Download Chapter 15 - coral reefs

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?
Chapter 15
Coral Reef Communities
Karleskint
Turner
Small
• 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
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
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
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
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
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
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
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
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
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
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
• Types of reefs:
•
•
•
•
Fringing
Barrier
Atoll
Patch
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
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
Types of Coral Reefs
• Atolls
–
usually elliptical, arise out of deep water and have a centrally-located lagoon
– Often eroded dead volcano
• Darwin’s theory of atoll formation:
– corals colonize shallow areas around newlyformed 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
Land
Sea
Reef
Fringing reef
Patch Lagoon
reef
Land
Reef
Sea
Barrier reef
Lagoon
Sea
Sea
Reef
Atoll
Patch
reef
Reef
Stepped Art
Fig. 15-11, p. 422
Types of Coral Reefs
• In addition, patch reefs can occur within
lagoons associated with atolls and barrier
reefs
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
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
Reef Structure
• Coral populations on reef front are massive domeshaped 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
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
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
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)
Comparison of Atlantic
and Indo-Pacific Reefs
• Greater sponge biomass in the Atlantic
• Pacific has giant clams and sea stars that
prey on corals
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
Coral Reef Ecology
• Photosynthesis on Reefs
– photosynthetic organisms: zooxanthellae, benthic
algae, turf algae, sand algae, phytoplankton,
seagrasses
– more dense than tropical ocean, with greater biomass
than reef animals
– associations of producers with other organisms assist
in efficient recycling, e.g., zooxanthellae with corals,
cyanobacteria with sponges
Coral Reef Ecology
• Reef productivity
– ratio of primary production to community
respiration = P-R ratio
• P = gross photosynthesis
• C = community respiration
– P-R ratio used to measure state of
development of a biological community
Coral Reef Ecology
• Reef productivity (continued)
– P-R ratio > 1 = primary production exceeds
respiratory needs
• biomass increases, excess biomass available for
growth or harvesting
– P-R ratio = 1 = steady state (climax)
• little biomass remains available for growth
– P-R ratios for coral reefs are typically close to 1
• high productivity balanced by high respiration
Coral Reef Ecology
• Reef productivity
– increases in productivity are often the result of
eutrophication
• eutrophication: nutrient enrichment
– eutrophication typically manifested as a
dramatic proliferation of algae
• if grazing doesn’t increase, algae can grow over
and smother corals
• This will happen with too much sewage and
fertilizer runoff
The Coral Reef Community
• Competition among corals and other reef
organisms
– fast-growing, branching corals grow over slowergrowing, encrusting or massive corals and deny
them light
– slower-growing corals extend stinging
mesenterial filaments from their digestive cavity
to kill faster-growing corals
– fast-growing corals can also sting and kill using
long sweeper tentacles with powerful
nematocysts
The Coral Reef Community
• Competition among corals and other reef
organisms (continued)
– Slower growing corals are more aggressive than
fast growing corals
– Massive corals are generally more shade
tolerant and are able to survive at greater
depths
– as a result…
• fast-growing, branching corals on many reefs
dominate upper, shallower portions
• larger, slower-growing corals dominate deeper
portions
The Coral Reef Community
• Competition among reef fishes
– High diversity
• coral reefs - marine habitats with greatest
diversity/abundance of fishes
• seems to defy competitive exclusion principle,
which suggests that no 2 species can occupy the
same niche
– 60-70% of reef fishes are general carnivores
– about 15% are coral algae grazers or omnivorous
The Coral Reef Community
• Competition among reef fishes (continued)
– hypotheses proposed to explain this:
• competition model: factors such as time of day or
night, size of prey, position in the water column, etc.
