The Building of A Reef

1. The Building of A Reef • Coral Growth - The Constructive Stage • Bioerosion - The Destructive Stage • Sediment Production and Redistribution Wilson Ramirez

2. Constructive Stage - Carbonate Production by Corals and other Organisms The most important processes in the marine system can be described by the formula: Ca++ + 2HCO3- <=> CaCO3 + CO2 + H2O The vigor with which aragonite will form is thus related to the abundance of free calcium (Ca++) and HCO3-. The addition of CO2 to water ultimately makes both of these available through the following process: CO2 + H2O <=> H2CO3 <=> H+ + HCO3- + Ca++ Free H+, left over from the calcification process lowers the pH (makes the solution acidic). Conversely, dissolution of carbonate will increase pH. The ability of various organisms to regulate pH within their tissues, and drive the reaction toward the precipitation of aragonite, may be an important factor in biologically-mediated calcification.

3. Bioerosion - Destructive Stage • While corals and coralline algae are capable of producing massive structures over time, most other organisms living in and on the reef counter that process in their quest for food (grazers) or shelter (borers). • This process was termed bioerosionby Neumann, and has subsequently been recognized as a major factor in both the biological and geological development of reefs. Grazers and Predators Borers Rates of Bioerosion

4. Bioerosion After Scoffin (1972)

5. Algae and Overgrowth • All dead surfaces of the reef are rapidly overgrown by a thin film of filamentous green algae. • These form broad algal turfs that are a favorite diet of many fishes and urchins. Some algae bore tiny but ubiquitous holes into the reef surface. • These endolithicalgaecan weaken the substrate, making it more susceptible to damage by grazers.

6. Grazers While some grazers (i.e. damselfish) selectively pluck turfs from the substrate, and actively "farm" the turfs within their territories most grazers are less selective. The algae are digested, and the remainder is passed through the gut, mostly as sand.

7. Predators Parrotfish bite off pieces of substrate and pass them through a rasping structure, the pharyngeal mill, which produces a mixture of algae and sediment. • Some predators feed on live coral. • Along the Great Barrier Reef, the Crown-of-Thorns starfish (Acanthaster plancii) has been the focus of national concern each time its population reaches epidemic proportions and devastates large areas of live coral. • In the Caribbean, coralliophyla (coral devoring snails) are becoming larger and more common and the number and size of fire worms is increasing. Both of these feed on coral.

8. DANGEROUS PREDATORS ?

9. Borers Boring into reefal rock by Clinoid sponge The upper core is of a relatively undisturbed reef coral. The lower core shows extensive boring by bivalves. Lithophega can reach 30 cm in length and, in isolated instances, over 50 individuals per cubic meter can be found within a patch of reef.

10. Rates of Bioerosion • The best estimates of bioerosion come from controlled experiments in both the laboratory and the field. • Based on these, grazers appear to be responsible for better than half of the bio­erosion in Caribbean reefs. • Ogden proposed a rate of 0.49 kg/m2-yr for a small reef system on the north side of St. Croix (Caribbean Sea). • This was computed from the amount of sediment produced by an "average" fish (determined by divers collecting "samples" from numerous fishes) multiplied by the number of defecations per fish and the number of fish on the reef.

11. Rates of Bioerosion • At many locations, urchins produce larger amounts of sediment (up to 5 kg/m2-yr; avg ~ 2kg/m2-yr, equally split between sand and mud. • The relative importance of sponge boring was determined for St. Croix by Moore and Shedd who measured rates averaging near 1.25 kg/m2-yr, with 90% of this being mud. Rates exceeding 4 kg/m2-yr are certainly possible.

12. Sediment Production & Redistribution • Sediment in the reef is derived from two primary sources. • The most important is bioerosion. • The other is the death and disintegration of skeletal remains of other organisms living on or around the reef. • Primary among these are molluscs, foraminifera and upright, carbonate-producing algae

13. Sediment Production & Redistribution • Character of the reef interior reflects a constant battle between coralgal (corals and algae) construction and subsequent degradation by bioeroders. • As soon as a coral dies, it is aggressively attacked by bioeroders. • When infauna die, their galleries are usually filled in by muddy sediment, which is in turn • Bound together by chemically precipitated cements. This process may be repeated many times over even a decade, leaving a fabric that is very complex and in some instances retains little evidence of the original coral.

14. Reef Accretion • The nature of a reef and the rate at which it accretes is the result of this complex interplay of factors; the term reef accretion is much more accurate that the more-commonly used reef "growth" in reflecting this constant battle between constructive and destructive processes.

15. Carbonate Budget • Recognizable coral comprises about 42% of the reef fabric; the remainder was made up of sediment (41%) and open void (17%). • Of the recognizable coral, only a small portion is usually in place. • In most reefs, the proportion of recognizable coral is much smaller. • As such, reefs are clearly not dominated by in-place and interlocking framework.

16. Carbonate Budget • Roughly half of the sediment produced by bioerosion is retained within the reef. • The remainder has been deposited in the sand channels that cross the deeper sections of the reef. • This sand is capable of accreting at a rate exceeding that of the intervening reef. • Thus, periodic export is necessary. • The volume moved from the channels under day-to-day conditions is insufficient to offset the in­balance predicted from the budget equation, and it has been proposed that storms are required to remove this excess.

