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Tropical Reefs: Coral/Algal
Diversity and Challenges
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Questions: Why are tropical reefs comparable to rain forests in diversity? Dissolved nutrients are scarce Productivity & diversity normally go together Why are tropical reefs disappearing?
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Increasing threats to coral
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Current bleaching threats
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What are the challenges?
Low nutrients dissolved in water Shallow habitats mean more intense wave action Long-term: sea level changes
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Tropical reefs are geologically important
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Reef ecology has long challenged biologists
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Dominant reef-building organisms
Small individuals, magnified in effect by large numbers Phylum Cnidaria Class Anthozoa Subclass Zoantheria Diverse algae, depositing CaCO3
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Cnidaria polyps: radial symmetry
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Cnidaria: tentacles for feeding
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Cnidaria: stinging cells on tentacles
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Not all corals deposit CaCO3
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“Hermatypic” corals create reefs in symbiosis with Symbiodinium
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Growth forms of colonies reflect tradeoffs
Access to light for symbiotic algae Issues of depth (growth upward) Issues of competition (shading) Risk of damage from turbulence in shallow seas Diverse growth forms reflect different balances
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Fast extension: linear growth form
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Extension + more surface area: branching
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Lattice: a subset of “branching” form
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Plate form: more resistant to turbulence
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Massive form: very wave-resistant
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Independent algae build reefs: crustose
Algae can contribute a large fraction of reef mass
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Independent algae: Green (Halimeda)
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Reefs: a matrix of solid coral and sand
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Reef diversity – some examples
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Ecological functional groups
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A larger scale view: reef building process
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Atoll structure
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Habitat diversity within an atoll
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