock lobster larvae and is an important management tool for the Western Rock Lobster commercial fishery. (CALM & MPRA 2005).

General use zones are areas where ecologically sustainable use is still a priority; however, activities such as fishing, aquaculture and petroleum exploration are permitted. All areas of Jurien Bay Marine Park not zoned as Special Purpose or Sanctuary are general use zones (CALM & MPRA 2005).

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Table 2-1 Zoning Category Allocations

Zoning Scheme Number of Zones Area of Percentage of Zones Marine Park Sanctuary Zone 10 3061 ha 3.7 % Special Purpose – Scientific 3 14037 ha 17 % Reference Special Purpose – 4 1427 ha 1.7 % Aquaculture Special Purpose – Shore 4 52 ha < 1 % Based Activities Special Purpose – Puerulus 1 57 ha <1 % Monitoring General Use 63742 ha 77 %

Conservation of the marine environment within the Marine Park is complemented by various adjacent land reserves along the mainland, such as the Nambung National Park and Wanagarren Nature Reserve, extending to the low water mark. Additionally, the islands within the Marine Park (eg. Boullanger Island) are ‘A Class’ nature reserves. Such adjoining areas are subject to integrated management between the Marine Park and the nature reserves (CALM & MPRA 2005).

There are four national parks and two nature reserves in the Jurien Bay region. Leseur National Park is located east of Jurien Bay in the northern sandplains. The Park consists of complex geology and exceptionally diverse flora, including a number of endemic species. Leseur National Park is one of the most significant areas for conservation in south western Australia, with several endangered species and at least 124 reliant bird species. Nambung National Park is located south of Cervantes, and encompasses the Pinnacles Desert, one of Western Australia’s best known tourist destinations. Nambung National Park also hosts an impressive wildflower display August – October. Two of the smaller national parks, Drovers Cave National Park and Stockyard Gully National Park, are located between the southern boundary of Leseur and the Jurien access road, and east of Leeman, respectively. Beekeepers and Southern Beekeepers nature reserves are located between Cervantes and south of Dongara and are representative of local flora and provide a resource for commercial production of honey (Marine Parks and Reserves Authority 2000).

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2.3 Oceanography

2.3.1 Regional Circulation

2.3.1.1 Leeuwin Current

The Leeuwin Current (Figure 2-7) is a strong (>1 ms-1), narrow (∼30 km) southerly current flowing adjacent to the West Australian coast for approximately 2000 km from the North West Cape, around Cape Leeuwin to the Great Australian Bight (Godfrey & Ridgway 1985). The Leeuwin Current is unique, being a poleward flowing eastern boundary current, distinct from the equator-ward flowing Benguela and Humboldt Currents off the west coast of southern Africa and South America (Pearce 1997). The Leeuwin Current occupies the latitude band of the eastern component of the south Indian Ocean sub-tropical gyre but is not a part of it. The north flowing West Australian Current is displaced further offshore.

The flow of the Leeuwin Current opposes the prevailing south-westerly winds. It is driven by the alongshore pressure gradient created by the connection of the Indian Ocean to the Pacific Ocean via the Indonesian Throughflow (Figure 2-7) (Godfrey & Ridgway 1985). Flow is strongest during autumn and winter and weakens during summer against the prevailing sea-land breeze systems which drive the summer north-flowing counter currents; the Capes Current in the south and Ningaloo Current further north (SRFME 2005).

The seasonal variability of the Leeuwin Current is coupled with a strong inter-annual variability associated with the El Niño Southern Oscillation (ENSO). El Niño years are associated with a weaker Leeuwin Current, while La Niña years pre-empt a stronger Leeuwin Current. During La Niña, the pressure gradient between the Pacific and Indian Ocean is greater, causing an increase in the alongshore pressure gradient driving the Leeuwin Current. Thus, annual strength of the Leeuwin Current can be measured using sea level observations.

The formation of eddies is fairly common within the Leeuwin Current flow, as meanders caused by current instability pinch off and form eddies. These are evident in satellite imagery of the Leeuwin Current. An eddy is generally formed every six – eight weeks and can persist for up to 3 months (Pattiaratchi 2005b). A particular eddy formation of significance to Jurien Bay is the persistent eddy which forms off the Abrolhos Islands. The formation of these eddies were observed in ‘Oceanography

26 Literature Review and circulation pattern of the Zeewijk Channel, Houtman Abrolhos Islands, Western Australia’ (Maslin 2005).

The presence of the Leeuwin Current affects the physical and biological characteristics of Western Australian shelf waters. Originating from the tropics, Leeuwin Current water is warm, low salinity and low in nutrients. Its presence also suppresses upwelling of cold, nutrient rich waters normally found on analogous eastern boundaries (Lourey et al. 2006). Atmospheric heat losses result in milder winters and greater rainfall than might otherwise be expected.

The Leeuwin Current affects the biological characteristics of the Western Australian coast by transporting tropical species further south than their geographic distribution would normally extend (Pearce 1997). Years of stronger Leeuwin Current typically exhibit greater Western Rock Lobster recruitment, but lower recruitment of species such as scallop and pilchard, which prefer cooler conditions. Southern Bluefin Tuna are known to migrate on Leeuwin Current flow.

While the biological impact of the Leeuwin Current in Jurien Bay is evident (diverse array of tropical marine flora and fauna), the impact on hydrodynamics has not been extensively investigated (Marine Parks and Reserves Authority 2000). While ‘tongues’ of Leeuwin Current water often penetrate across the continental shelf, transporting planktonic larvae and elevating local sea temperatures, it is assumed that the constricting effects of reefs, islands and the continental shelf restrict the effect of the Leeuwin Current on circulation and flushing in the nearshore lagoons (CALM & MPRA 2005).

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Figure 2-7 Schematic figure showing major currents of the eastern Indian Ocean, particularly the Leeuwin Current and the Indonesian Throughflow (SRFME 2005)

2.3.1.2 Capes Current

The Capes Current is a narrow, cool, equatorward boundary current which flows north along the coast of south-west Western Australia, shoreward of the 50 m depth contour during summer months (November through March). Flow is induced when strong northward wind stress caused by prevailing southerly winds overcomes the alongshore pressure gradient (Pearce & Pattiaratchi 1999). While the coast of Western Australia has no regular upwelling due to the presence of the Leeuwin Current, the Capes Current induces localized ‘pulses’ of upwelling during strong northward winds, bringing nutrients to the coastal zone. It also restricts the penetration of warm, oligotrophic Leeuwin Current water onto the shelf (Pearce & Pattiaratchi 1999). The Capes Current also plays an important role for fisheries in south-west Australia, assisting the transport of larvae and juveniles north along the WA coast. Of particular importance to the Jurien Bay region is the transport of the

28 Literature Review planktonic larvae of Western Rock Lobster northward toward juvenile nursery areas, allowing successful recruitment (Pearce & Pattiaratchi 1999).

2.3.2 Hydrodynamic Processes

The nearshore currents in Jurien Bay are predominantly influenced by the winds and exhibit distinct seasonality associated with seasonal variations in the coastal wind field (Sanderson 1997; D'Adamo & Monty 1997; Marine Parks and Reserves Authority 2000). Variations in currents occur on occasion due to the effects of tidal currents, sea bed topography, steering effects of islands, banks and reefs, passing weather systems and salinity, temperature and density gradients (D'Adamo & Monty 1997; Marine Parks and Reserves Authority 2000). Exchange between the coastal lagoons and the open ocean is generally restricted by the fringing limestone reef system (Sanderson 1997). Currents which are induced by forces independent of density (i.e. constant ρ) are barotropic flows, while currents induced by horizontal density gradients are baroclinic flows. Tides, wind driven flows and currents driven by waves are examples of barotropic flows.

While hydrodynamic forcing in Jurien Bay has been found to induce strong surface currents, areas in the deeper lagoon have been found to undergo limited flushing particularly during calm periods (CALM & MPRA 2005). Current observations obtained by D’Adamo & Monty (1997) found that surface currents ranged between 0.1 – 0.2 m/s (~1-2 % wind speed) while bottom currents ranged between 0.02-0.1 m/s (< 0.5 % wind speed). In the deep basins, currents were commonly observed moving in the opposite direction to the surface currents, or in a circulatory pattern.

2.3.2.1 Continental Shelf Waves

A continental shelf wave (Figure 2-8) is a long-period wave which propagates along a coastline after some forcing event. Continental shelf waves generally result from the passage of storms over a coastal boundary causing variation in atmospheric pressure and wind stress and forcing a ‘set-up’ of water along the coastline. The coastal boundary is the forcing mechanism for the wave, blocking the surface wind- induced Ekman flux and causing motion away from the coast (Gill & Schumann 1974; Pugh 2004).

Tropical cyclones in the north-west are a typical cause of the generation of continental shelf waves in Western Australia. Factors which affect the amplitude of the wave include the strength and path of the cyclone and local atmospheric

29 Literature Review pressure and wind conditions. The propagation speed is a function of bathymetry. Once generated, continental shelf waves propagate away from their source at a rate of 400-600 km per day and with a period of 5 – 10 days (Pattiaratchi 2005a). They have the potential to considerably affect local circulation, as they overcome the prevailing conditions induced by the summer sea breeze and cause northerly currents when sea level is decreasing and southerly currents when sea level is increasing (Pattiaratchi 2005a). Continental shelf waves are known to affect the circulation of Jurien Bay, with spectral analysis of 30-day summer current observations and 12-day winter current observations revealing the presence of continental shelf waves (Sanderson 1997).

Figure 2-8 Schematic of the propagation of a continental shelf wave (Pattiaratchi 2005a).

