Geoscience Reference
In-Depth Information
3.1 Amazon and Orinoco River Plume Monitoring
The Amazon is the world's largest river in terms of freshwater discharge (Milliman and
Meade
1983
; Perry et al.
1996
). It drains a large fraction of the South American continent,
discharging on average 1.55 ± 0.13 9 10
5
m
3
s
-1
of freshwater into the equatorial
Atlantic Ocean (Perry et al.
1996
). This is about 15 % of the estimated global river dis-
charge on an annual basis. The Amazon River is by far the largest single source of
terrestrial freshwater to the ocean and contributes about 30 % of total river discharge to the
Atlantic Ocean (Wisser et al.
2010
). The structure of the Amazon plume is strongly
influenced by a variety of physical processes, which are present on the northern Brazilian
shelf: the North Brazil Current (Flagg et al.
1986
; Richardson and McKee
1984
), trade
winds (Hellerman and Rosenstein
1983
) and strong currents associated with the tide
(Nittrouer and Demaster
1986
). These physical processes play a very significant role in the
dispersal and spreading of Amazon discharge (freshwater and suspended sediment) on the
northern continental shelf of South America.
Previous studies have shown that Amazon plume water can be traced offshore and
northwestward along the north Brazilian coast, covering most of the continental shelf from
11Sto5 N (Muller-Karger et al.
1988
,
1995
) into the Caribbean (e.g., Steven and Brooks
1972
; Froelich et al.
1978
; Hellweger and Gordon
2002
; Cherubin and Richardson
2007
),
and over 1,000 km eastward into the North Atlantic depending on the season. Beyond this
region, the Amazon's water has been traced northwestward into the Caribbean Sea and
eastward in the North Atlantic (Muller-Karger et al.
1988
,
1995
; Johns et al.
1990
; Hell-
weger and Gordon
2002
). Hydrographic surveys by Lentz and Limeburner (
1995
) revealed
that the Amazon plume over the shelf is typically 3-10 m thick and between 80 and
[200 km wide. Beyond the shelf, freshwater within the plume gradually attenuates with
depth as it travels away from the source, with a penetration depth of 40-45 m as far as
2,600 km offshore (Hellweger and Gordon
2002
; Hu et al.
2004
).
Both chlorophyll (Chl) concentration and primary productivity are the greatest in the
river plume-ocean transition zone, where the bulk of heavy sediments are deposited (Smith
and Demaster
1996
). The combination of riverine nutrient input and increased irradiance
availability creates a highly productive transition zone, the location of which varies with
the discharge from the river. High phytoplankton biomass and productivity of over
25 mg Chl-a m
-3
and 8 g cm
-2
day
-1
, respectively, are found in this transition region
(Smith and Demaster
1996
). Because of this, the North Brazil shelf acts as a significant
sink for atmospheric CO
2
(Ternon et al.
2000
).
The northwestern tropical Atlantic is also an area where another major river in the
world, the Orinoco, enters the ocean. The Orinoco River originates in the southern part of
Venezuela and discharges waters from about 31 major and 2000 minor tributaries into the
western tropical Atlantic. These waters are most of the time transported into the south-
eastern Caribbean sea, and during the rainy season, a larger but unquantified fraction of the
plume also flows east around Trinidad and Tobago into the Caribbean. The Orinoco is
considered to be the third largest river in the world in terms of volumetric discharge (after
the Amazon and the Congo), discharging an average of *3.6 9 10
4
m
3
s
-1
(Meade et al.
1983
; Muller-Karger et al.
1989
;V ¨r ¨smarty et al.
1998
). Low discharge occurs during the
dry season (January-May) and high discharge during the rainy season (July-October) as a
result of the meridional migration of the ITCZ.
The freshwater discharges from the Amazon and Orinoco Rivers spread outward into
the western equatorial Atlantic Ocean while continually mixing with surrounding salty
ocean surface water. The averaged geographical distribution of the low-salinity signatures