Environmental Engineering Reference
In-Depth Information
United States) CARIACO program. CARIACO (
Ca
rbon
R
etention
i
n
aC
olored
O
cean) consists of a monthly time series station in the eastern basin of the Cari-
aco at which hydrographic, nutrient and primary productivity measurements
have been made since November 1995. A suite of other measurements, includ-
ing a sediment trap mooring, roughly semiannual microbiological studies and
current meter measurements are also available from this site [24]. Although
early studies typically assumed that the chemistry of the Cariaco Basin was
in steady state or changed very slowly [27, 31], more recent work, including
that from the CARIACO program, has demonstrated that the system is quite
dynamic [29].
The location of the CARIACO station in the tropics, in an area of strong sea-
sonal upwelling, means that the chemistry of the water column and sediments
of this system are potentially very sensitive to climatic shifts as well as episodic
events. In this paper we present evidence of the seasonal and interannual vari-
ability of nutrients (nitrate, nitrite, phosphate, and silicate) and other species
in the Cariaco Basin between 1998 and 2004, as well as some information on
long-term trends in the deep water. We also discuss the factors that control both
the vertical and temporal variability of these parameters.
2. STUDY SITE
The Cariaco Basin is a pull-apart basin located on the continental shelf of
Venezuela in the trade wind belt. It is about 1400 m deep, is divided into two sub-
basins separated by a saddle rising up to about 900 m, and is nearly 150 km long
(E-W), but only about 50 km wide (N-S). It is located between the mainland of
Venezuela and a series of small islands (Isla Margarita, Tortuga) near the edge
of the continental shelf. The regional sill in this area is approximately 75-100
meters deep, but there are two deeper channels of about 135-150 m in the east
and west allowing exchange of Caribbean water to depth. Very little is known
about the exchange process, although Astor [3] have speculated that intrusions
over the eastern sill may be caused in part by impingement of Caribbean eddies
along the continental margin. The CARIACO time series site is located on the
northern side of the deepest part of the Eastern Basin at 10
o
30'N 64
o
40'W in
about 1400 m of water. Sampling is carried out monthly at this site using the
Venezuelan research vessel R/V Hermano Gines, operated by the Fundaci on
La Salle from Punta de Piedras, Isla de Margarita.
Because of the limited connection of the deep basin to the open Caribbean,
and because of the location of the basin within the core of seasonal upwelling
which occurs along the South American coast from Guiana to Columbia, the
waters of the Cariaco are oxygen depleted, and below about 300 m are sulfidic.
Primary production rates peak during the windy, upwelling season (roughly
January to April) and are relatively low during the rest of the year (although