2.3.2.2 Seiches

A seiche is a high frequency oscillation caused by a standing wave which forms in an enclosed water body in response to a forcing mechanism. Possible forcing mechanisms include internal and surface gravity waves, winds, atmospheric pressure disturbances and seismic activity. The dominant period of a seiche can be calculated using Merian’s formula for semi-enclosed basins;

T = 4L n gh where L = width of the enclosed water body g = gravity n = mode of oscillation h = water depth

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In Jurien Bay, the presence of the offshore limestone reef creates an enclosed lagoon inshore. Under strong wind conditions seiching may occur when set-up at the shoreline results in a standing wave oscillation (Pugh 2004; Beer 1997). Spectral analysis of current observations in Jurien Bay in 1997 showed high frequency east- west oscillations, suggesting the occurrence of seiching within the lagoon (Sanderson 1997). Seiching can result in the presence of strong, rapidly reversing currents (Pugh 2004).

Table 2-2 gives the fundamental longshore and cross-shore seiche periods for Essex Lagoon, assuming a width of 4 km and length of 4 km and an average depth of 10 m.

Table 2-2 Fundamental seiche periods for Essex Lagoon derived from Merian's Formula for semi- enclosed lagoons.

Longshore Period Cross-Shore Period Essex Lagoon 26.9 minutes 26.9 minutes

2.3.2.3 Tides

Tides are periodic and highly predictable movements of the ocean related in amplitude and phase to a periodic geophysical force. The dominant geophysical force is the interaction of gravitational force between the moon, earth and the sun, known as astronomical tides, and the centrifugal force of the motion of the earth in space (Pattiaratchi 2005a; Eliot 2004; Beer 1997). The equilibrium theory of tides provides a hypothetical explanation of tidal phenomena, assuming an infinitely deep ocean and instantaneous tidal response. The tidal generating forces of attraction between the moon, the sun and the earth and centrifugal force thus generate a tidal ‘bulge’ on either side of the earth. The high point of the tidal bulge is the high tide, while the low point is the low tide. According to this theory, any point on the earth’s surface will experience two equal high tides per day and two equal low tides per day (Pattiaratchi 2005a; Beer 1997).

There are a number of limitations of the equilibrium theory of tides. The slight tilt of the earth relative to the moon at 23.5 ° creates the diurnal inequality which results in a dominant semidiurnal tidal period of 12.42 hours. Additionally, the presence of land masses and an ocean of finite depth affect tidal prediction using equilibrium theory.

Spring and Neap tidal cycles occur on monthly time scales (every 29.5 days). Spring tides occur during the full and new moons, when the sun, moon and earth align, resulting in greater tide generating forces and higher tides. Neap tides occur during

31 Literature Review the first and last quarter, when the moon is at 90° to the sun resulting in lower tide generating forces and lower tides.

Any sequence of sea level or currents has a tidal component and residual:

U (t) = u p (t) + ur (t)

where up(t)=periodic component

ur(t)=residual component

The periodic component can be described using a cosine function with a particular amplitude and frequency:

u p (t) = H x cos(ω xt − g x )

where Hx=amplitude

ωxt=phase

2π gx=angular speed (related to the period) Tx = ω x

The periodic component of the tides up(t) is the sum of a number of tidal constituents related to the interactions between gravitational forces. There are over 100 tidal constituents. The more tidal constituents used in the equation, the greater the accuracy of tidal prediction, but the greater the complexity of the calculation. The dominant tidal constituents are shown in Table 2-3.

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Table 2-3 Dominant Tidal Constituents (Pattiaratchi 2005a; Beer 1997)

Name Symbol Period Semidiurnal Components (hours)

Principal lunar M2 12.42

Principal solar S2 12.00

Larger lunar elliptic N2 12.66

Luni-solar semi-diurnal K2 11.97 Diurnal Components

Luni-solar K1 23.93

Main lunar O1 25.82

Solar P1 24.07 Longer period tides (days)

Lunar fortnightly Mf 13.661

Lunisolar fortnightly Msf 14.765

Lunar monthly Mm 27.555

Solar semi-annual Ssa 182.621

Solar annual Sa 365.242

Form factor is used to define the relative importance of the diurnal and semi-diurnal tidal components.

⎡ H K + H O ⎤ F = ⎢ 1 1 ⎥ H + H ⎣⎢ M 2 S2 ⎦⎥ where F = 0 – 0.25 semi-diurnal tides

F = 0.25 – 1.5 mixed, mainly semi-diurnal tides

F = 1.5 – 3.0 mixed, mainly diurnal tides

F = >3.0 diurnal tides

Harmonic tidal constituents obtained from the Australian National Tide Tables for Jurien Bay in 2002 gave a form factor of 2.9, meaning Jurien Bay tides are mixed, mainly diurnal tides (Chua 2002). This result supports the literature reporting tides south of Exmouth on the Western Australian coast are predominantly mixed diurnal and microtidal (Sanderson 1997; Marine Parks and Reserves Authority 2000). Spring tidal range at Jurien Bay is around 0.5m, while neap tidal range is around 0.1m. At

33 Literature Review this magnitude, currents associated with tidal fluctuations in the region are not significant (Sanderson 1997).

2.3.2.4 Wave Climate

Wave climate is an important control on currents and circulation in shallow coastal waters. The wave climate of the coastal waters in the vicinity of Jurien Bay is dominated by swell waves with a significant wave height of around 1.5 m and periods ranging from T = 10 – 20 s. Incoming swell arrives mainly from the southwest, from distant temperate storms in the Indian and Southern Oceans (Sanderson 1997).

Seasonally, wave energy is generally low during the summer and autumn and more variable with greater magnitude during winter and spring (Sanderson 1997). Analysis of 2 years of Department for Planning and Infrastructure (DPI) 12 hourly wave buoy observations, collected off Jurien Bay, indicate seasonal contrast in wave direction and magnitude, with northwesterly swell in winter and southerly swell in summer. Simulation of surface waves using the Simulating Waves Nearshore (SWAN) wave model (version 4.0) indicate that hydrodynamic forces are strongly influenced by wave direction (SRFME Core Project 3 - Sediment Dynamics, SRFME 2005). Strong seasonality in the wave climate would hence suggest corresponding seasonality in current measurements.

2.3.2.5 Wind Stress

Wind blowing across the surface of the ocean generates turbulence at the surface. This turbulence causes a momentum flux to the surface water particles inducing a flow. This transfer of momentum is called wind stress. Wind stress, τ, is predicted using the following equation;

2 τ = ρ a C DW

where ρa = density of air

CD = drag coefficient

W = wind speed, 10 m above the water surface.

Wind stress is proportional to the square of the wind speed and acts in the direction of the wind. The surface layer is the upper layer of the water body where the friction from the wind surface acts. Within the surface layer, a velocity gradient known as the current shear develops. Beneath the surface layer, the wind effects are transferred

34 Literature Review through the water column through turbulence, convection and rotational effects (Pugh 2004; Beer 1997). With increasing depths, current speeds decrease and in some cases current speeds may reverse, relative to the surface direction (Pattiaratchi & Imberger 1991; Csanady 1973). This was occasionally observed in current observations in Jurien Bay by D’Adamo and Monty (1997), where observed current speeds were up to 10 times slower at the bottom and occasionally in the opposite direction. This type of circulation in a relatively deep basin, such as Essex Lagoon, may give rise to a topographic gyre formation.

2.3.2.6 Topographic Gyres

Topographic gyres are circulation patterns which form in shallow water bodies in response to wind forcing. They are characterized by a surface flow induced by wind stress balanced by a deeper return flow (Pattiaratchi & Imberger 1991). Weak recirculation gyres have previously been identified in Essex Lagoon (D'Adamo & Monty 1997), potentially explaining the incidence of opposing currents through the water column in Jurien Bay.

2.3.2.7 Coriolis Force and Rossby Number

The Coriolis force is an ‘apparent force’ resultant from the horizontal component of the earth’s excess centrifugal force. Coriolis acts on moving particles perpendicular to the direction of motion. In the southern hemisphere, Coriolis causes currents to be deflected to the left, while in the northern hemisphere, Coriolis causes currents to be deflected to the right. At the equator, the effect of Coriolis is nil (Beer 1997). The magnitude of the Coriolis force at a particular point on the earth’s surface can be measured using the Coriolis parameter;

f = 2Ωsinφu where Ω = the earth’s angular velocity (Ω = 7.2911x10-5 rads-1) φ = latitude of the point of interest (30°21.545’ at Jurien Bay) (Pattiaratchi 2005b)

Thus, the Coriolis force at Jurien Bay is 7.37x10-5.

The Rossby Number is a dimensionless number relating the ratio of eddy viscous forces to Coriolis forces for a given flow of a rotating fluid (Pattiaratchi 2005b). The value of the Rossby number defines the significance of the Coriolis force on moving waters. The Rossby number is defined by the following formula;

35 Literature Review

U R = O fL where U = characteristic velocity scale

L = characteristic length scale for the water body

ƒ = Coriolis force.

A Rossby number significantly greater than one indicates that fluid momentum is sufficient to overcome the effects of Coriolis, while a Rossby number less than one indicates that the fluid may undergo rotational transport (Fischer et al. 1979). The Rossby number for Essex Lagoon was calculated by Chua (2002) to be 0.679, indicating that Coriolis would have some effect on fluid motion in Jurien Bay.

