Environmental Engineering Reference
In-Depth Information
fraction of surface production and defecate primarily at the oxygen/sulfide
interface. Future studies will help resolve this issue.
Another possibility is that the maxima represent horizontal transport of
organic carbon from shallow areas. Temporally variable east-west gradients
in primary production are evident across the northern continental margin
of Venezuela (
http://seawifs.gsfc.nasa.gov/SEAWIFS.html
). Waters
east of the CARIACO station tend to be more productive [32,33]. Some intru-
sions of water into the Basin appear to originate in less productive Caribbean
regions to the northwest of Isla Margarita and are forced through the Canal de
la Tortuga (135 m) by mesoscale eddy circulation, although other (unknown)
processes also appear to cause intrusion events [2]. Therefore, lateral trans-
port at depth of significant amounts of relatively labile organic matter to the
CARIACO station is inconsistent with existing observations.
Major elemental composition of suspended organic matter may provide clues
to its provenance. This material becomes nitrogen-depleted in the upper 200
m, where C:N molar ratios increase from 8.6 in the surface to 15 above the
redoxcline (Fig. 3c). This is consistent with open ocean observations, where
C:N ratios in the epipelagic zone are lower than in the mesopelagic. At Station
ALOHA, for example, the median C:N ratio above 200 m is 6.4 (n = 533) which
is significantly lower (p
<
0.0001; Mann-Whitney Rank Sum test) than between
201 and 1044 m (7.0; n = 223), (
http://hahana.soest.hawaii.edu/hot/
hot jgofs.html
). Apparently N is preferentially remineralized and retained
in surface waters and particulate organic matter's nutritional quality diminishes
rapidly with depth [23,55,65]. In the Cariaco, however, ratios return to near
Redfield values at depths below 250 m (Fig. 3c). This trend suggests that POM
in the redoxcline is comprised of higher proportions of living cells or fresher
detritus than material in the overlying 100 m. In fact, estimates of prokaryotic
carbon biomass within the redoxcline accounted for 9-54% (
x
= 26%) of
the POC inventory during 14 cruises. Furthermore, the redoxcline supports
inventories of prokaryotes that vary from 26 to 329% of those in the oxic layer
(
x
= 94%; Table 2). Estimates of the C:N ratio in prokaryotes vary between 3.8
and 6.7 [12], so significant contributions from microbial biomass to organic
carbon inventories can offset high C:N ratios of nitrogen-depleted organic
matter in bulk measurements. Unlike Station ALOHA, particulate organic C:N
ratios in the oxic layer and underlying waters were not statistically different
during 79 CARIACO cruises (p
>
0.05; Mann-Whitney Rank Sum test); median
C:N ratios were 8.5 (n = 908) and 8.9 (n = 604), respectively. Elemental ratio
profiles suggest that particles below 250 m are on average no more N-depleted
than particles in oxic waters and are probably comprised of redoxcline-derived
microorganisms and byproducts (low C:N) along with surface-derived debris
(high C:N). The high median C:N ratios within the Cariaco Basin oxic layer
