Environmental Engineering Reference
In-Depth Information
Another factor which may be important in whether a bloom is maintained is
whether upwelling rates exceed the sinking rate of phytoplankton, which would
enable cells to remain longer in the mixed layer, thus increasing production.
Estimated upwelling rates
0.5md
−
1
were observed for 23 cruises, of which 16
exhibited high productivity. We conclude that the presence of strong upwelling,
and the upward transport of nutrients (including nitrite and nitrate) through a
mixed layer, results in conditions suitable for a bloom, especially when the wind
increases the turbulence and deepens the mixed layer. Fig. 2 (a, b) suggests
that high nitrite levels, as well as high nitrate levels, also contributed to the
occurrences of higher primary production.
≥
Oxygen.
Fig. 4 shows the contoured oxygen probe data for the period 1998
to early 2004. Oxygen concentrations decrease immediately below the euphotic
zone (top 35 to 50 m) and reach values of less than 20µM at depths of 200 to 250
m. The steepest gradient in oxygen concentration is typically between 100 and
200 m, but there is considerable annual and interannual variability. Upwelling
causes shoaling of oxygen isopleths (compare Fig. 3 and 4). We have observed
multiple occasions where small oxygen maxima were seen at depths as great
as 400 m, representing intrusions of oxygenated water from outside the basin
[3] and these maxima have been confirmed by discrete samples in some cases.
Some years (1997; 1998; 2001, 2002, late 2003) experienced much more intense
ventilation of deep layers than other years (1999-2000).
Figure 4.
Contours of oxygen from the continuous probe on the CTD at the CARIACO time
series site over time.
Nitrate.
In surface waters, nitrate concentrations are consistently low in the
upper 35-55 m, although occasionally during upwelling events, surface waters
contained more than 1 µM nitrate (Fig. 2a). During upwelling season, nitrate
contours move toward the surface. Nitrate concentrations increase rapidly below
the surface, reaching a maximum of up to 12 µM at about 160 m. Below this
depth, nitrate decreases to zero at about same depth oxygen reaches values
