Who, What, When, Where, Why, and How of Coral Reef Communities

1. Who, What, When, Where, Why, and How of Coral Reef Communities • Who (organisms) can be found in a coral reef community? • What is a coral reef community? • When can you find a coral reef community (is it seasonal)? • Where are coral reef communities found? • Why are coral reef communities so important? • How do coral reef communities interact with other ecosystems in the ocean?

2. Chapter 15 Coral Reef Communities

3. Let’s talk about primary productivity in the oceans - Photosynthesis by phytoplankton • Polar oceans are the most productive • Cold water holds more gases • Summer brings 24 hours of daylight for photosynthesis • Temperate waters are somewhat productive • Tropical waters are the least productive • That’s why there are nice clear, blue water • Coral reefs are like an oasis in the tropical water dessert

4. Coral Reef Communities • Coral reefs are highly productive, but occur in nutrient-poor waters • This is made possible by the symbiotic relationship between coral animals and zooxanthellae • These symbionts + algae form the basis of the community; other reef animals depend on these organisms

5. Organisms That Build Coral Reefs • Stony (true) corals are the primary organisms that deposit massive amounts of CaCO3 that compose most of the structure of coral reefs • Hermatypic: coral species that produce reefs, found in shallow, tropical waters and harbor zooxanthellae

7. Organisms That Build Coral Reefs • Ahermatypic: corals that do not build reefs, which can grow in deeper water from the tropics to polar seas • most do not harbor zooxanthellae • Coralline algae and organisms such as fire coral, deposit lesser amounts of calcium carbonate on reefs • fire corals important in Caribbean reefs • coralline algae important in structure of Pacific reefs

11. Organisms That Build Coral Reefs • Coral colonies • large colonies of small coral polyps, each of which secretes a corallite (cup of calcium carbonate) • the coral larva called a planula larva settles and attaches • a polyp develops, and reproduces by budding to form a growing colony • polyps’ gastrovascular cavities remain interconnected • a thin, usually colorful epidermis overlies the colony surface

12. Organisms That Build Coral Reefs • Coral nutrition • corals have evolved several strategies for obtaining food • symbiotic zooxanthellae • supply 90% of nutritional needs of stony coral • zooxanthella provide glucose, glycerol and amino acids • coral polyp provides a suitable habitat and nutrients, absorbed directly through the animal’s tissues • zooxanthellae remove CO2 and produce O2 • need of zooxanthellae for sunlight limits depths to which stony corals can grow

13. Organisms That Build Coral Reefs • Coral nutrition (continued) • corals as predators • tiny zooplankton or other small animals paralyzed by the cnidocytes (stinging cells in tentacles) are passed into the digestive cavity

14. Organisms That Build Coral Reefs • Coral nutrition (continued) • other sources of nutrition • mesenteric filaments (coiled tubes attached to the gut wall) can be extruded from the mouth to digest and absorb food outside the body • corals can feed off bacteria living in their tissues, which feed on dissolved organic matter directly from the water

15. Organisms That Build Coral Reefs • Reproduction in corals • Reproduction by fragmentation • in addition to budding, corals can also reproduce asexually by fragmentation • some branching corals are fragile and tend to break during storms • if they survive the storm, fragments can attach and grow into new colonies

17. Organisms That Build Coral Reefs • Reproduction in corals (continued) • Sexual reproduction in coral • Many species of coral are hermaphroditic, some have separate sexes • mostly broadcast spawners—release both sperm and eggs into the surrounding seawater • some are brooders—broadcast sperm, but retain eggs in the gastrovascular cavity • spawning is usually synchronous among Pacific reef species, but nonsynchronous among Caribbean species

21. Reef Formation • Involves both constructive and destructive phases • Bioerosion: the destructive phase of reef formation • boring clams or sponges attack exposed surfaces on the undersides of large corals • the coral stand weakens, then topples in a storm or ocean surge • accumulating debris smothers boring organisms, cracks are filled with CaCO2 sediments, and coralline algae cement it together

