Environmental Engineering Reference
In-Depth Information
3. RESULTS AND DISCUSSION
3.1 Surface Production
Waters overlying the Cariaco Basin are subject to strong seasonal oscilla-
tions in productivity driven by upwelling [32]. More than eight years of monthly
CARIACO observations have revealed that phytoplankton standing stocks usu-
ally remain low from June through December (
<
1.0µg chlorophyll
a
(Chl
a
)L
−
1
in upper 100 m) and reach concentrations as high as 8 µgChl
a
L
−
1
in January
through May. Annually integrated Chl
a
inventories in the oxic layer vary up
to 18-fold, from approximately 12 to 220 mg Chl
a
m
−
2
(Fig. 1a). In the upper
100 m, variance in Chl
a,
NPP, POC and PON are all significantly correlated
with one another (Table 1), suggesting that accumulation of particulate organic
matter is driven principally by local primary production. DOC concentrations
in the surface layer, however, do not covary with any other measured variable
(Table 1).
Standing stocks of the most numerous category of planktonic organisms -
prokaryotes (Bacteria and Archaea) - exhibit annual variations similar to Chl
a
. Inventories integrated over the surface oxic layer (
<
250 m) vary by as much
as 16-fold over an annual cycle (Fig. 1b). While most prokaryotes in this oxic
layer are presumed to be heterotrophic and reliant upon organic matter derived
from primary production in the upper 100 m [7], prokaryotic inventories do
not correlate (p
>
0.05) with Chl
a,
NPP, POC, PON nor DOC (Fig. 1, Table 1).
Results are the same whether depth integrations are confined to the upper 100m
or extended to 250 m. Furthermore, bacterial net production and abundances of
viruses and flagellated protozoa do not covary with phytoplankton production
or organic matter pools in any layer (Table 1). The poor temporal coherence
observed between organic matter pools (production) and microheterotrophic
variables (consumption) probably signifies that our monthly sampling is too in-
frequent to capture lagged responses of the microbial communities to variations
in organic matter pools.
3.2 Unique Depth Distributions
In most aquatic systems, fluxes of sinking particulate organic matter (POM)
decrease exponentially with depth and the POM changes in composition, be-
coming less labile and supporting ever-diminishing bacterial abundances [7,65].
Only about 6-9% of net primary production sinks to the redoxcline's upper
boundary as biogenic debris at Station CARIACO and is collected by its shal-
lowest sediment traps at
265-275 m [58]. The proportion of surface produc-
tion exported to this depth is similar to that seen in many open ocean systems.
However, prokaryotic abundances and activities in the Cariaco's redoxcline
(250-450 m) are significantly enriched relative to the overlying 50-100 m
∼
