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15 Sea Grass Beds, Kelp
Forests, Rocky Reefs, and Coral
Reefs
Notes for Marine Biology:
Function, Biodiversity, Ecology
By Jeffrey S. Levinton
Sea Grasses
• Sea grasses are marine angiosperms, or
flowering plants, that are confined to very shallow
water
• Extend mainly by subsurface rhizome systems
within soft sediment
• Found throughout tropical and temperate oceans
• Grow best in very shallow water, high light and
modest current flow
Sea Grasses
A bed of Zostera marina in Padilla Bay, Washington.
Blades of this sea grass are 50–100 cm high
Sea Grasses
Sea grass beds most easily colonize sediment after a successional
sequence featuring a previous colonizationd by seaweeds
Sea Grasses - Production,
Ecology
• High primary production, support a diverse group
of animal species
• Sea grass beds reduce current flow
• deter the entry of crab and fish predators from side
• May enhance growth and abundance of infaunal
suspension feeders near edge, although
phytoplankton may not penetrate far into bed
Sea Grasses - Grazing,
Community Structure
• Grazing on sea grasses variable: in temperate
zone, grazing on Zostera marina (eel grass) is
minimal
• In tropics, sea grass beds comprised of several
species that are grazed differentially because of
different toughness, cellulose content
• Green turtles nip leaf tips,which encourages
growth of more soft and digestible new grass
• Even tough grasses grazed by turtles, urchins,
dugongs. Green turtles have extended hindguts
with intestinal microflora, digesting cellulose
Sea Grasses - Grazing,
Community Structure
• Tropical sea grass beds diverse, often as many as
10 species, mixed with seaweed species
• Seaweeds and grasses grazed by a variety of
invertebrates, who also seek shelter among the
grass and seaweed
• Predators such as fishes, crabs, consume
invertebrates but no strong top-down effects by
predators
Sea Grasses - Decline
• Sea grasses very vulnerable to eutrophication phytoplankton shade sea grasses, strong reductions
of eel grass beds in North America
• Possible that overfishing results in reduced
grazing and overgrowth of epiphytes, which
smothers sea grasses
• Dredging, boat traffic, also causes decline of sea
grasses
• Disease important, fungus caused eelgrass
epidemic in 1930s, recovery, but other fungi are
now cause of sporadic diseases in tropical sea
grasses
Rocky Reefs - Kelp Forests
• Kelp forest - rocky reef complex found in cooler
coastal waters with high nutrients
• Kelp forest–rocky reefs are often dominated in
shallow waters by kelps and seaweeds and by
epifaunal animals in deeper waters animaldominated rocky reefs
• Switch from cover dominance by rapidly growing
seaweeds in shallow water to epifaunal animal
dominance in deeper water
Abundance of kelps, macroalgae (kelps plus
other seaweeds), and sessile invertebrates on a
transect with increasing depth, Friday Harbor,
Washinton State
Kelps and others dominating shallow water (1) Kelp Agarum
fimbriatum; (2) kelp Saccharina latissima; (3) crustose coralline
alga; (4) fleshy red seaweed
Some colonial invertebrates in deeper rocky reefs (5) sea squirt
Aplidium sp.; (6) sea squirt Didemnum sp.; (7) sea squirt
Metandrocarpa taylori
Rocky Reefs
• Abundant communities ofalgae and invertebrates,
often dominated by colonial invertebrates.
