Environmental Engineering Reference
In-Depth Information
less than 5 µM. Measurable nitrate was seen as deep as 300 m in 1998, rising
to about 250 m in late 2000. In January 2001, a deep intrusion occurred (see
oxygen plot) and maximum depths of nitrate penetration increased, but have
subsequently decreased again with time.
Nitrite.
Nitrite concentrations in the Cariaco Basin are always very low,
although there is frequently evidence for a primary nitrite maximum at the
base of the euphotic zone (Fig. 2b). Sampling resolution (with depth) for the
time series is not adequate to consistently see a secondary nitrite maximum
at the base of the suboxic zone, although this feature has been observed on
several occasions, and is clear in the detailed profile taken during the Jan 2004
microbiology cruise (Fig. 1). Deep nitrite maxima in Fig. 2b tend to be present
on dates when deep penetration of oxygen is apparent in Fig. 4.
Ammonium.
In oxic portions of the water column, ammonium concentra-
tions are usually quite low (Fig. 2c). Most times when ammonium is detected
at the surface are times when nitrate data suggest that upwelling is bringing
subsurface water into the mixed layer and when productivity is high. High
concentrations of ammonium from September to December 1999 were asso-
ciated with a period of heavy rains. River discharge during this period was
elevated well above typical levels and these months show the lowest salinity
for the whole time series, with a salinity having been observed for December
of 36.080 in the top 25 m. During this period the heavy coastal runoff may have
influenced ammonium supply to the basin. However, in Oct/Nov 1999, high
ammonium levels were seen throughout the water column which also might re-
flect problems with sample contamination. The depth of the 0.5µM ammonium
isopleth roughly coincides with the base of the suboxic zone or the upper part
of the anoxic layer. Based on contour plots for ammonium and nitrite, there
frequently are depths where low but measurable amounts of ammonium and
nitrite coexist (see below).
Phosphate.
Phosphate concentrations are always low in the upper 35-55 m.
Surface waters depleted in phosphate typically deepen late in the fall, while
shoaling of high phosphate water occurs between November and April (Fig. 2d).
Concentrations tend to increase relatively monotonically with depth, although
the contour plot shows a series of small deep minima between 250 and 350
m, usually in winter. In contrast to patterns reported for the Black Sea where
phosphate concentrations decrease almost to zero [6, 10, 21], deep minima
usually are relatively modest. However, we have coarse sample spacing below
250 m, so we may have missed sampling the depths of maximum phosphate
removal.
