E N V I R O N M E N T A L S U R V E Y OF M A T A I V A A T O L L , TUAMOTU A R C H I P E L A G O F R E N C H P O L Y N E S I A

A T O L L RESEARCH B U L L E T I N N O * 286

E N V I R O N M E N T A L S U R V E Y OF M A T A I V A A T O L L , TUAMOTU A R C H I P E L A G O F R E N C H P O L Y N E S I A

I S S U E D

B Y

T H E S M I T H S O N I A N I N S T I T U T I O N

WASHINGTON, D. C., U.S.A.

r l A Y

1985

CONTENTS

Page INTRODUCTION

PRESENTING MATAIVA ATOLL Geography

Background and p o p u l a t i o n Economy

GEOLOGICAL SETTING OF MATAIVA ATOLL Geomorphology of M a t a i v a

O u t e r r e e f a l and l a g o o n a l s e d i m e n t s

Sequence of t h e main g e o l o g i c a l e v e n t s a t M a t a i v a a t o l l

HYDROLOGICAL ENVIRONMENT C u r r e n t s

Water l e v e l

T e m p e r a t u r e and s a l i n i t y

T u r b i d i t y and l i g h t p e n e t r a t i o n D i s s o l v e d oxygen

N u t r i e n t s

PRIMARY PRODUCERS OF MATAIVA LAGOON P h y t o p l a n k t o n

B e n t h i c m a c r o f l o r a MATAIVA LAGOON FAUNA

Zooplankton C o r a l s M o l l u s c s

C r u s t a c e a n f a u n a

O t h e r m a r i n e i n v e r t e b r a t e s F i s h e s

CONCLUSION REFERENCES

TABLES AND ILLUSTRATIONS

I n a d d i t i o n t o t h e a u t h o r ' s own r e s e a r c h r e s u l t s , t h i s p a p e r i n - c l u d e s c o n t r i b u t i o n s by many c o l l e a g u e s , from u n p u b l i s h e d r e p o r t s , t h e s e s i n p r o g r e s s and o t h e r d a t a , which a r e h e r e g r a t e f u l l y acknow- l e d g e d . T h e s e c o l l a b o r a t o r s and t h e i r a f f i l i a t i o n s a r e l i s t e d below.

A l l a r e a l s o a t t a c h e d t o t h e C e n t r e d e l l E n v i r o n n e m e n t i n ~ o o r e a ( 1 ) . J . ~ e 1 1 ( 3 ) , F. Bourrouilh-Le

an(^) ,

J . d e ~ a u ~ e l a s ( ~ )

,

C. ~ a b r i e ( ~ )

,

R. ~ a l z i n ( ~ ) , M.

arme el in('),

L. ~ o n t a ~ ~ i o n i ( ~ ) , M. ~ o n t e f o r t e ( ~ )

,

(10) 0 . ~ d i n e t z ( ' ) , C. ~ a ~ r i ( ~ ) , M. ~ i c h o n ( ~ ) , J . P .

en on('),

M. R i c a r d

,

C e n t r e de l f E n v i r o n n e m e n t Antenne Museum-EPHE, BP 1 2 Moorea, P o l y n g s i e F r a n q a i s e

L a b o r a t o i r e de B i o l o g i e Marine e t M a l a c o l o g i e , EPHE, 55 r u e d e Buf f o n , 75005 P a r i s

-

F r a n c e

S c h o o l of b i o l o g i c a l S c i e n c e s , Macquarie U n i v e r s i t y , North Ryde, N.S.W. 2113, A u s t r a l i e

L a b o r a t o i r e de ~ 6 o d y n a m i q u e ~ U n i v e r s i t 6 d e Pau, 64000 Pau

-

F r a n c e L a b o r a t o i r e d e B i o l o g i e e t d l E c o l o g i e m a r i n e s , U n i v e r s i t 6 d e N i c e , 28 Avenue de V a l r o s e , 0 6 0 3 4 Nice Cededex

-

F r a n c e

S t a t i o n marine dlEndoume, Rue de l a b a t t e r i e d e s L i o n s , 13007 M a r s e i l l e

-

F r a n c e

U n i v e r s i t 6 F r a n q a i s e de l f 0 c 6 a n I n d i e n , B P 5 , 97490 S a i n t e C l o t i l d e I l e d e l a Rbunion

-

F r a n c e

James Cook U n i v e r s i t y , Queensland 4 8 1 1 , A u s t r a l i a

L a b o r a t o i r e d l E c o l o g i e a n i m a l e , U n i v e r s i t 6 dfOrlcSans, 45100 O r l 6 a n s

-

F r a n c e

L a b o r a t o i r e de Cryptogamie, Museum N a t i o n a l d f H i s t o i r e N a t u r e l l e , 1 2 r u e d e Buffon, 75005 P a r i s

-

F r a n c e

E N V I R O N M E N T A L S U R V E Y OF M A T A I V A A T O L L , T U A M O T U A R C H I P E L A G O F R E N C H P O L Y N E S I A

I N T R O D U C T I O N

Mataiva A t o l l , one of 84 i n French P o l y n e s i a , i s a s m a l l c o r a l i s l a n d a t t h e w e s t e r n e d g e of t h e Tuamotu A r c h i p e l a g o . T h i s a t o l l o c c u p i e s a p a r t i c u l a r p l a c e among t h e French P o l y n e s i a n a t o l l s s i n c e t h e d i s c o v e r y beneath t h e lagoon sediments of d e p o s i t s of phosphates soon t o be e x p l o i t e d . I n o r d e r t o e s t i m a t e t h e e n v i r o n m e n t a l e f f e c t s of such e x p l o i t a t i o n and p l a n a management scheme, numerous s t u d i e s have been c a r r i e d o u t s i n c e 1978 by many s c i e n t i f i c o r g a n i z a t i o n s : Antenne du Musdum N a t i o n a l e t d e 1'Ecole P r a t i q u e d e s Hautes Etudes e n P o l y n d s i e F r a n ~ a i s e , O f f i c e de l a Recherche S c i e n t i f i q u e e t Technique de0utre-Mer (ORSTOM), B. C. Research, I n s t i t u t d e Recherches Mddi'cales Louis Malardd (IRMLM)

,

C e n t r e N a t i o n a l pour 1 ' E x p l o i t a t i o n d e s Ocdans (CNEXO) and Commissariat $ I'Energie Atomique (CEA-LESE).

Because of t h e unusual and i n t e r e s t i n g r e s u l t s of t h e s e impact s t u d i e s , t h e Antenne Museum-E. P.H.E. h a s , s i n c e 1981, c o n t i n u e d t h e s c i e n t i f i c survey of Mataiva A t o l l . The geology and geomorphology of t h e a t o l l , t h e h y d r o l o g i c a l c h a r a c t e r i s t i c s of lagoon w a t e r s and t h e abundance and d i v e r s i t y of t h e marine f l o r a and f a u n a have been i n v e s t i g a t e d by a b o u t 20 s c i e n t i s t s . S e v e r a l c o n t r i b u t i o n s , i n c l u d i n g f o r example s t u d i e s of f i s h e s , c r u s t a c e a n s and p h y t o p l a n k t o n , a r e e x t r a c t e d from d o c t o r a l m a n u s c r i p t s .

Mataiva a t o l l has been chosen a s t h e s i t e of a post-Congress f i e l d t r i p of t h e F i f t h I n t e r n a t i o n a l C o r a l Reef Congress, t o be h e l d i n T a h i t i i n J u n e 1985.

*

Centre de llEnvironnement Antenne Musgum-EPHE, BP 12 Moorea, Polyn6sie Fran2aise

Laboratoire de Biologie Marine et Malacologie, EPHE, 55 rue de Buffon, 75005 Paris, France

F i g u r e 1: L o c a t i o n o f M a t a i v a a t o l l i n F r e n c h P o l y n e s i a and m o s a i c p r e p a r e d f r o m a e r i a l p h o t o g r a p h s .

0 , e

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PRESENTTNG MATAIVA ATOLL G e o g r a p h y

Mataiva a t o l l i s l o c a t e d 300 km n o r t h of T a h i t i , 14' 55' l a t . S and 148' 36' long. W. I t i s t h e westernmost a t o l l i n t h e Tuamotu Archipelago. T h i s s m a l l i s l a n d , 10 by 5 km, i s d i s t i n g u i s h e d by i t s u n u s u a l morphology: a wide a t o l l r i m , a l m o s t c o n t i n u o u s , a l l o w i n g o n l y

l i m i t e d o c e a n i c exchanges, and a r e t i c u l a t e d lagoon d i v i d e d i n t o numerous b a s i n s by a network of s l i g h t l y submerged p a r t i t i o n s ( F i g u r e 1 ) .

The c l i m a t e i s n o t u n l i k e t h a t e l s e w h e r e i n P o l y n e s i a , i . e . t r o p i g a l , h o t and humid, w i t h 2 s e a s o n s : one, r e l a t i v e l y d r y and c o o l (22-27 C) from A p r i l t o September, and t h e o t h e r , h o t and r a i n y (28-30 C) from October t o March. S i n c e t h e a t o l l has l i t t l e r e l i e f , r a i n f a l l r a r e l y e x c e e d s 2.5 m p e r y e a r , and t h e amount of s u n s h i n e i s h i g h (2500 hours p e r y e a r ) . Storms and c y c l o n e s a r e r a r e w i t h t h e e x c e p t i o n of 1983, when 3 c y c l o n e s h i t Mataiva. Trade winds a r e dominant, blowing from t h e e a s t e r n s e c t o r a t a n a v e r a g e s p e e d of 7 t o 10 m / s . The s w e l l i s g e n e r a l l y l e s s t h a n 4 m , coming mainly from t h e South.

