Transcript 15 Sea Grass Beds, Kelp Forests, Rocky Reefs, and Coral Reefs

15 Sea Grass Beds, Kelp
Forests, Rocky Reefs, and Coral
Reefs
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
©Jeffrey S. Levinton 2001
In Chapter 15 we will cover
• Kelp Forests
• Coral Reefs
Kelp Forests
• Dominated by brown seaweeds in the Laminariales
• Found in clear, shallow water, nutrient rich and usually
< 20°C, exposed to open sea
• Generally laminarian seaweeds have high growth rates,
often of the order of cm/d
• “Forests” can be 10-20 m high or only a meter in height
Laminaria kelp forest, as is often found in
New England
Diver in Macrocystis kelp forest, California
Complex life cycle
• Laminarian kelps have a complex life
cycle alternating between a large asexual
sporophyte and a small gametophyte
Microscopic, haploid
Male gametophyte
Diploid
sporophyte
Zoospores
Sori of
unilocular
sporangia
Antherozoids
Microscopic,haploid
Female gametophyte Egg
Sporophyte
development
fusion
Kelp Forests are Diverse
• Kelp forests have many species of
seaweeds, even if sometimes dominated
by one species
• Many invertebrate species present,
especially sessile benthic species living on
hard substrata - suspension feeders
common
Abundant benthic
invertebrates of an
Alaskan kelp forest
Kelp Forest Community Structure 1
• Herbivory - herbivorous sea urchins exert
strong effects on kelp abundance
Kelp Forest Community Structure 2
• Herbivory - herbivorous sea urchins exert
strong effects on kelp abundance
• Carnivory - in Pacific kelp forests, sea otter
Enhydra lutris can regulate urchin populations
Kelp Forest Community Structure 3
• Herbivory - herbivorous sea urchins exert
strong effects on kelp abundance
• Carnivory - in Pacific kelp forests, sea otter
Enhydra lutris can regulate urchin populations
• Result: trophic cascade. Add otters, have
reduction of urchins and increase of kelp
abundance. Reduce otters: kelp grazed down
by abundant urchins
Kelp Forest Community Structure 4
• Herbivory - herbivorous sea urchins exert
strong effects on kelp abundance
• Carnivory - in Pacific kelp forests, sea otter
Enhydra lutris can regulate urchin populations
• Result: trophic cascade. Add otters, have
reduction of urchins and increase of kelp
abundance. Reduce otters: kelp grazed down
by abundant urchins
• Recent history: Otters hunted to near
extinction, their recovery has strong impacts on
urchin/kelp balance
Sea otter, Enhydra lutris, a keystone species in
Pacific coast kelp forests
Sea otters
Urchins
Kelp
Trophic cascade in kelp forests. Increase of sea otters results
in reduction of urchins and an increase of kelp
Kelp Forest Community Structure 5
• Storms - can remove kelps, especially during El
Niño events when temperature is also warm
and nutrients in water are poor (all bad for
kelps)
• Storms can remove kelps, resulting in bare
bottoms known as barrens, which also can be
created by high rates of urchin grazing
Kelp Forest Community Structure 6
Alternative stable states:
• When kelp abundant, dominant California red
sea urchin* hides in crevices feeding upon drift
algae. Grazing on attached seaweeds not a
factor, so even though urchins are abundant,
kelps maintain dominance
• When kelps not abundant, urchins rove around
and graze down new kelp plants, maintaining a
barrens bottom
*Strongylocentrotus franciscanus
Alternative stable states in a California kelp forest
Kelp Forest Community Structure 7
Succession:
• Kelp forests are very dynamic but succession known in
Alaskan kelp forests dominated by Nereocystis
• Disappearance or reduction of urchins is followed by
recruitment of several kelp species
• Although Nereocystis is often an upper canopy species,
with fronds at the surface, it is often an annual and dies
back each year
• If urchins do not become abundant a species of
Laminaria gradually moves in and shades out other
seaweeds and comes to dominate
urchins
Costaria
Alaria
Desmarestia
Nereocystis
Laminaria
Successional sequence in an Alaskan kelp forest
Succession
towards
Laminaria
Storms
Maintain barrens
Kelp
Barrens
Urchins
Synthesis of possible transformations in a California kelp forest
Coral Reefs
• Geological Importance: massive physical
structures (1950 km Great Barrier Reef), islands
and archipelagos, old and well-preserved fossil
communities
• Biological Importance: High diversity, many
phyla, organisms with both very wide and
sometimes very localized geographic
distributions.
