Environmental Engineering Reference
In-Depth Information
distributions of POC/PON in the Cariaco almost never conform to this model
and frequently exhibit mid-water maxima within the redoxcline. For example,
the shallowest POC/PON maximum during CAR-74 coincided with the sub-
surface fluorescence peak and the deeper POC/PON maxima coincide with
light scattering peaks (Fig. 3c). These midwater peaks can account for sig-
nificant fractions of total suspended matter inventories in the Basin. Median
inventories of POC and PON integrated over the redoxcline (250-450 m) are
66
+
21 and 61
+
28% of inventories residing in the upper 250 m (Table 2). By
comparison, at Station ALOHA in the North Pacific Central Gyre, median POC
and PON inventories within the shallow mesopelagic zone (250-450 m) equal
27+5% and 25%+6% (
x
+ 1 S.D.) of the POC/PON measured in the upper
250 m (n = 55 cruises), respectively (
http://hahana.soest.hawaii.edu/
hot/hot jgofs.html
). This comparison clearly indicates that the Cariaco's
redoxcline is significantly enriched in POC relative to shallow mesopelagic
zones elsewhere in the world's ocean. The potential sources of these midwater
enrichments are explored below.
Integrated inventories of organic pools and microorganisms and activities within
the redoxcline (250-450 m) compared to the surface layer
Table 2.
(0-250 m). Expressed as ratio of
redoxcline: surface layer.
n
1
Minimum
Maximum
Mean
Median
POC
80
0.26
1.35
0.67
0.66
PON
79
0.30
2.02
0.68
0.61
DOC
47
0.19
3.02
0.83
0.75
Prokaryotes
82
0.26
3.29
0.94
0.74
Flagellates
29
0.29
11.11
2.78
2.19
Viruses
8
0.05
0.75
0.41
0.47
BNP
21
0.02
0.87
0.19
0.16
DCA
13
0.38
8.94
4.01
3.37
1
n = number of cruises included in statistics. Profiles from each cruise include 15-18 depths.
2
DCA = dark DIC assimilation.
3.3 Potential Sources of Midwater Enrichments
Profiles in the Cariaco suggest that particulate organic matter might be
introduced to the redoxcline through active translocation by migrating animals.
Acoustic scattering layers migrating from surface waters through the redoxcline
on a diel basis
have
been documented
repeatedly (R. Varela,
unpubl.).
Furthermore, codlet fish have been recovered from deep (
<
800 m) night-time
net tows in the Basin [3]. The behavior and physiology of these migrators are
poorly known. However, it seems improbable that they would forage a large
