68
Martin V. ANGEL
maximum advantage of the upwelling events. One of
the dominant species common to many upwelling areas
is the copepod Calanoides carinatus. Its life history
involves extensive ontogenetic migrations into deep
water, whereby it pre-seeds with its progeny bodies
of water that may ultimately be upwelled. Thus it is
able to take the fullest advantage of the ensuing bloom
of phytoplankton (Smith, 1984; Verheye, 1991). In
the Benguela Current region there are two euphausiid
species (Euphausia hanseni and Nyctiphanes capensis)
that also have life cycles that serve to maintain the
populations within the circulation cells involved in
the upwelling (Barange and Pillar, 1992). The peaks
in productivity resulting from the seasonality of the
upwelling events have an influence similar to that of
the seasonal cycles at temperate and subpolar latitudes.
There is a pulsing in the export of organic matter to
the neighbouring deep ocean, which, in turn, generates
seasonality in the deep communities.
There are marked differences in productivity between the various upwelling regions. These arise
because of differences in the nutrient concentrations of
the various water masses that are being upwelled. For
example, the water that upwells off California comes
from much the same subthermocline depths (c. 150 m)
as the water that upwells off Northwest Africa, but it
stimulates much higher productivity. Even within the
Northwest African upwelling region there are marked
differences in productivity to the north and south of
Cap Blanc. To the south the water upwelled is South
Atlantic Central Water, which is “older” and richer
in nutrients than the North Atlantic Central Water
upwelled to the north of the Cape (Gardner, 1977).
Another feature of upwelling regions is that the
concentrations of dissolved oxygen in the subthermocline water tend to be lower than normal. This is
because microbial degradation of the copious quantities of organic matter sedimenting from the surface
uses up much of it. Generally there is an oxygen
minimum at the permanent thermocline, at a depth of
about 1000 m where the waters are the “oldest”. In
the Atlantic, dissolved oxygen concentrations in the
oxygen minimum are mostly in excess of 2 ml °
−1 , and
do not restrict the ranges of pelagic species. Beneath
some of the upwelling regions, however, the inputs
of organic matter can be large enough for all the
dissolved oxygen available in the water to be used
up. A severe oxygen minimum occurs seasonally in
the Arabian Sea (e.g., Herring et al., 1998) and in
the Benguela Current region. Oxygen concentrations
fall so low that aerobic respiration is inhibited; to
survive there the organisms either have to rely on
anaerobic respiration, or to use alternative sources of
oxygen. One source is by reducing sulphate ions to
sulphide. Sulphides not only smell of bad eggs but
are also highly toxic, and if sulphide-rich water is
upwelled it can cause mass mortalities of fish and
marine invertebrates. In the Arabian Sea a strong
oxygen minimum develops seasonally as a response
to the upwelling induced by the south-west monsoon
at a time when the standing crop of organisms in
the water column is correspondingly high. Many of
the pelagic species inhabiting the sea area become
restricted to the upper 70–100 m. A notable example is
the swimming crab Charybdis smithii, which swarms at
the surface at night. During the day it stays within the
well-oxygenated water of the wind-mixed layer, where
it is more vulnerable to predation by tuna, although
elsewhere and in other seasons it migrates down to
daytime depths of 200–300 m (Van Couwelaar et al.,
1997). Other diel migrants, including several species of
fish, decapods and euphausiids, are able to tolerate the
low oxygen conditions and migrate down to spend the
day deep within the oxygen minimum, possibly taking
refuge from their usual array of predators (Herring
et al., 1998).
BIOPHYSICS AND OCEANIC FOOD-WEBS
In shallow seas where the seabed is illuminated and
the substratum is stable fixed plants can grow; a
substantial proportion of the primary production is
then produced by macroalgae and a small range of
higher plants such as mangroves and sea-grasses. The
relatively large size and/or high concentration of plant
biomass enable many of the grazers and browsers
similarly to be large in size; even some quite sizeable
fish are herbivorous. Additionally, the large plants
provide three-dimensional microhabitats, analogous in
structure to forest environments on land. Variations in
the local geology and differences in exposure to waves
and currents, suspended sediments, and differing tidal
regimes and run-off from land, create many fine-scaled
habitat mosaics in the littoral and sublittoral zones,
each of which supports distinctive assemblages of
species and subcommunities. Similarly in shallow tropical waters, corals containing symbiotic photosynthetic
algae also create highly complex 3-dimensional finescaled microhabitats, which support a high diversity of
species.