• Often are very patchy, with alternations of rocks
dominated by rich invertebrate assemblages and
turf-forming calcareous red algae
• Subtidal rock wall patches of animals often are
short on space, suggesting the importance of
competition
Rocky Reefs
• Many invertebrates have lecithotrophic
larvae, which reduces dispersal distance,
increases patchiness
• Rocky reefs are grazed more intensely,
mainly by sea urchins, on horizontal
benches
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 centimeters/day
• “Forests” can be 10-20 m high or only a meter in height
A kelp forest in the Aleutian Islands, Alaska: Cymathere triplicata
(foreground); Alaria fistulosa (rear)
Complex Life Cycle
• Laminarian kelps have a complex life
cycle alternating between a large asexual
sporophyte and a small gametophyte
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
• Herbivory - herbivorous sea urchins
• Carnivory - 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
• In lower-latitude California kelp forests, a
larger diversity of predators beyond sea otters
exerts top-down effects
Sea otter, Enhydra lutris
Sea otters (O)
Urchins (U)
O
U
K
Kelp (K)
O
U
K
Evolutionary Consequences
of Herbivory
• North Pacific - otters reduce urchins - low
herbivory (0-2%/day) - relatively few
defenses evolved by kelps against herbivory
• Australasia - less predation on urchins results in higher urchin herbivory (57%/day) - phlorotannin concentrations in
kelps were on average 5-6 x of North
Pacific
Steinberg, Estes, Winter, 1995, Proc. Nat. Acad. Sci. 92: 8145-8
Kelp Forest Community Structure
• Effect of storms: remove kelp
• El nino: storms + warm water -> kelp mortality
• California kelp forests*: storms remove kelp,
urchins roam, and inhibit kelp colonization and
growth: barrens
• California kelp forests: if kelp growth is rich,
urchins stay in crevices and capture drift algae
• This leads to two alternating states: barrens
and kelp forest
*Harrold and Reed 1985 Ecology
Alternative stable states in a California kelp forest
See Harrold and Reed 1985 Ecology; Ebeling et al. 2004 Marine Biology
Kelp Forest Community Structure
Succession:
• Nereocystis - winner in succession in Pacific NWAlaska??
• Urchins die -> kelp recruitment - several species cooccur
• Although Nereocystis is often an upper canopy species,
with fronds at the surface, it is often an annual and dies
back each year
• Laminaria gradually shades out other seaweeds wins if
no dense urchin populations
Successional sequence in an Alaskan kelp forest
Synthesis of possible transformations in a California kelp forest
Kelp Forest Recap
• Clear, nutrient-rich, cool < 20 ºC
• Trophic cascade: otters, urchins, kelp (plus
Orca at top of chain in Alaska)
• Barrens versus rich kelp forest - stable
states, owing to urchin behavior
• Succession in Alaska - light competition
leading to dominance by Laminaria
Coral Reefs
• Geological importance: often massive physical
structures
• Biological importance: biological structure,
High diversity,
• Economic importance: shoreline protection,
harbors, fishing, tourism
Coral reef, north coast of Jamaica, Caribbean
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 belong to the Scleractinia,
have endosymbiotic algae known as
zooxanthellae; high calcification rate
• 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
• pH? Increasing ocean acidity a problem?
Coral Reef Biogeography
• 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 from massive
structures ( Great Barrier Reef), to small patches such
(Eilat, Israel)
• Atolls - horseshoe or ring-shaped island chain of islands
atop a sea mount
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
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 year
• Massive: Produce lots of calcium
carbonate but grow more slowly in linear
dimensions, about 1 cm per year
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
Corals - Biodiversity and Form
Diversity
• Coral species usually first identified on basis of
morphology
• Problem: coral species have a large degree of
morphological plasticity - variable growth
response to variation in water energy, light,
competitive interactions with other species
• Problem: nearly morphologically identical species
• Species now identified more with DNA
sequencing
Zooxanthellae
• Found in species of anemones, hermatypic
corals, octocorals, bivalve Tridacna, ciliophora
(Euplotes)
• Once considered as one species: Symbiodinium
microadriaticum but they are at least 10 distinct
taxa, not much correlation between coral and
zx, large genetic distance among species; see
Rowan and Powers 1992 PNAS
• 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
• Located in endoderm tissue
Picked up by larvae, juveniles by infection from environment;
Some strains reproduce faster than others (see Little et al.
2004 Science)
Zooxanthellae - Benefits?
• 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
are in well-circulated water
• Facilitate calcification - uptake of carbon
dioxide by zooxanthellae enhances calcium
carbonate deposition: inhibit photosynthesis
and calcification rate decreases
Zooxanthellae - Bleaching?
• Bleaching - expulsion of zooxanthellae
• Causes - stress (temperature, disease)
• Mechanisms - poorly understood - zooxanthellae
cells appear to die and are expelled
• Test among mechanisms with fluorochromes;
support for cell death under temperature stress
(Strychar et al. 2004 J. Exp. Mar. Biol. Ecol.)
