Environmental Engineering Reference
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probably reflect the limited vertical extent of the photic zone that produces low
C:N particles compared to the relatively broad remineralization zone.
3.4 Trophic Structure in the Redoxcline
Suboxia and anoxia are known to exclude most animals and to support rela-
tively simple food webs with fewer trophic levels than oxic systems [9,10,64].
Therefore, enriched prokaryotic inventories in the redoxcline might be hypothe-
sized to result from reduced grazing in oxygen-depleted waters. Mortality rates
have not been measured for prokaryotes within Cariaco's redoxcline. Nonethe-
less, variance in distributions of the primary bacterivores (flagellated and some
ciliated protozoa) imply significant grazing pressure on prokaryotes. Flagellate
cell size varied between 2 and 6 µm in diameter, typical of marine bacterivores.
Their abundance profiles are quite variable, sometimes exhibiting maxima in
the photic zone, coinciding with elevated prokaryotic numbers (e.g., Figs. 4a,
4b), but often not. Over the course of 29 cruises, flagellate abundances in the
oxic layer have covaried with BNP, but not with prokaryotic abundances (Table
3). We interpret depauperate inventories in surface waters as manifestations
of grazing pressure on the flagellates themselves. Multiple peaks in flagel-
late abundance are frequently evident through the redoxcline and abundances
usually diminish below 500 m (Fig 4b). In fact, flagellate inventories in the
redoxcline vary from 29 to 1111% (
x
= 278%) of those integrated for the oxic
layer (Table 2) and are positively correlated with abundances of prokaryotes
(Table 3).
The CARIACO database for ciliated protozoan distributions is more lim-
ited than for other members of the microbial food web. As was the case for
flagellates, ciliate depth distributions and abundances are variable, presumably
responding to bottom-up and top-down controls (Fig. 5). At least one peak in
total ciliate abundance is commonly observed in the vicinity of the O
2
/H
2
S
interface, sizable standing stocks exist in the redoxcline and ciliate numbers
approach detection limits below 500 m. Based on gross morphological charac-
teristics, composition of ciliate communities appears to vary widely with depth.
For example in samples from CAR-5, tintinnids, known herbivores, were con-
fined to the oxic layer (Fig. 6). Hypotrichs, known to be particle-associated
(thigmotactic), exhibited a strong maximum through the redoxcline and a small
peak in the photic zone. In contrast, depth profiles of aloricate oligotrichs
and peritrichs exhibited peaks within the redoxcline, but standing stocks were
smaller than in the photic zone. Ciliate cell size within the redoxcline varied
between 15 and 50 µm in length, so the community appeared to be populated
by bacterivores and nanoplanktivores. Sizable anaerobic ciliate inventories and
similar stratified distributions have been reported for the Black and Baltic Seas
as well [46,67].
