Atlantic Coastal Biome
261
(ii) A linear zone along and just seaward of the turbidity front, where primary production ceases to be light limited and chlorophyll accumulates (<25 g chl m
−3 ) in a
near-surface (<5 m) bloom while CDOM remains a significant fraction of observable “pigments.” It is possible that chlorophyll accumulation in the second zone is
enhanced by the upwelling of shelf water along the salinity front, as in an estuarine
circulation; in shelf water not directly influenced by the Amazon discharge, CDOM
is not present in unusually high concentration.
(iii) Seaward of the chlorophyll plume, still only in about 40–50 m depth, nitrate
becomes limiting and is always <05 M so that chlorophyll values are reduced to
02–06 mg chl m
−3 .
Currently, there appears to be very little evidence for the availability to algal cells of
nitrogen regenerated by either benthic microbial activity or water column metabolism.
Nevertheless, it has been suggested recently (Subramanian, pers. comm, 2005.) that during
periods of high discharge of both Amazon and Orinoco, diatoms having endosymbiotic
diazotrophic cyanobacteria dominate the autotrophs; these support high productivity and
sedimentation rates (>150 mg m
−2 d
−1 ) within the river plumes.
The coastally trapped discharge from the Amazon and the subsequent input from the
Orinoco modify the ecology of the eastern shelf of Venezuela and the Gulf of Paria (Bonilla
et al., 1993). Most of the nutrients discharged by the Orinoco remain in this area, where
benthic regeneration is very active, so that their contribution to the eastern Caribbean
is problematical, though the low-salinity signal of Amazon and Orinoco water can be
detected as far away as Puerto Rico. Significant amounts of nitrogen probably do not
survive the passage through the coastal ecosystems and the Gulf of Paria (unless recycled
nitrogen is entrained in the regional flow) so that the pigment feature so prominent in
the eastern Caribbean is probably not caused by riverborne nutrients, as suggested by
Müller-Karger et al. (1989), but rather is now thought to be caused by upwelling in the
southern Carioco Basin.
Because the varved sediments of the deep Carioco Basin offer such an exceptional
archive of past climate changes, extending back for 600,000 years, revealing changes in
North Atlantic Deep Water formation during the glacial periods, an unusual amount
of scientific effort has been put into understanding the pelagic processes in the water
column that are revealed in the varved sediments. Here, we have unrivaled sea truth
for the available sea surface chlorophyll images and very complete monitoring of the
variable wind regime and the water column processes thereby forced (Müller-Karger
et al., 2004). There is a very regular seasonal cycle in bloom area (and concomitant area
of reduced SST) between >10 000 km
2 in March or April and an order of magnitude
smaller in October; both shipborne measurement and satellite images deliver a single
period annually (January–May) of high productivity, the integrated annual productivity
varying from 372 to 650 g C m
−2 y
−1 over the 6 years 1996–2001. These measurements
have enabled a very accurate estimate to be made of the resultant flux into the varved
sediments at depth. Satellite images show that the cold water and high chlorophyll plumes
are directed toward the northwest from the coastal region from the east-west coastline
that lies to the west of the island of Trinidad. The sea truth investigations also assisted
in separating the effects of CDOM-laden Amazon shelf water passing between this island
and the mainland from the chlorophyll associated with cold water at the coast a little to
the west.
Regional Benthic and Demersal Ecology
The benthic habitat on the Amazon shelf is anomalous because of the unusually high rate
of deposition of riverborne organic particulates that occurs here seasonally; during the
261
(ii) A linear zone along and just seaward of the turbidity front, where primary production ceases to be light limited and chlorophyll accumulates (<25 g chl m
−3 ) in a
near-surface (<5 m) bloom while CDOM remains a significant fraction of observable “pigments.” It is possible that chlorophyll accumulation in the second zone is
enhanced by the upwelling of shelf water along the salinity front, as in an estuarine
circulation; in shelf water not directly influenced by the Amazon discharge, CDOM
is not present in unusually high concentration.
(iii) Seaward of the chlorophyll plume, still only in about 40–50 m depth, nitrate
becomes limiting and is always <05 M so that chlorophyll values are reduced to
02–06 mg chl m
−3 .
Currently, there appears to be very little evidence for the availability to algal cells of
nitrogen regenerated by either benthic microbial activity or water column metabolism.
Nevertheless, it has been suggested recently (Subramanian, pers. comm, 2005.) that during
periods of high discharge of both Amazon and Orinoco, diatoms having endosymbiotic
diazotrophic cyanobacteria dominate the autotrophs; these support high productivity and
sedimentation rates (>150 mg m
−2 d
−1 ) within the river plumes.
The coastally trapped discharge from the Amazon and the subsequent input from the
Orinoco modify the ecology of the eastern shelf of Venezuela and the Gulf of Paria (Bonilla
et al., 1993). Most of the nutrients discharged by the Orinoco remain in this area, where
benthic regeneration is very active, so that their contribution to the eastern Caribbean
is problematical, though the low-salinity signal of Amazon and Orinoco water can be
detected as far away as Puerto Rico. Significant amounts of nitrogen probably do not
survive the passage through the coastal ecosystems and the Gulf of Paria (unless recycled
nitrogen is entrained in the regional flow) so that the pigment feature so prominent in
the eastern Caribbean is probably not caused by riverborne nutrients, as suggested by
Müller-Karger et al. (1989), but rather is now thought to be caused by upwelling in the
southern Carioco Basin.
Because the varved sediments of the deep Carioco Basin offer such an exceptional
archive of past climate changes, extending back for 600,000 years, revealing changes in
North Atlantic Deep Water formation during the glacial periods, an unusual amount
of scientific effort has been put into understanding the pelagic processes in the water
column that are revealed in the varved sediments. Here, we have unrivaled sea truth
for the available sea surface chlorophyll images and very complete monitoring of the
variable wind regime and the water column processes thereby forced (Müller-Karger
et al., 2004). There is a very regular seasonal cycle in bloom area (and concomitant area
of reduced SST) between >10 000 km
2 in March or April and an order of magnitude
smaller in October; both shipborne measurement and satellite images deliver a single
period annually (January–May) of high productivity, the integrated annual productivity
varying from 372 to 650 g C m
−2 y
−1 over the 6 years 1996–2001. These measurements
have enabled a very accurate estimate to be made of the resultant flux into the varved
sediments at depth. Satellite images show that the cold water and high chlorophyll plumes
are directed toward the northwest from the coastal region from the east-west coastline
that lies to the west of the island of Trinidad. The sea truth investigations also assisted
in separating the effects of CDOM-laden Amazon shelf water passing between this island
and the mainland from the chlorophyll associated with cold water at the coast a little to
the west.
Regional Benthic and Demersal Ecology
The benthic habitat on the Amazon shelf is anomalous because of the unusually high rate
of deposition of riverborne organic particulates that occurs here seasonally; during the