The Building of A Reef ï± Coral Growth - The Constructive Stage ï±...

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Transcript of The Building of A Reef ï± Coral Growth - The Constructive Stage ï±...

  • Slide 1
  • The Building of A Reef Coral Growth - The Constructive Stage Bioerosion - The Destructive Stage Sediment Production and Redistribution Wilson Ramirez
  • Slide 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 ++ + 2HCO 3 - CaCO 3 + CO 2 + H 2 O The vigor with which aragonite will form is thus related to the abundance of free calcium (Ca ++ ) and HCO 3 -. The addition of CO 2 to water ultimately makes both of these available through the following process: CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - + 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.
  • Slide 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 bioerosion by 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
  • Slide 4
  • After Scoffin (1972) Bioerosion
  • Slide 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 endolithic algae can weaken the substrate, making it more susceptible to damage by grazers.
  • Slide 6
  • Grazers The algae are digested, and the remainder is passed through the gut, mostly as sand. 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.
  • Slide 7
  • Predators 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.fire worms Parrotfish bite off pieces of substrate and pass them through a rasping structure, the pharyngeal mill, which produces a mixture of algae and sediment.
  • Slide 8
  • DANGEROUS PREDATORS ?
  • Slide 9
  • The upper core is of a relatively undisturbed reef coral. The lower core shows extensive boring by bivalves. Boring into reefal rock by Clinoid sponge Borers 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.
  • Slide 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/m 2 -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.
  • Slide 11
  • Rates of Bioerosion At many locations, urchins produce larger amounts of sediment (up to 5 kg/m 2 -yr; avg ~ 2kg/m 2 -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/m 2 -yr, with 90% of this being mud. Rates exceeding 4 kg/m 2 -yr are certainly possible. At many locations, urchins produce larger amounts of sediment (up to 5 kg/m 2 -yr; avg ~ 2kg/m 2 -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/m 2 -yr, with 90% of this being mud. Rates exceeding 4 kg/m 2 -yr are certainly possible.
  • Slide 12
  • S ediment 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
  • Slide 13
  • S ediment 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. 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.
  • Slide 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.
  • Slide 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.
  • Slide 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 in balance predicted from the budget equation, and it has been proposed that storms are required to remove this excess. 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 in balance predicted from the budget equation, and it has been proposed that storms are required to remove this excess.
  • Slide 17
  • 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: P g - P n = Sed p - Sed s, where P g = gross carbonate production P n = net carbonate production (including reincorporated sediment) Sed p = sediment produced within the reef Sed s = sediment stored within the reef channels (sediment not reincorporated) Carbonate budget
  • Slide 18
  • Gross production (P g ) 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/m 2 -yr for whole reefs and from 2.1 to 8.9 kg/m 2 -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 Gross production
  • Slide 19
  • 4 jellyfish; 4 grasses; 4 corals; 4 molluscs -> 16 species 3 mollusc species; 2 coral species 16 species -> 5 species Taphonomic Effects Start with 4 of each The Taphonomic Filter
  • Slide 32
  • Which morph will disappear faster? How will organism loss be different?
  • Slide 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? Evolution - different organisms Soft bodies gone Skeletal organisms filtered Reduced diversity Harder to