provide each species with a unique niche (hence, no
competition)
• predation disturbance model: assumes competition,
but suggests that the effect of predation or other
causes of death keep populations low enough to
prevent competitive exclusion
The Coral Reef Community
• Competition among reef fishes (continued)
– hypotheses proposed to explain this:
• lottery model: assumes competition occurs, but
suggests that chance determines which species of
larvae settling from the plankton colonize a
particular area of the reef
• resource limitation model: suggest that available
larvae are limited and that limitation prevents fish
population from ever reaching the carrying
capacity of the habitat
The Coral Reef Community
• Effect of grazing
– reef is a mosaic of microhabitats with different
levels of grazing and different algal communities
– grazing of larger, fleshier seaweeds permits
competitively inferior filamentous forms or
coralline algae to persist
– herbivory decreases with depth
– damselfish form territories where they exclude
grazers and permit abundant algal growth
• provides habitat for small invertebrates
• overgrows corals; fast-growing, branching corals are
most successful near damselfish
The Coral Reef Community
• Effect of predation
– predation of sponges, soft corals and
gorgonians provides space for competitively
inferior reef corals
– small invertebrates are almost all well hidden
or camouflaged, indicating the prevalence of
predation in the reef
The Coral Reef Community
• Symbiotic relationships on coral reefs
– cleaning symbioses
• cleaner wrasses, gobies, etc. feed on parasites of
larger fishes
• cleaning organisms set up a cleaning station
– Other symbiotic relationships
•
•
•
•
clownfishes and anemones
conchfish and the queen conch
gobies and snapping shrimp
crustaceans and anemones
Evolutionary Adaptations of Reef Dwellers
• Adaptive behaviors to avoid predation
– invertebrates hide during the day and forage
at night
– producing a poisonous coating of mucus
– burying the body in sand to hide
– inflating to appear larger
– hiding at night when nocturnal predators are
active
Evolutionary Adaptations of Reef Dwellers
• Structural adaptations for feeding
– cnidocytes (stinging cells) of cnidarians aid in prey
capture
– radioles (hair-like) appendages of Christmas tree
worms are used to capture phytoplankton
– non-bivalve mollusks use radula to graze algae
– mantis shrimp have extremely sharp forward
appendages
– snapping shrimp use sound to defend territory and
stun prey
– crinoids (feathers stars) use basket of mucus to feed
Evolutionary Adaptations of Reef Dwellers
• Protective body covering
– tough, defensive exteriors help animals avoid
predation, but can limit mobility and growth
• Role of color in reef organisms
– color for concealment and protection
– Many invertebrates have colors and stripes
that allow them to blend in with the
environment
Evolutionary Adaptations of Reef Dwellers
• Role of color in reef organisms (continued)
– brilliant color of many fish actually helps them to
blend in with colorful background of the reef
– other types of camouflage
• body shape
– warning coloration
• e.g., lionfish
– other roles of color
• defending territories
• mating rituals
Threats to Coral Reef Communities
• Effect of physical changes on the health of
coral reefs
– hurricanes and typhoons topple and remove
coral formations
– El Niño Southern Oscillation (ENSO)
• changes winds, ocean currents, temperatures,
rainfall and atmospheric pressure over large areas
of tropical and subtropical areas
• can cause massive storms
Threats to Coral Reef Communities
• Why are coral reefs important?
– protect coast from high surf conditions
– remove large amounts of carbon dioxide from
water and air
– provide habitat for a huge diversity of
invertebrates and fish
– economical value, many people earn living by
collecting and processing reef products
– important place of recreation
– have potential for harvesting pharmaceutical
products
Threats to Coral Reef Communities
• Effects of human activities
– Destructive fishing practices
• overfishing, i.e., eliminating grazers, allows algae
to overgrow reefs
• poisonous chemicals used to capture fish also
poison corals
• explosives used to stun and capture fish can cause
massive destruction to coral
• bottom trawling for fish also destroys coral
structures
Threats to Coral Reef Communities
• Effects of human activities (continued)
– Coastal development
• produces runoff containing nutrients, pesticides,
toxic wastes
• increases sedimentation and changes patterns of
water flow
Threats to Coral Reef Communities
• Effects of human activities (continued)
– Other human activities
• coral mined for use as bricks, road-fill, cement
component
• removed to make jewelry
• inexperienced snorkelers and boaters damage
reefs
Threats to Coral Reef Communities
• Effects of human activities (continued)
– effects of human-induced climate change in
atmosphere
• increased levels of carbon dioxide from burning of
fossils fuels primary cause of ocean warming
• causes corals to become stressed and more
susceptible to coral bleaching and disease
Threats to Coral Reef Communities
• Effects of human activities (continued)
– coral bleaching
• a phenomenon by which corals expel their symbiotic
zooxanthellae
• most often associated with warming of the ocean
water by ENSO or global warming
• if the stress is not too severe, corals may regain
zooxanthellae and recover
• if the stress is prolonged, corals may fail to regain
zooxanthellae and die
Threats to Coral Reef Communities
• Effects of human activities (continued)
– coral diseases
• black band disease: a distinct dark band of
bacteria migrates across the living coral tissue,
leaving behind a bare white skeleton
• white pox: characterized by white lesions and
caused by Serratia marcescens
– other coral diseases:
• white band disease
• white plague
• CYBD (Caribbean yellow band disease) or yellow
blotch disease