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15 Sea Grass Beds, Kelp Forests, Rocky Reefs, and Coral Reefs
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  1. 15Sea Grass Beds, Kelp Forests, Rocky Reefs, and Coral Reefs Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton

  2. 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

  3. Sea Grasses A bed of Zostera marina in Padilla Bay, Washington. Blades of this sea grass are 50–100 cm high

  4. Sea Grasses Sea grass beds most easily colonize sediment after a successional sequence featuring a previous colonizationd by seaweeds

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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 animal-dominated rocky reefs • Switch from cover dominance by rapidly growing seaweeds in shallow water to epifaunal animal dominance in deeper water

  10. Abundance of kelps, macroalgae (kelps plus other seaweeds), and sessile invertebrates on a transect with increasing depth, Friday Harbor, Washinton State

  11. Kelps and others dominating shallow water (1) Kelp Agarum fimbriatum; (2) kelp Saccharina latissima; (3) crustose coralline alga; (4) fleshy red seaweed

  12. Some colonial invertebrates in deeper rocky reefs (5) sea squirt Aplidium sp.; (6) sea squirt Didemnum sp.; (7) sea squirt Metandrocarpa taylori

  13. 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

  14. 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

  15. 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

  16. A kelp forest in the Aleutian Islands, Alaska: Cymathere triplicata (foreground); Alaria fistulosa (rear)

  17. Complex Life Cycle • Laminarian kelps have a complex life cycle alternating between a large asexual sporophyte and a small gametophyte

  18. 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

  19. Abundant benthic invertebrates of an Alaskan kelp forest

  20. 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

  21. Sea otter, Enhydra lutris

  22. Sea otters (O) Urchins (U) Kelp (K) O U K O U K

  23. 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 (5-7%/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

  24. 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

  25. Alternative stable states in a California kelp forest See Harrold and Reed 1985 Ecology; Ebeling et al. 2004 Marine Biology

  26. Kelp Forest Community Structure Succession: • Nereocystis - winner in succession in Pacific NW-Alaska?? • Urchins die -> kelp recruitment - several species co-occur • 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

  27. Successional sequence in an Alaskan kelp forest

  28. Synthesis of possible transformations in a California kelp forest

  29. 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

  30. Coral Reefs • Geological importance: often massive physical structures • Biological importance: biological structure, High diversity, • Economic importance: shoreline protection, harbors, fishing, tourism

  31. Coral reef, north coast of Jamaica, Caribbean

  32. 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

  33. 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

  34. 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?

  35. 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.

  36. 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

  37. Origin of Atolls

  38. 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

  39. Polyp of a scleractinian coral

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

  41. Hermatypic vs. Ahermatypic Corals • Hermatypic: Reef framework building, have many zooxanthellae, hi calcification • Ahermatypic: not framework builders, low calcification

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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)

  47. 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

  48. 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.)