B a c k g r o u n d a n d p o p u l a t i o n

Europeans d i s c o v e r e d Mataiva i n 1819: i t was t h e n named L a z a r e f f I s l a n d by B e l l i n g s h a u s e n . The numerous a r c h e o l o g i c a l d i s c o v e r i e s b e a r w i t n e s s t o a more o r l e s s permanent pre-European s e t t l e m e n t . Mataiva t h e n belonged t o t h e 8 i n d e p e n d e n t kingdoms of t h e w e s t e r n Tuamotu, which formed t h e c u l t u r a l and l i n g u i s t i c a r e a of Mihiroa. However, t h i s i s l a n d was o f t e n u n i n h a b i t e d , p a r t i c u l a r l y a f t e r i t s people were massacred i n

t h e 1 8 t h c e n t u r y by t h e i n h a b i t a n t s of Anaa a t o l l .

The p r e s e n t p o p u l a t i o n on Mataiva i s of r e c e n t o r i g i n . Around 1940, r e p r e s e n t a t i v e s from v a r i o u s f a m i l i e s of t h e n e i g h b o u r i n g a t o l l , Tikehau, who were e x p l o i t i n g Mataiva's c o p r a , d e c i d e d t o s e t t l e t h e r e . The a t o l l was d e c l a r e d autonomous i n 1950 and i n 1971 was a s s o c i a t e d w i t h t h e community of Rangiroa ( J a r r i g e e t a l . , 1978).

I n 1983, t h e p o p u l a t i o n of Mataiva numbered 183. I t i s a p o p u l a t i o n of predominantly younger g e n e r a t i o n (55.8% a r e under 20 y e a r s o l d ) , s l i g h t l y unbalanced (63.4% being f e m a l e ) and very f l u c t u a t i n g because of i t s p r o x i m i t y t o T a h i t i . The r e l a t i o n s h i p , w i t h Tikehau and Makatea, t o t h e l a r g e r community of Rangiroa, e x p l a i n s t h e f r e q u e n t p o p u l a t i o n s u r g e s t o t h e s e i s l a n d s , whether temporary o r permanent. For example, of t h e 172 p e r s o n s e s t a b l i s h e d on Mataiva r e c o r d e d i n a s u r v e y t a k e n i n 1964 o n l y 4 1 were s t i l l t h e r e i n 1983.

Economy

F i s h i n g a n d c o p r a p r o d u c t i o n a r e t h e two t r a d i t i o n a l means of s u p p o r t on t h e Tuamotu A t o l l s . These two n a t u r a l r e s o u r c e s a r e enough t o i n s u r e t h e s u b s i s t e n c e of t h e i n h a b i t a n t s and t o a l l o w t h e s u r p l u s t o be commercially e x p l o i t e d . Being c l o s e t o T a h i t i , t h e main market, Mataiva

g l o r i e s i n i t s p r i v i l e g e d p o s i t i o n ; moreover, i t w i l l be a b l e t o add t h e y i e l d from t h e e x p l o i t a t i o n of phosphates t o i t s f u t u r e revenue.

Fishing,

.

On a t o l l s g e n e r a l l y , t h e s e a p r o v i d e s a b o u t 70% of t h e p r o t e i n s consumed, mainly through f i s h , b u t a l s o s e a food ( s h e l l f i s h and c r u s t a c e a n s ) c o l l e c t e d a l o n g t h e r e e f , and t u r t l e s which a r e c o n s i d e r e d a g r e a t d e l i c a c y . F i s h i n g t e c h n i q u e s vary: s p e a r s ("pupuhil'), r o d s ( " a i r a " ) , l i n e s o r n e t s a r e used, b u t commercial f i s h e s a r e above a l l c o l l e c t e d i n f i s h t r a p s , t h e n s o l d t o p a s s i n g s c h o o n e r s . These f i s h t r a p s a r e l o c a t e d i n t h e p a s s and n e x t t o t h e hoa (ocean-lagoon c h a n n e l s ) and p a s s i v e l y c a t c h r e e f f i s h e s . The amount of t h e c a t c h v a r i e s g r e a t l y

t h r o u g h o u t t h e y e a r and p r o v i d e s around 45 t o n s a n n u a l l y .

Copra. An e s s e n t i a l a t o l l r e s o u r c e , c o p r a o c c u p i e s a l a r g e m a j o r i t y of t h e p o p u l a t i o n t h r o u g h o u t t h e whole y e a r : t h e upkeep of t h e c o c o n u t g r o v e , h a r v e s t i n g and d r y i n g of t h e n u t s and t h e e x p o r t t o T a h i t i . Due

t o t h e s i z e of i t s emerged r e e f r i m , Mataiva i s a n i m p o r t a n t p r o d u c e r w i t h a n a n n u a l h a r v e s t of n e a r l y 450 t o n s , i . e . a b o u t 5% of t h e t o t a l p r o d u c t i o n i n the Tuamotu Archipelago. U n f o r t u n a t e l y , t h i s p r o d u c t i o n was c o m p l e t e l y e l i m i n a t e d a s a r e s u l t of t h e 1983 c y c l o n e s and f o r a t

l e a s t two y e a r s , t h e main a c t i v i t y h a s been devoted t o t h e r e s t o r a t i o n of t h e d e v a s t a t e d coconut p l a n t a t i o n s .

Phosphates. The phosphate d e p o s i t r e p r e s e n t s t h e f u t u r e - - a 1 though a t t h e oment h y p o t h e t i c a l - - w e a l t h of Mataiva. I t e x t e n d s f o r a p p r o x i m a t e l y 5 km ?2 a n d r e p r e s e n t s 1 5 m i l l i o n s t o n s o f o r e , 1 0 o f w h i c h c a n b e e x t r a c t e d . I t s e x p l o i t a t i o n , f o r e c a s t t o l a s t 10 t o 15 y e a r s , w i l l n e c e s s i t a t e t h e employment of a b o u t 200 p e o p l e . Such a p r o j e c t w i l l r e s u l t i n complete upheaval i n t h e a t o l l ' s morphology and t h e way of l i f e f o r i t s i n h a b i t a n t s , b u t s h o u l d n o t do s o t o t h e d e t r i m e n t of i t s t r a d i t i o n a l r e s o u r c e s . One hopes t h a t Mataiva w i l l n o t e x p e r i e n c e t h e same f a t e a s i t s n e i g h b o u r i n g i s l a n d , Makatea, which was p r a c t i c a l l y d e s e r t e d f o l l o w i n g t h e e x p l o i t a t i o n of i t s d e p o s i t s .

GEOLOGICAL SETTING OF MATAIVA ATOLL

The morphological and s e d i m e n t o l o g i c a l c h a r a c t e r i s t i c s of Mataiva a t o l l a r e r a t h e r d i f f e r e n t from t h o s e e n c o u n t e r e d i n most o t h e r a t o l l s of French P o l y n e s i a . These s i n g u l a r i t i e s a p p e a r t o be a consequence of unusual and l a t e g e o l o g i c a l e v e n t s which c a u s e d , among o t h e r t h i n g s , t h e f o r m a t i o n of a r e t i c u l a t e d lagoon and d e p o s i t s of e x p l o i t a b l e phosphates.

I n p a r t i c u l a r , t e c t o n i c u p l i f t of t h e NW Tuamotu a t o l l s p r e s u m a b l y r e s u l t e d from t h e l o a d i n g e f f e c t s of t h e nearby v o l c a n i c complex. A c r u s t a l moat h a s developed p e r i p h e r a l t o T a h i t i , Moorea and Mehetia volcanoes. Beyond t h e o u t e r edge of t h e moat, f l e x u r i n g ( b u c k l i n g ) h a s developed a n a r c h which h a s been u p l i f t e d by a b o u t 10 m e t e r s ( P i r a z z o l i and Montaggioni, i n p r e s s ) .

Geomorphology of Mataiva

When approaching Mataiva by p l a n e , one i s immediately s t r u c k by 2 morphological c h a r a c t e r s : a wide r e e f r i m and a p a r t i t i o n e d lagoon.

The emerged reef ring i s 200 t o 1500 m wide, i . e . somewhat wider t h a n found i n t h e o t h e r a t o l l s of t h e Tuamotu Archipelago. T h i s r e e f r i m i s a l m o s t c o n t i n u o u s , broken o n l y by some c h a n n e l s ( h o a ) on i t s s o u t h e r n c o a s t and a s m a l l a t y p i c a l p a s s i n t h e NU. The N and E c o a s t l i n e s form a s i n g l e i s l e t (motu).

