2700: Coral Reefs

coral reefs

formed by hermatypic (stony) corals

• characterized by having zooxanthellae in tissue

• without it they can’t create complex structures 9calcium carbonate skeleton)

geological old and and large structure (largest biologically produced structures, visible from space)

structural complexity = high diversity

economic importance

• tourism

• supports lots of fish

• nursery grounds for commercially important fish

coral distribtution

most productive ecosystems

most endangered ecosystems

0.17% of oceans surface

1000 spp of fish

100000 spp of inverts

1/10 worlds total fish harvest

billions of $$ tourist trade/yr

500 million ppl rely on reefs

by coastlines

hard stony corals

hermatypic (reef building)

calcareous cups secreted by polyps

polyps 1-10 mm in diameter

zooxanthellae (1-5 million cells/cm²)

• photosynthesis and respiration on a smaller scale

all polyps interconnection

• colony

• all support each other

• all separate entities

• if some part of coral destroyed, colony can stay alive

coral symbiosis

sunlight goes into zooxanthellae

• photosynthesis

• excreted organic matter and O2

• respiration in coral makes H2O and CO2 that goes back into zooxanthellae to assist photosynthesis

• coral polyp is over the zooxanthellae

requirements

• warm, clear, tropical waters

• light and salinity of normal seawater

coral reef requirements

hard substrate

narrow temp range (between 18-29 celsius (23-25 optimal))

optimal light - <50 m

low turbidity - essential for zooxanthellae

ocean salinity - ~35 psu

limited wave action

narrow pH range

typical coral growth forms

diversity of forms indicative of competition for space and light

very competitive environments

plate like

foliaceous (leaf-like)

columnar

massive

branching

free-living

encrusting

plate like coral

maximize photosynthesis potential

right below sunlight

columnar coral

finger shaped

massive coral

mounds

slow growing

needs a lot of calcium carbonate in center

robust, less storm damage

branching coral

pretty ones

most fast growing

lots of opportunity for polyps and capture

fragile

in caribbean has a lot of these

encrusting coral

increasing photosynthetic potential

low profile prone to predation by sea stars and grazers

other structures forming reef organisms

coralline algae (also produce calcium carbonate)

soft corals

• sea whips and sea fans

other cnidarians such as hydrozoans or anemones

sponges

bryozoans

coralline algae forming reef structures

help cement reef to substrate

create chemical cues for coral to settle

help protect against wave action

important in reefs

soft corals forming reef structures

gorgonians

very giant fans

prone to wave action and storm damage

sponges forming reef structures

help cement coral rumble into substrate

barrel sponges

coral reefs accrete

similar to mangroves and sea grasses

trap sediment in between hard coral structures

over time building up hard substrate around it, perpetuates further reef growth

types of coral reefs

1. fringing reefs

2. barrier reefs

3. atolls

fringing reefs

on coasts or islands

continental shelf right next to shoreline

prime sunlight and diff factors for coral growth that make corals directly along land mass

young reef, hugs edges of shoreline

simplest form of reef

starting pt for next reef cycles

on an island that just formed, reef hugs islands

barrier reefs

some sort of erosion or sinking of the island (natural)

what used to hug shore now farther away

reef mostly separated from land mass

structure of a lagoon between reef and land mass

great barrier reef

• began forming 18 million yrs ago

• lots of erosion on land around it

• potentially 8000 yr old coral

  • young bc of glacial cycles

Meso-American Barrier reef system

• 900 km from Cancun to Honduras

atolls

eventually island center erodes away over millions of yrs

sinks below sea level

corals continuously build reef with accretion

still have ring of corals where island once was

accreting and building at same rate that island was sinking

central lagoon with a ring around it

coral reefs exhibit vertical zonation

some species do best with diff biological ad physical stresses

shallower waters have higher wave energy but higher light

some species thrive higher up with the higher wave action

coral reefs productivity is driven by the coral-zooxanthellae relationship

many organisms feed directly on corals or their products

• mucus

• eggs

• larvae

• contributes to bioerosion of reefs

trigger fish

parrotfish

• very important

• being overfished

butterfly fish

eats polyps with teeth

many organisms rely on reef structure

alternative stable states in Jamaican coral reefs

algae smothers reefs

species of sea urchin

• keystone species on coral reefs

• big herbivores

• kept algae in check by eating it

in 1980’s, some water-borne pathogen went through caribbean and infected sea urchins

pops of urchins collapsed

• made reefs collapse

food webs of coral reefs

producers

consumers

coral and coral mucus feeders

plankton feeders

predators

everything goes back to detritus

• inputs from mangroves, sea grass beds, estuaries

producers in coral reefs

2 levels

• seaweeds, coralline algae, photosynthetic bacteria

• corals/zooxanthellae

consumers

grazers

detritus feeders

coral and coral mucus feeders

plankton feeder

coral grazers

fish

urchins

snails

chitons

coral and coral mucus feeders

fish

sea stars

crabs

detritus feeders

sea cucumbers

worms

amphipods

soft corals

plankton feeders

sea fans

fish

feather stars

predators

fishes

squid

snails

coral reef biogeography

pacific and atlantic provinces were once united by connection across Tethys sea 10 million yrs ago

many Caribbean coral species went extinct as the atlantic ocean broadened

pacific ocean is old, environmentally stable

• has supported more coral species

corals grow very slowly

corals grow <15 cm/yr, most ~ 3 cm

how to measure coral growth

label with radioactive calcium

spike driven into coral; measure subsequent addition of skeleton

use of dyes (ex. alizarin red); created reference layer in coral skeleton

natural growth bands

corals help us recreate paleoclimate

18O2 us a great proxy of paleoclimate (and a stable isotope)

• heavier bc more neutrons

• rained out first in climate bc heavier

• evaporates less than regular O2 bc heavier

warm conditions on planet result in more evaporation and precipitation

• more record of 18O2

• can see what’s going on in water column through geological history

O2 evaporates first

18O2 rained out first, doesn’t evaporate as much

key is temp dependance

measuring isotope gives us an idea of temp in geological history

• ice cores

• geological sediment cores

very important

biodiversity makes reefs a stressful place

biological interactions in reef

competition

predation and grazing

disturbance

larval recruitment

disease

competition

shading

overgrowth

space

allelopathy

dominance by pocillopora damicornis probably reflects competitive superiority; diversity (H’) increases in areas where P. damicornis where the environment is not ideal

predation and grazing

coral predators (crown-of-thorns starfish)

grazers (surgeon fish, parrotfish, urchins)

large predators

disturbance

wave action

major storms

human caused

larval recruitment

mass spawning

space limitations

settling behavior

disease

can cause mass mortality of some species (black sea urchin, coral diseases)