Environmental Engineering Reference
In-Depth Information
4.4 Silicate
A representative profile for Si is shown in Fig. 9 (R/V Knorr 172-05 Stn 7, E.
Yakushev, unpublished data). Silica concentrations are low in the surface (
5
µM) and increase smoothly from 50 µM to 100 µM across the suboxic zone.
The highest values in the deep water reach 400 µM (not shown but data are on
Knorr 2003 web site).
Tugrul et al. [72] and Humborg et al. [20] demonstrated that the silicate
concentrations in the wintertime surface waters of the Black Sea decreased
by 60% from 1969 (R.V. ATLANTIS cruise) to 1988 (R.V. KNORR cruise).
They suggested that these changes were due to a two-thirds reduction in the
input of silicate from the Danube due to dam construction in the early 1970s.
The observed decrease in silicate inventory in the oxic layer may have been
responsible for dramatic shifts in phytoplankton species composition from
diatoms to coccolithophores and flagellates [20]. Temporal changes in the
distribution of silicate in the anoxic zone have not been discussed much because
only there are only a few time points available.
Comparison of the Atlantis II 1969 and Knorr 1988 data sets suggest that the
concentration of silicate in the deep water increased by a factor of two between
those two cruises [25]. This increase in the inventory of silicate appears to be
too high to reflect real changes in the inventory of dissolved silicate. However,
no analytical or methodological problems appear to exist. Data since 1991 has
been obtained by the same analytical procedures which were inter-calibrated
during the CoMSBlack and NATO “TU-Black Sea” International Programs
[21]. The opposing temporal trends in the silica concentrations of the oxic and
anoxic layers may represent real variations in the downward flux of biogenic
silica.
4.5 Carbonate System Parameters
The carbonate system properties undergo large changes with depth reflect-
ing the oxidation reduction reactions that influence the magnitude of total CO 2
(DIC) and alkalinity [16]. An example of data obtained during the 2001 R/V
Knorr cruise is shown in Fig. 10 versus density (Hiscock and Millero, unpub-
lished data). DIC is slightly less than 3000µmol kg 1 at the surface. It increases
with depth to the top of the suboxic zone where it is relatively constant. It then
starts to increase again after the appearance of sulfide. Total Alkalinity is fairly
uniform at 3550 µmol kg 1 until the sulfide zone where it starts to increase. pH
(which reflects the relative magnitudes of DIC and alkalinity) starts at 8.2 at the
surface and then decreases to about 7.45 in the suboxic zone. It then increases
slightly in the sulfide zone.
Total CO 2 and alkalinity are key parameters because they reflect the net effect
of all the oxidation-reduction reactions on the carbon and proton balances. In
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