Conspicuous features of organic reefs

ATOLL RESEARCH BULLJZl'IN

No.

46

Conspicuous f e a t u r e s of organic r e e f s by

J. I. Tracey, Jr., P. E. Cloud, Jr. and K. 0. Emery

Issued by

THE PACIFIC SCIENCE BOARD

National Acadeny of Sci.encns-Nati.ona1. Research Council Washington, D. C.

August 15,

1955

Conspicuous Features of Orgnnic Reef

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by

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J. I, Tracey, Jr., P. E. Cloud, Jr., and

K.

0. Emery

Introduction

. . . . .

~ 6 o ~ e r i t i G e s t u d i e s of ~ i c i f i c reef?, e s p e c i a l l j r ' i n t h e l a s t 10 years, have brought together oceanographers, b i o l o g i s t s , and geologists,

each dependent on :.he work of others and each t o some. degree inconven- ienced by t h e d i v e i s i t y of names used t o denote p a r t s o f r e e f s . The nomen- c l a t u r e of even -21s co~,spicuous f e a t i r e s of r e e f s hes varied widely.

Although t h e fiel;: i s much t o o a c t i v e f o r s t a b i l i z a t i o n in d e t a i l t o be p r a c t i c a b l e , agreement 'is d e s i r a b l e f o r t h o s e f e a t u r e s of reef and lagoon t h a t appear t o be g e n e r a l l y present and t h e r e f o r e e s p e c i a l l y importalt i n reef studies. The f o l l o w i n g list i s p u r e l y u t i l i t a r i a n and d@scri&ive;

it

i s not intenced a s a comprehensive ter'minology with established p r i - o r i t y of usage, ^, .

T$eirqmediateis$inplus t o t h i s paper was provided by D. P. Abbott, Marston Bates, F..i!., Bayer, and R. R. Harry of t h e P a c i f i c Science Board's

1953 I f aluk A t o l l team, :who worked, up a s e t of names and discussed them with 'hacey a t I f a l u k and Guam.' Discussion of t h e s e and other terms by Tracey, Cloud, and Emery a t Manila i n November 1953 led t o t h e c i r c u l a t i o n of a provisional d r a f t of t h e present paper t o 1 7 colleagues f o r review and c r i t i c i s m . The . r e p l i e s were generally favorable and t h e comments t h a t were received on s p e c i f i c f e a t u r e s aid terms helped g r e a t l y i n pre- paring t h e f i n a l revision,

The following l i s t , 'then,, s e t s f o r t h what we consider t o be t h e most widely d i s t i n c t i v e zones o r f e a t u r e s of r e e f s , i l l u s t r a t e d by a hypothetical cross-section; of an a t o l l . ( f i g . 1 ) . The zonation of b a r r i e r r e e f s (which a r e separated by a lagoon from, p r e e x i s t i n g land) and of f r i n g i n g r e e f s (which border p r e e x i s ' t i ~ g . land)' i s c l o s e enough t o t h a t of a t o l l r e e f s s o t h a t most of: t h e games are.. genei.al1.y applicable.

. .

Discus&on ^{. . ,}

. .

An a t o l l . c o n s i s t s o f

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ring-shaped organic reef t h a t encloses a lagoon i n which t h e r e is. no ~ y e e x i , s t i n g . l a n d , and which i s surrounded by t h e open sea.: ,The, pr,imary d i s t i n c t i o n s t o be drawn a r e 'between outer slope, reef, : i s l a p i , &agoon, a;nd, .smal&er , ~ , : . . . . reef . 'stxi.ict,ure's ^. within t h e lagoon.

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U. S. Geological Survey, Washington 25, ^{D. C.}

U. S. Geological Survey, University of Southern California, C a l i f .

Channels and passes between ocean and lagoon a r e a l s o of primary importance.

Most of t h e d i f f i c u l t y with names, e s p e c i a l l y f o r those who must r e l y on c o l l e c t i o n s and data gathered by others, l i e s i n boundaries between t h e p r i n c i p a l zones. The following t e r m a r e suggested, with purposely broad d e f i n i t i o n s :

1. Outer s l ~ . - - t h e s t e e y l y descending o u t e r slope of t h e reef below t h e dwindle point of abundant l i v i n g c o r a l and c o r a l l i n e algae, which i s o r d i n a r i l y a t about 10 fathoms.

2. Reef f r o n t .--the upper seaward f a c e of t h e 'reef, extending above t h e dwindle p o i n t of'abundant l i v i n g c o r a l and

c o r a l l i n e algae t o t h e r e e f edge. This zone commonly in- cludes a s h e l f , bench, or t e r r a c e t h a t slopes t o 8-15 f a t h o m , a s well as t h e l i v i n g , wave-breaking f a c e of t h e reef. The t e r r a c e may be an eroded surface or may be veneered with organic growth. The l i v i n g r e e f f r o n t above t h e t e r r a c e i n some places i s smooth and steep; i n other.

places it i s cut by grooves separated by r i d g e s t h a t

together have been c a l l e d groove and spur systems, forming . ^' comb-tooth patterns. If t h e t e r r a c e i s broad and w e l l

'. defined it may be w e l l t o designate it a s e p a r a t e reef zone.