A t o p o g r a p h i c c r o s s - s e c t i o n ( F i g u r e 2) a l l o w s a p r o p e r u n d e r s t a n d i n g of t h i s a t o l l ' s morphology. I t r e v e a l s a marked dissymmetry between t h e N end S c o a s t s of t h e a t o l l - - t h e N and NE c o a s t s a r e narrow (200-500 m) and of r e l a t i v e l y h i g h a l t i t u d e (+ 6 m), whereas t h e s o u t h e r n c o a s t s a r e

lower and wider (1000-1500 m). I n t h e same way, t h i s asymmetry c o n t i n u e s t o t h e i n n e r s l o p e , which i s wider (300 m) on t h e N c o a s t t h a n on t h e s o u t h e r n one (50 m). T h i s dissymmetry i s e a s i l y accounted f o r by t h e v i o l e n t storms coming from t h e N w i t - h t h e r e s u l t a n t a c c u m u l a t i o n of s t o r m r i d g e s on t h e N c o a s t , w h i l e t h e l a g o o n muds a r e t r a n s p o r t e d southwards where t h e y c o n t r i b u t e t o t h e s u p r a t i d a l a c c r e t i o n of t h e s o u t h e r n motu. T h i s h i g h energy s e d i m e n t a t i o n c o n t r a s t s w i t h t h a t which p r e v a i l s i n a "normal" m e t e o r o l o g i c a l s i t u a t i o n , where t h e i n f l u e n c e of t h e S E tradewinds f a v o u r s r a t h e r t h e widening of t h e n o r t h e r n i n n e r s l o p e .

The outer reef, of v a r i a b l e width ( 150-500 m)

,

i s c h a r a c t e r i s e d by 2 fundamental f e a t u r e s :

-

The p r e s e n c e of a r e e f f l a t f l a g s t o n e , more o r l e s s eroded, l o c a t e d below and behind t h e a l g a l r i d g e , where b i o d e t r i t i c s e d i m e n t a t i o n i s weak o r n o n - e x i s t e n t .

-

The p r e s e n c e of a n a l g a l r i d g e i n t h e p r o c e s s of chemical e r o s i o n ; remnants of a r a i s e d , f o s s i l r i d g e a p p e a r i n some p l a c e s ; c r u s t s of l i v i n g c o r a l l i n e a l g a e a r e s p o t t y . C o n s t r u c t i o n i s q u i t e i n a c t i v e and t h e r e l i e f i s i n t h e p r o c e s s of d e g r a d a t i o n .

The lagoon morphology _{i s} t h e most o r i g i n a l c h a r a c t e r i s t i c of Mataiva a t o l l . I t i s a r e t i c u l a t e d o r p a r t i t i o n e d lagoon, made up of a b o u t 70 p o o l s of v a r y i n g s i z e s (100 m t o o v e r 2 km), w i t h a n a v e r a g e d e p t h of 8 m. The s h a l l o w s which s e p a r a t e t h e s e p o o l s a r e under 0.1-0.8 m of w a t e r and form a network o v e r 200 km long.

T h i s s o r t of r e t i c u l a t e d lagoon seems t o be t h e o n l y one i n French P o l y n e s i a b u t i t s morphology i s r e m i n i s c e n t of c e r t a i n r e e f zones of Bora Bora ( S o c i e t y A r c h i p e l a g o ) o r of Ponape ( C a r o l i n e I s l a n d s ) , which a l s o c o n s i s t of such lagoon p o o l s e a c h i s o l a t e d from t h e o t h e r s . Surveys conducted i n t h e lagoon r e v e a l t h a t t h i s r e t i c u l a t i o n seems d i r e c t l y r e l a t e d t o p a l e o t o p o g r a p h y , most l i k e l y a p r e - P l e i s t o c e n e s u r f a c e , b u r i e d u n d e r 1 0 - 1 5 m o f p r e s e n t - H o l o c e n e s e d i m e n t s . T h i s p a l e o t o p o g r a p h y i n £ l u e n c e s t h e p r e s e n t topography and makes up t h e framework of t h e a t o l l ' s g e n e r a l morphology. However, Holocene c o r a l remnants l o c a t e d 30-40 cm above mean lagoon l e v e l , d e t e r m i n e t h e morphology of t h e lagoon c o a s t and form some s m a l l i s l e t s i n t h e e a s t e r n and s o u t h w e s t e r n p a r t s of t h e lagoon.

Outer reef a 1 and lagoonal sediments

Grain-size analyses of bottom s u r f a c e samples have o n l y been made on t h e f r a c t i o n l a r g e r t h a n 40 vm. Two t e x t u r a l p a r a m e t e r s (mean s i z e , Mz

and s o r t i n g , So) have been g r a p h i c a l l y determined from t h e c u m u l a t i v e f r e q u e n c y curve. There a p p e a r s t o be a s h a r p d i f f e r e n c e between t h e r e e f a l and l a g o o n a l s e d i m e n t s ( F i g u r e 3 ) . On t h e r e e f r i m , t h e s e d i m e n t s a r e medium t o c o a r s e s a n d s o r g r a n u l e s (MZ= 0.67-3.45 mm), u s u a l l y w e l l s o r t e d (So = 0.76-0.91), e x c e p t i n t h e w e s t e r n s e c t o r (So- 2.18). I n a l l c a s e s , f r a c t i o n s s m a l l e r t h a n 0.5 mm do n o t exceed 25%. On t h e c o n t r a r y , sediments i n t h e lagoon a r e f i n e o r v e r y f i n e s a n d s , t o muddy s a n d s (10-50% of f r a c t i o n s

<

40

y),

o r t o sandy s i l t s ( o v e r 50%

<

40 ym). The s o r t i n g i s u s u a l l y good (So = 0.28-1.48). Higher p e r c e n t a g e s of s i l t s have been found on t h e s h o a l s s i d e s o r a t t h e bottom of t h e b a s i n s , mainly i n t h e N and E s e c t o r s of t h e a t o l l (50-80% of s i l t s ) .

The composition of sediments r e v e a l s 2 main s o u r c e s , c o r a l s and C o r a l l i n a c e a e . Green c a l c a r e o u s a l g a e (Halimeda), F o r a m i n i f e r a and m o l l u s c s can a l s o make up a n i m p o r t a n t p a r t of t h e sediment. C r u s t a c e a , s e r p u l i d s , sponges and Bryozoa a r e a l s o found. F i g u r e 3 shows t h e g r a i n s i z e and c o m p o s i t i o n of s e d i m e n t s i n d i f f e r e n t p a r t s of t h e o u t e r r e e f and lagoon. The i m p o r t a n t f e a t u r e s t o emphasise a r e t h e abundance of F o r a m i n i f e r a on t h e s o u t h e r n o u t e r r e e f , t h e g r e a t v a r i a t i o n of p e r c e n t a g e s of Halimeda and F o r a m i n i f e r a i n t h e lagoon, and t h e b e t t e r r e p r e s e n t a t i o n of C o r a l l i n a c e a e and m o l l u s c s i n t h e lagoon t h a n on t h e o u t e r r e e f .

S e q u e n c e o f the main geological events at Mataiva atoll G e o l o g i c a l i n v e s t i g a t i o n s by s u b s u r f a c e d r i l l i n g showed t h e dominant i n f l u e n c e of t h e pre-Holocene s u r f a c e s on t h e topography of Mataiva's r e e f s t r u c t u r e s . By r e f e r e n c e t o t h e g e o l o g i c a l h i s t o r y of o t h e r French P o l y n e s i a n a t o l l s and p a r t i c u l a r l y t h a t of Makatea ( ~ o n t a g g i o n i , 1 9 8 5 ) , t h e g e o l o g i c a l h i s t o r y of Mataiva c a n be summed up a s f o l l o w s :

During t h e M i o - P l i o c e n e ( ? ) a p l a t f o r m - l i k e r e e f developed. T h i s o l d r e e f i s a t p r e s e n t emerging a l o n g t h e N c o a s t i n t h e upper p a r t of t h e ocean s i d e beach. I n t h e c e n t r a l and s o u t h e r n p a r t s of t h e a t o l l , i t i s b u r i e d under a l a y e r ( a few t o 30 m deep) of Holocene d e p o s i t s . During s e v e r a l s p e l l s of emergence, t h i s o l d r e e f underwent s e v e r e m e t e o r i t i c s o l u t i o n and p a r t i a l d o l o m i t i z a t i o n .

During a l a t e r s t a g e ( P l i o c e n e ? ) , t h e c a v i t i e s of t h e k a r s t t h u s formed were f i l l e d up w i t h a phosphate d e p o s i t . The c o n t r a s t i n f a c i e s b e t w e e n t h e r o c k s o f t h e o l d r e e f a n d t h e d e p o s i t d e m o n s t r a t e s t h e t o t a l l y d i f f e r e n t c o n d i t i o n s of d e p o s i t i o n f o r t h e phosphate. However, a t p r e s e n t i t i s n o t p o s s i b l e t o o f f e r a r e l i a b l e r e c o n s t r u c t i o n of t h e d e p o s i t i o n environment of t h e phosphates. Three a l t e r n a t i v e s may be proposed f o r t h e i r o r i g i n : on weakly c o n s o l i d a t e d c a r b o n a t e r o c k s ( h i g h r e s i d u a l p o r o s i t y ) , p h o s p h o r i t e s may have been formed by ( a ) a1 t e r a t i o n and d i a g e n e s i s of s e a b i r d s excrements and bones, ( b ) a l t e r a t i o n of marine o r g a n i c m a t t e r , d e p o s i t e d under low oxygen c o n d i t i o n s , ( c ) p o s t - d e p o s i t i o n a l a 1 te r a t i o n of d r i f t v o l c a n i c m a t e r i a l t h a t accumulated a s s o i l s .