17. Carbonate budget The general model for Jamaican reefs proposed by Land represents a milestone in reef-budgeting studies and has served as the basis for most subsequent attempts: Pg - Pn = Sedp - Seds, where Pg= gross carbonate production Pn= net carbonate production (including reincorporated sediment) Sedp= sediment produced within the reef Seds= sediment stored within the reef channels (sediment not reincorporated)

18. Gross production Gross production (Pg) is the total amount of carbonate produced on a reef over some period of time. In a sense, it represents the "potential accretion rate" of a reef before bioerosion and sediment export are considered. Estimates of gross production generally range between 0.8 and 1.4 kg/m2-yr for whole reefs and from 2.1 to 8.9 kg/m2-yr for specific reef zones. At any one time, the reef surface is occupied by some percentage of live coral, dead and algal-covered surface and loose sediment. Carbonate production by live coral will depend on total cover by each species and its depth-dependent growth rate

19. Surveying Coral ReefsBasic photographic survey The technique we use employs use of a Nikonos underwater camera with a 28mm lens for photography of the bottom from a vertical distance of 1.2m (giving a photo area of 70x100 cm). The size of the quadrat used is 70x100 cm for an area of 0.7m2. This is smaller than the standard meter square quadrat but it reflects the dimensions of the photo print and a transect of 10 quadrats can be taken in the field in the time of recording a single meter quadrat by inspection. Color negative film (Kodak 200ASA) is used; it is developed and printed as 4x6" prints.

20. How Fast? Coral Growth Rate ~ 1 - 30 cm/ year (10 - 300 m/1000 yrs) Reef Accretion = 1 - 10 m/1000 yrs (1-3 is “average”)

21. Problems with Ancient Reefs 1. Evolution

22. http://:www.ucmp.berkeley.edu/porifera/poriferafr.html/

23. Evolution Reef builders weren’t always corals How do we compare a modern coral reef to an ancient stromatoporoid reef? Corals Molluscs Sponges Stromatoporoids Algae/bacteria

24. Our Best Guess Reinhold Leinfelder FUNCTIONAL MORPHOLOGY Stromatoporoid Devonian Seas

25. The guild concept • Fagerstrom (1991) identified five basic “guilds” to place reef organisms: • Constructors – The constructors provide the building blocks of the reef, whatever their ultimate fate • Binders – constructors can be overgrown and bound together by algae, forams and other members of the binder guild • Bafflers – are those organisms that affect accretion by interrupting the flow of water, thereby encouraging sedimentation • Destroyers – include grazers and borers that break down the primary framework in various ways • Dwellers – passive inhabitants that contribute to the ecologic diversity of the reef but often have little to do with the actual accretionary process, except to help “fill in the spaces” within the reef interior

26. Some organisms are extinct How do we understand organisms that no longer exist Halucenogenia Conway Morris (1977)

27. Problems with Ancient Reefs 2. Taphonomy

28. What is taphonomy? Rapid Sedimentation Burial Karla Parsons-Hubbard

29. Taphonomy is…. • Everything between death and fossilization • Decay • Disarticulation • Dissolution • Abrasion • Overgrowth • Ubiquitous removal

30. It is affectd by… • Organisms involved • Platy vs massive vs branching • Organism mix • Resistance to loss • Processes that occur • Biological • Physical • Chemical • Rate • Time

31. Taphonomic Effects Start with 4 of each The “Taphonomic Filter” 3 mollusc species; 2 coral species 16 species -> 5 species 4 jellyfish; 4 grasses; 4 corals; 4 molluscs -> 16 species

32. Which morph will disappear faster? How will organism loss be different?

33. The End Result • Evolution - different organisms • Soft bodies gone • Skeletal organisms “filtered” • Reduced diversity • Harder to discuss interactions • More “apparent” (?) stability • Modern - ancient comparisons?

34. So, what does a fossil reef tell us? • What is left • When those fauna were first deposited (?) • How “true” is the history left behind?

35. Reef Framework ? Hubbard et al., 1997 and 2000

36. Location Percent Recovery Shallow-water reefs: Buck Island (north reefs) 29 Buck Island (south reefs) 18 Buck Island Bar 20 Tague Bay (St. Croix) 4 Salt River Canyon (St. Croix) 6 Mid-depth reefs: Puerto Rico (Island reefs) 16 Puerto Rico (mud mounds) 6 Puerto Rico (mid-shelf) 24 Deeper-water reefs: Salt River (St. Croix) 32 Cane Bay (St. Croix) 32 Lang Bank (St. Croix - north) 30 Lang Bank (St. Croix - south) 20 Puerto Rico (shelf edge) 14 Average: Shallow-water reefs 14 Mid-depth and Deeper-water reefs 25 Grand Average19 Recovery of coral in cores

37. Internal structure of a reef Seismic section across a shelf edge reef at La Parguera, Puerto Rico and the results of a core hole drilled on the inner of two ridges A Holocene reef about 15 m thick accumulated over the older Pleistocene reef (age 30,780 yBP) The present surface of the reef is covered with a thin layer of massive coral living at a water depth of 20 m Average Accretion rate of the reef 0.27 cm/yr

38. Basic concepts • True “reefs” need not be comprised of in-place and interlocking framework • Process and not product are the common denominator that we seek in linking modern carbonate buildups to examples in the rock record