102
Chapter 6: Biomes: The Primary Partition
deep as 50 m. Where irradiance at the sediment surface is >5% of surface irradiance,
we may find that the rate of growth of benthic diatoms is equivalent to phytoplankton
production in the overlying water mass.
To understand the biological processes by which pelagic organic matter is consumed
and transformed in the benthic ecosystem, and how nutrients arising from this process are
reinjected into the planktonic ecosystem, we shall have to have to understand the outlines
of active material transfer within the benthos. This is not simple to generalize, because
specialized feeding strategies are greatly more numerous and greatly more specialized
than within the pelagic ecosystem. Moreover, their study has not been central to the
thrust of biological oceanography in recent years and the information we require is widely
scattered. However, some generalizations can be made concerning relative rates of these
processes on continental shelves. With the use of benthic respiration chambers, it has
been shown that activity of macro-benthos and meio-benthos are mutually exclusive
spatially, depending on the nature of the sediments, principally their relative content
of soft organic material. Compartmentalized, macrofauna accounts for as little as 25%
of all respiration, whereas microbiota account for as much as 45% of total community
respiration. Further, relative activity diminishes into deep water, other things being equal,
and this trend continues into the deep sea. From 10 m to 100 m, the rate of benthic
respiration generally falls by as much as an order of magnitude.
The nature and amount of settling organic material on continental shelves determines
the response of the benthic ecosystem, which under most circumstances very rapidly
remineralizes it and ensures its burial, sometimes its deep burial by the activity of the
constituent macrofauna. The C/N ratio of settling material varies seasonally and is higher
than in the open ocean, but always much lower than the ratio of riverborne debris from
vascular plants (where C/N < 100) so the material itself is much more labile. A settling
phytoplankton bloom has a C/N ratio of around 7, which is close to the ratio of living
phytoplankton, but the slower rate of settlement in post-bloom, stable conditions is
of order C/N = 10. So not only does the supply of organic material to the benthos
pulse, following the phytoplankton calendar, but it differs in its composition seasonally.
This pulsed pattern is enhanced by the well-known general and probably near-linear
relationship between export production (or the percentage of cells produced that sink)
and total production rate.
Although in most models of benthic-pelagic coupling this simple vertical flux is
invoked as the principal input, in reality the energetic horizontal water movements over
shallow water induces a much greater horizontal flux, and resuspension. Thus, one cannot
assume that there is a more than general relationship between water column production of
organic material and the response of the benthic community spatially below. In addition,
the existence of a permanent or summer thermocline may act to isolate the two ecological
systems. Such hydrographic factors have significant influence on the manner in which
benthic organisms are distributed on the seabed and their response to production rates
in the overlying water column. Where conditions are unusually favorable for the local
supply of organic material by some combination of physical processes, benthic biomass
may be highly specialized and extraordinarily abundant: the slope off western Sweden
may support aggregations of the brittlestar Amphiura filiformis at densities of around
3000 m
−2 . This organism is facultatively both a suspension and a deposit feeder, and it
functions particularly well where physical conditions of turbulence, current speed, and
siltation rate are highly variable.
Chapter 6: Biomes: The Primary Partition
deep as 50 m. Where irradiance at the sediment surface is >5% of surface irradiance,
we may find that the rate of growth of benthic diatoms is equivalent to phytoplankton
production in the overlying water mass.
To understand the biological processes by which pelagic organic matter is consumed
and transformed in the benthic ecosystem, and how nutrients arising from this process are
reinjected into the planktonic ecosystem, we shall have to have to understand the outlines
of active material transfer within the benthos. This is not simple to generalize, because
specialized feeding strategies are greatly more numerous and greatly more specialized
than within the pelagic ecosystem. Moreover, their study has not been central to the
thrust of biological oceanography in recent years and the information we require is widely
scattered. However, some generalizations can be made concerning relative rates of these
processes on continental shelves. With the use of benthic respiration chambers, it has
been shown that activity of macro-benthos and meio-benthos are mutually exclusive
spatially, depending on the nature of the sediments, principally their relative content
of soft organic material. Compartmentalized, macrofauna accounts for as little as 25%
of all respiration, whereas microbiota account for as much as 45% of total community
respiration. Further, relative activity diminishes into deep water, other things being equal,
and this trend continues into the deep sea. From 10 m to 100 m, the rate of benthic
respiration generally falls by as much as an order of magnitude.
The nature and amount of settling organic material on continental shelves determines
the response of the benthic ecosystem, which under most circumstances very rapidly
remineralizes it and ensures its burial, sometimes its deep burial by the activity of the
constituent macrofauna. The C/N ratio of settling material varies seasonally and is higher
than in the open ocean, but always much lower than the ratio of riverborne debris from
vascular plants (where C/N < 100) so the material itself is much more labile. A settling
phytoplankton bloom has a C/N ratio of around 7, which is close to the ratio of living
phytoplankton, but the slower rate of settlement in post-bloom, stable conditions is
of order C/N = 10. So not only does the supply of organic material to the benthos
pulse, following the phytoplankton calendar, but it differs in its composition seasonally.
This pulsed pattern is enhanced by the well-known general and probably near-linear
relationship between export production (or the percentage of cells produced that sink)
and total production rate.
Although in most models of benthic-pelagic coupling this simple vertical flux is
invoked as the principal input, in reality the energetic horizontal water movements over
shallow water induces a much greater horizontal flux, and resuspension. Thus, one cannot
assume that there is a more than general relationship between water column production of
organic material and the response of the benthic community spatially below. In addition,
the existence of a permanent or summer thermocline may act to isolate the two ecological
systems. Such hydrographic factors have significant influence on the manner in which
benthic organisms are distributed on the seabed and their response to production rates
in the overlying water column. Where conditions are unusually favorable for the local
supply of organic material by some combination of physical processes, benthic biomass
may be highly specialized and extraordinarily abundant: the slope off western Sweden
may support aggregations of the brittlestar Amphiura filiformis at densities of around
3000 m
−2 . This organism is facultatively both a suspension and a deposit feeder, and it
functions particularly well where physical conditions of turbulence, current speed, and
siltation rate are highly variable.