Download Chapter 14: BENTHIC COMMUNITIES

Chapter 14: BENTHIC COMMUNITIES
Answer to End of Chapter Study Questions
1. What factors influence the distribution of organisms within a benthic community? How are
these distributions described? Why is random distribution so rare?
Benthic organisms live on or in the ocean bottom. Some benthic creatures spend their
lives buried in sediment, others rarely touch the solid seabed; most attach to, crawl over, swim
next to, or otherwise interact with the ocean bottom continuously throughout their lives. Their
distribution through space is determined by their needs and by the nature of their interactions
with their environment. Each of the physical and biological factors discussed in Chapter 12
plays a role in the distribution of these organisms.
The most common pattern for distribution of benthic organisms is small patchy
aggregations, or clumps. Clumped distribution occurs when conditions for growth are optimal
in small areas because of physical protection (in cracks in an intertidal rock), nutrient
concentration (near a dead body lying on the bottom), initial dispersal (near the position of a
parent), or social interaction. A random distribution implies that the position of one organism in
a bottom community in no way influences the position of other organisms in the same
community. A truly random distribution indicates that conditions are precisely the same
throughout the habitat, an extremely unlikely situation except possibly in the unvarying benthic
communities of abyssal plains. Uniform distribution with equal space between individuals, such
as the arrangement of trees in orchards, is the rarest natural pattern of all.
2. What are algae, and how are they different from plants? Are all algae seaweeds? How are
seaweeds classified? Which seaweeds live at the greatest depths? Why?
Algae is a collective term for autotrophs possessing chlorophyll and capable of
photosynthesis but – unlike plants – lacking vessels to conduct sap.
Not all algae are seaweeds: the single-celled diatoms and dinoflagellates discussed
earlier are unicellular algae.
Seaweeds are classified by the presence of accessory pigments, colored compounds in
their tissues. These accessory pigments (or masking pigments) are light absorbing compounds
closely associated with chlorophyll molecules. Accessory pigments may be brown, tan, olive
green, or red; they are what give most marine autotrophs, especially seaweeds, their
characteristic color. Multicellular marine algae are segregated into three divisions based on
their observable color. The green algae, with their unmasked chlorophyll, are the Chlorophyta,
the brown algae Phaeophyta, and the red algae Rhodophyta. Phaeophytes are most familiar to
beachcombers, and rhodophytes the most numerous.
Rhodophytes can live in surprisingly deep water. They excel in dim light because their
sophisticated accessory pigments absorb and transfer enough light energy to power
photosynthetic activity at depths where human eyes cannot see light. The record depth for a
photosynthesizer is held by a small rhodophyte discovered in 1984 at a depth of 268 meters
(879 feet) on a previously undiscovered seamount in the clear tropical Caribbean.
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3. What problems confront the inhabitants of the intertidal zone? How do you explain the
richness of the intertidal zone in spite of these rigors? Which intertidal area has larger
numbers of species and individuals: sand beach or rocky? Why?
The problems of living in the intertidal zone are formidable. The tide rises and falls,
alternately drenching and drying out the animals and plants. Wave shock, the powerful force of
crashing waves, tears at the structures and underpinnings of the residents. Temperature can
change rapidly as cold water hits warm shells, or as the sun shines directly on newly exposed
organisms. In high latitudes, ice grinds against the shoreline, and in the tropics, intense
sunlight bakes the rocks. Predators and grazers from the ocean visit the area at high tide, and
those from land have access at low tide. Too much fresh water can osmotically shock the
occupants during storms. Annual movement of sediment onshore and offshore can cover and
uncover habitats.
In spite of these rigors, the richness, productivity, and diversity of the intertidal rocky
community—especially in the world's temperate zones—is matched by very few other places.
There is intense competition for space. One reason for the great diversity and success of
organisms in the rocky intertidal zone is the large quantity of food available. The junction
between land and ocean is a natural sink for living and once-living material. The crashing of
surf and strong tidal currents keep nutrients stirred and ensure a high concentration of
dissolved gases to support a rich population of autotrophs. Minerals dissolved in water running
off the land serve as nutrients for the inhabitants of the intertidal zone as well as for plankton in
the area. Many of the larval forms and adult organisms of the intertidal community depend on
plankton as their primary food source.
In rocky intertidal zones, another reason for the success of organisms is the large
number of habitats and niches available occupation (see Figure 14.8). The habitats of intertidal
animals and plants vary from hot, high, salty splash pools to cool, dark crevices. These
spaces provide hiding places, quiet places to rest, attachment sites, jumping-off spots, cracks
from which to peer to obtain a surprise meal, footing from which to launch a sneak attack,
secluded mating nooks, or darkness to shield a retreat.
In contrast, sandy beaches are ecological nightmares. Sand itself is the key problem.
