EPA Bulletin 215, technical report on mermaid sound

7/29/2019 EPA Bulletin 215, technical report on mermaid sound

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TECHNICAL REPORT ON SUSPENDEDMATTER IN MERMAID SOUND,

DAMPIER ARCHIPELAGO

M.J. FORDE

DAMPIER ARCHIPELAGO MARINE STUDY

DEPARTMENT OF CONSERVATION AND ENVIRONMENT1 MOUNT STREETPERTH WESTERN AUSTRALIA 6000


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ISBN 0 7309 0578 0


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1 .

2 .

3 .

4.

5 .

CONTENTS

INTRODUCTION1 .1 SCOPE1 .2 STUDY AREA1 .3 PHYSICAL ENVIRONMENT1 .4 PORT OPERATION, FACILITIES AND ASSOCIATED

INDUSTRIESWIND AND WAVE ENERGY2. 12 .2

2 .3

2 .4

INTRODUCTIONWIND2 . 2 . 12 . 2 . 2WAVES

Methods: wind measurementDiscuss ion

2 .3 .1 Methods2 . 3 . 2 D i scu s s i o nTROPICAL CYCLONES IN THE DAMPIER REGION2 . 4 . 1 Cyclone ev en t s2 . 4 . 22 . 4 . 3

Discuss ionConc lus ions

WATER MOVEMENT3 .13 .23 .3

INTRODUCTIONMETHODS: CURRENT MEASUREMENTRESULTS3 . 3 . 13 . 3 . 23 . 3 . 3

3 . 3 . 4

T i d a l c u r r e n t sLoca l wind - dr iven c u r r e n t sLarg e - s ca l e c o n t i n e n t a l s h e l fc u r r e n t sDens i ty c u r r e n t s

LIGHT ATTENUATION4 .1 INTRODUCTION4 .2 MATERIALS AND METHODS4 .3 RESULTS

4 .44 .5

4 . 3 . 1 V e r t i c a l l i g h t a t t e n u a t i o n andsuspended load4 . 3 . 2 S p a t i a l and t empora l v a r i a t i o n inl i g h t a t t e n u a t i o n

DISCUSSIONCONCLUSIONS

VARIATION IN SUSPENDED LOAD IN MERMAID SOUND,BASED ON SEDIMENT TRAP DATA5 .1 INTRODUCTION5 .2

5 .3

MATERIALS AND METHODS5 . 2 . 1 F i e l d ev a lu a t i o n o f a p p r o p r i a t e

s e d i m e n t t r ap a s p e c t r a t i o5 . 2 . 2 Design of mo n i t o r i n g programme5 . 2 . 3 Sample a n a l y s i s5 . 2 . 4 Su r f ace s e d i me n t s : sampl ing and

a n a l y s i sMONITORING SITES

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5 .4 RESULTS5 . 4 . 15 . 4 . 25 . 4 . 35 . 4 . 4

Fie ld eva lua t ion of a p p r o p r i a t esed iment t r ap a s p e c t r a t i oVar i a t i o n of suspended load withh e i g h t above s e a - f l o o rTemporal v a r i a t i o n in suspendedloadS p a t i a l v a r i a t i o n in suspendedload

5 .5 DISCUSSIONSUMMARY

REFERENCESAPPENDIX 1

FIGURES1 . 12 .12 .22 .32 .42 . 5 a2 .5b2 .63 . 13 .2

3 . 3 a

3 . 3 b

4 . 14 .24 .3

The s tudy a r e a : Dampier Arch ipe lagoS i t e s o f w av er i d e r buoys dep loyed from May 1980to December 1982 .Monthly wind speed and d i r e c t i o n ro s e s fo rConzinc I s l and from September 1981 to June 1983.Diurna l wind p a t t e r n s r ecorded a t ConzincI s l an d in December 1982.D i u rn a l wind p a t t e r n s r ecorded a t ConzincI s l an d in June 1983 .S p e c t r a l a n a l y s i s of waver ider buoy d a t a fromnear Wi thne l l Bay and Legendre I s l a n d , June 1981.S p e c t r a l a n a l y s i s of waver ider buoy d a t a fromnea r Withnel l Bay and Legendre I s l a n d , March 1982.T r a j e c t o r i e s of t r o p i c a l cyclones ' T r i x i e ' , ' J a n e ''Lena' and 'Ch loe ' .Locat ions o f Nei l Brown acous t i c c u r r e n t met e r s .Computer model s im u la t i o n o f s p r i n g ebb t i d a lc u r r e n t s , Dampier Arch ipe lago (Adapted from M i l l s ,1985) .Hydrograph ic s e c t i o n s th rough Mermaid Sound showingd e n s i t y , t emp er a t u r e and s a l i n i t y c o n t o u r s ,29 June 1982.Hydrograph ic s e c t i o n s th rough Mermaid Sound showingd e n s i t y , t emp er a t u r e and s a l i n i t y co n to u r s ,12 December 1982 .Sampl ing s i t e s fo r l i g h t a t t e n u a t i o n d a t a .The r e l a t i o n s h i p between l i g h t a t t e n u a t i o nc o e f f i c i e n t and t o t a l suspended mass in March 1983 .The r e l a t i o n s h i p between l i g h t a t t e ~ u a t i o nc o e f f i c i e n t and suspended mass of o r g an i c m at t e rin March 1983.

4 .4 Contour maps o f l i g h t a t t e n u a t i o n c o e f f i c i e n t srecorded in Mermaid Sound between A p r i l 1982 andMarch 1983.

4 .55 . 15 .2

Change in l i g h t a t t e n u a t i o n a t Nelson Rocks overa s i x - h o u r p e r i o d , 28 June 1982.Sediment t r a p s i t e s in Mermaid Sound.Sediment t r ap assembly .

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5 .3 The c o l l e c t i o n r a t e s of sed iment t r a p s withd i f f e r e n t a s p e c t r a t i o s a t he igh t s r an g i n g from0 .5 m to 8 .5 m above th e s e a - f l o o r a t S i t e 11.

5 . 4 Pe r cen t ag e of mean weight o f organ ic m a t e r i a lin sed i men t t r a p s o f d i f f e r e n t as p ec t r a t i o s a tS i t e 11.

5 . 5 To t a l suspended load c o l l e c t e d in sed iment t r a p s0 .5 m and 6 m above the se a - f l o o r a t S i t e s 7 and

5 .6 Organic suspended load c o l l e c t e d in sed imentt r a p s 0 .5 m and 6 m above th e s e a - f l o o r a t S i t e s7 and 10.5 .7 Calcium carbona te suspended l oads c o l l e c t e d in

sed iment t r a p s 0 .5 m and 6 m above th e se a - f l o o ra t S i t e s 7 and 10.5 .8 Suspended r e f r a c t o r y loads c o l l e c t e d in sediment

t r a p s 0 .5 m and 6 m above the s e a - f l o o r a t S i t e s7 and 10 .5 . 9 Pe r cen t ag e of mean weigh t of o r g an i c m a t e r i a l insuspended l oads c o l l e c t e d in sed iment t r a p s 0 .5 mand 6 m above th e se a - f l o o r a t S i t e s 7 and 10.5.10 The r e l a t i o n sh i p between suspended load and meanwind speed a t Si t e 1 .5.11 The r e l a t i o n s h i p between suspended load and the

index fo r mean s p e c i f i c wave energy a t Si t e 1.5.12 The r e l a t i o n sh i p between suspended load and meanwind speed a t S i t e 8.5.13 The r e l a t i o n sh i p between suspended load and the

index fo r mean s p e c i f i c wave energy a t S i t e 8.5.14 The r e l a t i o n sh i p between suspended load and

mean wind speed a t Si t e 13.5 . l5 The r e l a t i o n s h i p between suspended load and theindex fo r mean s p e c i f i c wave energy a t Si t e 13.

5.16 Suspended l oads between Februa ry 1982 and March1983 a t S i t e 1. Peaks in January and March-Apri l1983 co in c id e with th e t iming of cy c l o n es ' J a n e 'and Lena .

5 .17 Suspended l oads between June 1982 and August 1983a t Si t e 13.

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695 .18 Suspended load between May 1982 and August 1983 a t 70

Si t e 8. The peak in March-Apr i l 1983 co in c id es witht he t iming of cyclones ' L e n a ' .

5 .19 Comparison of t o t a l suspended load in t r a p s , a t 720 .5 m deployed concur r en t ly a t S i t e s 10 and 11.

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TABLES

2 . 1

4 .1

4 .2

4 .3

5 . 1

5 .2

5 .3

5 .4a

5.4b

5 .5a

5 .5b

Monthly mean o f s i g n i f i c a n t wave h e i g h t s and wavep e r i o d s , and monthly index of s p e c i f i c wave energyder ived from waver ider d a t a fo r i n s h o re ando f f s h o r e s i t e s .Values o f t o t a l load and o r g an i c l o ad , d e r i v edfrom ten r e p l i c a t e samples t aken a t Nelson Rocks , 1March 1983 . to d e t e rmi n e ex p e r i men t a l e r r o r .Suspended l oads der ived from seaw a t e r samplesc o l l e c t e d in March 1983 and l i g h t a t t e n u a t i o nc o e f f i c i e n t s r ecorded c o n c u r r e n t l y .Suspended s o l i d and pigment l oads d e r iv ed fromDecember 1983 seaw a t e r samples , and l i g h ta t t e n u a t i o n c o e f f i c i e n t s r ecorded c o n c u r r e n t l y .Gra in - s i ze of core samples from th e su r f ace 50 mma t th e sed iment t r a p s i t e s . Cores c o l l e c t e d 1982 ,analysed 1983 .Muff l e oven a n a l y s i s of core samples from th es u r face 50 mm a t th e sed iment t r a p s i t e s .S i g n i f i c a n t r e l a t i o n s h i p s of suspended load ( t o t a land component p r o p o r t i o n s ) with mean wind speed(MWS) and mean s p e c i f i c wave energy (ME) a t t h r eesha l low s i t e s .S i g n i f i c a n t d i f f e r e n c e s between sha l low s i t e s inco n t en t of suspended l o ad .Pro p o r t i o n s (%) of th e o rg an i c , ca lc ium ca rb o n a t e sand r e f r a c t o r y co n t en t in th e mean suspended loada t each sha l low s i t e .S i g n i f i c a n t d i f f e r e n c e s of suspended l oads betweendeep s i t e s with p a i r ed a r r a y s .P r o p o r t i o n s (%) of th e o rg an i c , calc ium ca r b o n a t eand r e f r a c t o r y co n t en t in th e mean suspended loada t each of th e s i t e s with p a i r ed a r r a y s .

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ABSTRACT

This r e p o r t p r e s e n t s da t a from a s tudy (1981-1983) of th es p a t i a l and t empora l v a r i a t i o n s in l e v e l s of suspended m a t t e r inth e w a t e r column us ing sed imen t t r a p s in Mermaid Sound, DampierAr c h ipe l a go . Seasona l wind c o n d i t i o n s and wave energy reg imeswere a na ly s e d and r e l a t e d to suspended l oa ds to de te rmine thep o s s i b l e c a us e s of t he se v a r i a t i o n s . L igh t a t t e n u a t i o n was a l someasured and t r e n d s a s se s se d i n r e l a t i o n to suspended m a t t e r .Trap da t a were i n d i c a t i v e of the degree o f r e suspe ns i on ofs e d i me n t a t a s i t e and not o f n e t se d i m e n t a t i on r a t e s .

The compos i t ion of suspended m a t e r i a l was found to vary wi thh e i g h t above th e s e a - f l o o r . The or ga n i c f r a c t i o n was g r e a t e s tn e a r the w a t e r s u r f a c e whereas the ca lc ium c a r bona t e f r a c t i o nwas g r e a t e s t n e a r th e s e a - f l o o r . With in the s tudy a r e a suspendedl oa ds were found to be g r e a t e r in the sou t he r n s e c t o r of theSound i e i n s h o r e , t ha n in the nor t he r n s e c t o r ie o f f s h o r e .Leve l s were g e n e r a l l y h i ghe r in summer than in wi n t e r main ly asa r e s u l t of th e s e a s o n a l p r e v a i l i n g winds .

L igh t a t t e n u a t i o n va lues s uppor t e d t he se broad c onc l u s i ons ; th eo f f s h o r e a r e a s were a lways l e s s t u rb i d than t hose i n sh o re , whi lethe e n t i r e a r e a was more t u rb i d in summer than in w i n t e r . L igh tmeasurements a lo n e , however , should n o t be used as an i n d i c a t i o nof the suspended l oad . The compos i t ion o f m a t e r i a l in suspens ioni s more i m por t a n t to l i g h t a t t e n u a t i o n t han th e t o t a l dryw e i g h t ; th e critical f a c t o r s be ing or ga n i c co n ten t , g ra in s i zeand g r a i n c o l o u r . Dr edg ing caused widesp read de c r e a se in l i g h ta t t e n u a t i o n , p a r t i c u l a r l y over the summer of 1982-83 .