2.3.2.8 Ekman Veering

The concept of Ekman transport was discovered between 1893 and 1896, when Norwegian scientist and explorer, Fridtjof Nansen, observed that Arctic ice on the sea surface moved in a direction to the right of the direction of the wind, and not in the direction of the wind itself. Walfrid Ekman described this phenomenon in terms of hydrodynamic equations, so named Ekman Volume Transport. Ekman Transport is a function of the wind stress and the latitude (Pugh 2004) and results in the deflection of a barotropic surface current by Coriolis. Currents affected by Ekman Transport diminish in magnitude with frictional losses and are deflected marginally more with increasing depth, resulting in a spiral effect (Figure 2-9). Surface currents are deflected 45° to the direction of the wind, while the net transport occurs at 90° to the direction of the wind (Pattiaratchi 2005b).

Figure 2-9 Idealised Ekman Spiral in the Northern Hemisphere (Pattiaratchi 2005b).

36 Literature Review

The Ekman Depth is the depth of frictional influence and is the point in the water column where currents are moving in the opposite direction to that of the wind. Ekman Depth can be calculate using the following equation;

2A D = π Z E ρf

where AZ = vertical eddy coefficient (assume 40 kg/m/s)

ρ = density of seawater = 1025 kg/m3

ƒ = Coriolis Force

The depth of Ekman influence for Jurien Bay is therefore 102.2 m. This indicates that while Ekman transport may affect currents in Jurien Bay, reversal of currents at the seabed is due to some force other than Ekman transport.

2.3.3 Physical Properties

Physical properties of the water column are significant due to the potential to influence circulation via baroclinic flows. Baroclinic current flows arise in response to horizontal density gradients in the ocean. Density changes occur in response to interactions between salinity, temperature and pressure. The four properties are linked according to the equation of state (Mellor 1996).

2.3.3.1 Sea Temperature

Temperature observations obtained in the 1990s found that winter lagoon temperatures were generally cooler than that of the open shelf, while in summer lagoon temperatures were generally warmer (Pearce et al. 1999; D'Adamo & Monty 1997). The shallow coastal waters heat quickly during the summer due to solar radiation and atmospheric heat input and cool quickly during winter due to atmospheric heat loss. Additionally, during the winter, the Leeuwin Current flows down the coast, but is deflected away from it, leaving a strip of cool water along the inner shelf (Pearce 1997). Average inshore temperatures in Jurien Bay peak in March (22 - 24 °C) drop to a minimum generally in August and October (< 17°C) (Pearce et al. 1999). Temperature observations collected in January/February 1997 showed a generally small vertical temperature difference, which became a significant difference during periods of calm.

37 Literature Review

2.3.3.2 Salinity

The salinity of sea water refers to the concentration of dissolved salts in one kilogram of water, measured in parts per thousand. Salinity in the ocean varies according to freshwater inputs from rivers, runoff and precipitation and through surface evaporation. Despite this, salinity is relatively constant for ocean water, with an average value of around 35 ppt (Pond & Pickard 1983; Mellor 1996). Salinity in Jurien Bay generally ranges between 35-37 ppt (Marine Parks and Reserves Authority 2000). The intermittently flowing Hill River does not supply significant freshwater flow into the Jurien Bay coastal waters. There is potential during warm, still periods for high surface salinities to develop due to evaporation, however, during the summer, when this is most likely to occur, the prevailing sea breeze will most likely break down any significant salinity stratification. Salinity sampling conducted during summer 1997 revealed a vertical salinity gradient of a maximum of 0.3 ppt. Average salinity reading over approximately one week from 28th January 1997 – 6th February 1997 was 36.60 ppt (D'Adamo & Monty 1997).

2.3.3.3 Stratification and Mixing

The occurrence of stratification can have important implications for the circulation patterns in Jurien Bay. A significant density difference between the inshore lagoons and the offshore ocean water can inhibit flushing which is already restricted by bathymetry. As well as this, under periods of stratification, dense seawater can sink to the bottom of the deep basins and become trapped for extended periods of time (Marine Parks and Reserves Authority 2000). Two of the main conditions under which stratification may occur is through atmospheric heating of the surface layer and the addition of low salinity water from rivers and estuaries (Pugh 2004). As the only river input on the Jurien coast is the barred Hill River estuary, with minimal riverine input, the main mechanism for the development of stratification in Jurien Bay is atmospheric heating.

Salinity and temperature profiles obtained by CALM have indicated a potential for stratification. Under strong sea breeze conditions (typical throughout spring and summer) and storm conditions (periodic during winter) strong currents and efficient vertical mixing will eliminate any weak stratification which may form. Temperature observations collected in summer 1997 showed a generally small vertical temperature difference. However, on occasion, particularly during autumn under calm to moderate wind conditions (≤ 7m/s), stratification can develop and persist for periods of up to 3 weeks (D'Adamo & Monty 1997). This can have important

38 Literature Review implications for management of contaminants and nutrients discharged to the nearshore zone (D'Adamo & Monty 1997; Marine Parks and Reserves Authority 2000).

39 Methods

3 Methods

The purpose of this section is to describe the field methods and equipment used to acquire the data presented in this report and to outline the analytical techniques used to assess and present the data.

3.1 Data Collection

The data included in this report was gathered during winter in 2002, and during summer and autumn in 2006. The field study obtained current velocity and direction using an Acoustic Doppler Current Profiler (ADCP) and water level and in-situ sea temperature as ancillary data. Complementary data for the period of field study, obtained from various government agencies, include wind velocity and direction, wave heights and sea level observations, satellite imagery, synoptic charts, aerial imagery and bathymetry.

3.1.1 Currents

The 2002 winter current measurements were obtained by James Chua (Chua 2002). The purpose of gathering the data was to determine the speed and direction of the current profile in the vicinity of Jurien Bay and to use the field data to validate model simulations of currents in Jurien Bay. An ADCP was deployed on 30th July and retrieved on 12th August. The instrument was deployed in Essex Lagoon (30° 21.545’ S 115° 01.558’E) in 9.7 m of water (Figure 1-2). This location was selected to observe the characteristics of bottom currents in deeper water as well as being identified as an area susceptible to periods of stratification and poor flushing (Chua 2002).

The 2006 summer/autumn current measurements were obtained to allow seasonal comparison with the 2002 data. The ADCP was deployed on 27th February and retrieved on 20th March. This period was assumed to represent typical summer conditions and will be considered as such for the remainder of this study. However, the March data could provide a useful insight into the prevailing current conditions during autumn. The location of the instrument was roughly 550 m north-north-west of the original 2002 location in Essex Lagoon (30°21.25’S 115°01.35’E) in 10.1 m of water (Figure 1-2).

40 Methods

3.1.1.1 Acoustic Doppler Current Profiler

An ADCP is a eulerian (fixed frame) device used to measure current velocity and direction. The primary uses of ADCPs are measurement of vertical profiles of horizontal components of ocean current velocity. Secondary applications include measuring the vertical velocity component, relative backscatter (echo intensity) and to estimate surface waves, wind speed and direction and water level variation (Van Haren 2000).

The Doppler Effect1 is the principle behind the operation of the ADCP. An acoustic beam is emitted from the transducers, and the Doppler shift in the wavelengths of sound reflected off particles within the water column (e.g. sediment, plankton) is measured by the ADCP. A key assumption here is that the particles in the water column are moving at the same average velocity as the surrounding water. The shift in frequency is directly proportional to the speed at which the object is moving. Four beams are used to calculate a three dimensional velocity profile. The multiple beams each measure different components of velocity. Hence in order to measure east- west, north-south and upward velocities, three beams are required. The fourth beam is used to calculate the pitch and roll of the instrument by comparing the difference in velocity across layers of constant depth (R.D. Instruments 1996; Van Haren 2000). Currents adjacent to a hard reflector, for example the surface or seabed, cannot be measured due to ‘reflections of leak energy traveling in side lobes at more acute angles than the main beam’ (Van Haren 2000).

The instrument used for both the 2002 and 2006 field study was a RD Instruments ‘Workhorse Sentinel’ 600 kHz Acoustic Doppler Current Profiler; a small, cylinder shaped instrument with internal power supply and data storage (Figure 3-1 (b)). For the 2006 study, the ADCP was mounted on a triangular, weighted steel frame on the seabed and marked using a surface float. Assembled, the entire setup weighed approximately 80 kg. The transducers were mounted approximately 0.7 m from the seabed, with a blanking distance of 0.5m and beam angle of 20°. The first depth cell (bin) was 1.73 m from the sea bed. For the 2002 data, the depth cell size was 0.2 m vertically with an initial depth of 1.23 m and for the 2006 data the depth cell size was 0.5 m vertically. Observations were obtained at a frequency of 2 Hz over a period of 30 seconds and averaged every 5 minutes, yielding a daily total of 288 observations.

1 The changes in observed sound pitch resulting from relative motion. The Doppler shift is the difference between the stationary frequency and the moving frequency.

41 Methods

Velocity is measured with an accuracy of ±3 mm/s. To prevent biofouling over the sample period, a Vaseline and chili powder mix was applied to the transducer heads.

Figure 3-1 (a) ADCP moored in Essex Lagoon February/March 2006; (b) ADCP before deployment.

3.1.2 Sea Temperature

In addition to measuring current velocity and direction, the ADCP has a temperature sensor mounted on the transducer head to measure the in-situ water temperature, that is, the depth at which the instrument is moored. The precision of this sensor is ± 0.4° C. While the data obtained is of limited spatial significance, it is useful for assessing the temporal variability of water temperature and the relationship to the measured currents.

3.1.3 Navigation/Position Fixing

GPS coordinates for the 2006 sample location were obtained using the Magellan GPS 300 satellite navigator using WGS84 datum. This instrument has a positional accuracy of approximately 15 m. Coordinates for the 2002 sample location were

42 Methods sourced from Chua, 2002 and were obtained using navigational equipment on the deployment vessel.

3.1.4 Winds

Wind velocity and direction data were obtained for 2002 and 2006 from the Bureau of Meteorology (BOM) in order to correlate the currents with the winds. The data covers all dates for both periods of ADCP deployment. Data is recorded in three hour intervals from 12:00 am midnight to 6:00 pm daily. 12:00 pm midday data is missing from the summer data as observations were not taken at this time. The winter data is missing several observations over the entire collection period. The erratic nature of the data and the inconsistent data points, as well as the relatively large sample intervals is a major limitation of this data.

