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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. ©Jeffrey S. Levinton 2001. In Chapter 15 we will cover. Kelp Forests Coral Reefs. Kelp Forests. Dominated by brown seaweeds in the Laminariales

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15 sea grass beds kelp forests rocky reefs and coral reefs l.jpg

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

Notes for Marine Biology: Function, Biodiversity, Ecology

By Jeffrey S. Levinton

©Jeffrey S. Levinton 2001


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In Chapter 15 we will cover

  • Kelp Forests

  • Coral Reefs


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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


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Laminaria kelp forest, as is often found in

New England


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Diver in Macrocystis kelp forest, California


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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

Antherozoids

Microscopic,haploid

Female gametophyte

Sori of

unilocular

sporangia

Egg

fusion

Sporophyte

development


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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


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Abundant benthic

invertebrates of an

Alaskan kelp forest


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Kelp Forest Community Structure 1

  • Herbivory - herbivorous sea urchins exert strong effects on kelp abundance


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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


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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


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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


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Sea otter, Enhydra lutris, a keystone species in

Pacific coast kelp forests


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Sea otters

Urchins

Kelp

Trophic cascade in kelp forests. Increase of sea otters results

in reduction of urchins and an increase of kelp


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Kelp Forest Community Structure5

  • 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


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Kelp Forest Community Structure6

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


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Alternative stable states in a California kelp forest


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Kelp Forest Community Structure7

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


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urchins

Costaria

Alaria

Desmarestia

Nereocystis

Laminaria

Successional sequence in an Alaskan kelp forest


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Succession

towards

Laminaria

Storms

Maintain barrens

Kelp

Barrens

High urchin density

Lower urchin density

Increased roving behavior

Encourages sedentary behavior

Urchins

Synthesis of possible transformations in a California kelp forest


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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


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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


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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


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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


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Coral Reef Biogeography 1

  • Current division between Pacific and Atlantic provinces


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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


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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.


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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


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Origin of Atolls


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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


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Tentacle

Mouth

Digestive

Filament

Pharynx

Septum

Septum

Gastrovascular

Cavity

Basal plate

Polyp of a scleractinian coral


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Closeup view of expanded polyps of Caribbean

coral Montastrea cavernosa


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Hermatypic vs. Ahermatypic corals

  • Hermatypic: Reef framework building, have many zooxanthellae, hi calcification

  • Ahermatypic: not framework builders, low calcification


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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


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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


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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


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Zooxanthellae - Benefits?1

  • Nutrition - radiocarbon-labeled carbon taken up by zooxanthellae and transported to coral tissues (note corals usually also feed on microzooplankton)


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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


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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


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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


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Mass Spawning on Coral Reefs 1

  • Most corals have planktonic gametes


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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


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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


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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


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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


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Caribbean depth zonation


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Biological Interactions 1

  • Competition - shading, overgrowth, interspecific digestion, sweeper tentacles, allelopathy(?)

Acropora palmata

Overtopping

Montastrea annularis


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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)


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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


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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


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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)


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Interspecific Competition1

  • 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)


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Interspecific Competition2

  • Observation by Judith Lang

Scolymia lacera - supposed ecological variants placed

next to eachother: bare zone established after

mesentarial filaments extruded through polyp wall


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Interspecific Competition3

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


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Predation and Grazing1

  • 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


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Predation and Grazing2

100

1990s

80

Jamaican Coral

Reefs

60

Percent algal cover

40

20

1970s

0 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


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Predation and Grazing3

  • 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


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Predation and Grazing4

  • Explanations for Crown-of-thorns starfish outbreaks?

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)


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The End


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