I n s ho r e wind speed , wave energy and suspended load were alls i g n i f i c a n t l y c o r r e l a t e d , i n d i c a t i n g t h a t l o c a l wind c o n d i t i o n sr e g u l a t e d wave energy which in t u rn r esuspended bottoms e d im e n t s . By c o n t r a s t , o f f s h o r e , th e suspended load d id no tc o r r e l a t e with mean wind speed a l t hough i t d id c o r r e l a t e withwave energy . Thus , a combina t ion o f wind - induced waves as w e l las s we l l energy was l i k e l y to have caused r e suspe ns i on and

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t h e r e f o r e v a r i a t i o n s in suspended load in the no r t he r n s e c t o r .Ove r r i d i ng t h ese t r en d s a re th e e f f e c t s of c yc lone s whichg e n e r a t e both s we l l and wind energy . Cyclones were observed tocause l a r g e - s c a l e , s h o r t - t e rm r e suspe ns i on of se d i m e n t s ,c o n s i d e r a b l e damage to be n t h i c communi t i es , the g e n e r a t i o n ofc a l c a r e o u s rubb le and c o n s i d e r a b l e r e l o c a t i o n of se d i m e n t s .

ACKNOWLEDGEMENTS

I g r a t e f u l l y acknowledge th e generous s u p p o r t and encouragemento f Dr V.W. T a lbo t a t a l l s t a g e s th rough t h i s s t udy . He a l s ogave ad v ice , g en e ra l guidance and c o n s t r u c t i v e c r i t i c i s m s onthe r e p o r t . Dr D.A. M i l l s i s t hanked fo r pr ov i d i ng d a ta and fo rp rep a r in g the first d r a f t of Chap te r 3 . Dr R.G. Ch i t t l eb o ro u g h ,Dr J .R . Ottaway, Ms E. Moore, Mr C . J . Simpson and o t h e r s t a f fof the Depar tment of C ons e r va t i on and Environment d i scu s sed th ework a t v ar io u s s t a g e s , a s s i s t e d with f i e l dwor k and advi sed onthe p rep a ra t i o n of v ar io u s d r a f t s .

Assoc i a t e P r o f e s s o r A. J . McComb and s t a f f a t the Cent re fo rWater Research , U n i v e r s i t y of Western A u s t r a l i a , a dv i se d on th en a tu re of the f i e l d programme, c r i t i c a l l y read p a r t s of them a nusc r i p t , and gave t e c h n i c a l and l o g i s t i c a l su p p o r t . Dr R.Dunlap , Chemis t ry Depar tmen t , U n i v e r s i t y of Weste rn A u s t r a l i a ,Ms E. Moore and Dr J . R . Ottaway c o n s t ru c t i v e l y ed i t ed th epe nu l t i m a t e d r a f t . Mr B. St e wa r t , Mr A. Berman and Ms E. Mooreprepared t h i s r e p o r t fo r p u b l i c a t i o n .

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1. I NTRODUCJ..!.ON

1 .1 SCOPE

Mermaid Sound, an a rea of 400 km 2 in th e n o r th -wes t of WesternA u s t r a l i a (F ig . 1 . 1 ) , i s bounded by the mainland and th e i s l a n d so f th e Dampier A r ch i p e l ag o . The C o as t a l Waters Branch of th eDepar tment of Conserva t ion and Envi ronment under took a s e r i e s ofr e sea r ch p r o j e c t s to i n v e s t i g a t e th e marine ecosys tems o f t h i sr eg ion (Ch i t t l eborough , 1983; Gordon, 1983; M i l l s , 1985;Simpson, 1985; Ta lb o t , 1985; Talbo t and Creagh , 1985a ,b ) withth e o b j e c t i v e of managing and minimis ing human impact s .

I n i t i a l obse rva t ions of wate r t u r b i d i t y in Mermaid Soundi n d i c a t e d marked v a r i a t i o n from c l e a r ocean ic water s a t thenor the rn p a r t o f th e Sound to very t u r b i d c o n d i t i o n s near th emainland. C o i n c i d en t with t h i s t u r b i d i t y range i s a change inth e b i o t i c communi t ies . For example , th e o u t e r r e e f suppor t sex tens ive co ra l communi t ies dominated by ac ro p o r id s( eg . A. hyac inthus ) and p o c i l l o p o r i d s ( eg . P.damicornis ) ,whi le f u r t h e r i n to the Sound massive c o r a l s s u ch . a s Favia andFav i t e s a re dominan t . Turb id i ty and sediment load a rei mp o r t an t f a c t o r s in cora l r e e f development and they i n f luences p ec i e s compos i t ion . Endean (1976) has shown t h a t th e f a m i l i e sAcropor idae and Poci l lopo t idae a re very s u s c e p t i b l e to sed imentd e p o s i t i o n , whi le many massive s p ec i e s a r e more t o l e r a n t . Thus aknowledge of v a r i a t i o n s in suspended m at t e r i s i mp o r t an t inunders tand ing b i o t i c communi t ies .

Th i s s tu d y , i n i t i a t e d in September 1981, examined n a t u r a ls p a t i a l and t empora l v a r i a t i o n in suspended mat t e r in MermaidSound, and i n v e s t i g a t e d :

th e r e l a t i o n s h i p between suspended m at t e r andv e r t i c a l l ig h t a t t e n u a t i o n ,th e r e l a t i o n s h i p between marine sed iments andsuspended l o ad , andt he env i ronmenta l f a c t o r s which determine suspendedl o ad s .

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116 30'

EaglehawkIsland

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116 30'

Indian

0 SkmLl--L--L-"- '-- ' '

9..... ~Island Malus 0 tJslands f1 V'

n Goodwynv Olsland

Figure 1.1 The study area; Dampier Archipelago.

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116 45'

KEYLoading facilitiesParker Point2 East Intercourse Island3 Mistaken Island4 Materials offloading facility-

5 Island berth turning basinDepth contour in metres

11 45'

Nickol

Bay

Locality MaptJ


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1 .2 STUDY AREA

The Dampier Archipe lago was formed dur ing th e l a s t p o s t g l a c i a lmarine t r a n s g re s s i o n about 7000 years ago (Lawson & T re l o a r ,1983) . Res u l t an t f l o o d in g of the h in t e r l an d drowned thelow-ly ing va l l eys and p l a i n s l eav in g rocky high p o i n t s exposedas i s l ands and emergent s h o a l s . Th e dominant bedrock of thea rea i s gid ley granophyre , be ing an i n t r u s io n along the b asa lunconformi ty of the precambrian Fo r t e scu e group (Kr iewald t ,1964) . Medium-gra ined redd i sh- fawn l imes tone occurs mainly inth e o u t e r a reas of th e Archipelago (Biggs & Denman, 1981) .These d e p o s i t s , t h a t may be remnants o f a co as t a l dune systemof th e p l e i s t o c e n e epoch, form an o u t e r submarine escarpmentand assoc ia t ed emergent rocks and i s l a n d s (Lawson & T re l o a r ,1983) .

Th e inshore sed iment s of the Archipelago a re composed mainly ofc lay-s i zed mat e r i a l with high s i l i c a co n t en t der ived from r i v e rd i sch a rg e and shore e ro s i o n . Freshwate r d i sch a rg e to th eDampier Archipe lago i s from the i n f r eq u en t f low of the Mai t landRiver , and i s u s u a l l y assoc ia t ed with cyc lon ic r a i n f a l l . Thesed iment s become p ro g res s i v e l y co a r s e r (Lawson & T re l o a r , 1983)and i n c reas e in calc ium carbonate co n t en t with d i s t an ce fromthe mainland , as a r e s u l t of decreas ing t e r r igenous i n p ~ t andi nc rea s ing biogen ic d e p o s i t i o n . Recent (Holocene) d ep o s i t s ofcalc ium carbonate have minimal i ron s t a i n i n g , forming w h i t e grey beaches , fo redunes and main dunes . Rel i c (P le i s tocene )ma te r i a l forms weathe red , o x i d i s ed , redd i sh-c ream d ep o s i t sf u r th e r in land and in i s o l a t ed a reas of reworking with in theArch ipe lago (Biggs & Denman, 1981; Lawson & T re l o a r , 1983) .1 .3 PHYSICAL ENVIRONMENT

The a rea i s a r id and t r o p i c a l , with an average annual r a i n f a l lof approx imate ly 290 mm (Dept . of the I n t e r i o r , 1972) and anannua l ev ap o ra t io n r a t e of 3734 mm (Bureau of Meteorology,unpubl i shed da ta , 1980) . The per iod of h i g h e s t r a i n f a l l i sfrom Janu ry to March and i s assoc ia t ed with t r o p i c a l cyc lone

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a c t i v i t y . The annual range in sea t empera tu re i s 19 - 320.These l i m i t s a re c lose to the lower and upper t o l e r a n c e l e v e l sof many t r o p i c a l marine organisms (Woodside PetroleumDevelopment , 1979) . The t i d a l range i s moderate (4 .5 m): t i d e sa re sem id iu rna l . Curren t speeds a re g e n e r a l l y g r e a t e r inof f shore a rea s and ch an n e l s . T id a l ex cu r s io n may be as much as10 km under normal s p r i n g t ide c o n d i t i o n s . Wind ene rgy andwate r movement of the reg ion a re d i s cu s s ed in d e t a i l inChapters 2 and 3 r e s p e c t i v e l y .

1 .4 PORT OPERATION, FACILITIES AND ASSOCIATED INDUSTRIES

The sh e l t e r e d water s o f the A r c h i p e l a g o , t h e r e l a t i v e l y deepwater access to the mainland and th e sa f e anchorage of MermaidSound, have f a c i l i t a t e d development o f the p o r t and a s s o c i a t e di n d u s t r i e s wi th in the a r e a .

Hamersley I ron Pty Ltd was first to c o n s t r u c t a load ingf a c i l i t y , a t Par k e r P o i n t (F ig . 1 . 1 ) , which began ope ra t ions in1966. A second f a c i l i t y , l oca ted on Eas t In t e rcourse I s l and(F ig . 1 . 1 ) , s t a r t e d expor t ing i r on o re in March 1972. Toprov ide access to t h i s i s l and from th e main land , a so l i d fillcauseway was c o n s t r u c t e d . Dredging was r equ i r ed to c o n s t r u c tsh ipp ing channel s t o t he se t e rmina l s ; 17 x 10 6 m3 of dredgesp o i l were r e l o c a t e d wi th in Mermaid Sound, between 1965 and1977. Fur the r dredg ing , to widen and deepen s e c t i o n s of thesechannel s t o f a c i l i t a t e l a r g e r ore c a r r i e r s , r e su l t e d in 0 .716 x10 6 m3 of s p o i l being r e loca t ed in 1981-82 .

Dampier S a l t Pty Ltd was the n ex t major i n d u s t r i a l company inth e a r e a . C o n s t ru c t i o n began in 1967 and th e first h a r v e s t ofcommon s a l t (sodium c h l o r i d e ) occurred in 1971. A load ingf a c i l i t y was c o n s t r u c t e d on Mistaken I s l and (F ig . 1 . 1 ) and wasconnec ted to th e mainland by a so l i d fill causeway i n c o r p o r a t ing Eas t Mid - In t e rco u r s e I s l a n d .

The most r e c e n t major i ndus t ry in th e a rea has been n a t u r a l gasdevelopment by Woodside Offshore Pe t ro leum Pty L td . This i s

4


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still in the development s t a g e , but four d redg ing ope ra t ionshave been under t aken to da te :

t o permi t access to the supply base , 1 .2 x 10 6 m3 o fsed iments were dredged from near P h i l l i p Po in t andKing Bay (F ig . 1 .1 ) and d ep o s i t ed onshore behind abund. Sp o i l overf low r e s u l t e d in the b u r i a l and deathof 1 .2 ha of mangroves ad jacen t to th e bund in May1982.

a s s o c i a t e d with th e c o n s t r u c t i o n o f th e m a t e r i a l so f f l o a d i n g f a c i l i t y (MOF) (F ig . 1 . 1 ) 0.14 x 1 0 6 m3 ofs p o i l were r e l o c a t e d 1 .5 km nor th of th e dredges i t e .

fo r th e cons t ruc t ion of a submarine t r u n k l i n e (F ig .1 .1 ) . Blas t i ng and c l am s h e l l dredg ing r e s u l t e d in0.28 x 10 6 m3 of s p o i l be ing dredged and s i d e c a s t500 m between December 1981 and Ju ly 1983. The l a t e rper iod i nvo lved b a c k f i l l i n g and th e dumping of 0.026x 10 6 m3 of bedding m a t e r i a l (0-50 mm ag g reg a t e ) and0.94 x 10 6 m3 o f so l i d f i l l i n g (0-600 mm c rushed rock)over the p i p e l i n e ,

fo r th e cons t ruc t ion of th e i s l and t u r n i n g bas inof f shore from Withnel l Bay (F ig . 1 .1 ) between 1December 1982 and 21 Februa ry 1983 which involveddredg ing and onshore depos i t ion of 0 .55 x 1 0 6 m3 ofs p o i l in Noname Creek (F ig . 1 . 1 ) .

