226
Amanda W.J. DEMOPOULOS et al.
predicted to occur in the extreme southwest region of
the ocean (see Gage and Tyler, 1991).
Bottom-water oxygen
Oxygen concentration in the bottom water can be
a controlling factor in the preservation of organic
carbon and benthic fauna assemblages. For example,
at depths between ~100 and 1000 m, oxygen minimum
zones may develop below productive waters and coastal
upwelling zones, where the average annual flux of
organic matter to the seabed is high. The resulting
hypoxia can reduce abundance and biomass of many
benthic animals, altering species composition and
richness (Diaz and Rosenberg, 1995).
In the Northern Indian Ocean, an oxygen minimum
zone occurs between depths of 100 m and 1000 m,
where oxygen concentrations are <0.5 ml °
−1 . This zone
results from a combination of high surface productivity
driven by upwelling, inflow of oxygen-poor waters
from the Persian Gulf, Red Sea, and Banda Sea, and
slow deep-water circulation (Wyrtki, 1973). Such a
hypoxic layer at intermediate depths has significant
consequences for the quantity and quality of organic
matter reaching the deep sea from surface production.
Sinking flux of particulate organic carbon (POC)
Benthic organisms are fueled by sinking organic matter
from surface waters. Therefore, it is important to
evaluate what controls the flux of particulate organic
carbon to the deep sea. In general, regional flux of
particulate carbon decreases with depth and distance
from continents, and is directly controlled by overlying
primary production, the depth of the water column
(Suess, 1980; Smith and Hinga, 1983; Jahnke, 1996),
and the freshwater supply (Ittekkot et al., 1991).
Therefore, along continental slopes, where primary
production is high and the water column is shallow,
the flux of particulate organic carbon to the seafloor
is high over annual periods. Specifically, over the
Oman Margin of the Arabian Sea, particle flux is
strongly seasonal, with peaks during the Southwest
and Northeast Monsoons (Nair et al., 1989; Honjo
et al., 1999). High monsoonal primary production
(912.5 g C m
−2 y
−1 ), resulting from wind-induced mixing and nutrient injection into the euphotic zone, is
the main factor controlling the observed pattern of
particle flux (Burkill et al., 1993). For example, off
the Oman coast during the Southwest Monsoon, the
rate of sedimentation is approximately 365 g C m
−2 y
−1
at depths between 100 and 500 m (Burkill et al., 1993;
Pollehne et al., 1993). At 1500 m, the total annual flux
of particulate organic carbon drops to 53 g C m
−2 y
−1 ,
decreasing with depth to 23 g C m
−2 y
−1 at 3500 m
(Honjo et al., 1999). In general, particle fluxes during
the Southwest Monsoon are greater than during the
Northeast Monsoon (Honjo et al., 1999). Because of
the lack of upwelling, the spring inter-monsoon period
is the most oligotrophic season. Low sedimentation
rates are recorded during the inter-monsoon period,
corresponding to 6% of the total annual flux (Nair et al.,
1989). Therefore, there is high seasonal variability in
the total flux of particulate organic carbon, as a result
of monsoonal forcing.
Rates of primary production in the central abyssal
Indian Ocean, however, are similar to those of the
south Atlantic Ocean, which is characterized by low
production rates (Steeman Nielsen, 1975). For example,
in the oligotrophic gyre of the Indian Ocean, net
primary production is less than 109 g C m
−2 y
−1 . The
corresponding organic carbon flux in this region is
approximately 36.5 g C m
−2 y
−1 (Burkill et al., 1993;
Pollehne et al., 1993).
REPESENTATIVE DEEP INDIAN OCEAN HABITATS
Oxygenated slopes and basins on the Kenya
margin
An oxygenated slope and basin region occurs along
the margin off the coast of Kenya. Preliminary
investigations of the benthic fauna have been conducted
here, making this the best-studied region of its type in
the Indian Ocean.
Habitat and community description
Several rivers discharge terrigenous material onto the
Kenya shelf, and during monsoon periods the outflow is
large. The continental-shelf region of Kenya is narrow,
and the ocean is very deep near the coastline. Although
knowledge of the benthic communities is limited, some
data exist for densities and biomass of macro- and
meiofauna from sediments collected by a boxcorerrespirometer (belljar). Along the Kenyan Slope, the
densities and biomass of macrofauna (animals retained
on a 500 mm sieve) decrease with increasing depth to
1000 m. Macrofaunal densities decrease from 7590 individuals m
−2 at 500 m to 2960 individuals m
−2 at
1000 m, and the biomass decreases from 26.0 g C m
−2
Amanda W.J. DEMOPOULOS et al.
predicted to occur in the extreme southwest region of
the ocean (see Gage and Tyler, 1991).
