Environmental Engineering Reference
In-Depth Information
this date. Phosphate concentrations increase relatively steadily below the eu-
photic zone although there are some features near the interface in the suboxic
zone. Silicate concentrations increased monotonically through the interface.
4.2 Temporal Variability
Upwelling.
One of the strengths of a study which emphasizes reoccupation
of a single site is that temporal variability in parameters can be examined. In
Fig. 2, we present contours of the concentrations of nitrate, nitrite, ammonium,
phosphate and silicate in the upper 400 m of the Cariaco water column between
1998 and early 2004. We have chosen this depth range because our sampling
resolution is relatively high and because, based on other studies [3], we believe
that the primary variability due to climatological factors occurs in this portion
of the water column. The time series data (Fig. 2) show an annual pattern
of shoaling nutrient isopleths in the early part of the year (January through
May) and a deepening in summer and fall. Fig. 3 presents a plot representing
the variation over time of the integrated primary production, the depth of
mixed layer inside the Cariaco Basin, the depth at which nitrate begins to
increase (nitracline), and the estimated speed of upwelling. In this figure we
have approximated the depth of the nitricline as the depth at which the increase
in nitrate concentrations between adjacent depths is greater than 0.1 µM and
the depth of the mixed layer by the depth at which sigma-t difference between
the surface and a particular depth exceeds 0.125 kg m
−
3
. Rates of upwelling
are calculated by computing the change of depth of the 25.6 isopycnal between
two cruises. Since the cruises are approximately one month apart, the rates of
upwelling are not exact and may not reflect short term upwelling events
The variation of the integrated primary production with time (Fig. 3, bottom
panel) is important to the entire Cariaco ecosystem, and the analysis of the
effects of other parameters on that variation is therefore of particular interest.
Out of 69 cruises between 1998 and mid-2004, 18 had primary production above
2gCm
−
2
d
−
1
and 22 had productivities above 1.5 g C m
−
2
d
−
1
, representing
high values compared to the average production value of 1.4gCm
−
2
d
−
1
.For
15 of these cruises, the nitricline was located within the mixed layer (above the
solid line in Figure 3, middle panel). For several other dates (December 2000
and 2002 and July 2000), production increased above 1.5 g C m
−
2
d
−
1
even
though the nitricline was deeper than the base of the mixed layer. On these dates,
however, upwelling was strong enough to have raised the nutricline (by 20 m
between November and December of 2000, by 30 m between November and
December 2002, and by 20 m between June and July of 2000). In these cases,
although the nitricline did not reach the mixed layer, upward displacement of
water probably injected nutrients into the surface. Although we may not have