5


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- - - - - - - - - - - - - - - - - - - - - - - - - - - - 2. WIND AND WAVE ENERGY

2 . 1 INTRODUCTION

Winds and p res su re d i s t u r b a n c e s a c t i n g 6ver th e sea l ead to at r a n s f e r of energy from a i r t o wa te r . Some of t h i s energy i sc o n t a i n e d i n wave motion and su b se q u e n t l y may be expended byf r i c f i o r i a t th e s e a - f l o o r , p a r t i c u l a r l y in sha l low wa t e r s . Inth e p r o c e ~ s . s e a - f l o o r sed iments may be moved and th e f i n e rf r a c t i o n s placed i n suspens ion in the wa te r column. Theser esuspended sed iments can then be mobi l i sed by c u r r e n t s .

The manner in which m e t e o r o l o g i c a l p r o c e s se s i n t e r a c t to formth e observed s e a s o n a l wind c o n d i t i o n s of th e Archipe lago i sd i scu s sed in t h i s c h a p t e r . Av a i l a b l e wave d a ta a re ana lysed ande f f e c t s of long and s h o r t p e r i o d waves on th e Archipe lago a rec o n s i d e r e d . F i n ~ l l y , o c c u r r e n c e , e n e r g e t i c s and impact oft r o p i c a l c yc lone s a re examined with r e f e r e n c e to sed imen tmovement.

2.2 WIND

Local wind c o n d i t i o n s of th e P i l b a r a c o a s t , i n c lu d in g th eDampier Ar c h i p e l a g o , a re dominated by t h r e e m e t e o r o l o g i c a lproce s s e s :

M igr a t i on of high p res su re sys t ems ( a n t i - c y c l o n i c b e l t s )in to th e more n o r t h e r l y l a t i t u d e s in w i n t e r .

Format ion of monsoonal d ep res s io n s in summer.

Occur rence o f d i u r n a l l a n d / s e a breeze c o n d i t i o n s caused bythe d i f f e r e n c e in l and and sea t emp era tu res . This d i u r n a lp a t t e rn i s super imposed upon the winds caused by l a rg e -s c a l e p r e s s u r e s ys t e m s .

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- - - - - - - - - - - - - - - - - - - - - - - ~ ~ ~ ~ = =

116 30 '

Offshoresite 2) (

116 45'

Indian

20 25' 0 SkmL'-'--'--'--'-. . .! '

/ . ~ ' \ , r , ~ tt:JK.endrew 11 t?:;,'b-'Island

Dam pierArchipelago

~ , o ; f / 9 ~RosemaryIsland Malus O


16/110

KeyWind speed (ms1)- Over 13.5:

12.0-13.610.512.0 I9.0-10.57:5- 9.06.0- 7.54.5- 6. 03.0- 4.51.6- 3. 00.5- 1.5'

% - percent of data recovery

Jan. 1982

July 1982.

Jan. 1983

sFeb. 1982

Aug. 1982

Fob. 1983

Sept. 1981

Mar. 1982

Sept. 198295.1% 91.7%

Mar. 1983

Figure 2.2 Monthly wind speed and direction roses for Conzlnc Island from September 1981 to June 1983.8


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Nov. 1981 Dac. 1981Oct. 1981 ------------------- 88.2%/ ......_

/." "-,

June 1982Apr. 1982 May 1982

Dec. 1982

Oct. 1982 Nov. 1982

Apr. 1983May 1983 June 1983

9


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2 . 2 . 1 Methods: wind measurement

To measure wind in the Dampier Arch ipe lago , a Woeff le anemometerwas pos i t ioned 4 .5 m above th e h ighes t p o i n t of Conzinc I s land(F ig . 2 .1) from September 1981 to Ju ly 1983. The records wered i g i t i s e d to ob ta in mean values of wind speed and d i r e c t i o n ,fo r consecu t ive 30-minute per iods . These da ta were s to red oncomputer and t hen used to c a l c u l a t e monthly wind roses(F ig . 2 .2 ) .

2 .2 .2 Discuss ion

Wind p a t t e r n s in the Dampier Arch ipe lago a re s t rong lyi n f luenced by sea sona l m e teo ro log ica l c o n d i t i o n s ; however , ast he se cond i t i ons do no t f a l l nea t ly in to th e co n v en t io n a lseasons and as t h e r e i s v a r i a t i o n from year to year they wi l lbe d i scussed with r e fe rence to observed o ccu r ren ce .

Sep teabe r -Februa ry . During t h i s pe r iod , the b e l t of highpre ssu re sys tems i s l oca ted to the sou th , and monsoonaldepre ss ions gene ra te winds with a s t rong wes te r ly component(September to February 1982-83, Fig . 2 . 2 ) . The preva i l ingpa t t e rn of these winds was t y p i f i e d by December records for1982 (F ig . 2 .2 and 2 . 3 ) , in which 80% o f the wind con ta ined awes te r ly component . Super imposed upon t h i s pa t t e rn , g en e ra t edby th e monsoons, were d iu rna l l and / sea breezes , which enhancedwind speed dur ing the a f t e rnoon and reduced it dur ing the n i g h tand ea r ly morning (F ig . 2 .3 ) .

Wind speeds were g e n e r a l l y high dur ing t he se months , with 34%a t 7 .5 ms- 1 or g r e a t e r . Calm pe r iods (winds o f < 0 .5 ms-1) werer a re (F ig . 2 . 2 ) . In February 1982, th e wes te r ly p a t t e r nweakened, wind d i r e c t i on became var iab le and some s t rongof f shore breezes were ex p e r i en ced .

Marcb-Apr i l . Breakdown of the monsoonal summer p a t t e r n r e su l t e din a t r a n s i t i o n per iod in March and Ap r i l 1982, befo re thei n f luence of th e nor thward m ig ra t i n g a n t i - c y c l o n i c b e l t becamedominant . This pe r iod was c h a r a c t e r i s e d by f l u c t u a t i n g winds

10


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- - - - - ~ ~ - - - - ~ - ~ ~ ~ - ~ - - - - -

1982

36'16mV "' ~ / v v J ~ / \ ~ - ~ ~ . 1_\ ' ~ \ ,_[\,84 \ v v 1 1 : 1 : ! V ' 11

g;'+-;+ +1-tl-t-+-81+/1 t,l--"2\f-+-1+i +1-11-+1+I +-li-IH-1+I +1-tl-+1+I+1-tl-+-1 -1+1-+1-tl-+-1 -1 + 1 - t l - t l e - - + - I ~ + J - + ~ - t ~ - ~ + - 1-lt-+j-tl-;f-v+-1 +!-t1-1lf-+-1 -1 + 1 - l l - ~ t - - 1 +-:1+i ll 121/ I 2 12/ 1 2 I 4 /1 2 16/ 2 18/ 1 2 20/ 20ww(L([)0zH:I

2J2

8 / 1 2 1121/12 12 / 12 14 / 1 2 16/12

Figure 2.3 Diurnal wind patterns recorded at Conzinc Island in December 1982.18 / 1 2 2121/12

and some calm p e r i o d s . A p r i l 1983, however, was dominated bys t rong winds with an e a s t e r l y component. Hence, t he t iming anddura t ion o f t h i s low energy t r a n s i t i o n per iod v a r i e s from yearto yea r . Cyclonic s torm ev en t s may o c c u r in th e area fromNovember th rough to A p r i l , and a re d i s cu s s ed in Sec t i o n 2 .4May-July . The b e l t o f high pre ssu re sys tems moves to its mostn o r t h e r l y e x t e n t dur ing t h i s pe r iod , and becomes th e dominantm eteo ro lo g i ca l f e a t u r e of th e P i l b a r a r eg ion (Steedman andA s s o c i a t e s , 1982) . These sys tems d i r e c t an e a s t e r l y to s o u t h e r l ya i r s t r eam ac ro s s th e Pi lba ra c o a s t l i n e . The predominant windsobse rved over t h i s p e r io d , a t Conzinc I s l a n d , were from th es o u t h - s o u t h - e a s t in 1982 , while the e a s t e r l y component was moreobvious in 1983 . A d iu rna l l and / sea breeze was again e v i d e n t ,r e i n f o r c i n g s o u t h e r l y to e a s t e r l y winds in the morning andmodera t ing them in th e a f t e rn o o n and n i g h t (F ig . 2 .4 ) . Per iodso f l i g h t v a r i a b l e winds and calms , i n t e r s p e r s e d t h i s p a t t e r n .August . The t r a n s i t i o n per iod in 1982, from th e e a s t e r l y towes te r ly p a t t e r n , was obse rved dur ing Augus t . This per iod wast y p i f i e d by l i g h t winds (only 12% w e r e > 7 .5 ms- 1 ) , l ack ing adominant d i r e c t i o n a l component.

These conc lus ions concern ing s eas o n a l wind p a t t e r n s in theDampier r eg ion a re c o n s i s t e n t with t hose made by Woodside

11


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1 9 8 3

36"r:1274 ~;)H 18 " i\u f \ ~ \ ~ I(t 9 'Ha '8 / 6 1 0 / 6 1 2 / 6 1 4 / 6 1 6 / 6 1 8 / 6 2 0 / 6

ClwwQ_


21/110

g en e ra t ed swel l i s of ten o f s h o r t d u r a t i o n , a l thoughp o t e n t i a l l y s i g n i f i c a n t fo r b i o t i c communi t ies and sed imentd i s t r i b u t i o n . In w i n t e r , storm even t s i n the lower Ind ian Oceang en e ra t e sw e l l which i s a t t e n u a t e d in to a g e n e r a l l y low,c o n s i s t e n t , long p e r io d waveform as it approaches t he s tudya rea (Woodside Pe t ro leum Development Pty Ltd, 1980) . Wintersw e l l i s seen l o c a l l y as waves break ing on Hammersley Shoal , anon Cohen, Gidley and Angel I s l ands (F ig . 2 .1 ) .

2 . 3 . 1 Methods

Quan t i t a t i ve wave d a t a a r e a v a i l a b l e fo r th e Dampier r eg ion fo rthe per iod May 1980 - December 1982. These d a t a were c o l l e c t e dby R K Steedman and As s o c i a t e s fo r Woodside Offshore PetroleumPty Ltd (Steedman and Assoc ia t e s , 1983) .

Waver ider buoys were l oca ted a t two of f shore s t a t i o n s , onead jacen t to Legendre I s l an d and the o the r near the mouth of theHamersley I r o n ch an n e l . A t h i r d buoy was p laced a t an i n s h o res t a t i o n , near S t a r Rock west of Withnel l Bay (F ig . 2 .1 ) . Rawwave p r o f i l e s , sampled a t one second i n t e r v a l s fo r up to 20minutes , were c o l l e c t e d s e v e r a l t imes d a i l y . Each of thesep r o f i l e s was p r o ces sed and th e r e s u l t s p res en t ed as t ime s e r i e sof s i g n i f i c a n t wave h e i g h t , s i g n i f i c a n t wave pe r iod , averagezero c ross ing per iod and per iod o f peak s p e c t r a l o r d i n a t e . Forexample , Fi g u r es 2 .5a and 2.5b show der ived wave c h a r a c t e r i s t i c sof the i n s h o re and of f shore s i t e s fo r th e months o f June 1981and March 1982.

2 . 3 . 2 . Discuss ion

The wave da ta showed t h a t s i g n i f i c a n t wave h e i g h t s , and t h e r e -fo re wave energy d e n s i t i e s , were c o n s i s t e n t l y h i g h e r a t theof f shore s i t e s than a t th e s i t e with in ,Mermaid Sound (F igs .2 .5a and 2 . 5b) .

