sclerochronology: a data source for reef systems models

SCLEROCHRONOLOGY: A DATA SOURCE FOR REEF SYSTEMS MODELS R. W. Buddemeier

INTRODUCTION

Ecosystem m o d e l l i n g n e c e s s a r i l y d e a l s w i t h time-dependent d a t a : f l u x e s , r a t e s , c o n c e p t s of s t a b i l i t y , and s o f o r t h . T h i s a s p e c t of ecology s e p a r a t e s q u a n t i t a t i v e and p r e d i c t i v e s c i e n c e from i t s d e s c r i p - t i v e p r e c u r s o r s ; t h i s a r e a a l s o p r e s e n t s t h e g r e a t e s t e x p e r i m e n t a l d i f f i c u l t i e s . Few s y s t e m s have been t h o r o u g h l y monitored o v e r t i m e p e r i o d s comparable t o t h e l i f e t i m e s of t h e i r l o n g e s t - l i v e d major

components; s i m i l a r l y , i t i s u n u s u a l t o f i n d a d e q u a t e d a t a on a l l p h a s e s of s e a s o n a l , d i u r n a l , and o t h e r c y c l e s . A s a r e s u l t , much m o d e l l i n g i s done w i t h a n i m p l i c i t s t e a d y - s t a t e a s s u m p t i o n and w i t h assumed "average"

v a l u e s . Although t h e s e a s s u m p t i o n s r u n c o u n t e r t o t h e common o b s e r v a - t i o n s t h a t few systems a r e i n long-term e q u i l i b r i u m and e x t r e m e s a r e l i k e l y t o b e more i m p o r t a n t t h a n means i n s h a p i n g t h e long-term

c h a r a c t e r i s t i c s of a n ecosystem, i t i s a l i m i t a t i o n imposed by t h e

d i f f i c u l t y of d e v e l o p i n g a n a d e q u a t e d a t a b a s e from r e a l - t i m e o b s e r v a t i o n . R e t r o s p e c t i v e a n a l y s i s o f e n v i r o n m e n t a l c o n d i t i o n s o r organism

growth i s a f i e l d p i o n e e r e d by t h e d e n d r o c h r o n o l o g i s t s ; t h e p u r p o s e of t h i s p a p e r i s t o r e v i e w and a n a l y z e t h e e x t e n t t o which growth p a t t e r n s may p r o v i d e u s e f u l d a t a f o r m o d e l l i n g e f f o r t s i n c o r a l - r e e f ecosystems.

To b e o f maximum u t i l i t y , a n organism must show growth p a t t e r n s w i t h a known and r e l i a b l e t i m e b a s e , b e c a p a b l e of r e l a t i o n t o a s p e c i f i c l o c a t i o n o r environment, and have a l i f e t i m e (and p a t t e r n p e r i o d ) l o n g enough t o p r o v i d e a w o r t h w h i l e e x t e n s i o n o f f e a s i b l e r e a l - t i m e observa- t i o n s . Although many o r g a n i s m s show growth p a t t e r n s o f some s o r t , t h e s e c r i t e r i a r e d u c e t h e p r e s e n t l y a v a i l a b l e s e l e c t i o n t o two groups:

M o l l u s c s , p a r t i c u l a r l y t h e t r i d a c n i d clams (Bonham, 1965; P a n e l l a , 1 9 7 5 ) ; and h e r m a t y p i c c o r a l s . Of t h e s e , t h e c o r a l s a r e by f a r t h e more w i d e l y d i s t r i b u t e d and e x t e n s i v e l y i n v e s t i g a t e d , and w i l l b e t h e s o l e c l a s s of o r g a n i s m s s p e c i f i c a l l y d i s c u s s e d i n t h i s p a p e r . It s h o u l d b e k e p t i n mind, however, t h a t many of t h e o b s e r v a t i o n s may a l s o a p p l y t o o t h e r organisms whose growth p a t t e r n s remain t o b e i n v e s t i g a t e d .

