166 Productivity and Benthic Organisms
where the oxygen concentration becomes low enough, all higher organisms, and even
shell-bearing protists, will succumb, and only anaerobic bacteria remain. Such conditions - called dysaerobic when moderate, and anaerobic when severe - can produce
varved sediments (Fig. 3.14), since there is no disturbance by burrowing organisms.
Much can be learned from varves regarding climatic change, on the scale of decades
to millenia, especially with regard to changes in the supply of organic matter and
oxygen.
At the present time we have to go to certain special areas to study this phenomenon - fjords in Norway and Alaska, the Black Sea, the Santa Batbara Basin off
California. In the geologic past, however, when the poles were not icy cold and did
not therefore deliver oxygen-rich water to the deep ocean, conditions of oxygen
deficiency were widespread. Much of the petroleum we bum today was formed in
oceans with a low oxygen content, in regions where oxygen dropped below critical
values and where organic matter did not readily decay, therefore.
Basically, oxygen deficiency arises when oxygen demand is strong, and supply is
weak. For example, in the Black Sea the salt water filling the basin (through the
Bosporus, Chap. 4.3.5) comes from the Mediterranean and is covered with a layer of
freshwater brought in by the Danube, the Dnepr, and other rivers (Fig. 7.12). The
light freshwater forms a lid on the heavy deep water, cutting off exchange with the
atmosphere. Growth continues in the upper waters, delivering organic matter to the
water below. Here it is eaten by animals, which use up oxygen by respiration, and is
decayed by bacteria, also using oxygen. Thus, the deepest water becomes entirely
anaerobic.
A somewhat analogous process can be observed in organic-rich muds. Free
oxygen is present only in the uppermost layer of sediment, millimeter-thick (or centimer-thick when sandy). Burrowing animals must set up air-conditioning by pumping oxygen-rich water through their burrows - otherwise they must suffocate.
The geohistorian attempts to reconstruct the degree of oxygenation at the time and
place of deposition of a given sediment layer, from clues such as lamination or nature
of burrowing, and from chemical indicators such as sulfides and types of organic
matter present. The remains of certain benthic organisms also provide information
regarding the level of oxygenation (Fig. 6.9).
6.2 Benthic Life
6.2.1 Types. By far the greatest part of the sea floor is teeming with benthic organisms. Benthos which stays put is called sessile. All sponges, corals, brachiopods, and
bryozoans are sessile. Benthos which moves about is termed vagile. It can move
rapidly, like a startled crab, or slowly, like the sluggish sea urchins, starfish, most
bivalves, snails, and worms. Both groups have members living on the floor or on top
of other organisms (e. g., shells or kelp): the epifauna. Or they live hidden within
rocks and sediment: the infauna.
Compared with the 125000 marine species of epifauna there are only a paltry
30000 species of infauna. Why? Are there more niches in the epifaunal way of life,
where the oxygen concentration becomes low enough, all higher organisms, and even
shell-bearing protists, will succumb, and only anaerobic bacteria remain. Such conditions - called dysaerobic when moderate, and anaerobic when severe - can produce
varved sediments (Fig. 3.14), since there is no disturbance by burrowing organisms.
Much can be learned from varves regarding climatic change, on the scale of decades
to millenia, especially with regard to changes in the supply of organic matter and
oxygen.
At the present time we have to go to certain special areas to study this phenomenon - fjords in Norway and Alaska, the Black Sea, the Santa Batbara Basin off
California. In the geologic past, however, when the poles were not icy cold and did
not therefore deliver oxygen-rich water to the deep ocean, conditions of oxygen
deficiency were widespread. Much of the petroleum we bum today was formed in
oceans with a low oxygen content, in regions where oxygen dropped below critical
values and where organic matter did not readily decay, therefore.
Basically, oxygen deficiency arises when oxygen demand is strong, and supply is
weak. For example, in the Black Sea the salt water filling the basin (through the
Bosporus, Chap. 4.3.5) comes from the Mediterranean and is covered with a layer of
freshwater brought in by the Danube, the Dnepr, and other rivers (Fig. 7.12). The
light freshwater forms a lid on the heavy deep water, cutting off exchange with the
atmosphere. Growth continues in the upper waters, delivering organic matter to the
water below. Here it is eaten by animals, which use up oxygen by respiration, and is
decayed by bacteria, also using oxygen. Thus, the deepest water becomes entirely
anaerobic.
A somewhat analogous process can be observed in organic-rich muds. Free
oxygen is present only in the uppermost layer of sediment, millimeter-thick (or centimer-thick when sandy). Burrowing animals must set up air-conditioning by pumping oxygen-rich water through their burrows - otherwise they must suffocate.
The geohistorian attempts to reconstruct the degree of oxygenation at the time and
place of deposition of a given sediment layer, from clues such as lamination or nature
of burrowing, and from chemical indicators such as sulfides and types of organic
matter present. The remains of certain benthic organisms also provide information
regarding the level of oxygenation (Fig. 6.9).
6.2 Benthic Life
6.2.1 Types. By far the greatest part of the sea floor is teeming with benthic organisms. Benthos which stays put is called sessile. All sponges, corals, brachiopods, and
bryozoans are sessile. Benthos which moves about is termed vagile. It can move
rapidly, like a startled crab, or slowly, like the sluggish sea urchins, starfish, most
bivalves, snails, and worms. Both groups have members living on the floor or on top
of other organisms (e. g., shells or kelp): the epifauna. Or they live hidden within
rocks and sediment: the infauna.
Compared with the 125000 marine species of epifauna there are only a paltry
30000 species of infauna. Why? Are there more niches in the epifaunal way of life,