In Chapte r 5, th e degree of c o r r e l a t i o n between measures o fwave ene rgy and t r apped suspended m at t e r i s a s s e s s e d . Asmeasurements of suspended m at t e r were those of loadsaccumula ted ove r p e r io d s of th e o rd e r of one month, th e

13


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'z"''s(5 )

"s,,,

'z"''s"'

"s"'

IUTHNELL BAY SPECTRAL ANALYSIS

90 PERIOD OF PEAK SPECTRAL ORDINATE 30

20 20

10 10

0 - L ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ 0ZERO CROSSING PERIOD

WAVE PEAJOD

J SIGNifiCANT HAVE HEIGHT 32 2

JUN 19Bl

LEGENDRE ISLAND SPECTRAL ~ ~ Q L Y S J S

30 PERIOD OF PEAK SPECTRAL ORDINATE 30

:: j VERAGE ' , ' ~ " o ~ c _ " _ ' _ . _ ' _ ' " _ "__ _ ' _ " _ ' _ o _ o - - - ~ - - - - - - - - - - - - - - - - ~ " - - - [ ::[

2010

- - - " " - - 0

3 SIGNIFICANT HAVE HEIGHT '2 2

JUN 19Bt

Figure 2.5a Spectral analysis of waverider buoy data from near Withnell Bay and Legendre Island, June 1981.(Data from R.K. Steedman and Associates, 1983.)

14


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'z"'

's(S J

"s,,,

'z,,,

's]S ]

"s,,,

WITHNELl BIH SPEClRRL ~ N O L T S J S

30 PERIOD OF PERK SPECTRAL OAOJNATE[

30

200

::l= ROSSING PERIODPERIOD

3 SIGNIFICANT HAVE HEIGHT 32 Topical C}'Clone ' I an ' 2

MAR 1982

L E G E ~ O A E TSLRNO SPECTRAL RNRLTSTS

30 PERIOD OF PERK SPECTRAL ORDINATE 3020 20

ZERO CROSSING PEAJOO

WAVE PEAJOO

3 32 Trop ica l cyclone ' I an ' 2

,, I 5 [ 10 [HRA 1982

Figure 2.5b Spectral analysis of waverider buoy data from near Wlthnell Bay and Legendre Island, March 1982.(Data from R.K. Steedman and Associates, 1983.)

15


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f o l l o w i n g procedure was adopted to o b ta in an index o f waveene rgy over th e co r r e sp o n d i n g p e r io d s :

Monthly means of s i g n i f i c a n t wave h e i g h t s were d er i v ed fromd a t a p re s en t ed in Steedman and As s o c i a t e s (1983) and t h esemeans i n s e r t e d i n to a t h e o r e t i c a l ex p re s s io n fo r sp e c i f i cwave energy (US Army C o as t a l E n g i n e e r i n g Research C en t r e ,197 5) :

E e.g_ll28

Where p d e n s i t y of seaw a t e rg g r a v i t a t i o n a l a c c e l e r a t i o nh s i g n i f i c a n t wave h e i g h t

The sp e c i f i c wave ene rgy i s th e wave energy co n t a i n ed in aw a t e r column with a base o f 1 sq . m., given a t r a i n o fs i n u s o i d a l waves .

From th e c a l c u l a t e d d a t a p re s en t ed in Table 2 . 1 , th e i ndex o fwave energy i s seen to be 6-64 t imes g r e a t e r a t th e o u t e rs t a t i o n s than th e i n n e r one . The d i f f e r e n c e i s caused l a r g e l yTable 2.1 Monthly mean of significant wave heights and wave periods, and monthly index of specific wave energy.derived from waverider data for inshore and offshore sites.

Monthly 11ean Monthly ean Monthly index Energy RatioDate Locat ion of s ign i f i cant of s i gn i f i c an t of spec i f i c wave (outer : inner )wave he ight s ( ) wave 2er iod s (m) energll: (J- 2 )Nov. 1980 o1 0 . 70 4 .2 61 6 t i : lNov. 1980 I 0 . 28 2 .8 98May 1981 o2 0 . 8 2 4 .8 85 0 14 :1May 1 981 0 . 22 3 .6 60June 1981 o2 0 . 78 5 .2 768 64:1June 1981 I 0 . 10 5 .8 12Ju ly 1981 o2 1 . 03 4 .7 1 33 4 33:1July 1981 I 0 . 18 4 .6 40S ep t . 1981 o2 0.93 5 .8 1 088 18: lSept . 1981 0 . 22 3 .7 60Oct. 1981 o2 0 . 77 5 .5 746 16:1Oct. 1981 0 . 19 3 .2 46Feb. 1982 o2 0 . 76 4 .0 72 6 7: lFeb. 1982 0. 29 2 .8 10 6Mar. 1982 o2 0 .67 4 .0 56 4 10;1Mar. 1982 0 .21 2 .3 56Apr. 1982 o2 0 .47 4 .4 276 9 :1Apr-. 1982 I 0 .16 3. I 32Waverlder data fro a RK Steedaan an d Assoc ia te s , 1983o1 Offshore S i t e I - near Leaendre Is land )Outs ide Daapiero2 - Offshore Si te 2 - near aouth of Haaers ley Iron channe l )Archipelal loI - Inshore s i t e - near Withnel l Ba y - Inside Daapier Archipelaa:o

16


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by th e geometry of Mermaid Sound, with a long ax i s in anor th - sou th d i r e c t i o n (F ig . 2 . 1 ) , whi le th e predominan t windd i r ec t i o n s a re from th e e a s t or wes t . The Withne l l Bay s i t e(F ig . 2 .1 ) i s a l s o p r o t e c t e d from th e p e r s i s t e n t win te r swe l l ,which approaches th e Archipelago from a west n o r t h - w e s t e r l yd i r e c t i o n .

Wave energy i s l o s t between th e oute r and i n n e r s i t e s throughd i s s i p a t i o n by bot tom f r i c t i o n , invo lv ing th e movement ofse d i m e n t s , and the b reak ing of waves on th e shal low oute r r e e f sand i s l ands (F ig . 2 . 1 ) .

2 .4 TROPICAL CYCLONES IN THE DAMPIER REGION

Cyclones a re the most ene rge t i c even ts to a f f e c t t r o p i c a lr eg ions . These a re c lockwise ro t a t i o n a l (sou thern hemisphere ) ,low-pressure sys tems of t r o p i ca l o r i g in , with winds of a t l e a s tg a le fo rce (18 ms- 1 ) near th e c e n t r e . The energy i s der ivedmainly from sur f ace wate r t emp era tu re , and from h e a t r e lea sedwhen ocean ic wa te r vapour condenses to form c lo u d s . T ro p ica lcyc lones t h e r e fo r e oc c u r when wate r t e m p e r a t u r e i s h i g h e s t anda f f e c t the P i l b a r a co as t l i n e predominan t ly between December andAp r i l (Lourensz , 1981) . Rel iab le r eco rd s of cyc lone o c c u r r e n c e sand t r a j e c t o r i e s have been k e p t s in ce 1959, and in the 21 y ea r sto 1980 ten cyc lones passed wi th in 150 km of Dampier (Lourensz ,1981) .

An i n d i c a t i o n of th e wave energy g en e ra t ed by cyc lones can bega ined from o b s e r v a t i o n s of storm damage to c o r a l r e e f s in theDampier Archipe lago fo l lowing cyc lones p as s in g wi th in 150 km.

2 .4 .1 Cyclone even ts

T r o p i c a l cyc lone ' T r i x i e ' . T ro p ica l cyc lone ' T r i x i e ' formed on15 February 1975 and passed th rough th e Dampier Archipelago on18 Februa ry , b efo re c ross ing th e c o a s t a t Onslow, 180 km so u th west of Dampier , on 19 February ( ~ i g . 2 . 6 ) . This very severecyc lone had a minimum pressure of 925 mb and maximum recorded

17


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- - - - - - - - - - - - - - - - - - - - - - - - - - ~ - - - - - - - ..

'Jane' January 1983

Indian

Ocean

o 200kmI I

---------

D.ampier

TimorSea

'Chloe' February 1984~J

r /r-,IIIII

\\ '

WesternAustralia

' - - - - - - ~ ' T r i x l e ' February 1975

Figure 2.6 Trajectories of tropical cyclones 'Trixie', 'Jane', 'Lena' and 'Chloe'.

18

Wyndham


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winds ( t en min mean v a l u e ) of 52 .8Dampier , th e maximum g u s t was 63.9e i g h t hours th e wind was in excesspeak g u s t of 68 .9 ms- 1 (248 kmh- 1 )

ms- 1 (190ms- 1 (120of 25 ms- 1a t Onslow,

kmh- 1 ) . Atkmh- 1 ) . and fo r

(90 kmh- 1 ) . Afrom 'Tr ix ie '

i s th e h i g h e s t cy c lo n i c wind speed r e c o rd e d fo r A u s t r a l i a .

The Western A u s t r a l i a n Museum conduc ted s u rv e y s of cora l r e e f sin the Dampier A r ch i p e l ag o , with p a r t i c u l a r emphasis on t h o s enea r Kendrew I s l an d (F ig , 2 . 1 ) , between 1972 and 1974. In 1978,it was found t h a t th e p rev io u s ly r i c h c o r a l r e e f s nea r KendrewI s l an d had been marked ly d i s t u r b e d . Thi s was a t t r i b u t e d to acombina t ion of ' T r i x i e ' and a p o p u l a t i o n i nc r ea se of th e crownof t h o r n s , s t a r f i s h , Acanthas ter p lanc i (Marsh , 1978) . Twomass ive c o r a l c o l o n i e s (P la tygyra s i n e n s i s ) , each abou t 1 md i ame t e r and obse rved a l i v e on th e r ee f flat in 1974, werefound on th e beach in 1978 (Marsh, p e r s . comm., 1 9 8 1 ) .I n v e s t i g a t i o n of th e Angel I s l an d and Conzinc I s l an dcommuni t ies in 1978 r ev ea l ed s i g n s of s torm damage, a l sop o s s i b l y caused by ' T r i x i e ' (Marsh , p e r s . comm., 1984) ,

Tro p ica l c y c l o n e ' J a n e ' . T r o p i c a l cyc lone ' J a n e ' formed on 5J a n u a ry 1983, 800 km n o r t h - w e s t of P o r t Hedland, and c r o s sedth e c o a s t a t P a r d o o , 270 km n o r t h - e a s t o f Dampier on 9 January( F i g . 2 . 6 ) . Thi s sev e r e cyc lone had a minimum p res s u re o f 9 5 5 m band winds of 38 .6 ms- 1 (139 kmh- 1 ) r ecorded a t Po r t Hedland.Winds a t th e Conzinc I s l an d anemometer s i t e (F ig . 2 .1 )i n c rea s ed from 8 ms- 1 (29 kmh- 1 ) to 22 ms- 1 (80 kmh-1) o v e r30 h , and were ma i n t a i n e d a t t h a t speed fo r 18 h, b ef o r er a p i d l y r e t u r n i n g t o abou t 8 ms- 1 (29 kmh- 1 ) . The s t ro n g windsa t Dampier were from an e a s t e r l y , t u r n i n g to s o u t h e r l y ,d i r e c t i o n b e c a u s e t h e cy c l o n e , which had a c lockwise r o t a t i o n ,c r o s sed t h e c o a s t n o r t h - e a s t o f Dampier . E f f e c t s on th e c o r a lr e e f s of Nelson Rocks were e v i d e n t . Mass ive c o r a l s (Por i t e s sp)were broken from th e bot tom, and r o l l e d a v e r th e more f r a g i l eb ra n c h i n g c o r a l s such as Acropora formosa. Tubula r c o r a l s( e . g . A. hyac in thus ) of up t o 1 .5 m d i ame t e r had beeno v e r tu rn ed , and o t h e r s growing on l a r g e c o r a l heads 1 .5 m fromth e bot tom had o ld c o r a l d e b r i s t r apped on them. Desp i t e t he

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immediate mechan ica l damage which g en e ra t ed ca lca r eous rubb leand sed iment , new c o r a l growth had r e - e s t a b l i s h e d wi th in t h reemonths.

