Chapter 13
The fauna of deep-sea sediments
Seafloor habitats are termed benthic (adjective), an organism living in or on the
bottom is a benthont (noun), and the assemblages of organisms are termed benthos
(noun). The terms apparently are versions of the Greek “bathos” (βαθoς), meaning
depth. Benthic habitats share characteristics with both pelagic and terrestrial ones.
They are (more-or-less) solid substrates, like the land, but they are continuously
submerged in seawater. Thus, the basic physiological problems are the same as those
for pelagic ocean life, but the more two-dimensional aspect of a land habitat operates
as well. Benthic habitats grade downward in a series: intertidal, subtidal, bathyal
(continental slope depths), abyssal, and hadal (trenches). The solid Earth has two
principal surfaces, the continental shields above sea level, and mostly at the level of
steppe or lowland rainforest (∼300 m elevation), and the abyssal plains at depths of
about 4500 m. This deep-sea zone occupies about 60% of world ocean area (Fig. 1.6).
There are some rocky deep-sea sites, particularly at spreading centers, but most of the
ocean is underlain by sediment-covered bottom, 2000 to 5500 m below the surface
productive layer. Because of this areal dominance, and because sediment is easiest to
sample, we will focus on the benthos of deep-sea sediments. Many insights come
from comparisons to the more accessible subtidal and bathyal depths, so data from
them will be used as well.
(^) On bottoms within the euphotic zone there are often attached seaweeds, such as the
kelp forests from Baja California to Alaska. However, those are in a very narrow zone
next to the shore. The biota of the deep-sea floor depends upon food descending from
above. At the sediment surface, food accumulates in and can be gathered from a
narrow stratum. While only less than 1–2% of surface production reaches abyssal
bottoms, it still represents a moderately rich resource compared to the water column
just above.
(^) Physical conditions are less variable at abyssal depths than in other large habitats.
Temperatures are low, less than 3°C, but never low enough to freeze salt water.
Salinity is 33–35 PSS but varies even less within oceanic basins. Oxygen
concentration is mostly near saturation, greater than 4 ml liter−1, although, in places
where oxygen-minimum layers impinge on continental slopes, it can be much less.
There is no sunlight, and below about 1200 m even bioluminescence is minimal.
Hydrostatic pressure increases by about 1 atmosphere per 10 m depth, so at abyssal