Subsequent t o p h o s p h a t o g e n e s i s , Mataiva I s l a n d underwent a s l i g h t t e c t o n i c u p l i f t which a p p e a r s t o be confirmed by t h e a b s e n c e of d e p o s i t s l i n k e d t o t h e 120,000 B.P. h i g h s e a l e v e l , which i s found a t Moruroa

a t o l l between 7 and 11 m below t h e Holocene d e p o s i t s (Buigues, 1983).

The i s l a n d was l a t e r submerged and t h u s t h e phosphate d e p o s i t s and t h e r e s t of t h e o l d r e e f were r e c o l o n i s e d by c o r a l growth d u r i n g t h e Holocene t r a n s g r e s s i o n , a b o u t 6,000 y e a r s ago.

L a s t l y , between 5,000 and 2,000 B.P., c o r a l growth developed a t Mataiva a t a l e v e l s l i g h t l y h i g h e r t h a n t h e p r e s e n t one. The r e l i c t s of a l g a l r i d g e s found on t h e o u t e r r e e f (2,200

+

130 y r s ) and t h e P o r i t e s c o l o n i e s of t h e lagoon motu (5,210

+

130 y r s F a r e a t e l e v a t i o n s s i m i l a r t o t h e emerged b e a c h r o c k s , i n which- l i t h i f i c a t i o n p a t t e r n s i n d i c a t e a n e x p o s u r e p a t t e r n s t o . vadose d i a g e n e t i c environments d u r i n g t h e i r f o r m a t i o n (Montaggioni and P i r a z z o l i , 1984).

HYDROLOGICAL ENVIRONMENT

The h y d r o l o g i c a l c h a r a c t e r i s t i c s of Ma t a i v a a t o l l have a d i r e c t c o n n e c t i o n w i t h 2 main m a r p h o l o g i c a l f e a t u r e s : ( 1 ) a lagoon of reduced s i z e and d e p t h , whose s h o a l s f u r t h e r r e s t r i c t t h e volume ( 2 ) l i m i t e d b u t s t i l l e x i s t i n g r e l a t i o n s w i t h t h e ocean through hoa and p a s s . Conse- q u e n t l y , t h e physicochemical c h a r a c t e r i s t i c s of t h e lagoon w a t e r s , i f they show a c e r t a i n s p a t i a l homogeneity, p r e s e n t a n enormous temporal v a r i a b i l i t y , p r i n c i p a l l y d i c t a t e d by c l i m a t i c c o n d i t i o n s ( D e l e s a l l e ,

1982).

C u r r e n t s

The hoa's p o s i t i o n , c l o s e t o g e t h e r on t h e S s i d e o f t h e a t o l l and f a c i n g t h e dominant s w e l l s , i n r e l a t i o n t o t h a t of t h e p a s s on t h e o p p o s i t e s h e l t e r e d c o a s t , b r i n g s a b o u t a g e n e r a l c i r c u l a t i o n of t h e w a t e r s f r o m t h e S t o w a r d s t h e NW. O c e a n i c w a t e r s e n t e r t h e l a g o o n t h r o u g h t h e hoa, w h i l s t , i n t h e p a s s , t h e c u r r e n t i s most o f t e n o u t g o i n g ( F i g u r e 4 ) .

I n t h e lagoon, t h e p a r t i t i o n s slow down c o n s i d e r a b l y t h e water's c i r c u l a t i o n , and o n l y t h e wind-induced c u r r e n t s a r e measurable. However, because of t h e l a r g e r s i z e of t h e b a s i n s a l o n g t h e n o r t h e r n and s o u t h e r n c o a s t s , t h e flow of w a t e r may p r e f e r e n t i a l l y f o l l o w them, and a v o i d t h e a t o l l ' s c e n t r e . I n t h i s c a s e , t h e e a s t e r n p a r t of t h e lagoon, i s o l a t e d by a t r a n s v e r s a l s t r i n g of i s l e t s , a p p e a r s more c o n f i n e d .

Water level

The s m a l l amount of ocean- lagoon exchanges, added t o t h e r e l a t i v e l y reduced volume of t h e lagoon, e x p l a i n s how t h e w a t e r l e v e l c a n undergo c o n s i d e r a b l e v a r i a t i o n s . An example of such v a r i a t i o n s , r e c o r d e d d a i l y s i n c e 1979 n e x t t o t h e p a s s , i s g i v e n i n F i g u r e 5. T h e i r u s u a l a m p l i t u d e i s a b o u t a m e t r e , b u t c a n r e a c h o r s u r p a s s 1.5 m when a v e r y s t r o n g s w e l l o c c u r s . On t h e o t h e r hand, t h e s e v a r i a t i o n s a r e v e r y r a p i d , a rise of 40 cm i n 24 h o u r s i s n o t unusual. C o n s i d e r i n g t h e a r e a of t h e lagoon ( 2 00'

7

5

h a ) , such a n i n c r e a s e c o r r e s p o n d s t o a n e n t r y of w a t e r of a b o u t 10 m

,

i . e . 1/10 of t h e lagoon volume. The Lowering of t h i s l e v e l i s more g r a d u a l , around 10 cm i n 24 h, b u t c a n l a s t f o r p e r i o d s of 7 t o 10 d a y s , and t h u s r e a c h such a low l e v e l t h a t t h e c o r a l c o l o n i e s on t h e top of t h e p a r ti t i o n s emerge.

The g e n e r a l e v o l u t i o n of t h e l e v e l o v e r 5 y e a r s shows a c e r t a i n predominance of lower l e v e l s between J u l y and December, b u t w i t h o u t w e l l d e f i n e d c y c l e s .

The v a r i a t i o n s i n t h e lagoon l e v e l show t h a t Mataiva c a n n o t be c a l l e d a c l o s e d a t o l l . The e x i s t e n c e of a p a s s , a l t h o u g h a t y p i c a l and s h a l l o w , g u a r a n t e e s permanent o c e a n i c exchanges.

T e m p e r a t u r e a n d s a l i n i t y

Water t e m p e r a t u r e s i n t h e lagoon do no& v a j y much and g e n e r a l l y f o l l o w t h e a i r tem e r a t u r e s w i t h a maximum (29 -31

g

⁰ C ) i n t h e r a i n y s e a s o n and a minimum (25 -27 C) d u r i n g t h e d r y season. I f t h e r e i s a s l i g h t warming up of t h e s u r f a c e w a t e r s d u r i n g t h e day, s t i l l a marked t h e r m i c s t r a t i f i c a t i o n does n o t o c c u r a s c o u l d be expected i n t h e a b s e n c e of c i r c u l a t i o n .

T h i s homogeneity of t h e water column i s con£ irmed by s a l i n i t y l e v e l s which d i f f e r l i t t l e be tween s u r f a c e and bottom. Temporal v a r i a t i o n s i n s a l i n i t y a r e more marked. I n f a c t , i f t h e lagoon w a t e r s n e a r t h e hoa show s a l i n i t y v a l u e s u s u a l l y c l o s e t o t h o s e of t h e ocean (36 g / l )

,

i n t h e r e s t of t h e lagoon d r o p s i n s a l i n i t y (30.77 t o 33.89 g / l i n A p r i l 1981) o r i n c r e a s e s (36.56 t o 37.64 g / l i n October 1983) c a n be observed.

I f c l i m a t i c c o n d i t i o n s (heavy r a i n s o r long p e r i o d s of d r y n e s s ) d i r e c t l y i n f l u e n c e t h e s a l i n i t y of lagoon w a t e r s , t h e p h r e a t i c f r e s h w a t e r , h e l d i n t h e a t o l l f o u n d a t i o n , a l s o p l a y s a r o l e : t h e low s a l i n i t y l e v e l s o b s e r v e d i n A p r i l 1 9 8 1 w e r e m e a s u r e d a f t e r a month w i t h o u t n o t i c e a b l e p r e c i p i t a t i o n w h i l e o v e r 500 mm of r a i n had been r e c o r d e d t h e month b e f o r e .

T u r b i d i t y a n d l i g h t p e n e t r a t i o n

The lagoon w a t e r s of Mataiva always have a more o r l e s s pronounced milky appearance. S e c c h i d i s c measurements of w a t e r t r a n s p a r e n c y r e a c h 50 m i n t h e ocean, b u t do n o t exceed 7 m i n t h e lagoon n e a r t h e hoa and 2.5 m n e a r t h e p a s s . Quantum measurements i n d i c a t e a q u i c k a b s o r p t i o n of l i g h t w i t h d e p t h ( ~ i g u r e 6 ) . The a t t e n u a t i o n c o e f f i c i e n t deduced from t h e s e c u r v e s i s weaker n e a r t h e hoa (0.14) t h a n towards t h e n o r t h of t h e lagoon (0.28); t h e most t u r b i d w a t e r s (0.36) a r e found n e a r t h e p a s s . However, t h e amount of suspended m a t t e r i s n o t v e r y g r e a t : 6.7 t o 9 mg/l.