AGE AND GROWTH RATE DATA

It i s now w e l l e s t a b l i s h e d t h a t many r e e f c o r a l s h a v e s k e l e t a l d e n s i t y v a r i a t i o n s

Buddemeier

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e t a l . , Smith, 1974; Dodge

w i t h a n n u a l p e r i o d i c i t y (Knutson

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e t a l . , 1972;

1974; Moore and Krishnaswami, 1974; M a c i n t y r e and and Thomson, 1 9 7 4 ) ; s k e l e t a l d e n s i t y bands w i t h

Department of Hawaii, USA

Oceanography, U n i v e r s i t y of Hawaii, Honolulu,

l u n a r p e r i o d i c i t y (Buddemeier and K i n z i e , 1975) have a l s o been r e p o r t e d , b u t a r e n o t y e t e s t a b l i s h e d a s a r e l i a b l e i n t e r n a l chronometer. The d a i l y growth i n c r e m e n t s r e p o r t e d by Barnes (1972) a p p e a r t o b e t o o un- r e l i a b l e t o s e r v e a s more t h a n a secondary c o n f i r m a t i o n of o t h e r c h r o n o l o g i e s .

One of t h e most u n e q u i v o c a l u s e s of growth p a t t e r n s i s t h e d e t e r - m i n a t i o n of growth r a t e s , a g e s and a g e - s i z e r e l a t i o n s h i p s . The d i s t r i - b u t i o n of a g e s i n a community (among b o t h t h e l i v i n g and t h e dead b u t p e r s i s t i n g members) a s a f u n c t i o n of s p e c i e s and microenvironment p r o v i d e s a n o t h e r dimension f o r t h e s t u d y o f p o p u l a t i o n dynamics and community s t r u c t u r e development. The a b i l i t y t o e s t a b l i s h v a l i d age- s i z e r e l a t i o n s h i p s by r e p r e s e n t a t i v e growth p a t t e r n a n a l y s i s p r o v i d e s n o v e l o p p o r t u n i t i e s f o r p l a c i n g d e s c r i p t i v e s u r v e y s on a t e m p o r a l l y q u a n t i t a t i v e b a s i s .

Average colony growth r a t e s a r e a n o t h e r p a r a m e t e r of p o t e n t i a l i n t e r e s t f o r c o r a l r e e f system models. These r a t e s r e f l e c t c o m p e t i t i v e s t r a t e g i e s and t h e i r p r o b a b l e s u c c e s s , p r o v i d e a b a s i s f o r i n t r a -

community n i c h e comparison, and may b e compared w i t h t h e f a i r l y e x t e n s i v e l i t e r a t u r e on c o r a l growth r a t e s (Buddemeier and K i n z i e , 1976) t o p r o v i d e i n f o r m a t i o n a b o u t t h e g e n e r a l s u i t a b i l i t y of t h e growth environment o r s p e c i f i c , p o t e n t i a l l y g r o w t h - c o n t r o l l i n g , e n v i r o n m e n t a l f a c t o r s .

A d d i t i o n a l l y , growth r a t e s i n c o n j u n c t i o n w i t h s k e l e t a l d e n s i t y and composition a n a l y s i s p r o v i d e i n f o r m a t i o n on c a l c i f i c a t i o n r a t e s , and s k e l e t a l s t u d i e s may a l s o b e used t o d e t e r m i n e r a t e s of b i o e r o s i o n .

GROWTH PATTERN ANALYSIS

I n t e r e s t i n t h e p o t e n t i a l s i g n i f i c a n c e of t h e observed v a r i a b i l i t y i n t h e a p p r o x i m a t e l y a n n u a l growth band dimension was s t i m u l a t e d by t h e example of dendrochronology, wherein s y s t e m a t i c p a t t e r n v a r i a t i o n s

have been used t o c o n s t r u c t g e o g r a p h i c a l l y e x t e n s i v e m a s t e r c h r o n o l o g i e s and t o r e c o n s t r u c t p a s t c l i m a t i c v a r i a t i o n s . The hope h a s t h u s been r a i s e d t h a t s c l e r o c h r o n o l o g y ( a n a l y s i s of e x o s k e l e t a l growth p a t t e r n s ) might p r o v i d e s i m i l a r c a p a b i l i t i e s i n t h e marine environment.

