Near Shore Benthic Habitats
I
Continental shelf habitats
A. Are affected by
1) Shallow depth
(i) Water mixing prevents stratification
(a) Wave action (felt down to depths of 650 ft)
(b) Tidal currents
(c) Benefits of mixing
(i) Nutrients from sediments are stirred up
(ii) Temperatures are kept homogenous
2) Proximity to land
(i) High levels of sediment deposition
(a) Particles are sorted by water motion, affects substrate type
(i) Turbulent areas consist of sand and gravel
(ii) Quiet areas consist of silt and mud
(ii) Sediments are a rich source of new nutrients
3) Both factors above contribute to the high levels of productivity in the
overlying water
4) Two types of habitats
(i) Soft-bottom
(ii) Hard-bottom
II Soft – bottom Communities
A. Most extensive type due to continuous deposition of sediments onto the
continental shelf
B. At first they appear featureless
1) No macroalgae because there is nothing to hold onto
2) Most primary productivity performed by phytoplankton in the overlying water
C. Trophic Structure
1) Most benthic organisms depend on detritus
(i) Sources of detritus include
(a) Material from rocky shore & rocky bottom communities
(b) Organic material in sediments
(c) Fecal material and other detritus form water column above
2) Strategies for detritus feeding
(i) Deposit feeding
(a) Ingest sediments & extract nutrients from particles
(b) Example include many polychaete worms and sea cucumbers
(ii) Suspension feeding
(a) Collect food from the water using one of two mechanisms
(i) Filter feeders use body structures to strain food out of the water
1. ex/ sea pens, feather duster worms
(ii) Passive suspension feeders trap food particles with mucus
1. ex/ sand dollars
3) Predation
(i) Predation keeps populations in check so there is little competition
(cropping)
(ii) Predation Strategies
(a) Surface feeding (ex/ fishes)
(b) Burrowers (whelks, moon snails)
D. Distribution of Organisms
1) Most organisms live on the sediment surface (epifauna) or burrowed in the
sediments (infauna)
(i) A specialized group of microscopic organisms called meiofauna live in
spaces between sediment particles (see p. 295)
2) The uniform nature of the environment would suggest either a uniform or
random distribution of organisms
3) Actually, the distribution is clumped or patchy for certain groups
4) Patchy distribution is influenced by
(i) Larval settlement patterns related to light and temperature
(ii) Particle size of sediments
(iii) Sediment stability
5) Among infauna particle size is especially important
(i) Deposit feeders prefer small particle rich in nutrients
(ii) Suspension feeders prefer coarser particles.
(iii) This separation is influenced by biological interactions
(a) Deposit feeders disturb and turn over sediments (bioturbation), fine
sediments to be re-suspended that clog the filters of suspension feeders
(b) Many suspension feeders build solid tubes that interfere with
burrowing
6) Predation can also contribute to patchy distribution
(i) Amphipods can prey unevenly on newly settled larvae
(ii) Large animals like walruses, rays and grey whales can remove large
numbers of organisms
(a) Some scientists have argued that grey whales are keystone predators
because their feeding pits create new habitat, thus enhancing species
diversity.
III Hard bottom Communities
A. Have a rocky substrate. May be extensions of rocky shore lines or isolated rocky
reefs.
B. Are similar to rocky intertidal communities
1) Have a permanent substrate
2) Structurally complex habitat provides many areas for specialization
3) High levels of competition for space
4) Intermediate disturbances by predators and grazers opens space for new
colonizers
C. Yet different from the intertidal
1) Desiccation is not an issue
2) Little apparent zonation in most areas
3) Have higher abundance and diversity of species
D. Kelp Beds and Forests
1) Communities that are dominated by large, fast growing brown algae.
2) The most recognized type in California formed by Giant Perennial Kelp
(Macrocystus pyrifera) but many other species contribute to or dominate kelp
forests
3) A kelp dominated community is considered a forest only if the kelp can grow
all the way to the surface to form a canopy as blades fan across the surface.
Otherwise the community is called a kelp bed.
4) Kelp are among the fastest growing organisms in the world
5) Kelp have specific environmental needs in order to sustain such rapid growth
(i) Nutrient rich water
(ii) Mixing of water to constantly refresh the nutrients
(iii) Cold water which contains more nutrients, plus most species degrade
when exposed to warm temps
(a) The El Nino of 1997-98 greatly thinned the CA kelp forests, but the
return of cold water the following year led to rapid recovery.
6) Community structure of kelp forests
(i) Vertical Structure
(a) The canopy forms in association with depths that experience little
wave disturbance along the bottom but still have sufficient light for psyn of newly established kelp
(i) The canopy reduces light significantly affecting the species below
(b) The understory consists of shorter algae that are adapted to lower light
levels
(c) The algal turf consists mostly of encrusting and branching coralline
red algae
(d) Each zone has its own respective animal (faunal) associations
(i) Many juvenile fish hide in the canopy
(ii) Adult fish occupy the more open spaces of the understory
(iii)Many invertebrates dominate the turf zone
(iv) The algae themselves also host a rich community of epifauna, esp.
in the holdfasts
(ii) Horizontal Structure
(a) The inshore zone lacks the dominating canopy kelps because of strong
wave action. Many understory kelp do well here.
(b) The canopy zone has the richest diversity due to the high structural
diversity
(c) The offshore zone is usually sparser because the accumulation of
sediments makes rock outcrops rare. Also the depth may exceed the
photic zone for new kelp.
7) Health and Stability of Kelp Forests
(i) Depends on the balance of community members, esp. grazers vs. predators
(ii) Sea urchins are the most important grazers of kelp because they attack the
base of the stipes, causing detachment of the remaining thallus
(iii) Population explosions of sea urchins can decimate entire community
resulting in “urchin barrens.”
(iv) Incidents of urchin barrens were more frequent in the decades after sea
otters were hunted to near extinction because otters are major predators of
urchins
(v) A study of kelp forests in Alaska indicates a correlation between otter
population size and kelp density
(vi) Lobsters (predators of urchins) and abalone (competitors for space)may
also affect urchin populations.
(vii) Sea otter population in Southern California is not recovering as rapidly as
might be expected. Exposure to disease transmitted through animal feces
& untreated sewage is affecting otters.