(a) A healthy colony of the Caribbean elkhorn coral Acropora
palmata (b) A bleached colony of the same species
Mass Spawning on Coral Reefs
• 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
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
• Competition - shading, overgrowth,
interspecific digestion, sweeper tentacles,
allelopathy(?)
Acropora palmata
Overtopping
Montastrea annularis
Biological Interactions
• 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)
Dominance by Pocillopora damicornis probably reflects
competitive superiority; diversity (H’, which increases with
number of species and evenness of distribution of numbers)
increases in areas where P. damicornis is less common
Interspecific Competition 1
• Goreau Paradox – calcification rate not
proportional to percent cover (e.g.,
buttress zone - massive coral Montastrea
annularis)
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, reefs lacking
top carnivores might exert strong effects but data is
spotty
• 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, inhibits
coral recruitment
• Fish grazing usually very important (parrot fish,
surgeon fish), affects seaweed growth
Predation and Grazing 2
100
1990s
80
Jamaican Coral
Reefs
60
40
20
0
1970s
20
40
60
80
100
Percent coral cover
The perfect storm: hurricanes plus Diadema Dieoff:
alternative stable states?
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
See Sapp, J. 1999, What is Natural? Oxford
University Press, NY.
Acanthaster planci
Predation and Grazing 4
• Explanations for Crown-of-thorns seastar outbreaks?
1. Blasting of harbors in WWII--> enhanced
larval settlement
2. Overfishing and overcollection of starfish’s main
predator, giant triton Charonia tritonus, overfished islands
Have more crown-of-thorns seastars*
3. Storms-->nutrient washout-->phytoplankton-->
starfish larval survival
*N.K. Dulvy et al. Ecology Letters 2004
7, 410-416
Climate Change and Coral Reefs
• Sea surface temperature warming:
Warming has increased over past century
and heating correlated with increased
bleaching events, e.g., summer of 2005 in
Caribbean
Relationship between percent coral cover that was bleached
in a number of Caribbean stations and mean maximum
temperature for the hottest week at a given locality in 2005
Climate Change and Coral Reefs
• Acidification: carbon dioxide addition to
atmosphere results in reduction of seawater pH
• Corals secrete aragonite, a relatively unstable form
of calcium carbonate; aragonite is hard to secrete
at much higher activities of Ca and carbonate
relative to other form, calcite
• Already evidence of lower skeletal density in one
Australian coral over time
Coral Bleaching and Disease
• Bleaching - expulsion of zooxanthellae discussed
already
• Disease widely ascribed to decline of corals
• Disease very poorly characterized (Koch criterion
of disease)
• The bacteria must be present in every case of the
disease.
• The bacteria must be isolated from the host with
the disease and grown in pure culture.
• The specific disease must be reproduced when a
pure culture of the bacteria is inoculated into a
healthy susceptible host.
• The bacteria must be recoverable from the
experimentally infected host
Coral Bleaching and Disease
• White Band disease - affects acroporid corals,
perhaps gram negative bacterium, not isolated yet
(reorganization of Caribbean communities?)
• White Plague - rapid degradation of corals, gram
negative bacterium, cultured in lab and infects
corals
• Black Band disease - affects, non-acroporid
corals, consortium of microorganisms, leads to
sulfide accumulation and toxicity to corals
Richardson, L. 1998 TREE 13: 438-443
Bleaching Impacts on Coral Fish
• Coral fish communities very diverse, dependent
often on crevices in coral colonies and patches
• Most evidence shows little effect of bleaching
events on coral-associated fish (Booth and Beretta
2002 Marine Ecology Progress Series)
• Perhaps because most coral-associated fish are
long lived, so impacts are underestimated (see
Bellwood et al. 2006 Global Change Biology)
Future of Coral Reefs?
• Organic pollution
• Habitat alteration (turbidity caused by
humans)
• Over fishing
• Global warming --> bleaching, acidification
• Intense storms appear to have a contributory
role in degradation of corals
Future of Coral Reefs? 2
• Decrease of coral cover worldwide
• Loss of specific groups, such as acroporids
in Caribbean, relative to other corals
• Increase of bleaching worldwide
• Increase of disease
• Increase of crown-of-thorns starfish
outbreaks
See Bellwood et al. 2004 Nature; Pandolfi, J. M. et al. 2005,
Science 307: 1725-6
The End