Switching Between Deposit and Suspension Feeding in Coastal Zoobenthos
89
ted infrared light intensity caused by the undulating body activity of the filterfeeding worm. Vedel et al. (1993, 1994) found that suspension feeding was
phytoplankton below the
A thin layer of phytoplankton drained water overlying the sediment,
especially on days when wind-induced mixing of the water column is low,
indicates that the grazing impact can be restricted (Riisgard et al. 1996b; Vedel
1998). Near-bottom reduction of phytoplankton caused by a dense population
of suspension-feeding Nereis diversicolor has been observed in the field by
Riisgard et al. (1996b). Water samples were simultaneously collected at different heights above the bottom where N. diversicolor were present, and it
appeared that a phytoplankton-reduced near-bottom water layer of 5-10cm
in thickness developed on calm days. That such depletion of phytoplankton in
near-bottom waters plays a significant role for this worm was demonstrated
in field-growth experiments performed with worms transferred to glass tubes
placed at different heights above the bottom (Riisgard et al. 1996b). A nearly
ten-fold increase in growth from elevating worms just 10 cm above the seafloor indicates that extremely meagre food conditions may be prevalent near
the seafloor due to intraspecific competition for food.
A diffusion model was used by Larsen and Riisgard (1997) to describe
the development of vertical phytoplankton profiles, over suspension-feeding
Nereis diversicolor in stagnant water. The model was based on sinks located at
inhalant openings, and Fick's law with an effective diffusivity that decreased
with distance above the bottom due to biomixing generated by exhalant and
inhalant feeding currents. For N. diversicolor, having inhalant and exhalant
openings flush with the sediment surface and only a moderate exhalant jet
velocity of about 1 cm S-I, the thickness of a well-mixed boundary layer was
limited by the low values of diffusitivity prevailing at heights greater than
about 5 cm above the bottom.
The development of 4-8 cm thick near-bottom phytoplankton-depleted
layers and the influence of temperature and wind-mixing have recently been
further investigated by Vedel (1998) who also tried to evaluate the ecological
implications of the dense population of Nereis diversicolor in the Odense
Fjord. The worm's filtration capacity of 3-14 times the water column per day
89
ted infrared light intensity caused by the undulating body activity of the filterfeeding worm. Vedel et al. (1993, 1994) found that suspension feeding was
phytoplankton below the
A thin layer of phytoplankton drained water overlying the sediment,
especially on days when wind-induced mixing of the water column is low,
indicates that the grazing impact can be restricted (Riisgard et al. 1996b; Vedel
1998). Near-bottom reduction of phytoplankton caused by a dense population
of suspension-feeding Nereis diversicolor has been observed in the field by
Riisgard et al. (1996b). Water samples were simultaneously collected at different heights above the bottom where N. diversicolor were present, and it
appeared that a phytoplankton-reduced near-bottom water layer of 5-10cm
in thickness developed on calm days. That such depletion of phytoplankton in
near-bottom waters plays a significant role for this worm was demonstrated
in field-growth experiments performed with worms transferred to glass tubes
placed at different heights above the bottom (Riisgard et al. 1996b). A nearly
ten-fold increase in growth from elevating worms just 10 cm above the seafloor indicates that extremely meagre food conditions may be prevalent near
the seafloor due to intraspecific competition for food.
A diffusion model was used by Larsen and Riisgard (1997) to describe
the development of vertical phytoplankton profiles, over suspension-feeding
Nereis diversicolor in stagnant water. The model was based on sinks located at
inhalant openings, and Fick's law with an effective diffusivity that decreased
with distance above the bottom due to biomixing generated by exhalant and
inhalant feeding currents. For N. diversicolor, having inhalant and exhalant
openings flush with the sediment surface and only a moderate exhalant jet
velocity of about 1 cm S-I, the thickness of a well-mixed boundary layer was
limited by the low values of diffusitivity prevailing at heights greater than
about 5 cm above the bottom.
The development of 4-8 cm thick near-bottom phytoplankton-depleted
layers and the influence of temperature and wind-mixing have recently been
further investigated by Vedel (1998) who also tried to evaluate the ecological
implications of the dense population of Nereis diversicolor in the Odense
Fjord. The worm's filtration capacity of 3-14 times the water column per day