T h i s s t r o n g d e c r e a s e i n l i g h t i n t h e w a t e r i s caused by v e r y f i n e , s i l t y , c a l c a r e o u s p a r t i c l e s .

D i s s o l v e d o x y g e n

L e v e l s of d i s s o l v e d oxygen v a r y v e r y l i t t l e (5.4 t o 7mm/l) and remain c l o s e t o , o r h i g h e r t h a n t h e l e v e l of w a t e r s a t u r a t i o n , depending on t e m p e r a t u r e and s a l i n i t y . Such v a l u e s i n d i c a t e a c o n f i n e d environment where p h o t o s y n t h e t i c organisms p l a y a major r o l e i n w a t e r oxygenation.

Nutrients

Lagoon w a t e r s of a t o l l s a r e u s u a l l y c o n s i d e r e d t o be o l i g o t r o p h i c because of t h e a b s e n c e of c o n t i n e n t a l i n £ l u e n c e s . Mataiva lagoon w a t e r

,

on t h e c o n t r a r y , c o n t a i n s h i g h and v a r i a b l e c o n c e n t r a t i o n s of d i s s o l v e d n u t r i e n t s , e s p e c i a l l y n i t r a t e s and s i l i c a t e s (0.10-13.99 u a t g N-N0311, 0.3-16.5 u a t g S i l l ) . H e t e r o g e n e i t y i s v e r y marked between d i f f e r e n t a r e a s o f t h e l a g o o n a n d d i f f e r e n t p e r i o d s of m e a s u r e m e n t s , b u t n o w e l l - d e f i n e d p a t t e r n c a n be r e c o g n i z e d from t h e d i s t r i b u t i o n of t h e v a l u e s .

T h e c o n £ i n e m e n t o f t h e l a g o o n w a t e r , i t s l a c k o f d e p t h a n d i t s d i v i s i o n i n t o numerous p o o l s , a p p e a r t o be f a c t o r s t h a t a l l o w such h i g h and v a r i a b l e c o n c e n t r a t i o n s . Moreover, t h e m i g r a t i o n of n u t r i e n t s from t h e o c e a n i c deep l a y e r s and t h e v o l c a n i c s u b s t r a t u m through t h e c o r a l f o u n d a t i o n , a s shown i n Moruroa a t o l l ( R o u g e r i e e t a1.,1982) and i n Takapoto a t o l l ( R o u g e r i e , 1983), might be a n i m p o r t a n t c o n t r i b u t i o n t o t h e enrichement of t h e w a t e r s . Although we have no i n f o r m a t i o n a b o u t t h e p o r o s i t y of t h e c o r a l f o u n d a t i o n of Mataiva, i t i s p r o b a b l y n o t l e s s t h a n on Takapoto c o n s i d e r i n g t h e g e o l o g i c a l e v e n t s ( u p l i f t s , k a r s t i £ ic a t i o n ) undergone by Mataiva A t o l l .

PRIMARY PRODUCERS OF MATAIVA LAGOON

Phytoplankton

The p h y t o p l a n k t o n of t h e lagoon w a t e r s i s of r e l a t i v e l y low s p e c i f i c d i v e r s i t y . S i x c l a s s e s of a l g a e c a n be i d e n t i f i e d , Diatomophyceae, Dinophyceae, Chlorophyceae, Cyanophyceae, Cryptophyceae and Chrysophyceae ( C o c ~ o l i t h o ~ h o r i d e a e ) , b u t w i t h few s p e c i e s ( T a b l e A ) .

The diatoms dominate t h e phy t o p l a n k t o n i c f l o r a by t h e d i v e r s i t y of t h e e x i s t i n g s p e c i e s , p a r titularly t h e g e n e r a Mas t o g l o i a , t y p i c a l of t r o p i c a l s e a s , and Navicula. The s c a r c i t y of s t r i c t l y p l a n k t o n i c forms such a s R h i z o s o l e n i a , Chaetoceros o r T h a l a s s i o s i r a must be noted. These a r e found o n l y n e a r t h e hoa, where o c e a n i c w a t e r s e n t e r t h e l a g o o n . The p o p u l a t i o n s a r e m a i n l y made up o f p h y t o p l a n k t o n i c s p e c i e s . D i n o f l a g e l l a t e s a r e a l s o r e p r e s e n t e d by s p e c i e s from calm and s h a l l o w environments : Gymnodinium, Gonyaulax and Prorocentrum ( n o t a b l y

g.

ovum). The o t h e r c l a s s e s , e s p e c i a l l y Cryptophyceae and Chlorophyceae, a r e n o t of t e n found i n t h e p l a n k t o n of a t o l l lagoons. The p r e s e n c e , and, a t times t h e abundance, of some s p e c i e s such a s t h e g r e e n a l g a Pyramimonas i s a p e c u l i a r c h a r a c t e r i s t i c of Mataiva plankton.

The abundance of Mataiva's p h y t o p l a n k t o n v a r i e s from

lo3

t o 5 10 5 c e l l s p e r l i t r e . Q u a n t i t a t i v e l y t h e s m a l l - s i z e d phytof l a g e l l a t e s (10.30 vm) a r e of t e n dominant w h i l e t h e d i a t o m s a r e o n l y l o c a l l y abundant. A d i n o f l a g e l l a t e Gymnodinium and a c h l o r o p h y c e a e Pyramimonas sometimes r e p r e s e n t 90% of t h e c e l l s counted.

The p r e s e n c e and importance of phytof l a g e l l a t e s i n Mataiva p l a n k t o n a r e c h a r a c t e r i s t i c of a calm and s h a l l o w environment. However, b e c a u s e of t h e r i c h n e s s i n n u t r i e n t s of t h e w a t e r s , one might e x p e c t a h i g h

biomass i n t h e p l a n k t o n . On t h e c_oftrary c h l o r o p h y l l measurements g i v e low v a l u e s : 0.3 t o 1 ug c h l a 1 i n s u r f a c e w a t e r s , t o 5 n e a r t h e bottom. Never t h l e s s , t h e low p e f c e n t a g e of degraded pigments ( 155s i t a n 35%) a s w e l l a s t h e h i g h l e v e l of primary p r o d u c t i o n (23-90 mg cm d ), s o much h i g h e r t h a n v a l u e s u s u a l l y observed i n a t o l l s ( S o u r n i a e t R i c a r d , 1976; D e l e s a l l e e t a l . 1983), a r e i n d i c a t i v e of a r a p i d t u r n - o v e r i n t h e p h y t o p l a n k t o n i c p o p u l a t i o n s . Such a paradox between low biomass and s t r o n g primary p r o d u c t i o n c a n be p a r t i a l l y e x p l a i n e d by t h e w e a l t h of z o o p l a n k t o n i c p o p u l a t i o n s whose g r a z i n g c a n keep t h e p h y t o p l a n k t o n i c biomass a t a low l e v e l .

Benthic macroflora

The b e n t h i c m a c r o f l o r a i n Mataiva's lagoon i s c h a r a c t e r i s e d by a s m a l l number of s p e c i e s , of which o n l y a few a r e abundant ( T a b l e B ) . E s s e n t i a l l y , i t i s a h a r d s u b s t r a t u m f l o r a . Only a s e a - g r a s s , H a l o p h i l a c f . o v a l i s , forms e x t e n s i v e g r a s s b e d s on t h e sandy s h o a l s . A f i l a m e n t o u s g r e e n a l g a , Enteromorpha, c a n a l s o on o c c a s i o n form l a r g e masses on t h e n o r t h i n n e r s l o p e .

U n l i k e h i g h v o l c a n i c i s l a n d s , where brown a l g a e a r e t h e most abundant s p e c i e s , a t o l l f l o r a s a r e u s u a l l y dominated by g r e e n and r e d a l g a e . A t M a t a i v a , t h e g r e e n a l g a e a r e t h e m o s t a b u n d a n t a n d d i v e r s i f i e d ( 2 2

s p e c i e s ) , p a r t i c u l a r l y t h e g e n e r a Caulerpa and Halimeda. One of them, Halimeda ( o p u n t i a group) i s s t r o n g l y developed on Mataiva, an unusual s i t u a t i o n f o r a n a t o l l lagoon. Among t h e Rhodophyceae, t h e c r u s t o s e a l g a e a r e more numerous i n t h e lagoon and on t h e o u t e r r e e f ; however, g e l i d i a l t u r f s a r e w e l l developed on t h e lagoon's dead c o r a l s and on t h e o u t e r reef's f l a g s t o n e . Cyanophyceae make up t h e 3 r d c l a s s i n t h e a l g a l complement; i f t h e u s u a l e x t e n s i v e b l u e - g r e e n f o r m a t i o n s of a t o l l lagoon bottoms a r e n o t found i n Mataiva, t h e s e a l g a e do however form a d i s c r e t e f e l t on dead c o r a l s and on H a l o p h i l a l e a v e s ; t h e i r development i s a l s o i m p o r t a n t on beach e d g e s , where they form a n a l g a l mat a t times v e r y t h i c k , and i n t h e b r a c k i s h ponds of t h e a t o l l r i m .

