Environmental Engineering Reference
In-Depth Information
Figure 9.
Representative profiles for oxygen, sulfide (left) and for Si and PO
4
(right) from the
center of the western gyre during R/V Knorr 172-05, March 2003, Station 7, CTD 7.
the suboxic layer was insufficient to explain the phosphate minimum. Based on
sorption experiments conducted by [63] it may be difficult to explain the PO
4
minimum as due to sorption because the concentrations of the Fe/Mn oxides
are too low (on the order of 0.01-0.1 µM).
Another hypothesis suggested is that the decrease in phosphate in the suboxic
zone is due to its consumption by microbial chemosynthesis [63]. A maximum
of chemosynthesis occurs between the layers of the phosphate minimum and
maximum. The deeper maximum of phosphate could be explained by the rem-
ineralization of organic phosphorus. Yakushev et al. [74] observed an increase
of organic phosphorus (total phosphorus minus total phosphate) in this layer that
supported this explanation. In the deeper sulfidic layers sorption of phosphate
can occur on biogenic CaCO
3
particles [63].
Finally, there is spatial variability in the distribution of phosphate. New
data from the northeastern Black Sea (Yakushev, unpublished data) at the
coastal boundaries of the Rim current and at the marine sides of anticyclonic
eddies showed that the structure of the phosphate profile is different when
the hydrophysical dynamics are more intensive [11]. In these coastal regions
(where there are stronger currents and eddies) the upper phosphate minimum
was more pronounced in summer period but was not present during the winter.
The deep maximum was always present.
Thus, there are multiple mechanisms for the origin of the PO
4
minima and
maxima. The details of the processes that form the phosphate anomalies are
still uncertain.