To d a t e , r e s u l t s have n o t been c o m p l e t e l y c l e a r - c u t ; w h i l e i n d i v i d u a l t r a u m a t i c e v e n t s have been shown t o b e r e c o r d e d i n c o r a l s k e l e t o n s

(Macintyre and Smith, 1974; Moore and Krishnaswami, 1 9 7 4 ) , o n l y a few a t t e m p t s a t c o r r e l a t i o n of growth p a t t e r n s w i t h e a c h o t h e r o r w i t h e n v i r o n m e n t a l r e c o r d s have been made. I n a p r e l i m i n a r y s u r v e y based on growth bands i n c o r a l s from Enewetak A t o l l , Buddemeier

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e t a l . (1974) found no m a s t e r p a t t e r n c o r r e l a t i o n . Dodge and Thomson (1974) showed d i m e n s i o n a l and d e n s i t y p a t t e r n c o r r e l a t i o n s i n c o r a l s from Bermuda;

t h e s e two s e t s of r e s u l t s s u g g e s t t h a t e x t e r n a l growth c o n t r o l and p a t t e r n c o r r e l a t i o n s a r e more l i k e l y t o b e documented i n environments which a r e m a r g i n a l i n terms of t h e optimum c o n d i t i o n s f o r c o r a l growth and which show s u b s t a n t i a l v a r i a b i l i t y . The q u e s t i o n of what environ- m e n t a l p a r a m e t e r s c o n t r o l growth p a t t e r n f o r m a t i o n remains a s u b j e c t f o r d e b a t e and e x p e r i m e n t a t i o n . A l l e r and Dodge (1974) have shown t h a t

s e d i m e n t a t i o n can a f f e c t t h e a v e r a g e growth r a t e s of c o r a l s ; Weber and White (1974; i n p r e s s ) have demonstrated s i m i l a r e f f e c t s f o r t e m p e r a t u r e i n f i e l d s t u d i e s , a s have Clausen and Roth (1975) i n t h e l a b o r a t o r y . Buddemeier (1974) h a s s u g g e s t e d t h a t s e a s o n a l l i g h t v a r i a t i o n s may be a f a c t o r i n l o c a t i o n s where w a t e r t e m p e r a t u r e s remain w i t h i n t h e optimum r a n g e throughout t h e y e a r . I n g e n e r a l , t h i s i s an a r e a o f r e s e a r c h which shows promise, e s p e c i a l l y i n m a r g i n a l o r s t r e s s e d environments, b u t i n which much more work w i l l b e r e q u i r e d t o d e t e r m i n e whether c o r a l growth p a t t e r n s can s e r v e a s more t h a n a q u a l i t a t i v e i n d i c a t o r of non- s p e c i f i c e n v i r o n m e n t a l c o n d i t i o n s .

SKELETAL COMPOSITION

I n t e r p r e t a t i o n o f t h e i s o t o p i c , m i n e r a l o g i c a l , and chemical composi- t i o n o f c o r a l s k e l e t a l c a r b o n a t e i n terms of t h e c o n d i t i o n s of t h e e x t e r n a l environment a t t h e time of d e p o s i t i o n i s a t once one of t h e most promising and most problem-ridden u s e s of s c l e r o c h r o n o l o g y . I f s k e l e t a l composition can b e c a l i b r a t e d i n terms of ambient c o n d i t i o n s , t h e n band-dated samples would p e r m i t r e c o n s t r u c t i o n of e n v i r o n m e n t a l r e c o r d s ; and t h i s i n f o r m a t i o n might p e r m i t f u r t h e r r e f i n e m e n t i n i n t e r - p r e t a t i o n of t h e band p a t t e r n v a r i a t i o n s .

D e t r i t a l i n c l u s i o n s w i t h i n t h e s k e l e t o n have been used t o i d e n t i f y t i m e s and n a t u r e s of a l t e r a t i o n s i n t h e s e d i m e n t a t i o n p a t t e r n (Barnard e t a l . , 1974; Moore and Krishnaswami, 1 9 7 4 ) . It a l s o seems l i k e l y t h a t - -

c o r a l s f u n c t i o n a s r e l i a b l e s a m p l e r s f o r many d i s s o l v e d m a t e r i a l s i n t h e w a t e r . T h i s f a c t h a s b e e n most thoroughly demonstrated f o r v a r i o u s n a t u r a l and a r t i f i c i a l r a d i o n u c l i d e s (Knutson and Buddemeier, 1973;