The d i s t r i b u t i o n of a l g a e i n t h e lagoon i s f a i r l y homogeneous. Few s p e c i e s a r e p r e s e n t s i m u l t a n e o u s l y . The s p e c i f i c d i v e r s i t y and abundance i n c r e a s e c o n s i d e r a b l y n e a r t h e zones of w a t e r exchange w i t h t h e ocean (hoa and p a s s ) . On t h e o u t e r r e e f f l a t , t h e eroded f l a g s t o n e i s covered w i t h G e l i d i a l e s t u r f , w h i l e t h e brown a l g a P o c o c k i e l l a v a r i e g a t a i s dominant on i t s o u t e r zone. Near t h e r e e f f r o n t , so£ t a l g a e common t o t h i s zone ( M i c r o d y c t i o n , Dyc t i o s p h a e r i a c a v e r n o s i , ~ e o m e r c s van b o s s e ) a p p e a r , a s w e l l a s c r u s t o s e c o r a l l i n a c e a e , P o r o l i t h o n onkodes, C h e v a l i e r i c r u s t a sp. But they n e v e r form a n a l g a l r i d g e - t y p i c a l of t h e r e e f f r o n t of a n a t o l l , and t h e i r mass always remain p o o r l y developed.

Thus, Mataiva's marine f l o r a i s t h a t of a c l o s e d environment, w i t h few s p e c i e s , some of which a r e abundant. I t i s homogeneous and g r a d u a l l y changes n e a r t h e ocean exchange zones. Although n o t a r i c h f l o r a , i t s v i t a l i t y i s demonstrated by t h e p r e s e n c e of numerous young s h o o t s . The e x i s t e n c e of l a r g e g r a s s b e d s and t h e unusual development of Halimeda ( o p u n t i a t h e d i s t i n c t i v e c h a r a c t e r i s t i c s of t h i s f l o r a .

MATAIVA LAGOON FAUNA

Zooplankton

The zooplankton of Mataiva a t o l l i s mainly composed of t y p i c a l , l a g o o n a l h o l o p l a n k t o n i c s p e c i e s where 6 s p e c i e s of Copepods, 2 s p e c i e s of C h a e t o g n a t h s , 1 A p p e n d i c u l a r i a , 1 0 s t r a c o d and 1 Decapod S e r g e s t i d a r e dominant. S u r p r i s i n g l y , t h e meroplankton i s e x t r e m e l y r a r e , 5% of t h e t o t a l p l a n k t o n , whereas i t s t r u e c o n t r i b u t i o n i s u s u a l l y 35-65%. T h i s m e r o p l a n k t o n i s made up o f c r u s t a c e a n l a r v a e (Stoma t o p o d s , Decapod R e p t a n t i a and N a t a n t i a ) , f i s h eggs and l a r v a e , and F o r a m i n i f e r a . I n o t h e r r e s p e c t s , w h i l e a l l of t h e Mataiva zooplankton s p e c i e s a r e r e c o r d e d from o t h e r a t o l l s , t h e a b s e n c e of groups o c c u r r i n g i n a t o l l s open t o t h e ocean, e.g. P t e r o p o d s , S a l p s , D o l i o l i d s and some Copepods, i s noteworthy.

The zooglankton biomass i s much hi.?her t h a n i n t h e nearby ocean:

300-500 mg/m i n t h e lagoon, 10-18 mg m i n t h e ocean.

The d i s t r i b u t i o n of t h i s biomass i s v e r y h e t e r o g e n e o u s .

H o r i z o n t a l l y , t h e w e s t e r n and s o u t h e r n s e c t o r s of t h e a t o l l a r e a b o u t 3 times r i c h e r t h a n t h e e a s t e r n and n o r t h e r n a r e a s . V e r t i c a l l y , a ve y marked d i u r n a l s t r a t i f i c a t i - y e x i s t s between t h e s u r f a c e (80 mg m-') and t h e bottom (2 000 mg m ) of t h e lagoon. T h i s phenomenon has been observed i n o t h e r a t o l l s : Rangiroa ( ~ i c h e l , 1971), Mururoa ( ~ i c h e l , 1969), Takapoto (Renon, 1977), B i k i n i (Johnson, 1949).

The ocean-lagoon exchanges through t h e hoa and t h e p a s s a r e v e r y i m p o r t a n t : e n t r y of r e e f p l a n k t o n , mainly meroplankton, and o u t p o u r i n g of lagoon p l a n k t o n through t h e p a s s , i n q u a n t i t y a b o u t 35 t i m e s h i g h e r t h a n w h a t e n t e r s t h e l a g o o n . The l a g o o n t h u s c o n s t i t u t e s a n e x t r e m e l y p r o d u c t i v e environment, g r e a t l y e n r i c h i n g t h e nearby ocean.

The abundance and c o m p o s i t i o n of Mataiva's zooplankton a r e somewhat unusual :

F i r s t l y

,

i t s biomass i s on t h e a v e r a g e 2 to-+ times g r e a t e r t h a n i n o t h e r P o l y n e s i a n a t o l l lagoons : 300 t o 500 mg m $ t Mataiva, 50 t o 150 mg m-3 a t Moruroa (Renon, u n p u b l . ) , 47 t o 6 1 mg m- a t Takapoto (Ranon,

1977). The confinement of w a t e r s o n l y p a r t i a l l y e x p l a i n s t h i s pheno- menon, s i n c e Takapoto, a c l o s e d a t o l l , i s p o o r e r i n zooplankton.

S q o n d 1-y

,

t h e s c a r c i t y of m e r o p l a n k t o n i c forms c h a r a c t e r i s t i c of Ma taiva's zooplankton might be r e l a t e d t o t h e d e p a u p e r a t e b e n t h i c f a u n a , hence t o l i g h t e r g r a z i n g and may a c c o u n t i n p a r t f o r a r i c h biomass.

fin all^,

t h e n u t r i t i o n a l b a s i s of zooplankton p o p u l a t i o n s remains d i f f i c u l t t o d e f i n e . The p h y t o p l a n k t o n biomasses a r e n o t i n e q u i l i b r i u m i t h t h e zooplankton abundance, a l t h o u g h t h e t u r n o v e r r a t e of phyto-

p l a n k t o n a p p e a r s v e r y h i g h ; however, t h e s e s t o n p a r t i c l e s , which s u p p o r t b a c t e r i a l development, may be d i r e c t l y used by t h e zooplankton.

A s i s o f t e n t h e c a s e i n c l o s e d a t o l l l a g o o n s ( C h e v a l i e r e t D e n i z o t , l 9 7 9 ) , t h e s p e c i f i c d i v e r s i t y of Mataiva lagoon c o r a l s i s e s p e c i a l l y low. Only 28 s p e c i e s have been r e c o r d e d i n t h e lagoon ( T a b l e C ) . The a r e a s of maximum d i v e r s i t y (12-14 spp.) have been found a t s t a t i o n s on t h e N and S c o a s t s of t h e a t o l l . T h i s r e l a t i v e v a r i e t y i s r e l a t e d t o hydrodynamic c o n d i t i o n s , due t o t h e n e a r n e s s of t h e hoa and t h e g e n e r a l flow of t h e w a t e r a c r o s s t h e n o r t h e r n and s o u t h e r n edges of t h e lagoon. I n f a c t , o n l y t h o s e s t a t i o n s removed from t h e hoa and t h e p a s s have a low p o p u l a t i o n .

The c o v e r r a t e by s c l e r a c t i n i a n s i s g e n e r a l l y v e r y poor. The

c o l o n i e s a r e mainly l o c a t e d on t h e t o p s of t h e lagoon p a r t i t i o n s , a l o n g t h e edges of t h e pools. P o r i t e s l o b a t a , forming m i c r o a t o l l s on t h e s h o a l s , and Acropora t o r t u o s a , more g e n e r a l l y c o v e r i n g t h e pool s i d e s , a r e t h e 2 most commonly r e p o r t e d s p e c i e s . Other s p e c i e s , Montipora a e q u i t u b e r c u l a t a and Z e p t a s t r e a p u r p u r e a , a r e widely d i s t r i b u t e d , b u t form a poor c o v e r because of t h e s m a l l n e s s of t h e i r c o l o n i e s .

The c o r a l fauna's v i t a l i t y seems t o be v e r y poor and dead c o l o n i e s a r e numerous. The p e r c e n t a g e of l i v i n g c o r a l s , which i s a l m o s t 30% n e a r t h e hoa b u t t a p e r s t o 0 i n t h e e a s t e r n p a r t of t h e a t o l l , i s d i r e c t l y connected w i t h hydrodynamic c o n d i t i o n s

.

However, t h e s i z e of t h e dead c o l o n i e s of P o r i t e s and Acropora i n d i c a t e s a n a c c i d e n t a l o r i g i n f o r t h i s c o n d i t i o n . The combination of s e v e r a l e n v i r o n m e n t a l f a c t o r s , such a s a low w a t e r l e v e l a l o n g w i t h much r a i n o r i n t e n s e s u n s h i n e , i s l i k e l y t o i n d u c e , i n c e r t a i n p a r t s of t h e lagoon, h y d r o l o g i c a l c o n d i t i o n s incom- p a t i b l e w i t h c o r a l s u r v i v a l , t h u s c a u s i n g massive d e a t h .