3.1.5 Aerial Imagery

Aerial imagery (Figure 1-2) was supplied courtesy of the Department of Conservation and Land Management (CALM) and the Department of Land Information (DLI). The image is a georectified mosaic, flown on the 29th April 2004 at a scale of 1:25,000. The ground resolution is 0.5 m with an absolute accuracy of ± 5-10 m and a yellow filter applied for improved water penetration. The sampling locations and other features of interest were added to the figure using ArcGIS version 9.1.

3.1.6 Bathymetry

Data used to create the bathymetric representation of Jurien Bay was obtained by the Department for Planning and Infrastructure using an acoustic depth sounder mounted on a boat. The data files were digitalized using ArcView GIS 3.2a into a 50 m x 50 m output grid with four vertical layers and ten second time steps. Distance between the points was interpolated by averaging the 12 closest points using an inverse distance weighted grid (Chua 2002). The resultant image (Figure 2-4) shows the shore parallel reef lines, spits and tombolos and the deeper, partially enclosed basins of Essex and Favourite Lagoons.

3.1.7 Satellite Imagery

Satellite images were obtained from CSIRO Marine and Atmospheric Research (www.cmar.csiro.au, CSIRO 2006). The CSIRO images are high resolution maps of sea surface temperature, available freely from the website from October 2004. The purpose of obtaining this information was to make linkages between the observations

43 Methods of temperature from the ADCP with the regional sea temperatures over the deployment period.

3.1.8 Wave and Sea Level Observations

In addition to the water level data obtained from deployment of the ADCP, sea level observations and wave heights were obtained from DPI’s coastal data centre (http://www.dpi.wa.gov.au/imarine/coastaldata/1313.asp). DPI collects and analyses sea level and wave data at various locations along the Western Australian coast. Sea level is recorded at 5 minute intervals from a tide gauge located within the Jurien Boat Harbour relative to Mean Sea Level (MSL) (see Figure 1-2). Sea, swell and total significant wave heights2 are recorded at hourly intervals from the Jurien Bay wave rider buoy, located approximately 12 km offshore from the Jurien Boat Harbour, beyond the limits of the aerial photograph shown in Figure 1-2 (DPI 2006). ‘Total’ refers to the total wave height; a combination of sea and swell. ‘Swell’ refers to the smooth, long period waves which arrive at the coast generated by distant storms. ‘Sea’ refers to choppy, short period, locally generated waves.

3.1.9 Synoptic Charts

Synoptic charts were obtained from the Bureau of Meteorology (www.bom.wa.gov.au) which supplies archived Mean Sea Level Pressure charts for Australia from December 1999 onward. The charts display pressure systems affecting the Western Australian coastline which provides perspective to the wind, sea and swell climate at any particular time.

3.2 Data Analysis

Each of the data analysis methods was conducted on both the 2002 winter and the 2006 summer current data, in order to characterize the difference in prevailing conditions.

3.2.1 Currents

There are a number of options for representing time series water column current data, according to the desired analyses. For this study the data was used to create velocity magnitude plots for north-south and east-west components at the surface and the bottom of the water column; current roses for each data period were created

2 Significant wave height is defined as the average of the highest 1/3 of wave observations (Pond & Pickard 1983).

44 Methods for the entire water column and a ‘colormap plot’ was created in order to conceptualize the current velocity and direction time series for the entire period.

Velocity magnitude plots were created in Matlab using standard plotting techniques for the north-south and east-west components of velocity at the surface and bottom, summer and winter. Represented as a time series, they reveal the variations in current magnitude according to direction and depth. Note that the northerly and easterly components of velocity are considered positive, while the southerly and westerly components of velocity are considered negative.

Current roses were generated using Matlab in order to illustrate the prevailing conditions over the study period. Current roses are essentially a histogram plot of current direction and velocity and hence indicate the most dominant conditions. As is convention for currents, the current rose indicates the direction in which the currents flow. For the winter data, 40 bins were plotted covering the water column from 1.23 m to 9.23 m. For the summer data, 13 bins were plotted covering the water column from 1.73 m to 8.23 m.

A continuous ‘colormap plot’ was created in Matlab using the plot function ‘pcolor’ in order to conceptualize the speed and direction of currents over the entire water column in time series for both summer and winter. The advantage of this type of plot is that it represents all aspects of the data simultaneously; time series, velocity and direction and depth. In order to reduce the frequency of observations, and the resultant ‘pixelated’ effect of the plot, current observations were hourly averaged.

3.2.2 Temperature

The in situ temperature observations obtained from the ADCP were plotted as a time series in Matlab using standard plotting techniques, in order to illustrate the temporal trend over the period of data collection.

3.2.3 Analysis of Wind

The wind speed and direction for summer and winter were plotted as a time series in Matlab using standard plotting techniques, and as a wind rose histogram similar to the current rose histograms produced.

Due to the inconsistent nature of the wind data, in order to create the time series, the missing points were interpolated by averaging the adjacent values. While this is an unsatisfactory method of predicting winds, especially considering the size of the

45 Methods intervals, it allowed a continuous plot to be generated and did not change the overall shape of the plot. It also allowed the wind data to be matched with the current data. Due to the sampling resolution available, wind speeds are assumed to be constant for three hours between observations.

Wind roses were generated using Matlab in order to illustrate the prevailing conditions over the study period as a histogram. In contrast to the current rose, the wind rose shows the direction from which the wind originates, in accordance with convention.

3.2.4 Spectral Analysis

Spectral analysis using Fast Fourier Transform (FFT) was used to identify dominant periods in the north-south and the east-west components of the current data at the surface and bottom of the water column, as well as the water levels obtained by the ADCP. The Fourier Transform of a time series gives a spectrum which shows the variance of energy at particular frequencies. Obtaining information about the distribution of different frequencies of sea level disturbance is useful for describing a set of observations (Pugh 2004). The Fourier theorem states that a continuous variable, ζ(t), measured over a finite duration, D, may be represented in the interval t = -D/2 to t = +D/2 as the sum of a number of sinusoidal components, each with an integral number of waves in the time D.

1 ∞ ζ (t) = a0 + ∑[an cos(2πnt / D) + bn sin(2πnt / D)] 2 n=1 where n is an integer.

2 +D / 2 an = ζ (t)cos(2πnt / D)dt D ∫−D / 2

2 +D / 2 bn = ζ (t)sin(2πnt / D)dt D ∫−D / 2

The set of values an and bn is termed the ‘Fourier Transform’ of ζ(t) (Tucker & Pitt 2001).

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4 Results

The following is a presentation of the data collected from the ADCP, as well as the ancillary data collected to complement the discussion.

4.1 Currents

Both periods of deployment of the ADCP yielded high quality, complete data sets. Pitch and roll, which determine the stability of the instrument over the mooring period and therefore the accuracy of the observations, were negligible.

Figure 4-1 shows the N-S and E-W components of velocity for summer 2006, for the surface and bottom of the water column. The N-S figure indicates a majority of northerly current components for both the surface and the seabed, while the E-W figure indicates a majority of westerly current components for both surface and seabed. Surface current speeds reach a maximum of 270 mm/s, a minimum of 0 mm/s and a mean of 51.4 mm/s. Seabed current speeds reach a maximum of 205 mm/s, a minimum of 1 mm/s and a mean of 51.4 mm/s. In general, peaks in current velocity occur in the northerly direction. Bottom currents are generally lower in magnitude than surface currents, with the largest differences observed at distinct peaks in surface velocity. There is a particularly strong period of north-westerly surface currents occasionally tending north-easterly from around the 9th March, which is evident to a small extent in the seabed currents.

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Figure 4-1 Current velocity in N-S, E-W components for Jurien Bay – 27th February – 20th March 2006

Figure 4-2 shows the N-S and E-W components of velocity for winter 2002, for the surface and bottom of the water column. The N-S figure indicates a clear majority of southerly current components, while the E-W figure indicates a clear majority of easterly current components. Surface current speeds reach a maximum of 269 mm/s, minimum of 0 mm/s and a mean of 71.1 mm/s. Seabed current speeds reach a maximum of 136 mm/s, minimum of 0 mm/s and mean of 41.6 mm/s. In general, peaks in current velocity are in the easterly direction. Seabed currents are consistently lower in magnitude and variability than surface currents.

Occasionally, there are instances where it is evident that surface and bottom currents are flowing in opposite directions, for example on the 31st July, surface currents are flowing in a northerly direction, while bottom currents are flowing in a southerly direction. This observation is consistent with remarks made by D’Adamo and Monty (1997) regarding the presence of opposite surface and bottom flows.

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Figure 4-2 Current velocity in N-S, E-W components for Jurien Bay – 30th July – 12th August 2002

Current roses for summer and winter at all depth intervals from the surface to the seabed are shown in Appendix 1.1. Figure 4-3 and Figure 4-4 show the surface and seabed current roses for February and March 2006. At the surface, the prevailing current direction is north-north-westerly and around 40 % of all observations range west-north-westerly through northerly. These also account for the greatest current magnitude, peaking at 150-200 mm/s. Currents in all other directions account for up to 7 % each of all observations with small magnitudes of a maximum 100-150 mm/s. At the seabed, the prevailing current direction is north-westerly with significant proportions of north-north-westerly and west-north-westerly currents. Only a very small percentage of observations reach the peak of 150-200 mm/s observed at the surface. The percentage of currents in all other directions is small, with a noticeable peak in observations in the south-westerly to south-south-westerly direction. While most of the non-prevailing currents do not exceed 50-100 mm/s, the south-westerly through south-south-westerly currents reach 100-150 mm/s. The most pronounced difference between the surface currents and the bottom currents is the lower magnitudes in current speed at the seabed, and the slight difference in prevailing conditions. However, in general, the current characteristics for the whole profile are quite similar.