Many sand grains have sharp pointed edges, so rushing water turns the beach surface into a
blizzard of abrasive particles. Jagged grit works its way into soft tissues and wears away
protective shells. A small organism's only real protection is to burrow below the surface, but
burrowing is difficult without a firm footing. When the grain size of the beach is small, capillary
forces can pin down small animals and prevent them from moving at all. If these organisms are
trapped near the sand surface, they may be exposed to predation, to overheating or freezing,
to osmotic shock from rain, or to crushing as heavy animals walk or slide on the beach. As if
this weren't enough, those that survive must contend with the difficulty of separating food from
swirling sand and the dangers of leaving telltale signs of their position for predators or being
excavated by crashing waves. A few can run for their lives—some larger beach-dwelling crabs
depend on their good eyesight and sprinting ability to outrace onrushing waves. To these
horrors must be added all the usual problems of intertidal life discussed above. Not
surprisingly, very few species have adapted to wave-swept sandy beaches!
4. Which benthic marine habitat is the most sparsely populated? Why?
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Life on the deep ocean floor is more plentiful and obvious than in the bathypelagic water
above, but as bottoms go, the density of life at great depths tends to be low. The main difficulty
to be overcome is lack of food. There is, of course, no photosynthetic primary productivity at
these aphotic depths, and chemosynthetic productivity is usually limited to the richly inhabited
rift vents. Organisms on most of the deep bottom must be content with dust-sized scraps
falling—sometimes for miles—from the productive waters at the ocean surface.
Understandably, there is little to eat, and few large organisms at the table.
5. If tropical ocean generally supports very little life, why do coral reefs contain such
astonishing biological diversity and density?
The key to the problem lies in the association between the coral polyp and its resident
zooxanthellae. These single-celled plant-like organisms facilitate the rapid biochemical
deposition of calcium carbonate into the coral skeleton. The microscopic zooxanthellae carry
on photosynthesis, absorb waste products, grow, and divide within their coral host. The coral
animals provide a safe and stable environment and a source of carbon dioxide and nutrients;
the zooxanthellae reciprocate by providing oxygen, carbohydrates, and the alkaline pH
necessary to enhance the rate of calcium carbonate deposition. The coral occasionally
absorbs a cell, "harvesting" the organic compounds for its own use. The zooxanthellae are
captive within the coral, so none of their nutrients are lost as they would be if the zooxanthellae
were planktonic plants that could drift away from the reef. Instead nutrients are used directly by
the coral for its own needs. The cycling of materials is short, direct, quick, and very efficient.
The success of the coral encourages other successful colonizations with short nutrient
recycling pathways, and the entire community thrives in an oceanic version of "waste not, want
not."
6. Explain Charles Darwin's classification scheme for coral reefs? Is the classification still in
use?
In 1842, Charles Darwin classified tropical reef structures into three types: Fringing
reefs, barrier reefs, and atolls (see Figure 14.14). We still use this classification today.
As their name implies, fringing reefs cling to the margin of land. Fringing reefs form in
areas of low rainfall runoff primarily on the lee (downwind side) of tropical islands. The greatest
concentration of living material will be at the reef's seaward edge where plankton and clear
water of normal salinity are dependably available. Most new islands anywhere in the tropics
have fringing reefs as their first reef form. Barrier reefs are separated from land by a lagoon.
They tend to occur at lower latitudes than fringing reefs, and can form around islands or in
lines parallel to continental shores. The outer edge—the barrier—is raised because the
seaward part of the reef is supplied with more food and is able to grow more rapidly than the
shore side. As you would expect, conditions and species within the lagoon are much different
from those of the wave-swept barrier. The calm lagoon is often littered with eroded coral debris
moved from the barrier by storms. An atoll is a ring-shaped island of coral reefs and coral
debris enclosing, or almost enclosing, a shallow lagoon from which no land protrudes. Coral
debris may be driven onto the reef by waves and wind to form an emergent arc on which
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coconut palms and other land plants take root. These plants stabilize the sand and lead to
colonization by birds and other species. This is the tropical island of the travel posters.
7. What is the primary source of biological energy in rift vent and cold seep communities?
There is a very small amount of light produced by physical phenomena at some of the
vents, but this is insufficient for photosynthesis, and sunlight does not penetrate this far.
Instead, primary productivity proceeds by chemosynthesis, the production of usable energy
directly from energy-rich inorganic molecules in the environment. The usual energy-rich
molecule is hydrogen sulfide, often present in water escaping from the geologically active
vents or seeps. Specialized bacteria convert carbon dioxide to carbohydrate molecules using
energy stored in the covalent bonds of hydrogen sulfide. This energy-binding process replaces
photosynthesis in the world of darkness.
8. How can whale fall communities act as “stepping stones” between habitats for rift
organisms?
Studies of fallen whale skeletons have shown the presence of sulfur-oxidizing
chemosynthetic bacteria. As sulfide produced by these bacteria diffuses out of the bone,
planktonic larvae of vent organisms might sense its presence, settle, grow, and reproduce.
With luck, their offspring might drift to another whale fall and repeat the process. After many
steps a new or newly active vent would be reached.
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