Tro p ica l cyclone ' L e n a ' . Tro p ica l cyclone 'Lena ' formed on 5Ap r i l 1983, 850 km n o r th n o r th -wes t of Po r t Hedland , andc ro s s ed th e coas t a t Cape F l o u r i n , 120 km n o r t h - e a s t of Dampieron 8 Ap r i l (Fig 2 .6 ) . This much l e s s i n t ense cyclone had aminimum pre ssu re of 994 .5 mb, and winds of 42.5 ms-1 (153 kmh-1)a t Po r t Hedland . An anemometer pos i t ioned on the WoodsidePe t ro leum Development Pty Ltd t e s t p i l e near Withnel l Bay(F ig . 2 .1 ) r ecorded no winds i n excess of 10 ms-1 (36 kmh-1) .Inspec t ion of th e r e e f s a t Nelson Rocks a f f e c t e d by cyclone' J a n e ' , showed no v i s i b l e damage a f t e r ' L e n a ' .T r o p i c a l cyclone ' C b l o e ' . Tro p ica l cyclone 'C h lo e ' formed on26 Februa ry 1984, 300 km nor th of Derby, and crossed the c o a s ta t Cape Lamber t , 50 km e a s t of Dampier (F ig , 2 .6 ) . This i n t ensecyclone had a minimum pre ssu re of 962 mb, winds of 41 .7 ms- 1(150 kmh-1) , and g u s t s of 61 .1 ms-1 (220 kmh- 1 ) , r ecorded a tCape Lamber t . Ins t rument f a i l u r e preven ted the Bureau ofMeteoro logy from ga in ing accura te wind s t r e n g t h s a t Dampier;however , a t 2400 h on 28 February , t he wind was repor ted a t 17ms- 1 (61 kmh-1) . At 0913 h on 29 Feb r u a r y , g u s t s of 42 ms- 1(151 kmh-1) were r e p o r t e d , and a t 0500 h on 1 March of 15 ms- 1(54 kmh-1) were being ex p e r i en ced . These winds , l i k e those of' J a n e ' , were from an e a s t e r l y t u rn ing sou ther ly d i r e c t i on . Then e t e f f e c t s on th e r e e f s a t Nelson Rocks appea red to ben e g l i g i b l e , with no observed damage, and , a p p a r e n t l y , littlegene ra t ion of new ca lca r eous sed imen ts .

2.4 .2 Discuss ion

Tro p i ca l c y c l o n e ' T r i x i e ' has had marked, and p o s s i b l y long- te rm,e f f e c t s on marine communi t ies o f th e Dampier Arch ipe lago . Thef u l l e x t e n t of t he se e f f e c t s i s unknown. Independen ti n v e s t i g a t i o n in 1983 by Veron ( A u s t r a l i a n I n s t i t u t e of MarineScience) f a i l e d to f ind any developed c o r a l communit ies , or the

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r ema ins o f dead ones on S a i l f i s h Reef (Veron , p e r s . comm.,1 9 8 4 ) . T he r e f o r e , it seems l i k e l y t h a t th e c o r a l communi t i e s ,which were p r e s e n t on t h a t r e e f in 1974, were de s t r oye d bye i th e r ' Tr i x i e ' or by a combinat ion of ' Tr i x i e ' and the crownof t ho r ns s t a r f i s h , Acanthas ter p lanc i . The c a l c a r e ous r ema inso f t hos e c o r a l s may have been removed by ' T r i x i e ' , or sub s e que n t wave a c t i o n and c o n t r i b u t e d t o t he s e d i me n t s of th ea r e a . A r e p o r t o f an a ppa r e n t i n c r e a s e in c o r a l r ubb l e on th eKendrew I s l an d r e e f flat in 1978, ove r t h a t observed in 1974(Marsh , pe r . comm., 1984) , i n d i c a t e s t h a t some of th e brokenc o r a l was washed i n to th e sha l low a r e a s .

This obv ious damage to th e c o r a l r e e f s i s a r e s u l t of th e waveenergy a s s o c i a t e d with c y c l o n e ' T r i x i e ' . Winds from ' T r i x i e 'a re th e s t r o n g e s t r ecorded in th e a r e a , and , a d d i t i o n a l l y , th ec y c l o n e ' s pa th would have maximised th e e f f e c t o f th e windsupon th e Ar c h ipe l a go . P a s s ing th rough the Arch ipe lago andmoving p a r a l l e l to th e c o a s t , th e c yc lone ge ne r a t e d windsp e r p e n d i c u l a r to the c o a s t l i n e . The o u t e r a r e a s , i nc l ud i ngKendrew I s l an d and S a i l f i s h Reef , a r e unp r o t e c t e d from windinduced waves wi th n o r t h e r l y components , such as t hos e causedby ' T r i x i e ' .

In c o n t r a s t , tr o p i c a l c yc lone s ' J a n e ' , ' L e n a ' , and 'Ch l oe ' a l lc r os s e d th e c o a s t to th e n o r t h - e a s t of th e Ar c h ipe l a go , be f o r ed i s s i p a t i n g in th e h i n t e r l a n d . ' L e na ' was no t a s e ve r e c yc lone ,d id no t cause s t r o n g winds a t Dampier and caused no observede f f e c t s on th e l o c a l mar ine communi t i es .

T r op i c a l c yc lone s ' J a n e ' and ' C h loe ' were i n t e n s e and bo thge ne r a t e d winds o f s i m i l a r d i r e c t i o n . Those from 'Ch l oe ' weremore i n t e n s e a t Dampier . The n e g l i g i b l e e f f e c t s from 'Ch l oe 'i n d i c a t e t h a t damage t o t he b io t a by ' J a n e ' was n o t caused byth e l o c a l wind c o n d i t i o n s . The majo r d i f f e r e n c e between t he setwo c yc lone s was th e a r e a of o r i g i n : ' J a n e ' formed nor t h - we s to f Dampier , whi le 'Ch l oe ' formed to th e n o r t h - e a s t . TheArchipe lago would , t h e r e f o r e , have been a f f e c t e d by s w e l lene r gy from ' J a n e ' , whi le s we l l from 'Ch l oe ' would have

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a f f e c t e d the e a s t e r n shores of Dolphin I s l a nd and the BurrupPe n i nsu l a ( F i g . 2 . 1 ) .

2 . 4 . 3

T r op i c a l c yc lone s form h igh energy , l o c a l l y ge ne r a t e d waves ,and a l s o t r a i n s of swe l l wind waves . Both of t he se wave t ype shave a f f e c t e d th e Dampier Ar c h ipe l a go , and may r e s u l t inc a t a s t r o p h i c d i s t u r b a n c e of the be n thos . The d e s t r u c t i o n ofc a l c a r e o u s s t r u c t u r e s , such as c o r a l s , r e s u l t s in th e format ionof c a l c a r e ous rubble and s e d im e n t s .

The e f f e c t a c yc lone has upon th e Arch ipe lago i s dependen t uponth e i n t e n s i t y of the cyc lone and the approach d i r e c t i o n .Cyclones which p a s s to seaward o f , and c l o s e to , theArch ipe lago a re th e most d i s r u p t i v e of Ar c h ipe l a go mar inecommunit ies .

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3. WATER MOVEMENT

3 .1 INTRODUCTION

Water movement and c i r c u l a t i o n in and around th e DampierArch ipe lago i s i n f l u en ced by t i d e s , wind , cy c lo n i c a c t i v i t y ,r e g i o n a l c o n t i n e n t a l s h e l f c u r r e n t s and s e a s o n a l fo rma t i o n ofwate r d e n s i t y s t r u c t u r e .

Areas of s t ro n g and weak c u r r e n t s a re i d e n t i f i e d . The maximaa re g e n e r a l l y a s s o c i a t e d with peak s p r in g t i d e s , ando c c a s i o n a l l y with ext reme wind c o n d i t i o n s . The l o n g e r - t e r mc i r c u l a t i o n of wa te r through th e Arch ipe lago i s though t to bede te rmined p r i n c i p a l l y by seaso n a l winds (Chap te r 2 ) .

In a d d i t i o n to the r e fe ren ced s o u rces , th e i n f o r ma t i o np res en t ed here i s d er i v ed from f i e l d m easurem ent s of w a t e rc u r r e n t s , wate r t emp er a t u r e and s a l i n i t y conduc ted by th eDe p a r tme n t of Conserva t ion and Envi ronment .

3 .2 METHODS: CURRENT MEASUREMENT

Nei l Brown a c o u s t i c c u r r e n t mete rs were mounted on t a u t l i n e ,U-shaped moor ings or suspended from b o t t o m - r e s t i n g t r i p o d s( F i g . _3.1) . Deployment p e r io d s ranged from ten days to t h r e emonths . T r i v a n e drogues o f a r ea 3 . 5 m2 and minimal windage weret r ack ed u s i n g s t an d a rd su rvey ing t e c h n i q u e s . Tempera tu re s a l i n i t y measurements were made with a Yeo-ka l Auto l ab Po r t ab l eSa l i n i t y -Tem p era tu re B r i d g e , Model 602.

A two dimens iona l numer ica l hydrodynamic model ( M i l ls , 1985)was ap p l i ed t o t h e w e s t e rn A r ch i p e l ag o . S i mu l a t i o n s were madeo f s p r in g t i d a l c u r r e n t s , w i n d -d r i v e n f lo ws and th e f l u s h i n g ofc o a s t a l b as i n s by c o n t i n e n t a l s h e l f c u r r e n t t h r o u g h - f l o w .

G eograph icR l names and i n s t r u men t l o c a t i o n s r e f e r r e d to in t h i sch ap t e r a re shown in Fi g u re 3 . 1 .

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116 30 '

20 25'

EaglehawkIsland

20 43'

116 30'Figure 3.1

0

Indian

Skm

0 ' 9

c , ~ '

116 45'Hammerr:leyShoal---- ~0-------:-

JC Sea BuoyNelson Rocks

.:,V"?--.. ~Rosemary

Island Malus O tJ x CourtenayShoalBuoyIs lands..,V ~Vn GoodwynV Olsland Highc:?Poinl ~

24

)(Beaconws

KEY Current meter locations.-Jo_...Depth contour in metres.

116 45 '

Nicka/

Bay

Locality MaptJ


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3 .3 RESULTS

3 . 3 . 1

The dominan t t i d e s o f th e Nor th -W es t S h e l f a re s e m i - d i u r n a l .With in Mermaid Sound, th e mean s p r i n g r ange o f th e t i d e i s 3 .52m and th e mean neap r ange i s 0 .93 m ( d e r i v ed from harmonica n a l y s i s of one y e a r ' s t i d e h e i g h t s a t W i t h n e l l Bay, Pu b l i cWorks Depar tment , W.A.: u n p u b l i sh ed ) .

The s e m i - d i u r n a l t i d a l c u r r e n t s c o n t r i b u t e most to th e t o t a li n s t a n t a n e o u s w a t e r movements in th e r eg io n . However, s incet h e s e c u r r e n t s r e v e r s e d i r e c t i o n tw ice e v e r y t i d a l c y c l e , t heya d v e c t w a t e r p a r t i c l e s l e s s t h a n a b o u t 10 km, which i s muchl e s s than th e l eng th o f th e A r c h i p e l a g o 60 km).

The model led peak ebb c u r r e n t v e l o c i t y f i e l d fo r a mean s p r in gt i d a l r ange i s d e p i c t e d in Figure 3 . 2 . The d i s t r i b u t i o n o f th epeak f l o o d c u r r e n t s i s very s i m i l a r , e x c e p t r ev e r sed ind i r e c t i o n .

, , , , , , , , , , , , , , , , , , , , , ~, , , , , , , , , , , , , , , , , , , , , ~

0 6km- - 0.5ms- 1Figure 3.2 Computer model simulation of spring ebb tidal currents, Dampier Archipelago (adapted from Mills, 1985).

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On the i n n e r c o n t i n e n t a l s h e l f , in wate r dep ths of abou t 40 m,the d i r e c t i o n of p ~ a k ebb c u r r e n t i s abou t 300 , and t h a t ofpeak f lood abou t 120 . The t i d a l c u r r e n t s dev ia te littleen te r ing and l e av in g Mermaid S t r a i t and th e area betweenRosemary I s l and and Enderby I s l a n d . However , th e wate r i sfo rced to tu rn a l mo s t pe rpend icu la r as it f l ows i n to and ou t ofMermaid Sound.

The a r e a s o f s t r o n g e s t t i d a l c u r r e n t a re g e n e r a l l y found towardth e ou te r e x t r e m i t i e s of the Archipelago and occur when th e w a t e rl e v e l i s f a l l i n g or r i s i n g most r a p i d l y .

Immedia te ly n o r th of Cape Legendre , peak sp r ing t i d a l c u r r e n t sr each 0.5- 0 .6 ms-1. Along th e seaward r each es of th eHamersley I ron s h ip p in g ch an n e l , from Courtenay Buoy to SeaBuoy, peak sp r ing t i d a l c u r r e n t s a re i n excess of 0 .4 ms- 1 anda t t imes exceed 0 .5 ms- 1 . The c u r r e n t s over the shal low r e e fc r e s t and r e e f f l a t s immedia te ly wes t of Nelson Rocks haves i m i l a r sp eed s . Pass ing to the n o r t h - e a s t e r n s ide of GoodwynI s l and i s a deep channe l which has t i d a l f low speeds o f up to0 .7 - 0 .8 ms- 1 . Comparable speeds a re ach ieved in the channe lbetween Eaglehawk I s l and and Enderby I s l a n d . Tid a l speeds ofup to 0 .5 ms- 1 may o c c u r in th e v i c i n i t y o f Southwes t Ree f .

