Environmental Engineering Reference
In-Depth Information
plankton samples, and so far have no explanation of why the sinking material
is so depleted in P in a situation where denitrification is active and where the
upwelled waters appear to contain nutrients in relatively balanced ratios.
5.2 Suboxic/Transition Zone
Phosphate distributions in the Cariaco Basin differ from the “classic” patterns
typically reported for the Black Sea and often assumed to hold for other anoxic
basins. The contour plot shows little sign of the pattern seen in the Black Sea
of a strong minimum immediately above the onset of sulfide and a phosphate
maximum in the upper portions of the anoxic zone. However, in the one cruise
for which we have detailed samples across the interface (Fig. 1), we did see a
minimum in phosphate just above the depth sulfide was first detected. In the
Black Sea such features have been attributed to phosphate scavenging above
the interface onto iron oxides and phosphate release as the iron oxides dissolve
after settling into the anoxic zone [6]. The Cariaco minimum, although present,
is much less pronounced than in the Black Sea where concentrations have been
reported to be as low as 0.1 µM. The absence of clear minima in the contour
plot (Fig. 2) suggests any such feature in the Cariaco Basin must be fairly thin.
Even though phosphate scavenging in the suboxic zone of the Cariaco Basin
seems to be much less intense in the Cariaco than in the Black Sea, it may occur.
On a number of occasions, small phosphate minima are seen in the vicinity of
the interface and these could be due to scavenging onto a metal oxide phase,
perhaps formed during intrusions. In addition, biologically mediated phosphate
removal may occur at depths where chemoautotrophy is active. Coarse sample
spacing above the interface for most cruises suggests we may have missed
the most intense minimum as well. Comparison of the oxygen and phosphate
time series plots shows that phosphate minima tend to occur at times when
deep oxygen maxima are present. The Cariaco Basin water column has similar
or higher levels of dissolved iron compared with the Black Sea (250-500 nM
in the Cariaco [18], compared to 300 nM in the Black Sea [23]), but oxygen
injection occurs over a much thicker layer in the Cariaco due to a much more
gradual density gradient. Recently, Konovalov has suggested that Mn-oxides
(
>
8 µM in the Black Sea; [23]; maximum of 600 nM in the Cariaco; [18])
may be more important in scavenging phosphate in the Black Sea (Konovalov
personal communication).
Another major question about nutrients in the Cariaco is whether nitrite and
ammonium co-occur. A number of recent studies ( [9], [11] among others) have
suggested that the anammox reaction (NO
2
−
+NH
4
+
-
>
2N
2
+2H
2
O) may be
very important in suboxic regions in systems ranging from the Arabian Sea to
the Black Sea. However, the possibility of anammox in the Cariaco has not yet
been examined. In Fig. 1 we present the nutrient distributions for two cruises