Bottom-water oxygen
Oxygen concentration in the bottom water can be
a controlling factor in the preservation of organic
carbon and benthic fauna assemblages. For example,
at depths between ~100 and 1000 m, oxygen minimum
zones may develop below productive waters and coastal
upwelling zones, where the average annual flux of
organic matter to the seabed is high. The resulting
hypoxia can reduce abundance and biomass of many
benthic animals, altering species composition and
richness (Diaz and Rosenberg, 1995).
In the Northern Indian Ocean, an oxygen minimum
zone occurs between depths of 100 m and 1000 m,
where oxygen concentrations are <0.5 ml °
−1 . This zone
results from a combination of high surface productivity
driven by upwelling, inflow of oxygen-poor waters
from the Persian Gulf, Red Sea, and Banda Sea, and
slow deep-water circulation (Wyrtki, 1973). Such a
hypoxic layer at intermediate depths has significant
consequences for the quantity and quality of organic
matter reaching the deep sea from surface production.
Sinking flux of particulate organic carbon (POC)
Benthic organisms are fueled by sinking organic matter
from surface waters. Therefore, it is important to
evaluate what controls the flux of particulate organic
carbon to the deep sea. In general, regional flux of
particulate carbon decreases with depth and distance
from continents, and is directly controlled by overlying
primary production, the depth of the water column
(Suess, 1980; Smith and Hinga, 1983; Jahnke, 1996),
and the freshwater supply (Ittekkot et al., 1991).
Therefore, along continental slopes, where primary
production is high and the water column is shallow,
the flux of particulate organic carbon to the seafloor
is high over annual periods. Specifically, over the
Oman Margin of the Arabian Sea, particle flux is
strongly seasonal, with peaks during the Southwest
and Northeast Monsoons (Nair et al., 1989; Honjo
et al., 1999). High monsoonal primary production
(912.5 g C m
−2 y
−1 ), resulting from wind-induced mixing and nutrient injection into the euphotic zone, is
the main factor controlling the observed pattern of
particle flux (Burkill et al., 1993). For example, off
the Oman coast during the Southwest Monsoon, the
rate of sedimentation is approximately 365 g C m
−2 y
−1
at depths between 100 and 500 m (Burkill et al., 1993;
Pollehne et al., 1993). At 1500 m, the total annual flux
of particulate organic carbon drops to 53 g C m
−2 y
−1 ,
decreasing with depth to 23 g C m
−2 y
−1 at 3500 m
(Honjo et al., 1999). In general, particle fluxes during
the Southwest Monsoon are greater than during the
Northeast Monsoon (Honjo et al., 1999). Because of
the lack of upwelling, the spring inter-monsoon period
is the most oligotrophic season. Low sedimentation
rates are recorded during the inter-monsoon period,
corresponding to 6% of the total annual flux (Nair et al.,
1989). Therefore, there is high seasonal variability in
the total flux of particulate organic carbon, as a result
of monsoonal forcing.
Rates of primary production in the central abyssal
Indian Ocean, however, are similar to those of the
south Atlantic Ocean, which is characterized by low
production rates (Steeman Nielsen, 1975). For example,
in the oligotrophic gyre of the Indian Ocean, net
primary production is less than 109 g C m
−2 y
−1 . The
corresponding organic carbon flux in this region is
approximately 36.5 g C m
−2 y
−1 (Burkill et al., 1993;
Pollehne et al., 1993).
REPESENTATIVE DEEP INDIAN OCEAN HABITATS
Oxygenated slopes and basins on the Kenya
margin
An oxygenated slope and basin region occurs along
the margin off the coast of Kenya. Preliminary
investigations of the benthic fauna have been conducted
here, making this the best-studied region of its type in
the Indian Ocean.
Habitat and community description
Several rivers discharge terrigenous material onto the
Kenya shelf, and during monsoon periods the outflow is
large. The continental-shelf region of Kenya is narrow,
and the ocean is very deep near the coastline. Although
knowledge of the benthic communities is limited, some
data exist for densities and biomass of macro- and
meiofauna from sediments collected by a boxcorerrespirometer (belljar). Along the Kenyan Slope, the
densities and biomass of macrofauna (animals retained
on a 500 mm sieve) decrease with increasing depth to
1000 m. Macrofaunal densities decrease from 7590 individuals m
−2 at 500 m to 2960 individuals m
−2 at
1000 m, and the biomass decreases from 26.0 g C m
−2