Some of the i n t e r - i s l a n d ch an n e l s a re a l s o s u b j e c t to s t rongf lows ; fo r example , Fly ing Foam Passage . The few c u r r e n tmeasurements t h a t have been made in t h i s channe l su g g es t t h a tpeak sp r ing t i d a l f lows exceed 2 ms- 1 . The c u r r e n t f loodsnor thwards and ebbs t o t he s o u th . Spr ing t i d a l ex cu r s io n sa long t h i s c h a n n e l may be l a rge enough to exchange Nickol Bayand Mermaid Sound wate r s . The t i d a l c u r r e n t regime of MermaidSound i s i n f l u en ced up to a t l e a s t 5 km from th e sou thernopening of Fly ing Foam Passage . At a l o c a t i o n 1 km n o r th ofConzinc I s l a n d , and 3 km due wes t o f th e Passage mouth, peakebb v e l o c i t y on s p r i n g t i d e s i s n o r th -wes t a t abou t 0 .5 ms- 1 ,whereas th e cor respond ing f lood v e l o c i t y t o t he sou th s o u th e a s t does n o t e i c e ~ d 0.25 ms- 1 (Woodside Petroleum Development ,1979) . This e n e r g e t i c , d i r e c t i o n a l l y - b i a s e d c u r r e n t regime i s

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l i k e l y to have a s i g n i f i c a n t i n f l u en ce on sed iment t r a n sp o r t .

In th e channe l be tween West Lewis I s l and and Enderby I s l a n d ,maximum t i d a l c u r r e n t s reach 0 .6 ms- 1 . Between Malus I s l andsand High Po in t , s p r i n g t i d a l c u r r e n t s of 0.45 ms- 1 a rea t t a i n e d .

The l o c a l i t i e s d es c r ib ed above ex p e r i en ce s t rong t i d a l c u r r e n t s ;however, eq u a l l y s i g n i f i c a n t to sed iment t r a n s p o r t andd ep o s i t i o n a re r eg io n s with low c u r r e n t v e l o c i t i e s . Some oft h ese a re l o ca t ed a d j a c e n t to a r e a s with h igh wa te r c u r r e n tf l ow.

Weak t i d a l s t r eam s , t h a t r a r e l y exceed 0 .2 ms- 1 , occur t o t hesou th -wes t of Cape Legendre , on and i n s i d e Hammersley Shoa l .Along th e n o r th e rn s ide o f Malus I s l a n d s , and between th en o r th -wes t e rn end o f t h i s i s l and group and Rosemary I s l a n d ,peak sp r ing t i d a l c u r r e n t s r a r e l y exceed 0 .2 ms-1 . S i m i l a r l y ,weak t i d a l c u r r e n t s occur in an a rea between th e s o u th -wes t e rnshore o f Goodwyn I s l and and th e n o r th e rn shore of EnderbyI s l an d . The i n n e r r each es of Mermaid Sound and Mermaid S t r a i tundergo weak t i d a l o s c i l l a t i o n s with speeds o f up t o 0 .2 ms-1;however , t i d a l c u r r e n t s s t r en g th en in th e main channe l co n n ec t i n gt h ese two w a t e r ex p an ses , with peak c u r r e n t s o ccu r r i n g c lo s e tot imes of high and low w at e r .

Res idua l wa te r movement a s s o c i a t e d with th e t i d a l s t reams i sg e n e r a l l y low: of th e o r d e r of 0.01 ms- 1 . A n o tab l e ex cep t i o nto t h i s i s th e p r e v i o u s l y - d e s c r i b e d , d i r e c t i o n a l l y - b i a se dregime o ccu r r i n g w i t h i n , and a d j a c e n t to , t he F ly ing FoamPassage .

3 . 3 . 2 Local wind-dr iven c u r r e n t s

Local ly o ccu r r i n g wind modi f i es th e motion of th e A rc h i p e l a g ow at e r s , first in a t h in s u r f a c e l a y e r , and a f t e r seve ra l hourswi th in the e n t i r e w a t e r body. Elev a t i o n s o r d ep re s s io n s ofwate r l eve l ad jacen t to c o a s t l i n e s g ive r i s e to wa te r su r f ace

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s lo p es and p r e s s u r e g r a d i e n t f o r c e s . These , in co n j u n c t i o n withth e ap p l i ed wind s t r e s s and bottom f r i c ti o n s t r e s s , d e t e rmi n eth e r e s u l t a n t windinduced t r a n s p o r t .

Wind-dr iven model s i m u l a t i o n s and c u r r e n t mete r measurements(Woodside Pe t ro l eum Development 1979; Lawson and T r e l o a r , 1981;M i l l s , Depar tment of Conserva t ion and Envi ronment , unpubl i shedd a t a ) su g g es t th e e x i s t e n c e of tw o main modes of w i n d -d r i v e nc i r c u l a t i o n .

P r e v a i l i n g s p r in g and summer winds from th e s o u t h and wes t d r i v ewate r eas tward th rough Mermaid S t r a i t and between Rosemary andEnderby I s l a n d s , whi le wa te r in Mermaid Sound i s f o r cednor thwards t owards th e seaward e n t r a n c e .

E a s t e r l y and n o r t h e r l y winds i nduce sou thward f low i n to andt h rough Mermaid Sound and d r iv e wate r wes tward in between th ei s l a n d s of th e A r ch i p e l ag o . These winds may o c c u r o c c a s i o n a l l yin a s s o c i a t i o n with cyclones dur ing summer and autumn, o r as awin te r p a t t e r n of d i u r n a l l y f l u c t u a t i n g winds (Chap te r 2 ) .

Examinat ion o f c u r r e n t me te r r eco r d s shows t h a t th e e f f e c t ofwind s t r e s s on w a t e r movement i s most marked abou t t imes o f neapt i d e . This i s p a r t l y a r e l a t i v e e f f e c t , owing to the weaknessof th e neap t i d a l c u r r e n t s ; however, th e energy impar ted bywind to wate r i s more e f f i c i e n t l y t r ans fo rmed i n to ad v ec t i v ek i n e t i c energy a t t imes o f neap t i d e s , when th e f low l e s st u r b u l e n t and energy d i s s i p a t i o n , by bottom f r i c t i o n , o c c u r s a ta l e s s e r ra t e .

St ro n g (>10 ms- 1 ) p e r s i s t e n t winds enhance peak s p r in g t i d a lc u r r e n t s by 0 .5 to 0 .1 ms- 1 (Lawson and T r e l o a r , 1981) .Modera te to s t ro n g p e r s i s t e n t winds (>10 ms- 1 ) d i s t o r t o r comp l e t e l y n u l l i fy e th e neap t i d a l c u r r e n t r e v e r s a l s , r e s u l t i n g ina l a rge n e t ex cu r s i o n o f wate r wi th peak speed o f abou t0 .2 ms- 1 DCE c u r r e n t r e c o r d s ) .

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As examples of extreme fo r c ing , t he passages of t r o p i c a lcyclones ' I a n ' and ' J a n e ' n ea r t h e A r ch i p e lag o , in March 1982and January 1983 r e sp e c t i v e l y , were examined. ' I a n ' brough ts o u t h e r l y winds > 20 ms- 1 (72 kmh- 1 ) on 5 March, before s lowlyswinging nor the r ly and modera t ing to 7 ms- 1 (25 .2 kmh- 1 ) by 8March. Curren t s r ecorded 2 m above the s e a - f l o o r , in the 18 m(dep th ) channe l between Malus I s land and West Lewis I s l a n d ,f lowed c o n s i s t e n t l y to th e west fo r 36 h, befo re t u r n i n g e a s tfo r a f u r t h e r 12 h . C u r r en t speeds peaked a t 0 .4 ms- 1 in bothd i r e c t i o n s and th e n e t c u r r e n t run was 18.2 km to the wes t .Thi s e v e n t o ccu r r ed with in a few days of neap t i d e s .

The winds dur ing ' J a n e ' were i n i t i a l l y e a s t - n o r t h - e a s t a t10 ms- 1 (36 kmh- 1 ) on 7 Jan u a r y , befo re i n c r e a s i n g to morethan 20 ms- 1 (72 kmh- 1 ) two days l a t e r . C u r ren t s r ecorded 4 mabove th e s e a - f l o o r near th e sh ipp ing Beacon W5, i n i t i a l l yf lowed sou th fo r 46 h, fo l lowed by a 26 h n o r t h e r l y r e v e r s a l .Curren t speeds o f about 0 .35 ms- 1 were r ecorded in bothd i r e c t i o n s , and t h e n e t c u r r e n t run was 16.6 km. As on th eprev ious occas ion , t he se abnormal c u r r e n t s were measured with ina few days of neap t i d e s .

3 . 3 . 3 Large - sca le c o n t i n e n t a l s h e l f c u r r e n t s

These c u r r e n t s a re thought to be dr iven p r i n c i p a l l y by ap e r s i s t e n t sea l e v e l g r a d i e n t a long the Western A u s t r a l i a nc o a s t l i n e (Godfrey & Ridgway, 1984) . Holloway & Nye (1984) ,gave monthly averages of c u r r e n t s measured a t seve ra l l o c a t i o n son t he sou thern p a r t of the North-West S h e l f dur ing 1982-83 .The most s i g n i f i c a n t f ea tu re was a s o u th -wes t f lowing c u r r e n t( though t t o be p a r t o f th e Leeuwin C u r r en t ) of g r e a t e s t s t r e n g t hand dura t ion from Februa ry to J u l y . Weakening or shor t - t e rmr e v e r s a l s of th e c u r r e n t may occur dur ing p e r io d s of s t ro n gsou th -wes te r ly winds. On th e inner s h e l f , th eexpec ted to be s i m i l a r , with monthly speeds of

f low reg ime i s0 .0 1 -0 .1 ms-1

and a predominan t ly wes te r ly f low; however, th e f requency anddura t ion o f wind-dr iven r e v e r s a l s i s l i k e l y to be g r e a t e r thanfo r th e o u t e r s h e l f .

29

I

11


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Numerica l model s i m u l a t i o n s , assuming a s t ead y l ong - sho r ep res su re g r a d i e n t to d r i v e s h e l f c u r r e n t s , sugge s t t h a t th eDampier Archipe lago wa te r s a r e p a r t i a l l y s h e l t e r e d from th ei n f l u e n c e of c u r r e n t s on th e s h e l f . The model p r e d i c t e d t h a tw a t e r f lows t h rough the i nne r p o r t i o n of Mermaid Sound a tr educed s pe e ds ; l e s s than 25% of th e c u r r e n t speeds on th es h e l f .

3 . 3 . 4 Dens i ty e f f e c t s

Seasona l v a r i a t i o n in a i r t e m pe r a t u r e , combined wi th annua le xc e s s of p o t e n t i a l e va po r a t i on over r a i n f a l l in the Dampierr e g i o n , r e s u l t s in s p a t i a l and t empora l v a r i a t i o n s in theh y d r o l o g i c a l s t r u c t u r e of t he Ar c h ipe l a go . From A p r i l to August ,wate r s n e a r th e mainland a re both c oo l e r and more s a l i n e t hant hose o f f s h o r e ( F i g . 3 . 3 ) . This de nse r , i nne r w a t e r l e a ve s th eSound as ne a r - bo t tom g ra v i t y f low a long the d e e p e s t p a r t s ofthe o u t e r ba thymet ry , moving c oun t e r to a windinduced in f lowof s u r f a c e wa te r .

Dur ing Oc tobe r to March, th e i nne r w a t e r i s more s a l i n e bu twarmer than th e o f f s h o r e wate r s ( F i g . 3 .3 ) , which d i s p l a y ad en s i t y s t r u c t u r e more t y p i c a l of t h a t below t he s e a sona lc o n t i n e n t a l s h e l f t he r m oc l i ne . The p r e v a i l i n g sou t h - we s t windsof t h i s pe r i od f o r c e th e i n n e r w a t e r toward th e nor t he r n andseaward bounda r i e s of the Sound, l e a v i ng as a low d en s i t ys u r f a c e l a y e r .

30


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g1

i 0]ll 20"'..

g1

29 June 1982.

~ . 0 . u .u 19.8..==== : : : : : ~ : ~ H W H H.. : = ; = = = = : = ~ = ~ H m ; H m l ~ ~ l l 1 1 H H H 1 ; ;;mmmmm

KeyStations located on atransect from Dampierrunning approximatelydue north to the 35 mcontour beyondthe Sound.