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Figure 4-3 Summer surface current histogram for Jurien Bay – 27th February – 20th March 2006.

Figure 4-4 Summer seabed current histogram for Jurien Bay – 27th February – 20th March 2006.

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Figure 4-5 and Figure 4-6 show the surface and seabed current roses for July and August 2002. The prevailing current at the surface is south-easterly, with other dominant directions ranging southerly through easterly. The peak current magnitude is south-easterly at 150-200 mm/s while southerly through easterly currents peak at 100-150 mm/s. All other current directions account for very minor proportions of total currents and rarely exceed 50-100 mm/s. At the seabed, the prevailing currents range almost equally south-westerly through southerly with current speeds peaking at 100-150 mm/s. All other significant current directions have some southerly component, while those with a northerly component account for a very small proportion, with current speeds not exceeding 50-100 mm/s. In general, in contrast to the summer currents, there is a significant difference in current characteristics between surface and bottom during the winter. Current magnitude is much smaller near the bottom and prevailing direction swings westerly at the bottom in reference to the surface.

Figure 4-5 Summer surface current histogram for Jurien Bay – 30th July - 12th August 2006.

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Figure 4-6 Summer seabed current histogram for Jurien Bay – 30th July - 12th August 2006.

Additional current representation plots are presented in Appendices 9.1 through 9.4. Figure 9-1 and Figure 9-2 show the ‘colormap’ representation of hourly averaged summer and winter currents respectively plotted alongside the winds for the same period. The summer data confirms that currents are predominantly northerly and westerly and shows only occasional variation in velocity and direction through the water column. The north-south current component plot displays alternating periods of northerly and southerly current direction of 1-2 days duration. This persists until around 9th March, where average current velocity increases and remains predominantly northerly for the remainder of the record. The east-west current component plot displays predominantly westerly currents with a ‘striping’ effect of easterly currents daily, possibly resulting from the afternoon summer sea breeze.

The winter ‘colormap’ plot confirms that currents flow predominantly southerly and easterly and in general displays far greater variability than the summer data. At several points in the time series, the direction of current flow between the surface and bottom is opposing. This is consistent with observations shown in Figure 4-2 and those of D’Adamo and Monty. The north-south current components display strong, prolonged periods of southerly currents, especially over 31st July – 1st August,

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4th – 6th August and 9th – 11th August. The east-west current components display several periods of particularly strong easterly currents over the record.

Figure 9-3 and Figure 9-4 display the non directional current magnitudes for summer and winter at four points through the water column. These plots illustrate the relationship of current magnitude throughout the water column and the peaks of velocity regardless of direction over time. In general, the magnitude of current velocity decreases toward the seabed. During the summer, the diurnal sea breeze current peaks are most influential in the surface currents and diminish throughout the water column. Figure 9-7 and Figure 9-8 show overlaid directional current magnitude at various points through the water column. The different colours highlight the variation throughout the water column.

4.2 Sea Temperature

Figure 4-7 shows the temperature readings obtained from the ADCP at the seabed during summer 2006. There are two major scales of change; the diurnal cycle of heating and cooling and the immediately obvious gradual heating and sudden cooling cycle. With the data available, a conservative estimate of the latter timescale is approximately ten days. The diurnal heating and cooling is the result of air-sea heat fluxes, and exhibits an early cooling to a daily minimum between 6:00 – 8:00 am, before peaking at a daily maximum in the early afternoon. The figure is dominated however, by the gradual heating to a maximum temperature of 25.8 °C on the 10th March before a rapid drop of around 2 °C over 3 days. For the remainder of the sampling period, the diurnal cycle fluctuates around just over 23 °C. Minimum temperature over the period is 22.8 °C with a mean of 23.9 °C.

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Figure 4-7 Seabed water temperature for Jurien Bay – 27th February – 20th March 2006

Figure 4-8 shows the temperature readings obtained from the ADCP at the seabed during winter 2002. As with the summer data, the diurnal heating and cooling cycle is again evident, however, exhibits generally higher daily ranges and less periodicity in daily minimum and maximum sea temperature. There is a general warming trend over the period; however, it is difficult to deduce anything from this trend due to the short interval of data available. Three distinct temperature peaks occur throughout the period, with a maximum peak of 19.3 °C. The minimum over the period is 16.6 °C with a mean of 17.7 °C.

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Figure 4-8 Seabed water temperature for Jurien Bay 30th July – 12th August 2002

4.3 Winds

Figure 4-9 and Figure 4-10 illustrates the wind velocities and directions respectively obtained during the summer 2006 and winter 2002 sample period. The red lines through the directional plot indicate 90°, 180° and 270°, or east, south and west respectively. While gaps in the data and the large sample intervals limit the applications of this data, there are still some fairly robust trends.

In Figure 4-9, a strong diurnal periodicity is evident in both wind speed and direction, as would be expected in the presence of a strong summer sea breeze cycle. Wind speeds over the first 10 days are relatively low, averaging around 3-4 m/s and peaking at around 8 m/s, predominantly south-westerly through south-easterly, with noteworthy periods of north through north-westerly winds. From around 9th March, winds are predominantly south-south-easterly and markedly higher than the preceding period averaging around 6-7 m/s with peaks around 12 m/s.

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Figure 4-9 Wind velocity and direction for Jurien Bay – 27th February – 20th March 2006 (BOM 2006a)

Figure 4-10 shows the wind velocities and directions respectively obtained during the winter 2002 sample period. As would be expected due to the absence of the summer sea breeze, there is much less diurnal periodicity within the data and far greater variability. There are three significant peak periods of wind velocity, on the 2nd, 4th-6th and 9th August, the latter two corresponding to north-west tending north-east wind direction. Average wind speeds during relative calm are around 3-4 m/s, while velocity peaks range between 7-12 m/s.

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Figure 4-10 Wind velocity and direction for Jurien Bay – 30th July – 12th August 2006 (BOM 2002)

Figure 4-11 is a histogram of the observed wind record over the summer period of ADCP deployment. There is a clear dominance of south-easterly winds (~ 27%) over a range of speeds from 0-12 m/s. A significant proportion of winds originates from the north (~16%) and from south-south-east (~15%) at speeds peaking at 6-9 m/s. Occasional winds originate from the NE and ESE, peaking at 6-9 m/s, while all other wind directions are fairly negligible.

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Figure 4-11 Wind velocity and direction histogram for Jurien Bay – 27th February – 20th March 2006 (BOM 2006a)

Figure 4-12 is a histogram of wind speed and direction for the duration of the winter 2002 ADCP deployment. Winds are predominantly offshore with a dominance of winds from the north-east (~16%) peaking at 9-12 m/s. Other significant wind observations originate south-south-easterly through to easterly, northerly and north- westerly at speeds peaking at 6-9 m/s. The north-westerly winds are characteristic winter winds, due to typical north-west storms. In general, the summer winds reach higher magnitudes than winter winds with less variability.

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Figure 4-12 Wind velocity and direction histogram for Jurien Bay – 30th July – 12th August 2002 (BOM 2002)

4.4 Satellite Imagery

Figure 4-13 and Figure 4-14 show the satellite sea surface temperatures for the Western Australian coast on the 5th March 2006 and the 16th March 2006, toward the beginning and end respectively of the summer ADCP deployment period (see Figure 1-1 for location of Jurien Bay). On the 5th March, the flow of the Leeuwin Current is evident along the coastline by the elevated sea temperatures shown in red and orange. The shelf zone immediately adjacent to Jurien Bay indicates sea temperatures of around 24° C. The directional flow arrows in the vicinity of Jurien Bay indicate a degree of onshore flow. Evidence of the cool, north flowing Capes Current is markedly absent from this image.

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Figure 4-13 Satellite sea surface temperature image of the Western Australian coast – 5th March 2006 (CSIRO 2006)

On the 16th March 2006, the Leeuwin Current is still evident by the elevated sea temperatures, however, appears to be less dominant. In contrast to the 5th March, the shelf zone adjacent to Jurien Bay in this image appears to exhibit sea surface temperatures of around 22° C. The warm, south flowing Leeuwin Current has been displaced further offshore by the cool Capes Current, originating from the south. An important point to consider when comparing Figure 4-13 and Figure 4-14 is that the colour legend is not the same scale for both days. Hence the cooling trend is slightly exaggerated.

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Figure 4-14 Satellite sea surface temperature image of the Western Australian coast – 16th March 2006 (CSIRO 2006)

4.5 Waves and Sea Level Observations

Figure 4-15 shows significant wave heights at Jurien Bay over the summer sample period. Swell waves clearly dominate the wave climate, closely following the total wave height curve, with sea waves only occasionally surpassing swell waves in magnitude. A clear peak occurs mainly in swell waves, but reflected slightly in sea waves around the 4th March, while another peak occurs around the 12th March, continuing through to around the 19th March. During this period, there is a clear increase in sea waves, several times equaling or surpassing the swell climate in magnitude. As you might expect with sea waves generated by local winds, the

61 Results summer sea breeze cycle is evident in the diurnal periodicity of sea wave magnitudes.

Figure 4-15 Wave heights for Jurien Bay – 27th February – 20th March 2006 (DPI 2006)

Figure 4-16 shows significant wave heights at Jurien Bay over the winter sample period. There exists a gap in the data over the 30th and 31st July due to instrument malfunction. As in the summer data, swell waves dominate the local wave climate, closely following the total wave curve. As you might expect, the diurnal periodicity evident in the summer sea wave climate is absent in the winter sea wave climate due to the absence of the sea breeze. There are three particularly evident peaks in both sea and swell over the period, around the 1st-2nd August, 4th-5th August and the 9th-11th August.