24. Types of reefs: • Fringing • Barrier • Atoll • Patch

25. Types of Coral Reefs • Fringing reefs • develop along shores of tropical/subtropical islands or continental landmasses • Of all reef types, most affected by human activities because of their proximity to land, develop right next to the land

27. Types of Coral Reefs • Barrier reefs • similar to fringing reefs but separated from the landmass and fringing reef by lagoons or deepwater channels • Great Barrier Reef is the world’s largest barrier reef

30. Types of Coral Reefs • Atolls • usually elliptical, arise out of deep water and have a centrally-located lagoon • Often eroded dead volcano

31. Darwin’s theory of atoll formation: • corals colonize shallow areas around newly-formed volcanic islands to form a fringing reef • the island sinks and erodes, and a barrier reef is formed about the island • the island sinks completely, leaving an atoll

36. Land Sea Lagoon Sea Land Lagoon Sea Sea Reef Reef Reef Fringing reef Reef Patch reef Barrier reef Patch reef Stepped Art Atoll Fig. 15-11, p. 422

37. Types of Coral Reefs • In addition, patch reefs can occur within lagoons associated with atolls and barrier reefs

38. Reef Structure • Different reef types share common characteristics • Reef front or forereef: portion of the reef that rises from the lower depths of the ocean to a level just at or just below the surface of the water, on the seaward side • drop-off: a steep reef-front that forms a vertical wall • spur-and-groove formation or buttress zone: finger-like projections of the reef front that protrude seaward; disperses wave energy and helps prevent damage

39. Reef Structure • Reef crest: the highest point on the reef and the part that receives the full impact of wave energy • where wave impact is very strong, it may consist of an algal ridge of encrusting coralline algae, lacking most other organisms, and penetrated by surge channels, grooves of the buttress zone • Reef flat or back reef: portion behind the reef crest • reef flat of fringing reefs ends at the shoreline • reef flat of atolls and barrier reefs descends into the lagoon

40. Reef Structure • Coral populations on reef front are massive dome-shaped brain corals and columnar pillar corals on intermediate slopes, below this region coral species form plate-like formations • Higher up on reef where wave energy is greatest, branching species of coral are found, e.g., elkhorn coral in Caribbean • In protected areas behind reef front, in shallow calm waters, small species of coral occur, e.g., rose, flower and star corals

45. Coral Reef Distribution • Major factors influencing distribution (corals are sensitive): • temperature – corals do best at 23-25o C • light availability – photosynthetic zooxanthellae need light, corals not found below 60 meters • sediment accumulation – can reduce light and clog feeding structures • salinity, corals absent from areas of massive freshwater outflow, e.g., the mouth of the Amazon • wave action – moderate wave action is beneficial, brings in oxygenated seawater, removes sediment that could smother coral polyps • heavy wave action during hurricanes can damage reef structure • duration of air exposure – can be deadly

47. Comparison of Atlantic and Indo-Pacific Reefs • Pacific reefs are older and have a greater depth of reef carbonates • Buttress zone is deeper on Atlantic reefs and coral growth may extend to 100 m down • Pacific coral growth rarely exceeds 60 m • Proportion of reef covered by corals may approach 100% on some Pacific reefs, but usually less than 60% on Atlantic reefs • Algal ridges more common in the Pacific because of wind and waves

48. Comparison of Atlantic and Indo-Pacific Reefs • Hydrozoan Millipora complanata (fire coral) is dominant on Atlantic reefs • similar species never dominate in the Pacific • Atlantic corals nocturnal (night); Pacific corals diurnal (day)

49. Comparison of Atlantic and Indo-Pacific Reefs • Greater sponge biomass in the Atlantic • Pacific has giant clams and sea stars that prey on corals

50. Coral Reef Ecology • Source of nutrients for coral reefs • land runoff for reefs close to land • That’s why too much runoff can bring unwanted nutrients • source of nutrients for atolls unclear (usually out in the middle of the ocean) • possible explanations: • nutrients accumulated over time are efficiently recycled • reef bacteria and filter feeders capitalize on nutrients from dissolved/particulate organic matter • nutrients are stored in the biomass of the community’s inhabitants