'E~ "

-.~"

~ "

12 December 1982.

Station Locations Station Locations Figure 3.3 Hydrographic sections through Mermaid Sound showing density, temperatur e and salinity contours.


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4. LIGHT ATTENUATION

4 .1 INTRODUCTION

V e r t i c a l a t t e n u a t i o n of l i g h t th rough wa te r i s caused by acombina t ion of a b s o r p t i o n and s c a t t e r i n g p r o c e s s e s . Watera lone a b s o rb s and r e f l e c t s l i g h t , b u t t h e s e e f f e c t s a re g r e a t l yenhanced by suspended phy top lank ton , i n a n i ma t e p a r t i c u l a t em a t t e r and d i s s o lv ed co loured m a t e r i a l (Ki rk , 1981) . D i s s o l v e dco loured m a t e r i a l has been assumed u n i mp o r t a n t in DampierA rc h i p e l a g o wate r s , as t h a t o f f r e s h wa te r o r i g i n (eg humics u b s t an ces ) f l o c c u l a t e s and p r e c i p i t a t e s under mar inec o n d i t i o n s (Duinke r , 1 9 8 0 ) .

Suspended m a t e r i a l i n d i r e c t ly a f f e c t s marine communi t ie s byi n c r e a s i n g l i g h t a t t e n u a t i o n , which l i m i t s pr imary p ro d u c t i o n .Li g h t l e v e l s l i m i t the dep ths to which b en th i c pr imaryp ro d u c e r s may o c c u r (Chapman & Chapman, 1973) . L imi ted l i g h tmay be the reason why macroa lgae and many c o r a l s p e c i e s of th eA rc h i p e l a g o have ranges conf ined to t h e s h a l l o w s ( W o o d s i ~ ePe t ro l eum Development , 1979) .

At p r e s e n t , th e l i g h t r equ i rement s of th e Arch ip e l ag o ' s pr imaryproducers are unknown. R ecen t r e s ea rch has shown t h a t pr imaryproducers in marine env i ronments d i s p l a y p h o to ad ap t a t i o n ss i m i l a r t o t hose found in terrestrial p l a n t s . For example ,c o r a l s on the Grea t B a r r i e r Reef have been shown to be adap tedto e i t h e r high (300 E i n s t e i n s m- 2 s - 1 ) o r low (100 ~E i n s t e i n s m-2 s -1 ) l i g h t l e v e l s (Cha lke r e t a l , 1983) . Pho toad ap t i o n s have a l so been found amongst th e a l g a e , with rangeso f 50-200 and 500-1000 E i n s t e i n s m- 2 s - 1 r ep o r t ed fo r shadeand sun l ov ing s p e c i e s r e s p e c t i v e l y (Raven & G l i d ew e l l , 1 9 7 5 ) .Less i s known of th e l i g h t r eq u i r emen t s of Sargassum andCau le rpa s p e c i e s , th e p r i n c i p a l ma c ro a l g a l s p ec i e s in th eA rc h i p e l a g o (Borowi tzka , p er so n a l communica t ion , 1984) . Larkum(1983) r e p o r t s t h a t n e i t h e r th e p h o t o s y n t h e t i c c a p a c i t y nor th ec o n t r i b u t i o n made t o p r imary p r o d u c t i v i t y , by t h ese p l a n t s , i sknown.

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In a d d i t i o n to t h i s i n d i r e c t e f f e c t of l i g h t a t t e n u a t i o n , th ep h y s i ca l presence of suspended m a t e r i a l can d i r e c t l y a f f e c tb i o t a . F i l t e r - f e e d i n g organ i sms a re d e p e n d e n t upon suspendedor ga n i c m a t t e r fo r food; however , suspended mat t e r can haveadverse e f f e c t s on b i o t a . For example , c e r t a i n c o r a l s a res u s c e p t i b l e to ex ces s iv e sed imen t l oa d . Lasker (1980) notedt h r e e mechanisms by which c o r a l growth i s a f f e c t e d by suspendedsed iment :

( i ) Plan u la s e t t l e m e n t i s preven ted by s e d i me n t i nunda t i onof s u i t a b l e s u b s t r a t e s .

( i i )

( i i i)

Cora l growth i s i n h i b i t e d by s e t t l i n g sed imen t becausesed imen t r e j e c t i o n r eq u i r e s energy .

Rapid s e t t l e m e n t of suspended m a t e r i a l can causei nunda t i on and s u f fo ca t i o n of c o r a l co l o n i e s .

These adverse e f f e c t s a re not l i mi t ed to c o r a l s . Bakus (1969)r e f e r s to suspended m a t e r i a l s t r e s s i n g both sponges and a s c i d i a n sby b u r i a l , cana l c logg ing and chamber c lo g g in g .

This c h a p t e r , however , w i l l be l i m i t e d to the r e l a t i o n s h i pbetween l i g h t a t t e n u a t i o n and suspended s o l i d s , and s p a t i a l andt empora l v a r i a t i o n in l i g h t a t t e n u a t i o n wi t h i n Mermaid Sound.

4 .2 MATERIALS AND METHODS

To de te rmine t he e xpe r i m e n t a l e r r o r in th e e s t i m a t i o n ofsuspended mass and or ga n i c c o n t e n t , a s e r i e s of te n wate r sampler e p l i c a t e s were c o l l e c t e d in August 1982. Some 50 litres ofwate r were c o l l e c t e d from ne a r Nelson Rocks , div ided in to 5litre subsamples , f i l t e r e d , f rozen and subse que n t l y a na l y se d .To a s s e s s th e r e l a t i o n s h i p between suspended m a t t e r and v e r t i c a ll i g h t a t t e n u a t i o n , a s e r i e s of wate r samples were t aken inc on j unc t i on with l i g h t a t t e n u a t i o n p r o f i l e s in March andDecember, 1983. In March, 15 samples were taken from sevens i t e s a t a depth of 1-2 m, and a no t he r 1-2 m sample was taken

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s i m u l t a ne ous l y with a 4-5 m sample a t Nelson Rocks. InDecember , 11 samples were t aken from t h r ee s i t e s a t two d e p t h s ,namely , 1-2 m and 7-8 m ( F i g . 4 . 1 ) .

wa te r samples were t aken in March 1983 us i ng a 5 litre Niscansampl ing b o t t l e a t 1-2 m de p th . Three litres were used fo rassessment of suspended load and organ ic co n t en t , whi le ther emain ing 2 litres were used fo r c h l o r o p h y l l ! and phaeophyt ind e t e r m i n a t i o n .

The 3 litre samples were f i l t e r e d t h rough pre -combus ted ,pre-weighed g l a s s f i b r e filters of pore s i z e 0 . 8 - 1 . 2 micron .Fol lowing f i l t r a t i o n , the filters were r i n sed with abou t 100 mlof de ion ized wa te r , to remove any s a l t r e s i d u e s , and f r oz e n .

Samples were l a t e r d r i ed to c o n s t a n t weigh t in a 120C co n s t an tt e m pe r a t u r e oven, be f o r e be ing weighed to 0.00001 g on aS a r t o r i u s d i g i t a l ba l a nc e (model 2004 MPB). Organic weigh t ofth e filtrate was de te rmined by l o s s on i g n i t i o n a f t e r h ea t in ga t 600C fo r 90 min in a muff l e fu rnace (Ki ln Manufac ture r s ,WA: model LFB-14) . To preven t con tamina t ion from c onde nsa t i on ,th e filters were cooled on a l l oc c a s i ons in a d e s i c c a t o r p r i o rto weigh ing .

The 2 litre samples were f i l t e r e d th rough i d e n t i c a l , bu tu n t r e a t e d , filters and f rozen p r i o r to a n a l y s i s f o r c h l o r o p h y l l! and phaeophyt in p igmen t s us ing ace tone e x t r a c t i o n method( S t r i c k l a n d & Pa r sons , 1972) .The l i g h t p r o f i l e s done in c on j unc t i on with th e sampl ing fo rsuspended mat t e r were de te rmined us i ng a Licor -1888 i n t e g r a t i n gquantum meter , f i t t e d with a Licor-192S underwate r quantums e n s o r , measu r ing the p h o to sy n th e t i c photon f l u x d e n s i t y (PPFD)in the 400-700 ~ m range . L i g h t i n t e n s i t y r e a d i n g s were t akena t 1 m i n t e r v a l s , s t a r t i n g a t 1 m depth .r e f e ren ce r e a d i ng was a l s o t a ke n .

34

An a bove - su r f a c e


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The a t t e n u a t i o n o f PPFD i s approx imated by th e eq u a t i o n

Q

where Q and Q0 a re th e downward photon f l u x in th e p h o to s y n th e t i cwaveband a t depth z metres and a t th e su r f ace r e s p e c t i v e l y andwhere k i s th e ( n a t u r a l l ogar i thm) v e r t i c a l l i g h t a t t e n u a t i o nc o e f f i c i e n t ( u n i t s a re in quan ta m-2s- 1 ; Kirk , 1977) . Thev e r t i c a l a t t e n u a t i o n of l i g h t through n a t u r a l w a t e r s with depthi s t h e r e f o r e ap p r o x i ma t e l y e x p o n e n t i a l . On t h i s b a s i s , a t t e n -u a t i o n c o e f f i c i e n t s were c a l c u l a t e d by r e g r e s s i n g th e n a t u r a ll ogar i thm of th e PPFD a g a i n s t dep th , th e s lope o f th e l i nebe ing th e v e r t i c a l l i g h t a t t e n u a t i o n c o e f f i c i e n t . This a t t e n -u a t i o n c o e f f i c i e n t was used as an index of t u r b i d i t y and comparedwith th e suspended l o ad s o b t a i n ed u s i n g t h e f i l t r a t i o n method.

A s e p a r a t e s e r i e s o f l i g h t p r o f i l e s were t aken a t 23 s i t e s ,ex t en d i n g from th e headwate rs of th e Sound to a s i t e 7 .5 kmnor th of Sea Buoy (F ig . 4 . 1 ) , on s ix o ccas i o n s between A p r i l1982 and March 1983. Tempera tu re and s a l i n i t y p r o f i l e s weret aken in co n ju n c t i o n with t h e s e , us ing a Yeo-Kal Auto labP o r t a b l e S a l i n i t y - T e m p e r a t u r e Br idge , Model 602 (Chap te r 3 ) .Readings were t aken between 0930 h a n d 1630 h, dur ing neap t i d eand on c l o u d l e s s days in order t o minimise e n v i ro n me n t a lv a r i a b l e s (Ki rk , 1977) .

The a t t e n u a t i o n c o e f f i e n t s c a l c u l a t e d from t h e s e p r o f i l e s werep l o t t e d us ing a con tour ing computer package (SYMAPS) and thenexamined fo r s p a t i a l and t empora l v a r i a t i o n .

A f u r t h e r s e t of seven l i g h t p r o f i l e s was taken a t approx imate lyhour ly i n t e r v a l s between 0920 h and 1500 h a t Nelson Rocks on28 June 1982.

35


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116' 30'

20' 25'

EaglehawkIsland

43'

Indian

Ocean0 Skm

116' 45'/ ~l / 9 ~ ~ ~ ~ 1 : 1

KEYSiles sampled for Vertical profiles (1 lo 23)0 Suspended solids and light, March 1983& Suspended solids and ligh t, December 1983

..-3().-' Depth contour in metres

11 45 '

36

Nick of

Bay

locality Map8


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4 .3 RESULTS

4 . 3 . 1 V e r t i c a l l i g h t a t t e n u a t i o n and suspended load

The ten r e p l i c a t e s ana lysed fo r t o t a l load and organ ic con ten tto determine ex p e r im en ta l e r r o r d i s p l ay ed s t anda rd d e v i a t i o n sof abou t 15% of th e mean fo r both an a ly s es (Table 4 .1 ) . Of thel i g h t p r o f i l e s determined in March 1983, two e x h i b i t e danomalous values p robab ly because o f patchy cloud c o n d i t i o n s ;hence were d i s c a r d e d . Of the wate r samples ana lysed , th e twot aken s im ul t aneous ly , a t d i f f e r e n t dep ths a t Nelson Rocks, werevery s i m i l a r and were averaged fo r compar ison with th e l i g h tp r o f i l e (Table 4 .2 ) .

Table 4.1 Values oftotalload and organic load. derived from tan replicate samples taken at Nelson Rocks. 1 March 1983,to determine experimental error.