• Economic Importance: shoreline protection,
harbors, fishing in developing world, tourism
Coral Reefs
• Compacted and cemented assemblages of
skeletons and sediment of sedentary organisms
• Constructional, wave-resistant features
• Built up principally by corals, coralline algae,
sponges and other organisms, but also
cemented together
• Reef-building corals have symbiotic algae
known as zooxanthellae; these corals can
calcify at high rates
• Coral reefs are topographically complex
Coral Reefs - Limiting Factors
• Warm sea temperature (current problem
of global sea surface temperature rise)
• High light (symbiosis with algae)
• Open marine salinities usually
• Low turbidity - coral reefs do poorly in
near-continent areas with suspended
sediment
Coral Reefs - Limiting Factors 2
• Strong sea water currents, wave action
• Reef growth a balance between growth
and bioerosion
• Reef growth must respond to rises and
falls of sea level
Coral Reef Biogeography 1
• Current division between Pacific and Atlantic
provinces
Coral Reef Biogeography 2
• Current division between Pacific and Atlantic
provinces
• Strong Pacific diversity gradient: (1) diversity
drops with increasing longitude, away from
center of diversity near Phillipines and
Indonesia; (2) also a latitudinal diversity
gradient, with diversity dropping with
increasing latitude, north and south from near
equator
Coral Reef Biogeography 3
• Current division between Pacific and Atlantic
provinces
• Strong Pacific diversity gradient: (1) diversity
drops with increasing longitude, away from
center of diversity near Phillipines and
Indonesia; (2) also a latitudinal diversity
gradient, with diversity dropping with
increasing latitude, north and south from near
equator
• Historically, Pacific and Atlantic provinces
were once united by connection across Tethyan
Sea, which disappeared in Miocene, ca. 10
million years ago.
Reef Types
• Coastal reefs - wide variety of reefs that grow on the
shallow continental shelf, sometimes large massive
structures like the Great Barrier Reef, down to small
patches such as reef at Eilat, Israel
• Atolls - reefs in form of ring or horseshoe-shaped chain
of coral cays built up on open oceanic volcanic island.
Balance of sinking of island and upward growth of
coral reefs
Origin of Atolls
Reef-building (Hermatypic) corals
• Belong to the phylum Cnidaria, Class
Anthozoa, Order Scleractinia
• Secrete skeletons of calcium carbonate
• Are colonies of many similar polyps
• Can be divided into branching and
massive forms
• Have abundant endosymbiotic
zooxanthellae
Tentacle
Mouth
Digestive
Filament
Septum
Pharynx
Septum
Gastrovascular
Cavity
Basal plate
Polyp of a scleractinian coral
Closeup view of expanded polyps of Caribbean
coral Montastrea cavernosa
Hermatypic vs. Ahermatypic corals
• Hermatypic: Reef framework building,
have many zooxanthellae, hi calcification
• Ahermatypic: not framework builders,
low calcification
Growth forms
• Branching: grow in linear dimension
fairly rapidly 10 cm per y
• Massive: Produce lots of calcium
carbonate but grow more slowly in linear
dimensions, about 1 cm per y
Measures of coral growth
• Label with radioactive calcium
• Spike driven into coral; measure
subsequent addition of skeleton
• Use of dyes (e.g., alizarin red): creates
reference layer in coral skeleton
• Natural growth bands: e.g., seasonal
Zooxanthellae
• Found in species of anemones, hermatypic
corals, octocorals, bivalve Tridacna
• Considered as one species: Symbiodinium
microadriaticum
• Is a dinoflagellate: found in tissues without
dinoflagellate pair of flagellae, but can be put
in culture where flagellae are developed
• Found in corals within tissues (endodermal),
concentrated in tentacles
Zooxanthellae - Benefits? 1
• Nutrition - radiocarbon-labeled carbon taken
up by zooxanthellae and transported to coral
tissues (note corals usually also feed on
microzooplankton)
Zooxanthellae - Benefits? 2
• Nutrition - radiocarbon-labeled carbon taken up by
zooxanthellae and transported to coral tissues
(note corals usually also feed on
microzooplankton)
• Source of oxygen for coral respiration - maybe
not a major benefit, because corals are in
oxygenated water
Zooxanthellae - benefits? 3
• Nutrition - radiocarbon-labeled carbon taken up by
zooxanthellae and transported to coral tissues
(note corals usually also feed on
microzooplankton)
• Source of oxygen for coral respiration - maybe not
a major benefit, because corals are in oxygenated
water
• Facilitate release of excretion products - Again,
not likely to be a major benefit, because corals
in well-circulated water
Zooxanthellae - benefits? 4
• Nutrition - radiocarbon-labeled carbon taken up by
zooxanthellae and transported to coral tissues
(note corals usually also feed on
microzooplankton)
• Source of oxygen for coral respiration - maybe not
a major benefit, because corals are in oxygenated
water
• Facilitate release of excretion products - Again,
not likely to be a major benefit, because corals in
well-circulated water
• Facilitate calcification - uptake of carbon
dioxide by zooxanthellae enhances calcium
carbonate deposition: inhibit photosynthesis
and calcification rate decreases
Mass Spawning on Coral Reefs 1
• Most corals have planktonic gametes
Mass Spawning on Coral Reefs 2
• Most corals have planktonic gametes
• On Great Barrier Reef, reefs off of Texas: many
species of corals spawn at same time
Mass Spawning on Coral Reefs 3
• Most corals have planktonic gametes
• On Great Barrier Reef, reefs off of Texas: many
species of corals spawn at same time
• Facilitates gamete union, perhaps a mechanism
to flood the sea with gametes to avoid all being
ingested by predators
Mass Spawning on Coral Reefs 4
• Most corals have planktonic gametes
• On Great Barrier Reef, reefs off of Texas: many
species of corals spawn at same time
• Facilitates gamete union, perhaps a mechanism to
flood the sea with gametes to avoid all being
ingested by predators
• Facilitiates release of gametes at time when
currents are minimal and gametes can unite
Depth Zonation on Reefs
• Reefs dominated by different coral
species at different depths
• May be controlled by factors similar to
rocky shores, but not so well known, also
possible relationship to changing light
conditions
Caribbean depth zonation
Biological Interactions 1
• Competition - shading, overgrowth,
interspecific digestion, sweeper tentacles,
allelopathy(?)
