1. 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

2. In Chapter 15 we will cover
Kelp Forests
Coral Reefs

3. 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 m high or only a meter in height

4. Laminaria kelp forest, as is often found in
New England

5. Diver in Macrocystis kelp forest, California

7. 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

8. 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

9. Abundant benthic
invertebrates of an
Alaskan kelp forest

10. Kelp Forest Community Structure 1
Herbivory - herbivorous sea urchins exert strong effects on kelp abundance

11. 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

12. 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

13. 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

14. Sea otter, Enhydra lutris, a keystone species in
Pacific coast kelp forests

15. Sea otters
Urchins
Kelp
Trophic cascade in kelp forests. Increase of sea otters results
in reduction of urchins and an increase of kelp

16. 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

17. 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

18. Alternative stable states in a California kelp forest

19. 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

20. urchins
Costaria
Alaria
Desmarestia
Nereocystis
Laminaria
Successional sequence in an Alaskan kelp forest

21. Encourages sedentary behavior
Succession
towards
Laminaria
Storms
Maintain barrens
Kelp
Barrens
Lower urchin density
High urchin density
Increased roving behavior
Encourages sedentary behavior
Urchins
Synthesis of possible transformations in a California kelp forest

22. 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

25. 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

26. 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

27. 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

28. Coral Reef Biogeography 1
Current division between Pacific and Atlantic provinces

29. 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

30. 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.

31. 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

32. Origin of Atolls

33. 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

34. Tentacle
Mouth
Digestive
Filament
Pharynx
Septum
Septum
Gastrovascular
Cavity
Basal plate
Polyp of a scleractinian coral

35. Closeup view of expanded polyps of Caribbean
coral Montastrea cavernosa

36. Hermatypic vs. Ahermatypic corals
Hermatypic: Reef framework building, have many zooxanthellae, hi calcification
Ahermatypic: not framework builders, low calcification

37. 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

38. 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

39. 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

40. Zooxanthellae - Benefits? 1
Nutrition - radiocarbon-labeled carbon taken up by zooxanthellae and transported to coral tissues (note corals usually also feed on microzooplankton)

41. 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

42. 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

43. 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

44. Mass Spawning on Coral Reefs 1
Most corals have planktonic gametes

45. 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

46. 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

47. 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

48. 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

49. Caribbean depth zonation

50. Biological Interactions 1
Competition - shading, overgrowth, interspecific digestion, sweeper tentacles, allelopathy(?)
Acropora palmata
Overtopping
Montastrea annularis

51. 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)

52. 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

53. 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

54. 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)

55. 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)

56. 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

57. 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

58. 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

59. Predation and Grazing 2 100 1990s 80 Jamaican Coral Reefs
Percent algal cover 60 40 20
1970s
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

60. 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

61. Predation and Grazing 4
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)

62. The End