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Figure 4-16 Wave heights for Jurien Bay – 30th July – 12th August 2002 (DPI 2006)

Figure 4-17 shows the sea levels observed in Jurien Bay during summer sampling 2006. Initially the sea levels are quite erratic, with large ranges. From around 4th March through 11th March, a typical diurnal tidal cycle is evident with ranges of approximately 0.5 m. Sea level drops fairly significantly after 12th March, and a semi- diurnal tidal cycle is slightly evident in the curve, with a small peak and a small trough in addition to the main diurnal peak and trough. The relative drop in sea level is followed by a short period of very small sea level ranges gradually increasing to around 0.5 m on the 20th March.

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Figure 4-17 Sea Level Observations for Jurien Bay – 27th February – 20th March 2006 (DPI 2006)

Figure 4-18 shows the sea levels obtained from the ADCP over the period of summer sampling. At the beginning of the record, the sea level is quite low, before increasing sharply by around 0.6 m over the 27th and 28th February and oscillating over relatively small ranges for several days. Oscillations typical of a diurnal tidal cycle are then observed for just over a week, before sea level drops again fairly dramatically and undergoes daily oscillations over very small ranges. The ADCP curve is quite different from the tide gauge curve (Figure 4-17). While the average sea level displays similar trends, the diurnal sea level oscillation varies considerably. A possible reason for this could be the difference in sample points. The Jurien Bay tide guage is located in the sheltered Jurien Bay boat harbour, while the ADCP was in a comparably exposed area.

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Figure 4-18 Water level observations obtained from the ADCP – 27th February – 20th March 2006.

Figure 4-19 shows the sea levels observed in Jurien Bay for the winter 2002 sample period. Far more variability is evident in winter than summer, which is to be expected with the typical winter low pressure systems influencing sea level. For the first five days, sea level ranges are relatively low, around 0.4 m, before increasing to a peak of around 0.8 m, persisting for several days. From around 11th August, sea levels drop, with smaller ranges and a slight semi-diurnal tidal cycle is evident.

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Figure 4-19 Sea Level Observations for Jurien Bay – 30th July – 12th August 2002 (DPI 2006)

Figure 4-20 shows the water level observations obtained from the ADCP. These observations are very similar to the tide gauge observations. Small scale oscillations in sea level over the first two days of observations indicate rapid sea level change possibly due to seiching within the lagoon.

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Figure 4-20 Water level observations obtained from the ADCP – 30th July – 12th August 2002.

4.6 Synoptic Charts

The synoptic charts for the summer and winter period are shown in Appendix 9.1. The charts show the distribution of atmospheric pressure over Australia, which helps to identify causes of variations in wind, current, sea level and wave climate data.

For the summer period 27 February – 20 March 2006, the synoptic charts are mainly influenced by typical summer anticyclonic high pressure systems which are associated with fine, warm weather, morning easterly winds and the diurnal southerly sea breeze. Tropical Cyclone Emma is evident in the 28th February and 1st March charts (downgraded to a low pressure system on 1st March). On the 10th and 11th March a low pressure system with a cold front passes the south west of Western Australia.

For the winter period 30th July – 12th August 2002, the synoptic charts mainly display typical winter anticyclonic weather systems which are associated with relative calm, interspersed with the passing of winter low pressure systems to the south of Western Australia, associated with strong winds, rain and storm conditions. Three significant events occur over the period; on 1st August a low pressure system and associated

67 Results cold front pass south of Australia, impacting the coastline approximately as far north as Geraldton. On the 4th August, a low pressure system passes to the south of Australia with an associated cold front impacting the coast as far north as Shark Bay. On the 8th and 9th August a third low pressure system passes just to the south of Australia, again with an associated cold front.

4.7 Tropical Cyclone ‘Emma’

Toward the end of February, a low developed within an active monsoon trough to the north of North West Cape, Western Australia. The low reached tropical cyclone intensity and was named Tropical Cyclone ‘Emma’ on the 27th February. Emma crossed the Pilbara coast around midday on the 28th February (Figure 4-21), resulting in heavy rain in the vicinity of Karratha. TC Emma moved south-south-east resulting in strong winds, heavy rains and property damage (BOM 2006b).

Figure 4-21 Synoptic Image of Tropical Cyclone Emma (BOM 2006a)

4.8 Spectral Analysis

Spectral analysis for summer and winter currents, in north-south and east-west components for the surface and bottom of the water column, is shown in Figure 4-22 through Figure 4-26. Surface observations are shown in blue, while seabed observations are shown in red.

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Figure 4-22 shows spectral analysis of the north-south summer currents. At the surface, four distinct peaks occur, corresponding to periods of 3 days, 24 hours and 12 hours. These peaks are also found to occur for currents at the seabed. While higher frequency events are found at higher spectral density at the surface than the seabed, lower frequency events are found at equal or higher spectral density at the seabed.

Figure 4-22 Spectral Analysis of North-South surface and seabed currents for Jurien Bay – 28th February – 20th March 2006.

Figure 4-23 shows spectral analysis of the east-west summer currents. At the surface, four distinct peaks occur, at 5 days, 24 hours, 12 hours and 1.8 hours. Currents at the seabed also exhibit four distinct peaks, however, these are found at 5 days, 2 days, 24 hours and 12 hours. As with the north-south spectral densities, higher frequency events have higher spectral density at the surface, while lower frequency events have higher spectral density at the seabed, with the exception of the five day spectral peak.

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Figure 4-23 Spectral Analysis of East-West surface and seabed currents for Jurien Bay – 28th February – 20th March 2006.

Figure 4-24 shows spectral analysis of the water level data obtained from the ADCP during summer 2006. Three distinct peaks are observed, at periods of 24 hours, 12 hours and 1.8 hours. The implications of these findings will be further discussed in section 5.3.

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Figure 4-24 Spectral Analysis of ADCP Water Levels for Jurien Bay – 28th February – 20th March 2006.

Figure 4-25 shows the spectral analysis of the north-south winter currents. Four spectral peaks occur at the surface, at frequencies of 3 days, 24 hours, 12 hours and 1.8 hours. At the seabed, three spectral peaks occur, corresponding to frequencies of 5 days, 24 hours and 8 hours. There is a lesser influence of the diurnal and semi- diurnal frequencies than the summer time data due to the absent sea breeze. In general, a lower spectral density is observed for all frequencies at the seabed, with the exception of the 8 hour spectral peak observed.

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Figure 4-25 Spectral Analysis of North-South surface and seabed currents for Jurien Bay – 30th July – 12th August 2002.

Figure 4-26 shows the spectral analysis of the east-west winter currents. At the surface, three main spectral peaks occur at frequencies of 3 days, 24 hours and 12 hours. At the seabed, two main spectral peaks occur at frequencies of 24 hours and 9 hours. There is a generally lower spectral density at the seabed for the majority of frequencies.

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Figure 4-26 Spectral Analysis of East-West surface and seabed currents for Jurien Bay – 30th July – 12th August 2002.

Figure 4-27 shows spectral analysis for water levels measured by the ADCP in winter 2002. Three distinct peaks are observed at periods of 24 hours, 12 hours and 1.8 hours. The implications of these findings will be further discussed in section 5.3.

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Figure 4-27 Spectral Analysis of ADCP Water Levels for Jurien Bay 30th July – 12th August 2002.

74 Discussion

5 Discussion

5.1 Currents

Comparison of the current observations obtained for summer 2006 and winter 2002 reveals a distinct seasonality in current direction and magnitude. While summer currents are predominantly north-westerly, winter currents are predominantly south- easterly at the surface, tending south to south-westerly with increasing depth. Current direction during summer is closely correlated with the summer wind direction as illustrated by comparing the wind and surface current roses (Figure 4-3 and Figure 4-11). While winter winds are quite variable in direction, winter surface currents remain predominantly south-easterly. The dominance of southerly currents in general during winter is due to the steric height gradient of the Leeuwin Current forcing a predominantly southerly flow along the coast. South-easterly currents also correspond to the direction of wind stress of periodic northwesterly storms. In general, winter currents display far greater variability in both magnitude and direction, while summer currents display a distinct diurnal periodicity related to the summer wind field.

Analysis of the mean, maximum and minimum current velocities in summer and winter indicates that for the two periods surveyed, the magnitude of the highest surface current speed observed during summer at 270 mm/s is similar to the winter surface maximum of 269 mm/s. For surface currents, average magnitude is greater in winter (71.1 mm/s) than summer (59.6 mm/s), while at the seabed, average magnitude is greater in summer (51.4 mm/s) than winter (41.6 mm/s). Maximum winter seabed currents (136 mm/s) are significantly smaller than peak summer seabed currents (205 mm/s). In general, the variability in current speeds is low, which is characteristic of low energy, wind driven environments (Sanderson 1997). During summer there is smaller variation between surface and bottom current speeds than during winter, indicating a more uniform profile. In both summer and winter, current magnitudes are greatest at the surface and diminish slightly with depth. On occasion, summer currents at the seabed will flow in the opposite direction to those at the surface; while in winter, this happens quite frequently (see Figure 9-1 and Figure 9-2). This usually occurs during periods of lower wind speeds (≤ 5 m/s). At higher wind speeds, the magnitude and direction of currents through the whole profile are relatively constant. The above findings are supported by hydrodynamic modeling by D’Adamo and Monty (1997), which predicted the occurrence of weak recirculation gyres near Essex Lagoon.

