Environmental Engineering Reference
In-Depth Information
Keywords:
prokaryotes, protozoa, viruses, chemoautotrophs, redoxcline, anaerobes, produc-
tivity
1. INTRODUCTION
The Cariaco Basin is a tectonically-formed depression on Venezuela's north-
ern continental margin, reaching depths of nearly 1400 m and enclosing an
approximate volume of 5.2 x 10
12
m
3
below the 180 m isobath [40]. The Basin
is surrounded by a sill (90-150 m in depth) and its bathymetry confines lateral
water exchange primarily to the surface 100 m, although sporadic intrusions
of oxygenated Caribbean surface waters to depths of
>
310 m have been docu-
mented recently [2]. While sulfide concentrations in bottom waters are known
to vary over decadal time scales [43], the Cariaco Basin has been euxinic almost
continuously for the past 12.6 - 14.6 ky [25,38].
Stratified anoxic water columns have long been known to support multi-
ple layers of biological activity, variously fueled by oxygenic photosynthesis,
anoxygenic photosynthesis, chemolithoautotrophy and heterotrophy [18,20,-
21,28,49,50]. While somewhat of a simplification, for the purposes of this
study we will divide the Cariaco system into three major layers, oxic (
<
250
m), redoxcline (250-450 m) and anoxic (
>
450 m). The oxic layer is made up
of an aphotic sublayer, which is supplied with organic matter from the shal-
low euphotic zone (usually
<
75 m). The oxic layer may be comprised of
more than one water mass and is structurally and functionally similar to other
upwelling-prone coastal seas in the tropics and subtropics. The redoxcline is
the depth interval over which the chemical redox potential undergoes its largest
change and where the transition from oxic to sulfidic state occurs. Its precise
position and thickness vary, but generally it falls between 250 and 450 m in
the Cariaco. As in the Black Sea and other anoxic water columns, the Cari-
aco's redoxcline supports enriched inventories of prokaryotes, microbial ATP,
protozoa, enhanced cycling of redox-sensitive elements and chemoautotrophic
production [4,13,14,15,23,24,30,49,56]. The anoxic layer is characterized by
increasing concentrations of H
2
S, CH
4
,PO
4
3
−
and NH
4
+
and diminishing mi-
crobial activity with depth [14,42,56]. Linkages between processes in the upper
250 m and underlying waters in this system are poorly understood.
The present communication explores basic aspects of the microbial ecol-
ogy of the redoxcline of the Cariaco Basin and their relationship to surface
processes. Results are derived from monthly hydrographic/ productivity cruises
and seasonal process cruises staged by the cooperative US-Venezuelan CARI-
ACO program (
CA
rbon
R
etention
I
n
AC
olored
O
cean).
