Atlantic Coastal Biome
247
observed mesozooplankton abundance may be attributed to intense and localized consumption by their dense shoals; models of the relationship between Engraulis biomass,
sea surface chlorophyll and temperature, and Calanoides abundance show that this relationship is highly sensitive to parameters difficult to determine empirically, such as
predator-specific zooplankton mortality rates. The other side of this coin is that the
recruitment of these species is, in turn, controlled at least in large part by the intensity
of upwelling and the consequent strength of the plankton blooms.
The anomalously large year-class of Cape anchovy resulted from the late 1999 spawning
in the southern Benguela of an unexceptional adult population at a time when the
southeast wind anomaly (and hence upwelling intensity and negative SST anomaly)
was the strongest of the previous 40 years for which records exist (Roy et al., 2001).
Although this event raised as many questions as answers, the association between physical
forcing and subsequent recruitment is not in question; what is in question is why, in
this case, the previously established inverse relationship should fail. It had previously
been determined that strong upwelling was detrimental to anchovy year-class strength,
because it was thought that increased offshore advection would lead to increased loss of
eggs and larvae to the open ocean. However this may be, there has been a significant
long-term shift in relative abundance; since 1965–1966 the biomass of Engraulis has been
very significantly greater than of Sardinops—indeed, in the late 1980s by an order of
magnitude—although during the 1950s the biomass of the two species was essentially
equivalent.
Regional Benthic and Demersal Ecology
All linear shelves having intermittent but strong coastal upwelling risk benthic anoxia
as the sediments are supplied with DOC produced in the pelagial at rates faster than
this material can be oxidized by heterotrophic microbes. The Benguela system is no
exception, and widespread depletion of oxygen occurs, especially in the northern part of
the province; in exceptional events, such as in 1993–1994, benthic hypoxia may extend
the entire length of the shelf and may result in mass mortality of benthic invertebrates
and demersal fish, although species such as hake may be able to respond by actively
changing their distribution.
Demersal fish surveys, such as those of MacPherson and Gordoa (1992, 1996) and
Mas-Riera et al. (1999), reveal partitions of the benthic habitat primarily into shelf
and slope components and secondarily with relation to bottom type and bottom dissolved
oxygen and temperature. Once again, species replacement to ensure habitat partitioning
is very evident: Merluccius paradoxus and M. capensis inhabit southern and northern shelf
environments respectively: M. polli is a species of lower latitudes. The entire shelf/slope
habitat may be partitioned into six regions, of which the shelves north and south of
28
S will be of greatest interest to us here. The species replacement is striking between
these regions: in 1997–98, M. paradoxus formed 55–80% of the entire biomass in the
south, depending on season, and M. capensis about 75–80% of all biomass in the north.
What it is today, I am not prepared to guess. The biomass runners-up were, respectively,
Nematogobius barbatus in the north and Helicolenus dactylopterus in the south. Diversity
of each was found to decrease in regions of poorly oxygenated bottom water. The earlier
surveys of MacPherson and Gordoa performed in 1983–1990 suggested a somewhat
different partition and revealed significant between-year changes in biomass. M. paradoxus
was assigned by this analysis to a southern slope association, and of the “southern shelf”
species only Lepidopus caudatus was sufficiently abundant to bring significant biomass to
the total for the entire province. A general decrease of demersal biomass was recorded
during this period.
247
observed mesozooplankton abundance may be attributed to intense and localized consumption by their dense shoals; models of the relationship between Engraulis biomass,
sea surface chlorophyll and temperature, and Calanoides abundance show that this relationship is highly sensitive to parameters difficult to determine empirically, such as
predator-specific zooplankton mortality rates. The other side of this coin is that the
recruitment of these species is, in turn, controlled at least in large part by the intensity
of upwelling and the consequent strength of the plankton blooms.
The anomalously large year-class of Cape anchovy resulted from the late 1999 spawning
in the southern Benguela of an unexceptional adult population at a time when the
southeast wind anomaly (and hence upwelling intensity and negative SST anomaly)
was the strongest of the previous 40 years for which records exist (Roy et al., 2001).
Although this event raised as many questions as answers, the association between physical
forcing and subsequent recruitment is not in question; what is in question is why, in
this case, the previously established inverse relationship should fail. It had previously
been determined that strong upwelling was detrimental to anchovy year-class strength,
because it was thought that increased offshore advection would lead to increased loss of
eggs and larvae to the open ocean. However this may be, there has been a significant
long-term shift in relative abundance; since 1965–1966 the biomass of Engraulis has been
very significantly greater than of Sardinops—indeed, in the late 1980s by an order of
magnitude—although during the 1950s the biomass of the two species was essentially
equivalent.
Regional Benthic and Demersal Ecology
All linear shelves having intermittent but strong coastal upwelling risk benthic anoxia
as the sediments are supplied with DOC produced in the pelagial at rates faster than
this material can be oxidized by heterotrophic microbes. The Benguela system is no
exception, and widespread depletion of oxygen occurs, especially in the northern part of
the province; in exceptional events, such as in 1993–1994, benthic hypoxia may extend
the entire length of the shelf and may result in mass mortality of benthic invertebrates
and demersal fish, although species such as hake may be able to respond by actively
changing their distribution.
Demersal fish surveys, such as those of MacPherson and Gordoa (1992, 1996) and
Mas-Riera et al. (1999), reveal partitions of the benthic habitat primarily into shelf
and slope components and secondarily with relation to bottom type and bottom dissolved
oxygen and temperature. Once again, species replacement to ensure habitat partitioning
is very evident: Merluccius paradoxus and M. capensis inhabit southern and northern shelf
environments respectively: M. polli is a species of lower latitudes. The entire shelf/slope
habitat may be partitioned into six regions, of which the shelves north and south of
28
S will be of greatest interest to us here. The species replacement is striking between
these regions: in 1997–98, M. paradoxus formed 55–80% of the entire biomass in the
south, depending on season, and M. capensis about 75–80% of all biomass in the north.
What it is today, I am not prepared to guess. The biomass runners-up were, respectively,
Nematogobius barbatus in the north and Helicolenus dactylopterus in the south. Diversity
of each was found to decrease in regions of poorly oxygenated bottom water. The earlier
surveys of MacPherson and Gordoa performed in 1983–1990 suggested a somewhat
different partition and revealed significant between-year changes in biomass. M. paradoxus
was assigned by this analysis to a southern slope association, and of the “southern shelf”
species only Lepidopus caudatus was sufficiently abundant to bring significant biomass to
the total for the entire province. A general decrease of demersal biomass was recorded
during this period.