75 Discussion

5.1.1 Currents and Wind

The findings from this field study are supportive of prior observations that winds are the primary driving force for current flow in Jurien Bay (Chua 2002; D'Adamo & Monty 1997; Sanderson 1997). Maximum surface current speeds were found to be approximately 2% of the maximum wind speed during both summer and winter. Studies into the effect of the sea breeze on currents in Perth coastal waters found that currents respond to changes in wind speed almost instantaneously (Pattiaratchi et al. 1997). This observation is consistent with winds and currents in Jurien Bay, as illustrated by Figure 9-1 and Figure 9-2. It is particularly evident in the summer plot, with the daily onset of the sea breeze. The ‘striping’ effect in the plot is the response of currents to the change in the wind field and correlates closely with the peak in wind speed.

Figure 4-3, Figure 4-5, Figure 4-11 and Figure 4-12 show the surface current roses for summer and winter and the wind roses for summer and winter respectively. These indicate that the strength of correlation between wind and currents in summer is very high but more variable during winter. The dominant summer winds from the south- east drive the primarily north-westerly summer surface currents. The dominant direction of origin of winds in winter is spread almost evenly between the north-west, north-east and south-east directions; however the current rose indicates the currents are consistently southerly through easterly. Currents are potentially less influenced by non-storm winds in winter due to lower magnitudes than the summer sea breeze, and more influenced by other factors such as swell and the Leeuwin Current. Figure 9-2 illustrates the strength of correlation between winter storm winds on the circulation patterns. Model simulations conducted by Chua (2002) indicate that current movements are mainly controlled by storm activities during winter, which predominantly arrive from the north-west.

The distinct seasonality in the currents mentioned in section 5.1 arises from the seasonality of the wind field. Summer winds peak daily, as do summer current fields, while during winter, winds reach the magnitude of the summer sea breeze only under storm conditions and this is reflected in the current field (Figure 9-1 and Figure 9-2). These intermittent periods of high energy are interspersed with prolonged periods of low energy, which over extended periods of time could lead to stratification in the water column and limited mixing. This poses a potential problem for dispersal of nutrients and contaminants. Flushing times modeled in 2002 (Chua 2002), however indicated that winter flushing times were of the order of 2-5 days and therefore

76 Discussion adverse effects due to poor flushing are likely to be minimal. The key period of interest for inhibited flushing is autumn. With the cessation of the daily sea breeze and generally fine, calm conditions, mixing energy is likely to be diminished. While modeling studies predict flushing times of four days (Chua 2002), these are based on similar wind data to that obtained for this study and hence are not entirely reliable. The current characteristics during autumn in Jurien Bay deserve further attention, and although current data for this study was gathered in March, the presence of the summer sea breeze is still evident both in the wind and current fields.

An interesting trend occurs in the summer data, which will be further discussed in relation to sea temperature. Over the period of sampling, there are two distinct ranges in the wind data before and after the 9th March. Leading up to 9th March, winds speeds are relatively low with an average of 3.2 m/s and range in direction from the typical southerly winds through north-westerly to north-easterly winds. Following 9th March, wind speeds increase to an average of 5.4 m/s with a predominantly south to south-easterly origin. This increase is reflected in the increase in locally generated sea waves, as shown in Figure 4-15. The increase in wind speed also corresponds to a period of higher magnitude and predominantly northerly currents, a rapid drop in sea temperature, a relative increase in wave height and a relative decrease in sea level. These findings will be further discussed in section 5.2.

Major limitations in the consistency and completeness of the wind data affect the depth of analysis which can be performed. The assumption that wind is constant in three hour blocks (six hours from 9 am – 3 pm, 6 pm – 12 am) is unrealistic. Midday especially, is a crucial period for continuous data in summer as this covers the approximate time of commencement of the sea breeze. While not of concern for this study, if the current data were to be used as model validation (as in Chua, 2002), the model capabilities are restricted by the data.

5.1.2 Currents and Weather

The passing of synoptic systems and their associated weather has been found to influence the currents and physical properties in Jurien Bay. The influence of wind on currents has already been discussed; hence this section deals with non-specific aspects of weather.

Currents in winter are particularly susceptible to influence by the passing of weather systems. The three main peaks in winter winds evident in Figure 4-10 correspond to

77 Discussion the passing of synoptic low pressure systems and drive strong, prolonged southerly currents evident in Figure 9-2. Three analogous peaks are evident in the winter wave data. The wave climate has implications for current flow as it will affect current flow in the direction of wave propagation.

During the summer sampling period, the weather systems are mainly typical summer anticyclonic systems resulting in fine weather and the expected sea breezes. A peak in summer swell on the 4th March may be the influence of a distant storm and the current record shows a brief period of strong southerly currents. Tropical Cyclone Emma, which crossed the coast on the 28th February, has the potential to affect circulation via the generation of continental shelf waves. This will be further discussed in section 5.3.

5.2 Temperature

Temperature observations collected using the ADCP give some insight into the influence of external factors on the oceanographic characteristics in Jurien Bay. The summer temperature trend illustrated in Figure 4-7 illustrates that some forcing has caused a gradual increase in sea temperature followed by a dramatic drop in sea temperature. The peak in temperature, at 25.8 °C is substantially higher than the March average and maximum temperature (22.8 °C and 24.8 °C respectively) observed for Jurien Bay in the 1990s (Pearce et al. 1999). In order to put the trend into perspective with regional sea temperatures, the satellite images for the sampling period were obtained. In the days leading up to the temperature peak on 10th March, the sea temperature in the vicinity of Jurien Bay was around 24-25 °C (Figure 4-13). In general, shallower nearshore waters are likely to be slightly warmer than offshore waters due to the increased influence of air-sea heat fluxes.

There are two mechanisms by which sea temperature change can occur; by advection of water masses or by air-sea heat flux (Personal Communication, Pearce 2006). At this scale of change, it is unlikely that air-sea heat fluxes were the major cause of heating. While the Leeuwin Current has been previously accepted to have limited influence on nearshore oceanography in Jurien Bay, D’Adamo & Monty (1997) suggested that advection of warm Leeuwin Current water may affect the temperatures of nearshore waters. Figure 5-1 shows the influence of the Leeuwin Current on inner bay temperature loggers during winter, 1997. Figure 4-13 and Figure 4-14 show the satellite imagery for 5th March, prior to the peak in sea temperature and for 16th March, following the rapid drop in sea temperature. The

78 Discussion warm waters of the Leeuwin Current can be seen onshore near Jurien Bay in Figure 4-13, while in Figure 4-14 the waters off Jurien Bay are dominated by the cooler flow of the Capes Current, absent in the image from 5th March.

Figure 5-1 Winter sea temperatures – Jurien Bay, showing the influence of the Leeuwin Current (D'Adamo & Monty 1997)

There appears to be two possible driving forces for the incursion of Leeuwin Current water into Jurien Bay. The change in wind climate over the period of study was discussed in Section 5.1.1 as relatively low velocity, variable direction winds prior to 9th March followed by higher velocity, typical southerly winds beyond 9th March. The strong southerly winds typical in summer drive the flow of the cool Capes Current north along the WA coast, as well as upwelling of cool, bottom waters. This results in cooler sea temperatures than might otherwise be expected. In the satellite image for 5th March, the presence of the Capes Current is absent from the figure, possibly overcome by the Leeuwin Current and the lack of southerly winds driving the flow. A reduction in opposing wind stress results in intensification of the Leeuwin Current. In the satellite image for the 16th March, the flow of the Capes Current is once again apparent, and corresponds to cooler sea temperatures around Jurien Bay. The Capes Current restricts the penetration of Leeuwin Current water onto the continental shelf (Pearce & Pattiaratchi 1999) so a decrease in the prevailing summer winds which drive the current will reduce this restriction.

79 Discussion

The second possible driving force for the onshore flow of Leeuwin Current water is the presence of a persistent eddy which forms in the vicinity of the Abrolhos Islands periodically throughout the year. The eddy forms as a result of meanders of the Leeuwin Current, persists for four to six weeks before spinning off and reforming. The presence of this eddy results in the deflection of flow onshore to the south at Jurien Bay. The satellite images show a meander with onshore directional velocity around Jurien Bay (Figure 4-14). In the presence of such an eddy, as well as the diminished southerly winds driving northerly flow, it appears that the Leeuwin Current has the capacity to intrude within the shelter of the offshore reef system and influence local oceanography. In a study of Perth coastal waters, it was found that mesoscale features of the Leeuwin Current, for example meanders and jets, could influence adjacent continental shelf waters (Mills et al. 1996).

Leeuwin Current water could also be further driven onshore by a buoyancy flux of the warmer, less saline tropical water over cooler shelf water. This could result in a stratified surface layer, which has implications for baroclinic flow in Jurien Bay. While it is difficult to further investigate the effect of the Leeuwin Current on the nearshore of Jurien Bay with the data available, it is evident that there is some influence which warrants further study.

Three significant temperature peaks occur in the winter data (Figure 4-8). These peaks correspond with the presence of low pressure synoptic systems crossing the Western Australian coast. A possible explanation for the peaks in temperature would be an onshore flow induced by the weather system which transported warmer surface water onshore. This further illustrates how weather systems can influence physical properties, and therefore the hydrodynamics of Jurien Bay.