Martin V. ANGEL
maximum advantage of the upwelling events. One of
the dominant species common to many upwelling areas
is the copepod Calanoides carinatus. Its life history
involves extensive ontogenetic migrations into deep
water, whereby it pre-seeds with its progeny bodies
of water that may ultimately be upwelled. Thus it is
able to take the fullest advantage of the ensuing bloom
of phytoplankton (Smith, 1984; Verheye, 1991). In
the Benguela Current region there are two euphausiid
species (Euphausia hanseni and Nyctiphanes capensis)
that also have life cycles that serve to maintain the
populations within the circulation cells involved in
the upwelling (Barange and Pillar, 1992). The peaks
in productivity resulting from the seasonality of the
upwelling events have an influence similar to that of
the seasonal cycles at temperate and subpolar latitudes.
There is a pulsing in the export of organic matter to
the neighbouring deep ocean, which, in turn, generates
seasonality in the deep communities.
There are marked differences in productivity between the various upwelling regions. These arise
because of differences in the nutrient concentrations of
the various water masses that are being upwelled. For
example, the water that upwells off California comes
from much the same subthermocline depths (c. 150 m)
as the water that upwells off Northwest Africa, but it
stimulates much higher productivity. Even within the
Northwest African upwelling region there are marked
differences in productivity to the north and south of
Cap Blanc. To the south the water upwelled is South
Atlantic Central Water, which is “older” and richer
in nutrients than the North Atlantic Central Water
upwelled to the north of the Cape (Gardner, 1977).
Another feature of upwelling regions is that the
concentrations of dissolved oxygen in the subthermocline water tend to be lower than normal. This is
because microbial degradation of the copious quantities of organic matter sedimenting from the surface
uses up much of it. Generally there is an oxygen
minimum at the permanent thermocline, at a depth of
about 1000 m where the waters are the “oldest”. In
the Atlantic, dissolved oxygen concentrations in the
oxygen minimum are mostly in excess of 2 ml °
−1 , and
do not restrict the ranges of pelagic species. Beneath
some of the upwelling regions, however, the inputs
of organic matter can be large enough for all the
dissolved oxygen available in the water to be used
up. A severe oxygen minimum occurs seasonally in
the Arabian Sea (e.g., Herring et al., 1998) and in
the Benguela Current region. Oxygen concentrations
fall so low that aerobic respiration is inhibited; to
survive there the organisms either have to rely on
anaerobic respiration, or to use alternative sources of
oxygen. One source is by reducing sulphate ions to
sulphide. Sulphides not only smell of bad eggs but
are also highly toxic, and if sulphide-rich water is
upwelled it can cause mass mortalities of fish and
marine invertebrates. In the Arabian Sea a strong
oxygen minimum develops seasonally as a response
to the upwelling induced by the south-west monsoon
at a time when the standing crop of organisms in
the water column is correspondingly high. Many of
the pelagic species inhabiting the sea area become
restricted to the upper 70–100 m. A notable example is
the swimming crab Charybdis smithii, which swarms at
the surface at night. During the day it stays within the
well-oxygenated water of the wind-mixed layer, where
it is more vulnerable to predation by tuna, although
elsewhere and in other seasons it migrates down to
daytime depths of 200–300 m (Van Couwelaar et al.,
1997). Other diel migrants, including several species of
fish, decapods and euphausiids, are able to tolerate the
low oxygen conditions and migrate down to spend the
day deep within the oxygen minimum, possibly taking
refuge from their usual array of predators (Herring
et al., 1998).
BIOPHYSICS AND OCEANIC FOOD-WEBS
In shallow seas where the seabed is illuminated and
the substratum is stable fixed plants can grow; a
substantial proportion of the primary production is
then produced by macroalgae and a small range of
higher plants such as mangroves and sea-grasses. The
relatively large size and/or high concentration of plant
biomass enable many of the grazers and browsers
similarly to be large in size; even some quite sizeable
fish are herbivorous. Additionally, the large plants
provide three-dimensional microhabitats, analogous in
structure to forest environments on land. Variations in
the local geology and differences in exposure to waves
and currents, suspended sediments, and differing tidal
regimes and run-off from land, create many fine-scaled
habitat mosaics in the littoral and sublittoral zones,
each of which supports distinctive assemblages of
species and subcommunities. Similarly in shallow tropical waters, corals containing symbiotic photosynthetic
algae also create highly complex 3-dimensional finescaled microhabitats, which support a high diversity of
species.