To ta l load Organic load OrganicmgL-1 mgL-1 %

0.550 0 .404 73.40.584 0.430 73.60.702 0.424 60.40.680 0.430 63.20.596 0.402 67.50.622 0.538 68.50.678 0 .420 61.90.576 0.330 57.30.748 0 .436 58.30.488 0.330 67.6

x 0.622 0 .414 67.0SD 0.079 0.059 8 .9

When compared, the v e r t i c a l l i g h t a t t e n u a t i o n c oe f f i c i e n t s werefound to be s i g n i f i c a n t l y nega t ive ly c o r r e l a t e d to both t o t a ldry weigh t ( r 11 -0 .8 4 6 , p < 0 .001 ; Fig . 4 .2 ) and organ ic dryweigh t ( r 11 0 .790 , p < 0.002; Fig . 4 .3 ) . Pigment ana lyse swere n o t de termined fo r the March samples , th rough t e c h n i c a lovers igh t .

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;:m."(;~ 0uc.2;;c2;;:

0.0-0.02

-0.04

-0.06 ' ~-0.08

-0.10

-0.12

-0.14

-0.16

-0.18

-0.20

-0.22

-0.240.0

2Dry weight (mgl _,)

y =-15.02 X -0.496r = -0.846n = 13p < 0.001

3

The relationship between light attenuation coefficient and total suspended mass in March, 1983.

0.25

0.50 0.75

y = -0.167x - 0.02r = 0.79n = 13p < 0.002

1.00Organic dry weight (mgl - 1)

1.25

Figure 4.3 The relationship between light attenuation coefficient and suspended mass of organic matter In March, 1983.

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Table 4.2 Suspended loads derived from seawater sample collected in March 1983 and light attenuation coefficientsrecorded concurrently.

Date Station 1*1 Total load Organic load Organic LightI I mg L -1 rng L- 1 attenuationI I co ef f i c i en tI I4 March Conzinc Boat IT I 2 .0 6 0 .8 3 40 -0 .1 8Nonaae Rock I Tl 1 .4 0 ,7 3 5 0 .2 -0 . 17Conzinc West I Tl 1.82 0 .6 30 - 0 . 1 5I I March Conzinc East I Tl 2.64 0 .94 36 -0 . 17I I

9 March Ender by I Tl 1 .7 4 0 .6 5 38 - 0 . 1 2Nelson IT I 1 .2 6 0 . 81 64 - 0 . 1 6Nelson I B 1 .2 1 0 .8 66Nelson I Tl 1. 15 0.86 75 -0 . 11Noname Rocks IT I 3 .0 2 1 .1 3 38 - 0 . 2 2Conzinc East IT I 1; 53 0.62 40 -0 . 15I I

10 March Sea Buoy ITl 0 .7 2 0 .5 2 72 -0 .0 9zinc West I Tl 0 .9 2 0 .4 5 - 0 . 1Lone Hazard I Tl 0 .8 8 0 .5 7 65 -0 .08Nonae Rocks IB I 2.54 0 .91 36 -0 .1 7

T 2 to p (J froM top)B bottoM (1 from bottom)

When th e second s e r i e s o f sample s were t a ke n in Decemberblooms of th e b l u e - g r e e n a lga Trichodesmium were w i d e s p r e a d .These were n o t observed in March. The blooms were p a t c h i l yd i s t r i b u t e d i n t o dense s u r f a c e windrows, o r appeared to be w e l lmixed in a t l e a s t th e s u r f a c e wa te r . Comparison of th e v e r t i c a ll i g h t a t t e n u a t i o n c o e f f i c i e n t s with th e t o t a l suspended or ga n i cor p igmen t c o n t e n t of sampled w a t e r r e v e a l e d no s i g n i f i c a n tc o r r e l a t i o n s (Tab le 4 . 3 , page 4 2 ) .

4 . 3 . 2 ~ a t i a l and t empora l v a r i a t i o n in l i g h t a t t e n u a t i o n

The c on tou r maps of th e v e r t i c a l l i g h t a t t e n u a t i o n c o e f f i c i e n t sa r e p r e se n t e d ( F i g . 4 . 4 ) . Al though t e m pe r a tu r e and s a l i n i t ywere d e t e r mi n e d fo r s i t e s on s ix o c c a s i o n s , u n s a t i s f a c t o r yi n c i d e n t l i g h t c o n d i t i o n s l i m i t e d th e number of l i g h t p r o f i l e sa t d i f f e r e n t t i m e s . However , f rom th e c on tou r maps, a p e r s i s t e n tg r a d i e n t o f i n c r e a s i n g l i g h t a t t e n u a t i o n , from th e o u t e r s i t e st o t hos e c l o s e s t to th e base of th e Sound, can be s e e n . Thisp a t t e r n remained c o n s i s t e n t t h r oughou t th e e n t i r e sampl ingprogramme.

39


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.

------............: : : : : : : : : : : : : : :/ /2 ' : /..........

Light At tenua t ionHerma id SoundApdl 6, 19112

Sta tionNo.

' - .09' - .12' - .107 -.128 - .12, - .14

10 - . 1411 - .1312 - .14" - . 21" - .18" -. 24" -. 2917 - .17" -. 26" - . 2020 - . 2121 - . 18" -. 16" - .14

[Light Attenuat ion Coef f ic ien t'Her!Mid SoundI

' 12 1 1982

....... ..

...... 2

..

..1 t :n ion ..\ llo. ..-.04i - . 05 .- .05' - . 0 ~l - .06-.07' - .06+ ' -.0710 -.osl 11 - . 0612 - . 08! " - . 121 " -. 09l " -.07" -. 09l7 -. 13lB -. 10! 9 -. n20 -.112l - . l J" l7i " -. 13iII Light Attenuati .on Coeffi.c i.enti ~ I H m 3 i . d Souud' une " 198:.'L

St t i onNo.

1,'' - .10' - .09- .1 0'

- .1 0

' - .1310 - . 1211 - . ll12 - .1 2" - .11" - .1715 - .1 6

[L igh t Attenuat ion Coeff ic ient; H"rmaid Sound.: Harch 7, 1983

t7

:J.!i::I::l

Figure 4.4 Contour maps of light attenluation coefficients (m) recorded in Mermaid Sound between April, 1982 and March,1983.40


49/110

il 1iII lJI 14r-i 17flil

-.07- .07-.011- .08-.09- , LO-.09- . LO-. 08- .09-.15- .12-.12- . l2- . lJ

I .~ L i g h t Att"-nuation Coeff icient! !ermaid Sound' S e p t e m b e r l ) , 1962L-------------------------

ii ih I l l dW3'H.. . "ti :!!I!!

l : : : : : ~i i ' ~ ' H

i i ~ ' l !i:::::l:::::g:::::[ 1 ; ! ; 1 ~:::::=:1 ! 8 1 ~l : : : : : ~j ~ ~ ; ~ j!""'""":

~ ~ ~ ; Ii ~ f 1 1 !i ~ ~ ~ !H12H

; i u H ~

KeyAbsolute value range applying to each level.('maximum' included in highest level only)Minimum Maximum

-.04 -.05

-.05 -.07

-.07 -.OB-.OB -.09

-.09 -.11

-.11 -.12

-.12 - .13

-.13 -.14

-.14 -.16

-.16 -.17> .1 7Figure 4.4 Coni.

41

S ta t ion,o.- . o ~ o-.uG' -. 05' -.056 -.OB-. 06-.06

9 - .0611 -.07" -.12D -.}!1" -.])16 - . J l" -.1119 -. l l20 -.llo2 l - . lG

Lght A t ~ e n u a t i o n Coeff icientHermaid SoundO


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Table 4.3 Suspended solid and pigment loads derived from December 1983 seawater samples. and l ight attenuationcoefficients recorded concurrently.

Date ISt a t i on 1*1 Tota lI I loadI I I L- 13 Dec I ILone Hazard IT I 1. 99I B I 2 . 12Conz inc I s . IT I 2.02IB I 3 . 714 Dec I ILone Hazard ITII B IConz inc I s . IT II B IAngel I s . ITIIB I5 Dec I IAngel I s . IT II B IConz inc I s . IT I

I B ILone Hazard IT II B I

1 . 942.87.548.642.571 . 27

3 .182.032.272.643 .393.58

6 Dec IT I 1.93Conzinc I s . IBI 2.92Lone Hazard IT I 2. 00IB I 4 . 25Angel I s . ITI 1.96IB I 3 . 23

Organ icloadag L-1

1. 220.791 . 181 .4

0 .811.134.732.731.330.76

1 . 571 . 050 .970 . 9 21.181 . 210 . 8 30 . 9 90 . 8 31 . 3 30 . 8 31 . 21

Organic61.437 . 358 .637 .7

41 .940.562.731 .651 .66 0 . 0

49 .551 ,942.735.034 .733.842 .933 .84 1 . 631 .442 .437 .5

*T = t o p (1 f ro t op ) ; B ~ b o t t o m (7-8 dep th ) .

Chlo r ophy l l

0 .370.422.721 . 11

0 . 6 10.741 . 150 .910.730 .91

0 . 9 00 . 970 .810 .850 . 910 .900 .480 .610.541 . 030.730 .79

Phaeophy t in

0 .180.290 .080 .81

0.350.370 .760 .500 . 450. 45

0 .530 .480 .530.420 .450 .530 . 2 40.530 . 3 60 .610 . 450. 44

Lighta t t enua t i onc o e f f i c i e n t

- 0 . 13-0 .23

- 0 . 1-0 . 11-0 ,08

-0 .09- 0 . 11- 0 . 1 6

0 . ] 3-0 . 19- 0 . 1 8

The most obvious t empora l t r en d fo r th e a rea sampled, i n c lu d in gth e o u t e rmo s t s t a t i o n 7 .5 km n o r t h of th e mouth of th e Sound,was t owards g r e a t e r t u r b i d i t y in summer than in w i n t e r . D esp i t et h i s s e a s o n a l t r e n d , th e o u te r and i n n e r s i t e s r emained ,t h roughou t t he y ea r , r e l a t i v e l y c l e a r and t u r b i d r e s p e c t i v e l y .The t r a n s i t i o n a rea between th e o f f s h o r e and i n sh o r e c o n d i t i o n s ,in t e rms of l i g h t a t t e n u a t i o n , occur red in th e c e n t r a l r eg i o nbetween th e Malus I s l a n d s and t h e so u t h e r n end of Angel I s l an d(F ig . 4 . 1 )t u r b i d i t y .

Thi s a rea a l so had th e g r e a t e s t seaso n a l range in

One a rea of p e r s i s t e n t l y h i g h e r t u r b i d i t y o c c u r r e d n e a r ConzincI s l an d and Fly ing Foam Passage ( F i g . 4 . 1 ) . The December 1982l i g h t p r o f i l e s showed an o t h e r a rea of u n u su a l l y t u r b i d wate rn e a r W i t h n e l l Bay (F ig 4 . 4 ) . I t was th e s i t e of th e most t u r b i dwate r recorded d u r i n g th e s t u d y .

42


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The l i g h t p r o f i l e s t aken a t Nelson Rocks showed t h a t l i g h ta t t e n u a t i o n d e c re a s e d by a b o u t 60% o v e r th e s ix -h o u r p e r i o d .Th is change co i n c i d ed wi th an incoming t i d e , c a u s i n g a dep thi n c r e a s e of a p p ro x i ma t e l y 2 m over th e sampl ing p e r i o d( F i g . 4 . 5 ) .

-0.20 ---- Attenuation Coefficient-0.18 - - - - Predictedtide height-0.16

;: -0.14"~a; -0.12".2 -0.10;;..

5

"4

3 I:E"';;-0.08~ --- /_ ..... ""...,2 i=:E"' -0 .06:::; -----0.04-0.02

0 4 - - - - - ~ - - - - - - r - - - - - . - - - - - - . - - - - - - r - - - - - . - - - - - - . - - - - - - ~ o0 900 100 1100 1200 1300 1400 1500 1600

Time (h)Figure 4.5 Change in light attenuation at Nelson Rocks over a six hour period, 28 June, 1982.

4 .4 DISCUSSION

The r e p l i c a t e a n a l y s e s of su sp e n d e d l oad and o rg a n i c c o n t e n t o fw a t e r sa mp le s t aken a t Nelson Rocks gave s t an d a r d d e v i a t i o n s ofa b o u t 15%, and i n d i c a t e t h a t s i n g l e s p o t sa mp le s shou ld g i v er easo n ab l e e s t i m a t e s o f th e a c t u a l v a l u e s a t th e t ime o fs a m p l i n g . Th is co n c l u s i o n i s s u p p o r t e d by th e s i m i l a r v a l u e so f th e two sa mp le s c o l l e c t e d s i mu l t an eo u s l y a t d i f f e r e n t d e p t h sa t Nelson Rocks.

However , th e d i s p a r i t y be tween th e r e s u l t s from th e two sampl ings e r i e s , in March and December 19

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EPA Bulletin 215, technical report on mermaid sound

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