The i n h a b i t a n t s of t h e a t o l l r e p o r t e d t h e o c c u r r e n c e of such a n e v e n t i n November 1978, r e p e a t e d i n November 1980: w a t e r l e v e l oat 52 cm, extremely s t r o n g s u n l i g h t and w a t e r t e m p e r a t u r e s c l o s e t o 32 C f o r 10 days. The lagoon w a t e r s t u r n e d green-brown i n c e r t a i n p a r t s and produced a n a u s e a t i n g odour. I n May 1981, t h e l i v e c o r a l c o v e r was low t h r o u g h o u t t h e lagoon, exceeding 10% o n l y n e a r t h e hoa. Other measurements taken i n O c t o b e r 1 9 8 3 show a c o n s i d e r a b l e i n c r e a s e i n l i v e c o v e r . Such a

s i t u a t i o n i s o n l y p o s s i b l e a s a r e s u l t of o c e a n i c i n f l o w s , a l l o w i n g r e c r u i t m e n t of l a r v a e and a r e t u r n t o c o n d i t i o n s f a v o u r i n g t h e growth of

t h e s u r v i v i n g s p e c i e s .

The o u t e r r e e f c o r a l f a u n a i s much more d i v e r s i f i e d t h a n t h a t i n t h e lagoon ( F i g u r e 1 0 ) . These c o l o n i e s a r e o f t e n s m a l l o r v e r y e n c r u s t i n g . The eroded r e e f f l a t f l a g s t o n e i s l i t t l e c o l o n i s e d by c o r a l s which mainly d e v e l o p n e a r t h e r e e f f r o n t . Acropora and P o c i l l o p o r a a r e t h e 2 b e s t r e p r e s e n t e d g e n e r a .

Holluscs

The m a l a c o l o g i c a l fauna of Mataiva, w i t h 222 s p e c i e s r e c o r d e d ( T a b l e D), i s v e r y u n e q u a l l y d i s t r i b u t e d between t h e lagoon and t h e o u t e r r e e f .

On t h e o u t e r r e e f , 169 s p e c i e s (156 g a s t r o p o d s and 13 b i v a l v e s ) have been r e c o r d e d . T h i s f a u n a i s f a i r l y s i m i l a r t o t h o s e of o t h e r Tuamotu o u t e r r e e f s . N e r i t a p l i c a t a , Nodoli t t o r i n a l e u c o s t i c ta and L i t t o r i n e a

c o c c i n e a occupy t h e upper zone, where t h e s c a r c i t y of T e c t a r i u s g r a n d i n a t u s i s r a t h e r s u r p r i s i n g . The r e e f f l a t f l a g s t o n e h a r b o u r s n e a r l y 30 s p e c i e s of which a l a r g e m a j o r i t y belongs t o a c a r n i v o r o u s e p i f a u n a (Vasum, Conus, e t c . ) . A wide v a r i e t y of forms c a n be c o l l e c t e d , a l t h o u g h n o n e i s a b u n d a n t e x c e p t C y p r a e a m o n e t a . The f a u n a o f t h e f o r e r e e f i s dominated by Turbo s e t o s u s , a c t i v e l y e x p l o i t e d by t h e a t o l l ' s i n h a b i t a n t s . L i v e Drupa morum and Cypraea c a p u t - s e r p e n t i s a r e abundant t h e r e a s w e l l a s numerous s h e l l s coming from t h e o u t e r s l o p e , e v i d e n c e of i t s r i c h n e s s .

I n t h e lagoon, 77 s p e c i e s have been c o l l e c t e d : 55 g a s t r o p o d s and 22 b i v a l v e s . The s p e c i e s d i s t r i b u t i o n i s v e r y uneven: t h e r i c h e s t f a u n a o c c u r s n e a r t h e hoa i n t h e S of t h e a t o l l . Here, t h e more abundant s p e c i e s belong t o t h e e p i f a u n a (Cypraeidae, ~ u c c i n i d a e ) : up t o 20 Cyprea o b v e l a t a p e r m2 under c o r a l b l o c k s ; on t h e sandy s h o a l s , Cypraea moneta i s t h e o n l y macromollusc found. One b o r i n g s p e c i e s , Lithophaga cinnamomina, i n h a b $ t s t h e P o r i t e s c o l o n i e s , a t t h e r a t e of s e v e r a l dozen i n d i v i d u a l s p e r dm of t h e l i v i n g s u r f a c e . There i s a l s o a d i f f e r e n c e between t h e windward and leeward c o a s t s of t h e same r e e f :

-

^Arca

v e n t r i c o s a , P i n c t a d a m a r g a r i t i f e r a and T r i d a c n a maxima a r e more abundant on t h e leeward s i d e .

Moving away from t h e S of t h e a t o l l , t h e r e i s a c o n s i d e r a b l e d e c l i n e i n t h e m a l a c o l o g i c a l f a u n a . The sandy s h o a l s a r e occupied by a few Cardium fragrum beds. A s m a l l o y s t e r , C r a s s o s t r e a c u c c u l a t a , i s p a r t i c u l a r l y abundant on t h e b r a n c h e s of dead Acropora. The endofauna of bottom s e d i m e n t s i s a l s o u n u s u a l , c o n s i s t i n g of many s p e c i e s unknown e l s e w h e r e i n French P o l y n e s i a .

Along t h e lagoon and motu e d g e s , t h e m a l a c o l o g i c a l fauna shows l i t t l e d i v e r s i t y , o n l y 3 s p e c i e s being found t h e r e . The a b s e n c e of t h e N a s s e s , M i t r e s , Cones a n d T e r e b r e s u s u a l l y f o u n d i n t h i s z o n e i s s u r p r i s i n g .

On t h e l i t t o r a l f r i n g e of t h e motu, 3 s p e c i e s a p p e a r i n s u c c e s s i o n : L i t t o r i n a c o c c i n e a , N e r i t a p l i c a t a and Clypeomorus b r e v i s , w h i l s t t h e C e r i t h i d a e and Cypraeidae u s u a l l y p r e s e n t i n t h e upper l e v e l s a r e a b s e n t . I n t h e same way, t h e hoa c o n t a i n a n impoverished homogeneous mollusc f a u n a (8 s p e c i e s ) which i s c h a r a c t e r i s t i c of semif u n c t i o n a l hoa ( f u n c t i o n i n g - i n t e r m i t t e n t y ) .

Ma t a i v a ' s m a l a c o l o g i c a l f a u n a , w h i c h i n c l u d e s a b o u t 1 / 5 o f t h e s p e c i e s r e c o r d e d i n French P o l y n e s i a ( R i c h a r d , 1982), a p p e a r s t o be, on t h e whole, f a i r l y r i c h . However, t h e lagoon f a u n a i s , on t h e whole, poor, e s p e c i a l l y away from t h e zones under o c e a n i c i n f l u e n c e . Only a few

s p e c i e s (Lithophaga cinnamomina, C r a s s o s t r e a c u c c u l a t a ) a r e r e a l l y very abundant. However, t h i s m a l a c o l o g i c a l fauna i s unusual. because of t h e uncommon s p e c i e s found i n t h e s e d i m e n t s and on t h e r e e f f l a t s and t h e wide h e t e r o g e n e i t y of t h e p o p u l a t i o n s .

C r u s t a c e a n f a u n a

The c r u s t a c e a n fauna of Mataiva a t o l l i n c l u d e s a b o u t 100 s p e c i e s ( M o n t e f o r t e , 1 9 8 4 ) . Most s t u d i e s h a v e b e e n c a r r i e d o u t o n c r a b s ,

e s p e c i a l l y c o r a l - a s s o c i a t e d s p e c i e s , and on mud-shrimps, which a r e v e r y numerous on t h e sandy s h o a l s .

C r u s t a c e a i n h a b i t a l l environments i n Mataiva. Land c r a b s (Cardisoma c a r n i f e x ) a r e p l e n t i f u l i n t h e c o c o n u t g r o v e s , a s a r e h e r m i t c r a b s Coenobita p e r l a t u s . The c o c o n u t c r a b l a t r o , on t h e c o n t r a r y , i s r i b a b l y because of o v e r - c o l l e c t i n g

The sandy environments of t h e lagoon a r e d e n s e l y occupied by t h e mud-shrimps C a l l i c h i r u s armatus l i v i n g i n permanent burrows. D e n s i t i e s up t o 3 mud-shrimps p e r s q u a r e metre were observed on t h e muddy b o t t

S

m s of t h e b a s i n s ; they a r e lower i n t h e s h a l l o w w a t e r s (0.5 t o 1 ind./m ).

