Download Kelp Forests

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.
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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)
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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)
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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)
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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
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