Environmental Engineering Reference
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isotope fractionation and the specific rate of photosynthesis studied under nat-
ural conditions and in pure cultures of marine phytoplankton [35].
Detailed investigations of δ
13 C of POC were carried out at 9 stations in the
deep water zone (Fig. 5). As illustrated in Fig. 5, seasonal changes of δ
13 Cof
POC in the oxic zone are opposite to changes in the anoxic zone. In May, there is
a substantial decrease in the carbon isotopic composition of POC in comparison
with the end of March [24, 36, 50]. The opposite situation is observed at the
end of September when the quantity of POC in surface waters increases due
to active photosynthesis (Table 10). In general, POC concentrations decrease
significantly with depth. The isotopic composition of POC in the layer of
its maximum content differs from the isotopic composition of POC in the
photosynthetic zone. Specifically, in accordance with the data [58], at water
depths of 110 - 135 m at every of 4 stations a distinct increase in POC content
(up to 125 mg/l) was accompanied by 13 C depletion of POC compared to the
upper layers (Table 10, Footnote). Since considerable fractionation of carbon
isotopes during aerobic mineralization of organic matter does not occur [35],
main reasons for changes in δ
13 C of POC along the water column profile are
seasonal changes in photosynthetic rates (Fig. 5) and in the carbon isotopic
composition of phytoplankton biomass.
Table 10.
Particulate organic matter from Black Sea surface layers and its isotopic composition.
Date
Surface layer (0-1 m)
Layer 1-20 m
Reference
C org ,mgl 1
13 C, ‰ C org ,mgl 1
13 C, ‰
of sampling
δ
δ
May 1988
48
-25.5
93
-25.1
[36]
September-
148
-21.8
158
-21.9
[58]
October 1992
δ
13 C (average) POC in the layer 110 - 135 m (zone of the deep-sea POC maximum) = -24.1 ‰ (range
from -23.0 to -25.8‰) [58]
Since fractionation of carbon isotopes occurs not only during photosynthesis
by phytoplankton, but also during chemosynthesis and bacterial photosynthesis
[38], attempts to reveal this effect at the boundary between the oxic and anoxic
waters of the Black Sea were made. The carbon isotopic composition of POC
in the chemocline was studied in 1988 during the cruise of R/V “Knorr” [36].
Despite the fact that samples from the chemocline were collected in 3 m
intervals, changes in δ
13 C of POC at the upper boundary of the sulfide zone
and below were not revealed (Fig 6a). Other scientists who participated in this
cruise analyzed both, the distribution of microorganisms and their activity, at
the same depths where samples for carbon isotopic composition were collected.
Neither an increase in the quantity nor an increase in the CO 2 dark fixation rates
 
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