Acropora palmata
Overtopping
Montastrea annularis
Biological Interactions 2
• Competition - shading, overgrowth,
interspecific digestion, sweeper tentacles,
allelopathy(?)
• Predation and grazing - some common coral
predators (e.g., crown-of-thorns starfish),
grazers (e.g., surgeon fish, parrotfish, urchins)
Biological Interactions 3
• Competition - shading, overgrowth,
interspecific digestion, sweeper tentacles,
allelopathy(?)
• Predation and grazing - some common coral
predators (e.g., crown-of-thorns starfish),
grazers (e.g., surgeon fish, parrotfish, urchins)
• Disturbance - e.g., storms, hurricanes, cyclones
Biological Interactions 4
• Competition - shading, overgrowth,
interspecific digestion, sweeper tentacles,
allelopathy(?)
• Predation and grazing - some common coral
predators (e.g., crown-of-thorns starfish),
grazers (e.g., surgeon fish, parrotfish, urchins)
• Disturbance - e.g., storms, hurricanes, cyclones
• Larval recruitment - mass spawning, question
of currents and recruitment of larvae
Biological Interactions 5
• Competition - shading, overgrowth,
interspecific digestion, sweeper tentacles,
allelopathy(?)
• Predation and grazing - some common coral
predators (e.g., crown-of-thorns starfish),
grazers (e.g., surgeon fish, parrotfish, urchins)
• Disturbance - e.g., storms, hurricanes, cyclones
• Larval recruitment - mass spawning, question
of currents and recruitment of larvae
• Disease - spread by currents, can cause mass
mortality of some species (e.g., common black
sea urchin Diadema antillarum in 1980s)
Interspecific Competition 1
• Goreau Paradox – measured calcification
rates of many species on Jamaican reefs –
relative abundance on reef is not
necessarily explained by growth rates slower growing forms often dominate
(e.g., massive coral Montastrea annularis
is dominant of a depth zone, forming
large buttresses)
Interspecific Competition 2
• Observation by Judith Lang
Scolymia lacera - supposed ecological variants placed
next to eachother: bare zone established after
mesentarial filaments extruded through polyp wall
Interspecific Competition 3
Conclusion: Interaction is due to
interspecific competition by digestion
(variants are different species)
• Corals compete by rapid growth, shading,
interspecific digestion, sweeper tentacles.
• Slower growing forms have interspecific
digestion, sweeper tentacle defenses, which
allows them to hold place on the reef against
faster-growing competitors
Predation and Grazing 1
• Role of predation on reefs poorly known
• Caribbean: Urchin Diadema antillarum feeds both
on sea grasses surrounding patch reefs and on
algae on reefs. Experimental removal results in
strong seaweed growth. Disease in 1980s
eliminated most urchins and this resulted in
strong growth of seaweeds
Predation and Grazing 2
100
1990s
80
Jamaican Coral
Reefs
60
40
20
0
1970s
20
40 60
80
100
Percent coral cover
Die-off of Diadema: Seems to have flipped Jamaican reefs into
alternative stable state (also a result of storm damage). Instead of
rich coral cover, you now have poor coral cover and lots of algae
Predation and Grazing 3
• Pacific Ocean: Crown-of-thorns starfish
Acanthaster planci feeds on corals
• Outbreaks all over Indo-Pacific starting
in 1960s
• Formerly rare, they changed behavior:
herding instead of dispersed, changed
from nocturnal to diurnal in feeding
Predation and Grazing 4
• Explanations for Crown-of-thorns starfish
outbreaks?
1. Blasting of harbors in WWII, resulting in enhanced
sites for larval settlement
2. Overcollection by shell collectors of starfish’s main
predator, Giant triton Charonia tritonus
3. Storms, which wash out nutrients, stimulate
phytoplankton growth and enhance larval survival of
the starfish (some question this, as larvae can do well
Under starvation)
The End