For t h e whole l a oon, t h e e s t i m a t e d p o p u l a t i o n of 2.38 t o 4.76 10

9

i n d i v i d u a l s , i . e . a biomass of 285 t o 571 t o n s wet

C.

a r m a t u s r a n g e s from w e i g h t ( 1 0 0 t o 200 k g p e r h e c t a r e ) . C a l l i c h i r u s a r m a t u s i s a g r e a t sediment reworker, f e e d i n g on t h e mud which f a l l s i n t o i t s g a l l e r y , and g e n e r a t i n g c o n s i d e r a b l e d i s t u r b a n c e . I t has been e s t i m a t e d t h a t t h e upper c e n t i m e t r e of mataiva's bottom s e d i m e n t p a s s e s through t h e mud-shrimps' burrows 4 t o 9 t i m e s p e r y e a r . L i v i n g a l s o i n a burrow, t h e stomatopod L y s i o s q u i l l a m a c u l a t a ("varo") i s common and much p r i z e d because of i t s t a s t y f l e s h . A c r a b Calappa h e p a t i c a i n h a b i t s t h e s e sandy s u r f a c e s , b e i n g w e l l a d a p t e d t o t h i s b i o t o p e . The h a r d s u b s t r a t e s i n t h e lagoon, and e s p e c i a l l y P o r i t e s m i c r o a t o l l s , s h e 1 t e r a reduced f a u n a , dominated by 2 Xanthidae, C h l o r o d i e l l a n i g r a and Phymodius u n g u l a t u s , and 1 P o r t u n i d a e , Thalamita admete, more numerous n e a r t h e hoa.

The t r a n s i t i o n zones (hoa and p a s s ) and t h e o u t e r r e e f f l a t s h a r b o u r a much more abundant and d i v e r s i f i e d p o p u l a t i o n t h a n does t h e lagoon.

The hoa and t h e p a s s c o n t a i n many s p e c i e s of t h e o u t e r r e e f f l a t , which o c c a s i o n a l l y e n t e r t h e lagoon:

-

~ h ' l o r o d i e l l c y t h e r e a , Liomera - b e l l a , P i l o d i u s p u g i l . One a l s o f i n d s many j u v e n i l e s of o u t e r r e e f s p e c i e s : E t i s u s laevimanus, Thalamita and Lep t o d i u s . The G r a p s i d a e and P a g u r i d a e a r e dominant i n t h e s l i g h t l y sumserged zones.

On t h e o u t e r r e e f f l a t s , t h e f a u n a i s e q u a l l y a b u n d a n t a n d d i v e r s i f i e d , p a r t i c u l a r l y on t h e r e e f f r o n t ( T a b l e E ) . Many s p e c i e s , such a s t h e c r a y f i s h , ~ a l u n i r u s p e n i c i l l a t u s , c l i m b t h e o u t e r s l o p e a t n i g h t and a r e found on t h e c r e s t . The e x p o s u r e of t h e r e e f f l a t m o d i f i e s t h e c r u s t a c e a n fauna and i t s d i s t r i b u t i b n ; t h u s , P l a g u s i a s p e c i o s a i s a b u n d a n t on t h e w h o l e e x p o s e d r e e f f l a t b u t l e s s commonly f o u n d on s h e l t e r e d r e e f s and t h e r e l i m i t e d t o t h e r e e f f r o n t .

On t h e s e o u t e r r e e f s , t h e p r e s e n c e of l i v i n g c o r a l s i n d u c e s t h e e x i s t e n c e of a w e l l developed s y m b i o t i c f a u n a . T h i s c o n s t i t u t e s up t o 80% of t h e i n d i v i d u a l s a s s o c i a t e d w i t h a l i v i n g c o r a l . The Xanthidae among t h e Brachyoura and t h e Alpheidae among t h e N a t a n t i a , a r e most abundant. I n p e r t i c u l a r , c r a b s

of

t h e genus T r a p e z i a

(2.

s p e c i o s a ,

x.

b e l l a and

-

^{T. formosa)} r e s t r i c t e d t o t h e c o r a l P o c i l l o p o r a , a r e p a r t i c u l a r l y numerous i n t h e f r o n t a l zones.

O t h e r m a r i n e i n v e r t e b r a t e s

A t p r e s e n t , o n l y t h e major i n v e r t e b r a t e g r o u p s have been s t u d i e d . However, some i n t e r e s t i n g o b s e r v a t i o n s on o t h e r g r o u p s may be c i t e d .

.The sponges a r e abundant i n t h e lagoon under t h e empty T r i d a c n a s h e l l s o r c o v e r i n g t h e d e a d A c r o p o r a b r a n c h e s . Many s p e c i e s w i t h b r i g h t i s h c o l o u r s a r e p r e s e n t , b u t t h e c o l o n i e s a r e u s u a l l y s m a l l , e x c e p t f o r one b l a c k s p e c i e s , sometimes o v e r 20 cm h i g h , which i s found i n t h e lagoon and on t h e o u t e r r e e f s l o p e .

The echinoderms i n c l u d e few s p e c i e s b u t some of them a r e v e r y abundant. Such i s t h e c a s e , i n th e lagoon, of t h e b l a c k sea-cqcumber, Halodeima -9 a t r a w i t h d e n s i t i e s r e a c h i n g 1 o r 2 i n d i v i d u a l s p e r m-L. T h i s s p e c i e s i s known t o p r e f e r more o r l e s s c o n f i n e d environments ( S a l v a t , 1975; S a l v a t e t a l . ,1979). Another s e a cucumber, Cucumaria s p . , i s of t e n found n e a r t h e hoa under empty T r i d a c n a s h e l l s .

On t h e o u t e r r e e f , one may f i n d two o t h e r H o l o t h u r i a n s , t h e t h i n , 1-ong Synap t e s n e a r t h e beach, and t h e rounded, w h i t e s p o t t e d Bohadschia a r g u s on t h e r e e f f l a t f l a g s t o n e . But u r c h i n s a r e more abundant t h e r e .

-

Echinometra mathaei i s p r e s e n t i n t h e r e e f f l a t c a v i t i e s ; n e a r t h e r e e f f r o n t , t h e p e n c i l u r c h i n H e t e r o c e n t r o t u s m a m i l l a t u s i s o f t e n found a t t h e b a s e of t h e s p u r s , w h i l e t h e helmet u r c h i n , C o l o b o c e n t r o t u s p e d i f e r , whose morphology i s w e l l a d a p t e d t o r e s i s t wave a c t i o n , i s more abundant on t h e upper p a r t of t h e s p u r s , e s p e c i a l l y t h e r e l i c t a l g a l r i d g e of t h e swell-exposed s o u t h e r n r e e f f r o n t s .

Ascidians form v e r y d i s c r e t e c o l o n i e s and a r e n o t w e l l known i n French P o l y n e s i a . However, s y m b i o t i c a s c i d i a n s , a s s o c i a t e d w i t h g r e e n a l g a e ( P r o c h l o r o n ) , a r e p r e s e n t i n Mataiva lagoon on t h e dead Acropora b r a n c h e s , and a t t h e b a s e of t h e s p u r s on t h e o u t e r r e e f .

Fishes

The i c h t h y o l o g i c a l f a u n a of Mataiva lagoon i s poor, n o t o n l y i n number of s p e c i e s , b u t a l s o i n number o f i n d i v i d u a l s . Of t h e 115 s p e c i e s r e c o r d e d ( T a b l e F), o n l y 10 seem well e s t a b l i s h e d i n t h e lagoon and a r e i n e v i d e n c e a t p r a c t i c a l l y a l l t h e s t a t i o n s . These a r e u s u a l l y s m a l l i n d i v i d u a l s b e l o n g i n g t o

noc t u r n u s ) , t h e Pomacentridae (Chae todon a u r i g a ,

-

^C. ephippium) ( S c a r u s s o r d i d u s . S c a r u s SD.

-.

1.

^-

more o r l e s s c l o s e d a t o l l s , a r e

t h e Gobiidae ( ~ m b l y g o b i u s p h a l a e n a ,

&.

(Chromis c o e r u l e a ) , t h e C h a e t o d o n t i d a e

,

o r j u v e n i l e s of M u l l i d a e and S c a r i d a e Some - s p e c i e s , common t o t h e lagoons of n o t found i n Mataiva: Arothron h i s p i d u s , Chromis d i m i d i a t u s .

Near t h e hoa and t h e p a s s , t h e number of s p e c i e s r e p o r t e d i n c r e a s e s c o n s i d e r a b l y (40-52 s p e c i e s ) . G e n e r a l l y , t h e r e i s much h e t e r o g e n e i t y i n t h e d i s t r i b u t i o n and abundance of t h e f i s h p o p u l a t i o n s of Mataiva lagoon.

More d e t a i l e d s t u d i e s ( B e l l and G a l z i n 1984, G a l z i n 1985), c a r r i e d o u t i n 1981 and 1983, have demonstrated t h e c l o s e r e l a t i o n e x i s t i n g be tween t h e abundance and d i v e r s i t y of

-

² t h e f i s h f a u n a ( t o t a l num e r of s p e c i e s ,

-9

number of s p e c i e s 250 m

,

number of i n d i v i d u a l s 250 m ) and t h e l i v e c o r a l cover. Changes i n l i v e c o r a l c o v e r , e s t i m a t e d t o be a s s m a l l a s 0 t o

<

2% and

<

2 t o 2 t o

<

5%, produced s i g n i f i c a n t i n c r e s e s i n t h e t o t a l

-1

number of s p e c i e s and t h e number of i n d i v i d u a l s 250 m

.

On t h e o t h e r hand, t h e r e e f complexity, which i s t h e same f o r dead and l i v i n g c o r a l c o l o n i e s , i s w i t h o u t any i n f l u e n c e on f i s h p o p u l a t i o n s .