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

ReefsNotes for Marine Biology: Function, Biodiversity,

EcologyBy 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 inNew 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

Zoospores

Diploidsporophyte

Sporophytedevelopment

Antherozoids

Microscopic, haploidMale gametophyte

Eggfusion

Sori ofunilocularsporangia

Microscopic,haploidFemale 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 anAlaskan 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 resultsin 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

• 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

Alternative stable states:

*Strongylocentrotus franciscanus

Alternative stable states in a California kelp forest

Kelp Forest Community Structure 7

• 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

Succession:

urchins

Costaria

Alaria

Desmarestia

Nereocystis

Laminaria

Successional sequence in an Alaskan kelp forest

Successiontowards

Laminaria Storms

Maintain barrens

Barrens

Incr

ease

d rovin

g beh

avior

High ur

chin

dens

ityLower urchin density

KelpEncourages sedentary behavior

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

Polyp of a scleractinian coral

Mouth

Pharynx

Septum

Basal plate

Tentacle

DigestiveFilament

Septum

GastrovascularCavity

Closeup view of expanded polyps of Caribbeancoral 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 palmataOvertoppingMontastrea 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 placednext to eachother: bare zone established after mesentarial filaments extruded through polyp wall

Interspecific Competition 3Conclusion: 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

Die-off of Diadema: Seems to have flipped Jamaican reefs intoalternative stable state (also a result of storm damage). Instead of rich coral cover, you now have poor coral cover and lots of algae

0 20 40 60 80 100Percent coral cover

100

80

60

40

20

Per

cen

t al

gal c

over

1990s

1970s

Jamaican CoralReefs

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 settlement2. Overcollection by shell collectors of starfish’s main predator, Giant triton Charonia tritonus3. Storms, which wash out nutrients, stimulate phytoplankton growth and enhance larval survival ofthe starfish (some question this, as larvae can do wellUnder starvation)

The End