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

1. 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

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 10-20 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 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

17. 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

18. Alternative stable states in a California kelp forest

19. 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

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

21. 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

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