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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. Sea Grasses. Sea grasses are marine angiosperms, or flowering plants, that are confined to very shallow water
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15Sea Grass Beds, Kelp Forests, Rocky Reefs, and Coral Reefs Notes for Marine Biology: Function, Biodiversity, Ecology By Jeffrey S. Levinton
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
Sea Grasses A bed of Zostera marina in Padilla Bay, Washington. Blades of this sea grass are 50–100 cm high
Sea Grasses Sea grass beds most easily colonize sediment after a successional sequence featuring a previous colonizationd by seaweeds
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
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
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
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
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
Abundance of kelps, macroalgae (kelps plus other seaweeds), and sessile invertebrates on a transect with increasing depth, Friday Harbor, Washinton State
Kelps and others dominating shallow water (1) Kelp Agarum fimbriatum; (2) kelp Saccharina latissima; (3) crustose coralline alga; (4) fleshy red seaweed
Some colonial invertebrates in deeper rocky reefs (5) sea squirt Aplidium sp.; (6) sea squirt Didemnum sp.; (7) sea squirt Metandrocarpa taylori
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
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
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
A kelp forest in the Aleutian Islands, Alaska: Cymathere triplicata (foreground); Alaria fistulosa (rear)
Complex Life Cycle • Laminarian kelps have a complex life cycle alternating between a large asexual sporophyte and a small 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 an Alaskan kelp forest
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
Sea otters (O) Urchins (U) Kelp (K) O U K O U K
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
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
Alternative stable states in a California kelp forest See Harrold and Reed 1985 Ecology; Ebeling et al. 2004 Marine Biology
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
Synthesis of possible transformations in a California kelp forest
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
Coral Reefs • Geological importance: often massive physical structures • Biological importance: biological structure, High diversity, • Economic importance: shoreline protection, harbors, fishing, 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 belong to the Scleractinia, have endosymbiotic algae known as zooxanthellae; high calcification rate • 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 • pH? Increasing ocean acidity a problem?
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.
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
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
Closeup view of expanded polyps of Caribbean coral 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 year • Massive: Produce lots of calcium carbonate but grow more slowly in linear dimensions, about 1 cm per year
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
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
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
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)
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
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.)