Environmental Engineering Reference
In-Depth Information
Silica.
Silica concentrations in surface waters (1-7 m) were generally low
but measurable (Fig. 2e). Fig. 5 shows a plot of silica values at 1 and7mfor
1998-2004. In 1999, persistently high surface values were likely attributable
to enhanced terrestrial supply from land during the period of unusually high
precipitation and runoff mentioned above in the discussion of ammonium.
Other, shorter periods of high surface silica values may be due to other periods
of runoff, or with upwelling and deep mixing. However, silica concentrations
seldom were below 1 µM for extended periods, except in 1999 and 2001.
In contrast, phosphate and nitrate concentrations are often at or below the
detection limit in surface water. Previously, Ferraz-Reyes [14] reported that
diatoms are most abundant during the upwelling season (November to April),
and that cyanobacteria and dinoflagellates are more important during periods
of low winds. The CARIACO silica data suggest that silica is almost never
limiting at the CARIACO site.
N:P Ratios.
Fig. 6 shows contour plots of the ratio of total inorganic N
(NO
2
−
+NO
3
−
+NH
4
+
)toPO
4
. In the surface layers, this ratio is very low, but
is strongly influenced by the fact that both inorganic nitrogen and phosphate
concentrations are frequently at or near blank levels. At depths immediately
below the euphotic zone, the N:P ratio seldom falls below 14:1. At depths
below about 100 m the ratio decreases again, due to nitrate and nitrite removal
by denitrification.
5. DISCUSSION
5.1 Oxic Waters
Some of the nutrient time series data have already been published. Astor [2]
described the temporal variability of nitrate, nitrite, ammonium and phosphate
between November 1995 and October 2001. Thunell [35] reported on the
temporal variability of nitrogen isotopes as well as of nitrate and phosphate
in the surface layers of the Cariaco Basin for 1998 and 1999, finding that
the sinking flux of nitrogen in the Cariaco has an isotopic signature which was
controlled by the isotopic composition of nitrate in the thermocline, indicative of
complete consumption of nitrate in the surface. The longer time series reported
in the present study confirms the patterns seen over the shorter periods.
Water column data are consistent with this picture. In Fig. 5 we present data
for the top two depths sampled (1 and 7 m) during the time series. This graph
shows clearly the times at which nutrients are elevated in the surface waters,
usually during the first few months of the year. The vertical axes in the plots are
in the ratio 16 SiO
2
:16NO
3
:1PO
4
or roughly in the ratio that organisms take
up these compounds when nutrient concentrations are not limiting [7, 26]. From