Moore and Krishnaswami, 1974; Dodge and Thomson, 1974; Noshkin e t a l . , 1 9 7 5 ) , and t h e i m p l i c a t i o n i s s t r o n g t h a t many o t h e r t r a c e o r minor e l e m e n t s s h o u l d b e s i m i l a r l y "sampled." However, t h e mechanism f o r t h i s sampling remains u n c l e a r ; working w i t h some o f t h e same samples a n a l y z e d by Noshkin

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e t a l . ( 1 9 7 5 ) , Levy

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e t a l . (1975) found t h a t t h e heavy n u c l i d e s were c o n c e n t r a t e d i n " h o t s p o t s " r a t h e r t h a n d i s t r i b u t e d uni- formly through t h e s k e l e t a l band. T h i s c o u l d b e due t o i n c o r p o r a t i o n of d e t r i t a l p a r t i c l e s o r ( l e s s l i k e l y ) t o p a t c h i n e s s o f c a l c i f i c a t i o n i n t h e p r e s e n c e of r a p i d l y f l u c t u a t i n g c o n c e n t r a t i o n s of t h e d i s s o l v e d s p e c i e s . I n e i t h e r e v e n t , f u r t h e r i n v e s t i g a t i o n i s i n d i c a t e d b e f o r e

assuming t h a t s m a l l c o r a l samples n e c e s s a r i l y r e p r e s e n t s h o r t - t e r m ambient w a t e r c h a r a c t e r i s t i c s .

Other s k e l e t a l composition f a c t o r s which have been i n v e s t i g a t e d i n - i n c l u d e Sr/Ca r a t i o s (Weber, 1973; Houck, 1 9 7 5 ) , Mg/Ca r a t i o s (Weber, 1974a; Houck, 1 9 7 5 ) , c o n t e n t (Land e t a l . , 1975; Weber and Woodhead, 1970; Weber, 1974b; Walsh, 1975) and 1 8 0 c Z t e n t (Weber and Woodhead, 1970, 1972; Walsh, 1 9 7 5 ) . While t h e r e may b e s l i g h t e n v i r o n m e n t a l

e f f e c t s on S r and Mg c o n t e n t , i t seems s a f e t o s a y t h a t t h e s e p a r a m e t e r s promise t o y i e l d l i t t l e i n f o r m a t i o n of u t i l i t y t o m o d e l l e r s , a t l e a s t i n t h e n e a r f u t u r e .

The s t a b l e i s o t o p e s t u d i e s of c o r a l s a r e somewhat more c o n t r o v e r - s i a l . I t seems c l e a r t h a t t h e 180 c o n t e n t is r e l a t e d t o t h e w a t e r t e m p e r a t u r e a t t h e time of c a l c i f i c a t i o n and t h a t

13c

c o n t e n t i s r e l a t e d t o p h o t o s y n t h e t i c a c t i v i t y . However, t h e s e r e l a t i o n s h i p s a r e n o t s i m p l e , c o n s i s t e n t , o r even monotonic. P o s i t i o n w i t h i n t h e colony, c a l c i f i c a t i o n r a t e s on a m i c r o s t r u c t u r a l l e v e l , and o t h e r f a c t o r s a p p e a r t o a f f e c t i s o t o p i c c o m p o s i t i o n s ; and t h o s e t r e n d s which h a v e been e s t a b l i s h e d have u n c e r t a i n t y l e v e l s h i g h enough t o r e q u i r e a n a l y s i s o f a v e r y l a r g e number of samples i n o r d e r t o e s t a b l i s h r e a s o n a b l e c o n f i d e n c e l e v e l s .

Although n o t w i t h o u t promise, a l l s k e l e t a l c o m p o s i t i o n p a r a m e t e r s a p p e a r t o s u f f e r from t h e same c o m p l i c a t i o n s when viewed a s p o t e n t i a l e n v i r o n m e n t a l i n d i c a t o r s . The c a l c i f i c a t i o n p r o c e s s of t h e c o r a l s i s s u f f i c i e n t l y complicated and dynamic t h a t a l l of t h e s k e l e t a l

