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Chapter 4: Physical Control of Ecological Processes
reflecting the topography of the bottom terrain, filling hollows and bowls, until a plane
surface is formed. Within a few hours, the new sediments become dewatered and their
surface becomes rippled by subsequent water movement, the dimension of the ripples
depending on current speed and sediment particle size. In mature sediments, under rapid
water movement, ripples may become parallel sand waves or ridges several meters high,
arranged at right angles to mean flow. Resuspension occurs continuously, and sorting of
particles by their density follows, sand being deposited as ripples, and mud mainly in
the troughs between. Bedding of sediments of different density occurs also at tidal and
event scales. Bioturbation by burrowing benthic organisms powerfully modifies the ideal,
physically driven arrangement.
Terrigenous material delivered by rivers to the shelf comprises mineral sand particles
and organic material derived from the decay of terrestrial and estuarine vegetation. The
latter tends to remain incompletely oxidized in estuarine regions and close inshore,
whence the black, organic-rich, reducing and acidic muds that may dominate here where
tidal streams weaken. Pelagic organic material sinking to the shelf deposits comprising
fecal pellets, phytoplankton cells, and zooplankton carcasses is more completely oxidized
and forms blue or gray muds, often with high calcium content from the skeletons of
foraminifera. Sedimentary material of benthic origin comprises fragments of the shells of
mollusks, and of the exoskeletons of echinoderms and crustacea. These various material
are sorted and transported by coastal currents and tidal streams according to their relative
density and particle size to form the mosaic of bottom types we observe on continental
shelves. Where, as off California, the topography of the shelf is characterized by the
existence of deep basins and offshore islands and the input of terrigenous material is
negligible, the grain size and organic content of sediments follows the general pattern
of depth. Close inshore and in the shoal water around islands, grain size is large and
organic content low. Progressively into the deep basins and troughs, grain size decreases
and organic content increases.
Where, as in the many tropical regions, wave action at the coast is relatively slight and
input from rivers is significant, terrigenous mud deposits may occur close inshore, while
offshore muds are principally of pelagic origin. The distribution of sediments and water
types on the shelf determines, quite precisely, the composition of the benthic invertebrate
and demersal fish faunas on the continental shelves.
Thus, ecological processes on continental shelves are fragmented spatially, and predictable only from a specific knowledge of the forces that drive this fragmentation. It
is this problem that makes it so much more difficult to generalize satisfactorily about
the geography of ecological processes in coastal seas than in the open ocean. Research
into benthic ecology has, in recent decades, turned very largely toward an understanding of fluxes of energy within the invertebrate and microbial community, and between
this and the pelagic ecosystem. Consequently, the exploratory surveys characteristic of
benthic ecology of the first half of the 20th century are no longer done, and we are left
with adequate descriptions of the distribution of benthic species associations covering
only a very small fraction of the continental shelves of the world. From these, we must
extrapolate to the unknown regions.
Chapter 4: Physical Control of Ecological Processes
reflecting the topography of the bottom terrain, filling hollows and bowls, until a plane
surface is formed. Within a few hours, the new sediments become dewatered and their
surface becomes rippled by subsequent water movement, the dimension of the ripples
depending on current speed and sediment particle size. In mature sediments, under rapid
water movement, ripples may become parallel sand waves or ridges several meters high,
arranged at right angles to mean flow. Resuspension occurs continuously, and sorting of
particles by their density follows, sand being deposited as ripples, and mud mainly in
the troughs between. Bedding of sediments of different density occurs also at tidal and
event scales. Bioturbation by burrowing benthic organisms powerfully modifies the ideal,
physically driven arrangement.
Terrigenous material delivered by rivers to the shelf comprises mineral sand particles
and organic material derived from the decay of terrestrial and estuarine vegetation. The
latter tends to remain incompletely oxidized in estuarine regions and close inshore,
whence the black, organic-rich, reducing and acidic muds that may dominate here where
tidal streams weaken. Pelagic organic material sinking to the shelf deposits comprising
fecal pellets, phytoplankton cells, and zooplankton carcasses is more completely oxidized
and forms blue or gray muds, often with high calcium content from the skeletons of
foraminifera. Sedimentary material of benthic origin comprises fragments of the shells of
mollusks, and of the exoskeletons of echinoderms and crustacea. These various material
are sorted and transported by coastal currents and tidal streams according to their relative
density and particle size to form the mosaic of bottom types we observe on continental
shelves. Where, as off California, the topography of the shelf is characterized by the
existence of deep basins and offshore islands and the input of terrigenous material is
negligible, the grain size and organic content of sediments follows the general pattern
of depth. Close inshore and in the shoal water around islands, grain size is large and
organic content low. Progressively into the deep basins and troughs, grain size decreases
and organic content increases.
Where, as in the many tropical regions, wave action at the coast is relatively slight and
input from rivers is significant, terrigenous mud deposits may occur close inshore, while
offshore muds are principally of pelagic origin. The distribution of sediments and water
types on the shelf determines, quite precisely, the composition of the benthic invertebrate
and demersal fish faunas on the continental shelves.
Thus, ecological processes on continental shelves are fragmented spatially, and predictable only from a specific knowledge of the forces that drive this fragmentation. It
is this problem that makes it so much more difficult to generalize satisfactorily about
the geography of ecological processes in coastal seas than in the open ocean. Research
into benthic ecology has, in recent decades, turned very largely toward an understanding of fluxes of energy within the invertebrate and microbial community, and between
this and the pelagic ecosystem. Consequently, the exploratory surveys characteristic of
benthic ecology of the first half of the 20th century are no longer done, and we are left
with adequate descriptions of the distribution of benthic species associations covering
only a very small fraction of the continental shelves of the world. From these, we must
extrapolate to the unknown regions.