3 Seaward r e e f margin_.--the seaward edge of t h e r e e f f h t , marked i n places by an a l g a l ridge and c u t by surge _{. .}

channels, which a r e t h e landward extensions of t h e reef- '

:

f r o n t grooves. _{, .,. ..}

.. .

4. Reef flat.--the upper s u r f a c e of t h e r e e f , -commonly ^: exposed or awash a t lowest t i d e . The presence o f ' i s l a n d s on t h e f l a t m o d i f i e s t h e ecology of t h e r e e f ; t h e r e f o r e , an important d i s t i n c t i o n should be drawn between i s l a n d reef f l a t s , or f l a t s seaward from i s l a n d s , and i n t e r - . .

i s l a n d reef f l a t s or f l a t s between islands. ^{,. :.} The reef f l a t is commonly d i v i s i b l e i n t o outer and inner reef f l a t s , o r o u t e r , c e n t r a l , and i n n e r reef f l a t s ; but one "inner" or "outern zone m y not be t h e close

ecologic equivalent of another 5.nnertI or "outerrg zone.

On inter-island r e e f s , and on seaward reef f l a t s adjoining i s l a n d s , t h e outer zone i s .toward the'dcean, t h e inner i s toward lagoon o r shore. I n r a r e i n s t a n c e s a broad r e e f flat :on t h e lagoon shore o f a n i s l a n d m y be subdivided i n t o an outer.lagoon reef f l a t , near t h e lagoon reef edge, and an inner lagoon r e e f f l a t , near t h e island; but c a r e f u l d i s t i n c t i o n should be made

between i t s . p a r t s and those of reef f l a t s t h a t a b u t thc open sea. I f a r e e f f l a t i s not present on t h e lagoon s i d e of an i s l a n d , i t s place may be taken by a lagoon s h e l f , on which d e t r i t a l sediments predominate over organic growth.

5.

Seaward beach.--the seaward-facing beach of reef i s l a n d s .

6.

Lagoon beach.-the lagoonward-facing beach of reef islands.

7.

Lagoon reef mar&!&.--the lagoonward margin of t h e r e e f ; unlike t h e seaward reef margin, it i s not n e c e s s a r i l y defined by growth. I n some places, e s p e c i a l l y where i s l a n d s a r e present, t h e r e may be no lagoon reef margin a t a l l . I f t h e lagoon reef margin i s w e l l defined, a lagoon reef f r o n t may be present and even a lagoon

t e r r a c e , comparable t o t h e seaward reef f r o n t and t e r r a c e . If t h e lagoon reef margin i s poorly defined, a lagoon shelf m y s e p a r a t e lagoon slope from reef f l a t o r lagoon beach ( f i g . 2 ) .

8. &ocn slope.--the border zone of t h e lagoon t h a t slopes downward f r a n t h e lagoon reef margin o r lagoon beach t o t h e lagoon f l o o r .

9. Lazoon floor.--the undulating t o n e a r l y l e v e l f l o o r of t h e lagoon.

Ylinor organic prominences on lagoon f l o o r s or slopes, a l l broadly r e l a t e d , range from small mowds o r t a l l narrow pinnacles t o l a r g e masses, hundreds of f e e t i n diameter. Some p r e f e r t o use a s i n g l e term f o r a l l such f e a t u r e s , f o r example, c o r a l knoll, bioherm, or patch r e e f . The con- s i d e r a b l e physical and organic v a r i e t y of t h e s e f e a t u r e s is e c o l o g i c a l l y s i g n i f i c a n t , however, and should be indiceted i n some way. I n general it seems preferable t o use informal names t h a t w i l l describe both t h e doininant organism and t h e physical appearance. Examples are: a l g a l knoll; coral- a l g a l mound; millepore patch; kcropora t h i c k e t , etc.

Windward +

LAGOON ISLAND REEF

SEAWARD REEF MARGIN

LAGOON REEF F U T (Algol ridge on6 surge channels

rked dwindling of obundont g Cord + I0 and fothorno) coroline olgae,

FIGURE I. CONSPICUOUS FEATURES OF AN ATOLL AND ITS PERIPHERAL REEF Hypothetical sect~on, not to scale, shows principal features by capital lettering; other features,

subdivisions, and explanatory notes are indicated by lower case letters.

REEF LAGOON ISLAND REEF

SEAWARD LACOON

MARGIN r

FIGURE I. CONSPICUOUS FEATURES OF AN ATOLL AND ITS PERIPHERAL REEF Hypothetical sectlon, not to scale, shows principal features by capital lettering; other features,

subdivisions, and explanatory notes ore indicated by lower case letters.

SEA

LAGOON

FIGURE 2. COMMON SUBDIVISIONS 0 _UPPE REEF SURFACE

SEEN IN PLAN VIEW