c o n s t i t u e n t s move through s e v e r a l m e t a b o l i c a l l y c o n t r o l l e d s t e p s , o f t e n w i t h a s i g n i f i c a n t t i m e l a g between u p t a k e and d e p o s i t i o n (Cooksey and Cooksey, 1972; C h a l k e r , 1975). Environmental c o n d i t i o n s such a s l i g h t and t e m p e r a t u r e undoubtedly a f f e c t d i f f e r e n t s t a g e s o f t h e c a l c i f i c a - t i o n p r o c e s s i n d i f f e r e n t ways. The r e s p o n s e s a r e f u r t h e r c o m p l i c a t e d by s p e c i e s and i n d i v i d u a l d i f f e r e n c e s i n c o r a l s and by s p e c i f i c adap-

t a t i o n t o microenvironmental c o n d i t i o n s . A l l i n a l l , t h e s t a b l e i s o t o p i c composition of c o r a l s k e l e t o n s shows some promise and poten- t i a l , b u t a g r e a t d e a l of f u r t h e r work i s r e q u i r e d t o make s u c h measurements r e l i a b l y u s e f u l a s e n v i r o n m e n t a l i n d i c a t o r s .

SUMMARY AND CONCLUSIONS

From t h e f o r e g o i n g i t c a n b e s e e n t h a t r e t r o s p e c t i v e s t u d i e s of c o r a l s k e l e t o n s can d i r e c t l y and r a t h e r u n e q u i v o c a l l y p r o v i d e informa- t i o n on c o r a l a g e s and growth r a t e s , and on c e r t a i n a s p e c t s of w a t e r c h e m i s t r y and s e d i m e n t a t i o n a t t h e time of s k e l e t a l d e p o s i t i o n . I t seems l i k e l y t h a t growth p a t t e r n a n a l y s i s can y i e l d some i n d e x o f g e n e r a l e n v i r o n m e n t a l v a r i a b i l i t y and s u i t a b i l i t y f o r c o r a l growth.

I t i s p o s s i b l e t h a t t h e combination o f s k e l e t a l composition and growth p a t t e r n a n a l y s i s w i l l e v e n t u a l l y p e r m i t q u a n t i t a t i v e r e t r i e v a l of some t y p e s of e n v i r o n m e n t a l d a t a . An o p t i m i s t i c view of t h i s p o t e n t i a l i s p r e s e n t e d by Weber

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e t a l . (1975); however, t h i s p o s s i b i l i t y i s s t i l l f a r from b e i n g r e a l i z e d .

An i m p o r t a n t p o i n t i s t h e need f o r feedback o f i n f o r m a t i o n from t h o s e i n t e r e s t e d i n time-dependent r e e f ecosystem m o d e l l i n g t o t h o s e r e s e a r c h e r s i n t e r e s t e d i n growth p a t t e r n s t u d i e s of c o r a l s and o t h e r organisms. Viewed from a narrowly " c o r a l r e s e a r c h " p o i n t of view, t h e r e a r e many i n t e r e s t i n g problems; i f some of t h o s e problems h a v e b r o a d e r r a m i f i c a t i o n s t h a n o t h e r s , i t i s up t o t h e p o t e n t i a l u s e r s of

t h e d a t a t o communicate t h e i r n e e d s t o t h e p o t e n t i a l d a t a p r o d u c e r s .

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A l l e r , R. C.,and Dodge, R . E. 1974. Animal-sediment r e l a t i o n s i n a t r o p i c a l lagoon--Discovery Bay, Jamaica. J . Mar. Res. 32:209-232.

Barnard, L. A , , Macintyre, I . G.,and P i e r c e , J . W. 1974. P o s s i b l e environmental i n d e x i n t r o p i c a l r e e f c o r a l s . Nature 252:219-220.

Barnes, D. J . 1972. The s t r u c t u r e and f o r m a t i o n of growth r i d g e s i n s c l e r a c t i n i a n c o r a l s k e l e t o n s . P r o c . Royal Soc., London (B) 182: 331-350.

Bonham, K . 1965. Growth r a t e of g i a n t clam Tridacna g i g a s a t B i k i n i A t o l l a s r e v e a l e d by r a d i o a u t o g r a p h y . S c i e n c e 149:300-302.

Buddemeier, R. W. 1974. Environmental c o n t r o l s o v e r a n n u a l and l u n a r monthly c y c l e s i n hermatypic c o r a l c a l c i f i c a t i o n . pp. 259-267,in:

Proceedings of t h e Second I n t e r n a t i o n a l C o r a l Reef Symposium, Vol. 2. Great B a r r i e r Reef Committee, B r i s b a n e .

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