362
K.Reise
cause the shoreline to retreat landwards. Major destabilizers are burrowing
and deposit-feeding invertebrates, as well as fish, birds and mammals that dig
feeding pits. Direct consumption of stabilizers may also result in a destabilization of the sediment. The effects of stabilizers and destabilizers on sediment
layering and composition, erosion and accretion have cascading effects
throughout the biotic community and produce a distinct, dynamic patchiness
at the sedimentary shore.
Sediments are composed of a continuum of particle sizes where the larger
ones are mostly spherical and the smaller ones elongated and flat. While
the former behave as independent particles, the latter are attracted to each
other by Van der Waals forces and form cohesive particles. This cohesiveness
is substantively enhanced by microbes secreting organic material, termed
extracellular polymeric substances (Paterson and Hagerthey, Chap. 5). The
microbes may also bind larger sand particles to each other. At the surface of
shore sediments, it is the microphytobenthos that primarily increases the
cohesiveness. Major components are the diatoms which attach to sand grains
or move freely on muddy surfaces. Cyanobacteria often bind sediments with
their meshwork of filaments. Species diversity of microbes consolidating
sediments attains a maximum halfway between clean sand and pure mud.
Hydrodynamics are the major force determining sediment composition
and stability, but the microbes counteract and often generate cohesive sediments in spite of a strong flow by trapping and binding small particles, which
otherwise would become resuspended. Treating the physical and biotic components separately would inhibit our understanding of the sediment properties; this is a salutary lesion for all soft-sediment ecologists. Sediment
stability is also a function of the microphytobenthic species composition.
Diatom species differ with respect to their secretion activity and certain species of cyanobacteria may generate conspicuous, consolidated sedimentary
structures, including the laminated stromatolites also known from the earliest
history of life.
Cycles in the abundances of sediment stabilizers and destabilizers may
occur. On both sides of the northern Atlantic, benthic diatom growth proceeds in spring and then comes under grazing pressure by corophiid
amp hip ods in summer. As a consequence, sediment stability first increases
and then decreases, and thus accretion gives way to erosion. In late summer,
migrant wading birds arrive and feed on the amphipods. This partially
releases the diatoms from grazing pressure, and sediment stabilization is
achieved again. In winter, light limitation and higher hydrodynamic forces
hamper the diatoms, and then the sediment returns to instability. Eroding
conditions prevail until the following spring when diatoms again bloom
(Daborn et al. 1993; Cadee, Chap. 6).
Sediment reworking by bioturbators is important in the vertical transport
of particles. The latter can be quantified as the layer of sediment that is
K.Reise
cause the shoreline to retreat landwards. Major destabilizers are burrowing
and deposit-feeding invertebrates, as well as fish, birds and mammals that dig
feeding pits. Direct consumption of stabilizers may also result in a destabilization of the sediment. The effects of stabilizers and destabilizers on sediment
layering and composition, erosion and accretion have cascading effects
throughout the biotic community and produce a distinct, dynamic patchiness
at the sedimentary shore.
Sediments are composed of a continuum of particle sizes where the larger
ones are mostly spherical and the smaller ones elongated and flat. While
the former behave as independent particles, the latter are attracted to each
other by Van der Waals forces and form cohesive particles. This cohesiveness
is substantively enhanced by microbes secreting organic material, termed
extracellular polymeric substances (Paterson and Hagerthey, Chap. 5). The
microbes may also bind larger sand particles to each other. At the surface of
shore sediments, it is the microphytobenthos that primarily increases the
cohesiveness. Major components are the diatoms which attach to sand grains
or move freely on muddy surfaces. Cyanobacteria often bind sediments with
their meshwork of filaments. Species diversity of microbes consolidating
sediments attains a maximum halfway between clean sand and pure mud.
Hydrodynamics are the major force determining sediment composition
and stability, but the microbes counteract and often generate cohesive sediments in spite of a strong flow by trapping and binding small particles, which
otherwise would become resuspended. Treating the physical and biotic components separately would inhibit our understanding of the sediment properties; this is a salutary lesion for all soft-sediment ecologists. Sediment
stability is also a function of the microphytobenthic species composition.
Diatom species differ with respect to their secretion activity and certain species of cyanobacteria may generate conspicuous, consolidated sedimentary
structures, including the laminated stromatolites also known from the earliest
history of life.
Cycles in the abundances of sediment stabilizers and destabilizers may
occur. On both sides of the northern Atlantic, benthic diatom growth proceeds in spring and then comes under grazing pressure by corophiid
amp hip ods in summer. As a consequence, sediment stability first increases
and then decreases, and thus accretion gives way to erosion. In late summer,
migrant wading birds arrive and feed on the amphipods. This partially
releases the diatoms from grazing pressure, and sediment stabilization is
achieved again. In winter, light limitation and higher hydrodynamic forces
hamper the diatoms, and then the sediment returns to instability. Eroding
conditions prevail until the following spring when diatoms again bloom
(Daborn et al. 1993; Cadee, Chap. 6).
Sediment reworking by bioturbators is important in the vertical transport
of particles. The latter can be quantified as the layer of sediment that is