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II. Soft-bottom Subtidal Communities -- Much of the continental shelf is mud or sand -Tidal currents can be very strong -Water column is generally mixed; nutrients are also brought in by rivers -Waters over the continental shelf are highly productive so there is more food for the benthos. -There is sufficient light for plants The dominant large kelp Macrocystis (5 spp) Laminaria There is a predominance of infauna; some epifauna. Because there is nothing to hold on to, sessile forms are virtually absent (very few macroalgae) There is a close relation between particle size and distribution of organisms Physical Determinants of Kelp Distribution (40 spp) A. Cooler water temperatures Ecklonia ( 9 spp) Nereocystis ( 1 sp) B. Upwelling, high nutrient availability C. Hard bottom D. Light (limits deep and shallow habitat) E. Wave Intensity (displacement, breaking depth) http://www.sanctuarysimon.org/regional_sections /kelpForests/overview.php?sec=kf - Some to heights over 20-30 m - Rapid growth (50 cm/day) & productivity (0.4-9 g C/m2/day) - About 10% of production is consumed by herbivores; 90% enters the detrital food web Video Stratification (ex. Pacific N.A.) Macrocystis - Extensive three dimensional structure - Large numbers of potential habitats for fish etc. 1 Worldwide distribution of kelp beds IV. Kelp Beds, Sea Urchins and Predators Sea urchins are by far the most important herbivores in kelp beds Several species of Strongylocentrotus are especially important Explosive growth can lead to “fronts” that wipe out whole kelp beds, forming “urchin barrens” What forces are responsible for these shifts? (kelp bed vs. urchin barren)?? Variable Recruitment (due to food availability, warm temperatures, delivery by currents) Change in predation pressure Disease: Cause declines in urchin populations Behavior: sea urchins become more active Urchin barrens have been observed over a very wide geographical range Connectedness Web, West Coast of N.A. Comparison of kelp cover and sea urchin numbers at two sites in Alaska. Otters were not a factor at Shemya. A functional web includes strengths of interactions Urchin density Amchitka a emy , Sh sity den in h Urc Vegetation Amchitka Estes and Palmisano (1974) 2 indirect effect indirect effect Direct effect Direct effect Predator (otter) PHerbivore Herbivore urchin + urchin H- H- Basal species Basal species (kelp) -- 20/30 otters per Km2 eat 35000 kg/km2/yr or 3.8 kg/animal/day or 15-20 urchins/animal/day indirect effect Predator - A strong interaction that involves at least 3 trophic levels P- Predator - Strong influence on community Herbivore structure, production G- Direct effect in Alaska (orca) P- + - Two alternative stable states (“phase shifts”) (kelp) Predator (otter) P- + Herbivore + urchin HBasal species (kelp) Basal species “Trophic cascade” : multi-level trophic interaction whereby predators, by suppressing herbivores, significantly alter plant abundance. (Pinnegar et al. 2000, Polis et al. 1999). “Keystone species” : a species, often a predator, having a dominant influence on the composition of a community. (e.g. Paine, 1968) - A “Keystone predator” effect in a “trophic cascade” Estes et al. Marine Mammal Science, Vol 21: 169-172 (January 2005) 3 Current explanations Oceanographic Conditions • • • • • • Fisheries Keystone predators (even false ones) Phase shifts in community structure Trophic cascades Shifts in dominance Fishing down food webs Trophic level dysfunction Northwest Atlantic Predominant Maine coastal fisheries then and now 1927 Atlantic Cod “…so great a quantity of... great fish...called bacalaos ...that at times they even stayed the passage of the ship…” (John Cabot ca. 1500) 100-160 lbs Max 211 lbs 1998 < 50 lbs “Fishing Down Steneck and Carlton 2001 Marine food webs” The Gulf of Maine Showing “Cashes Ledge” Mt. Desert Island Cashes Ledge Geographic patterns of large predatory fishes at the Cashes Ledge Region (ARP NARP), islands of Matinicus (MAT) and Mt. Desert Rock (MDI), and at coastal sites (TOL and PEM) 4 Sea Urchin Fishery Lobster landings and coastal groundfish abundance in Maine indicating a possible contributions to booms in lobster populations • • • • • • Keystone predators (even false ones) Phase shifts in community structure Trophic cascades Shifts in dominance Fishing down food webs Trophic level dysfunction 5