5.3 Spectral Analysis

Spectral analysis of the current and water level data allows an insight into the dominant periods within the data and hence the forcing mechanisms. The dominant periods found in the data are attributed to seiching (1.8 hours), tides (12 and 24 hours), the sea breeze (24 hours) and meteorological forcing (3-5 days). For currents, the higher frequency events generally exhibited greater spectral density at the surface than the seabed, while the lower frequency events had higher or equal spectral density at the seabed. This occurs because the higher frequency events are induced mainly by winds and hence affect the surface currents most strongly. Lower frequency events are induced by external forcing which is likely to affect the entire

80 Discussion water column and not just the surface currents. Examples of such events include atmospheric pressure variation and continental shelf waves.

Seiching within Essex Lagoon is evident in the east-west components of summer currents and the north-south components of winter currents at frequencies of 1.8 hours. The influence of seiching is only found in the surface current plots, indicating that currents induced by seiching affect surface currents to a greater extent than seabed currents. This observation was also found in spectral analysis of currents in the Zeewijk Channel, Abrolhos Islands (Maslin 2005). The 1.8 hour period seiche is also evident in spectral analysis of the summer and winter water level data, due to the change in water levels induced by seiching. Seiching induces strong currents, increasing the mixing energy within the Lagoon, however is unlikely to assist flushing since currents are rapidly reversing back and forth.

The 12 and 24 hour spectral peaks represent the influence of tidal currents and the summer sea breeze. The influence of tidal currents was found in all summer and winter, east-west and north-south and surface and bottom plots at periods of 12 and 24 hours. The dominant diurnal tidal cycle is evident as the influence of the 12 hour period is generally significantly smaller than the 24 hour peak. During summer for east-west and north-south current components, the tidal period peaks are higher for seabed currents than for surface currents. This suggests that tidal currents have a comparatively greater influence on circulation at the bottom of the water column. This observation was also found in current analysis at Zeewijk Channel, Abrolhos Islands (Maslin 2005). Occasionally, tidal peaks appear slightly less or slightly greater than 12 or 24 hours. This is likely to be due to tidal inequality. The 24 hour tidal period peak is shared with the diurnal sea breeze induced currents. The size of the diurnal peak in the summer plots, when compared with the diurnal peak in the winter plots, is much greater. This indicates that the diurnal current signal during summer is dominated by the sea breeze and the influence of tidal currents is comparatively small. As with the seiche period, the 12 and 24 hour spectral peaks are also evident in spectral analysis of water level data for summer and winter.

Long period fluctuations in the current data of periods of 3 – 5 days represent the influence of meteorological forcing on currents in Jurien Bay. Similar long period fluctuations were identified in current data obtained in 1997 and were thought to represent the impact of anticyclonic high pressure systems and continental shelf waves (Sanderson 1997). While continental shelf waves are quite common along the coast of Western Australia, with periods of up to ten days, the length of sampling

81 Discussion periods (20 days in summer, 14 days in winter) is not sufficient to identify their presence with spectral analysis.

82 Conclusion

6 Conclusion

The circulation of Essex Lagoon, Jurien Bay is a low energy, wind dominated system with a distinct seasonality in current characteristics. This seasonality has implications for ecosystem management, as flushing and mixing must be considered on a seasonal basis and not assumed constant throughout the year. During the summer time, the strong sea breeze drives a characteristic current signal daily, which is usually constant through the entire water column. During the winter, strong current events are primarily associated with storm conditions which occur periodically throughout the season. Surface and bottom currents are found to regularly flow in opposite directions during the winter. The north-west prevailing currents during the summer contrast with the south-east prevailing currents during the winter.

While some of the summer data for this study was collected during March (autumn), the winds and currents were typical of summer. However, autumn is a particularly important time of the year in the understanding of the circulation patterns. During winter, currents were shown to respond strongly to storm events, exhibiting lower current speeds under non-storm conditions and the potential for recirculation via topographic gyres. Typical autumn weather, with an absence of sea breezes and of storm activity, could result in extended periods of calm and lessened circulation and mixing. If stratification results, more energy would be required to mix the water column and this could have implications for water quality if excess nutrients were present.

Other forcing mechanisms found to influence the circulation in Jurien Bay were seiching, tides, continental shelf waves and passing weather systems. Spectral analysis identified frequency peaks in the current data relating to seiching with a period of (1.8 hours) and longer period meteorological effects with periods of 3-5 days. Continental shelf waves could not be identified within the record due to the length of sampling, however are known to occur and significantly influence circulation. Passing weather systems were found to influence the wind and wave climate which in turn drives currents and circulation.

Investigation into temperature in conjunction with circulation found evidence that the Leeuwin Current influences the nearshore oceanography of Jurien Bay by advection. The two primary forcing factors thought to encourage advection of Leeuwin Current flow into Jurien Bay are the decrease of opposing southerly wind stress and the presence of an eddy in the Abrolhos Islands driving flow onshore. Under reduced

83 Conclusion southerly wind stress during summer the Leeuwin Current appears to come onshore near Jurien Bay and affect nearshore sea temperatures. This has implications for the density characteristics and therefore baroclinic flow in Jurien Bay. Increased temperatures could potentially lead to more stratification and an intrusion of buoyant surface waters overlying dense bottom waters increases the energy required to mix the water column. During autumn, with typically lower southerly wind stress and a stronger Leeuwin Current, circulation and mixing and mixing could be considerably restricted, especially during calm weather. This issue deserves further investigation.

Scenarios which could potentially result in inhibited flushing and water quality issues include extended periods of calm or atypically low sea breezes; uncharacteristic changes in the wind field; rapidly reversing currents causing recirculation, for example in the presence of a seiche or continental shelf wave and changes in the density field due to advection. A potential management strategy would be to determine the frequency of such events occurring and the likelihood of water quality issues arising in order to manage detrimental effects on the marine environment.

84 Recommendations

7 Recommendations

There are a number of aspects of the analysis of the oceanography of Jurien Bay which could be improved in the future. While a number of biological studies are being carried out, the physical aspect of the oceanography has not received the same focus. Recommendations for future work concern the quality of ancillary data, the seasonal coverage of the existing current data and the application of the existing data.

One of the major limitations of this study is the wind data. The data available from the Bureau of Meteorology is available only in three hour intervals and currently skips observations at 12:00 pm and 9:00 pm. The 12:00 pm observation is particularly crucial during summer since the summer sea breeze would come into effect over the six hour period between 9:00 am and 3:00 pm, resulting in a dramatic change in wind climate. Ideally, wind observations would be captured more regularly during the day at equal intervals. Availability of more data would greatly improve the analysis of the data and increase the modeling capabilities.

The temperature observations obtained from the ADCP provided interesting discussion points regarding the advection of the Leeuwin Current into the sheltered nearshore zone of Jurien Bay. A more focused investigation of the effect on physical water properties would be valuable. Obtaining similar temperature measurements over an improved temporal and spatial scale (throughout the water column and in numerous locations around Jurien Bay), as well as complementary salinity measurements, would allow stronger conclusions to be made about the implications for physical oceanography and the driving forces responsible.

Autumn has been identified as critical in the seasonal cycle in terms of stratification and mixing and obtaining specific current data during this period would provide better estimates of the nature of flushing and circulation than making assumptions based on winter and summer data. Measurements of temperature and salinity in conjunction with currents would aid in characterizing baroclinic aspects of the oceanography. Investigating the duration and frequency of periods which lead to low mixing would be a useful exercise.

Further studies using the existing data could provide further modeling, flushing and particle track estimates to complement existing studies (D'Adamo & Monty 1997; Sanderson 1997; Chua 2002).

85 References

8 References

Beer, T. 1997, Environmental Oceanography, 2nd Edition edn, CRC Press.

BOM, (2002), Bureau of Meteorology [Online], Australian Government, Available: www.bom.gov.au.

BOM, (2006), Bureau of Meteorology [Online], Australian Government, Available: www.bom.gov.au [10th April 2006].

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88 Appendices

9 Appendices

9.1 Currents and Wind

Figure 9-1 Current velocity and direction with depth and time at Jurien Bay – 27th February – 20th March 2006

89 Appendices

Figure 9-2 Current velocity and direction with depth and time at Jurien Bay – 30th July – 12th August 2006

90 Appendices

9.2 Current Magnitude

Figure 9-3 Non-directional current velocity with depth and time at Jurien Bay – 27th February – 20th March 2006.

91 Appendices

Figure 9-4 Non-directional current velocity with depth and time at Jurien Bay – 30th July – 12th August 2002.

92 Appendices

9.3 Current Roses

9.3.1 Summer

(a) (b)

(c) (d)

(e) (f)

(g) (h)

93 Appendices

(i) (j)

(k) (l)

(m)

Figure 9-5 Current roses for February/March 2006 for (a) 1.73m, (b) 2.23m, (c) 2.73m, (d) 3.23m, (e) 3.73m, (f) 4.23m, (g) 4.73m, (h) 5.23m, (i) 5.73m, (j) 6.23m, (k) 6.73m, (l) 7.23m, (m) 7.73m above the seabed.

94 Appendices

9.3.2 Winter

(a) (b)

(c) (d)

(e) (f)

95 Appendices

(g) (h)

(i)

Figure 9-6 Current roses for July/August 2002 for (a) 1.23m, (b) 2.03m, (c) 3.03m, (d) 4.03m, (e) 5.03m, (f) 6.03m, (g) 7.03m, (h) 8.03m, (i) 9.03m above the seabed.

96 Appendices

9.4 Current Components

9.4.1 Summer

Figure 9-7 Current velocity in N-S, E-W components at Jurien Bay – 27th February – 20th March 2006

97 Appendices

9.4.2 Winter

Figure 9-8 Current velocity in N-S, E-W components at Jurien Bay – 30th July – 12th August 2002

98 Appendices

9.5 Synoptic Charts

9.5.1 Summer

99 Appendices

100 Appendices

(BOM 2006a)

9.5.2 Winter

101 Appendices

(BOM 2006a)

102