43
Zooplankton foraging behaviors can then lead to the aggregation into regions
with reduced flows and this may result in a decrease in its horizontal dispersal
(McManus and Woodson 2012).
High gradients of water density and viscosity could act as a physical barrier to
certain organisms attempting to migrate across and could result in the accumulation of organisms within and below pycnoclines (Lougee et al. 2002), but there is
abundant evidence from observations on phyto- and zooplankton to support that
the pycnocline should be considered not only as a way station where sinking particles may aggregate, but also as a preferred habitat of a characteristic group of
planktonic organisms (Longhurst 1998).
5.2 The Sea Water-Sediment Interface
This interface is a preferred habitat for microorganisms (Fig. 5.1). Respiration in
the water-sediment interface is dominated by bacteria and they play a major role
in the decomposition of organic matter that reaches the sea bed. The supply of
organic matter to the bottom originates from primary production in the upper
layer. Most of the primary production that sinks is remineralized during its descent
and only a small proportion of the surface production arrives to the sea bed; nevertheless, this supply of organic matter is the major determinant of abundance and
growth of deep-sea benthic biota (Turley 2000).
Because the near bottom layer represents a boundary between two oceanic biotopes (pelagial and benthal), the animal populations living there belong to diverse
ecological groups. This environment is occupied by endemic species, derived
from downward extensions of pelagic planktonic populations, which are often
seasonal in nature, as well as infaunal species emerging into the water column,
often on diel cycles. The presence of organisms in this layer is determined by two
general factors: organism behavior and boundary layer hydrodynamics (Dauvin
and Vallet 2006).
On the sea floor, a boundary layer of reduced flow extends for tens of centimeters above the bottom. In very quiet waters nutritious organic matter accumulates
on the bottom, where it is most easily utilized by animals that ingest sediment
directly. At the other extreme of the bottom boundary layer energy spectrum,
strong currents erode the sediment, leaving little organic matter and making suspension feeding difficult because there is so much inorganic matter in suspension. For the intermediate stages, a gradient in abundance of suspension-feeding
organisms exists. As the suspension-feeding organisms deplete the concentration
of food particles near the sediment-water interface, moderate tidal currents generate enough turbulence in the bottom boundary layer to increase the supply of
food to the suspension-feeders by turbulent diffusion. Both current velocity and
bottom roughness influence vertical diffusion, which have the effects of increasing
the transport of food material trough this boundary layer (Legendre et al. 1986;
Mann and Lazier 2006). The weakness or lack of near-bottom currents in lakes has
5.1 The Pycnocline Interface
Zooplankton foraging behaviors can then lead to the aggregation into regions
with reduced flows and this may result in a decrease in its horizontal dispersal
(McManus and Woodson 2012).
High gradients of water density and viscosity could act as a physical barrier to
certain organisms attempting to migrate across and could result in the accumulation of organisms within and below pycnoclines (Lougee et al. 2002), but there is
abundant evidence from observations on phyto- and zooplankton to support that
the pycnocline should be considered not only as a way station where sinking particles may aggregate, but also as a preferred habitat of a characteristic group of
planktonic organisms (Longhurst 1998).
5.2 The Sea Water-Sediment Interface
This interface is a preferred habitat for microorganisms (Fig. 5.1). Respiration in
the water-sediment interface is dominated by bacteria and they play a major role
in the decomposition of organic matter that reaches the sea bed. The supply of
organic matter to the bottom originates from primary production in the upper
layer. Most of the primary production that sinks is remineralized during its descent
and only a small proportion of the surface production arrives to the sea bed; nevertheless, this supply of organic matter is the major determinant of abundance and
growth of deep-sea benthic biota (Turley 2000).
Because the near bottom layer represents a boundary between two oceanic biotopes (pelagial and benthal), the animal populations living there belong to diverse
ecological groups. This environment is occupied by endemic species, derived
from downward extensions of pelagic planktonic populations, which are often
seasonal in nature, as well as infaunal species emerging into the water column,
often on diel cycles. The presence of organisms in this layer is determined by two
general factors: organism behavior and boundary layer hydrodynamics (Dauvin
and Vallet 2006).
On the sea floor, a boundary layer of reduced flow extends for tens of centimeters above the bottom. In very quiet waters nutritious organic matter accumulates
on the bottom, where it is most easily utilized by animals that ingest sediment
directly. At the other extreme of the bottom boundary layer energy spectrum,
strong currents erode the sediment, leaving little organic matter and making suspension feeding difficult because there is so much inorganic matter in suspension. For the intermediate stages, a gradient in abundance of suspension-feeding
organisms exists. As the suspension-feeding organisms deplete the concentration
of food particles near the sediment-water interface, moderate tidal currents generate enough turbulence in the bottom boundary layer to increase the supply of
food to the suspension-feeders by turbulent diffusion. Both current velocity and
bottom roughness influence vertical diffusion, which have the effects of increasing
the transport of food material trough this boundary layer (Legendre et al. 1986;
Mann and Lazier 2006). The weakness or lack of near-bottom currents in lakes has
5.1 The Pycnocline Interface