Environmental Engineering Reference
In-Depth Information
However, it is unclear that the surface waters are sufficiently nitrogen de-
pleted to result in local nitrogen fixation. At times when nitrate and phos-
phate were detectable in the surface 50 m, N
∗
, defined as [NO
3
−
]+[NO
2
−
]
- 16[PO
4
3
−
] + 2.9 and representing the deviation of the inorganic nitrogen/-
phosphate ratio from the Redfield ratio, was always greater than 1.0 µMand
was frequently above 2.0 µM, suggesting the possible importance of nitrogen
fixation [16]. However, the waters at the base of the euphotic zone are rich
in both inorganic nitrogen and phosphorus ( [35]; Fig. 2). The ratio of total
inorganic nitrogen (nitrate + nitrite + ammonium) to phosphate for our study
period is shown in Fig. 6. At essentially all times, at the base of the mixed
layer, this ratio is at the theoretical Redfield ratio of 15 or 16:1. This water
is likely to be the major source of supply of nutrients to the surface waters,
which would mean that the phytoplankton populations receive N and P in the
ratio at which they use these elements. While the nitrogen isotope and N
∗
data
[35] suggest that nitrogen fixation may be at least regionally important, the
high levels of nitrate typically found at 35 to 50 m, together with the N:P ratio
of 15:1 in this water suggests that nitrogen fixation within the waters of the
Cariaco Basin itself is unlikely to be an important source of fixed nitrogen.
Consistent with this,
Trichodesmium
, a nitrogen fixer commonly found in the
open Caribbean, is relatively rare in the Cariaco surface waters (Varela, personal
communication).
Figure 6.
Temporal variability of Total Inorganic N:P ratio at the CARIACO site. The heavy
line represents an N:P ratio of 16:1.
Surprisingly, N:P ratios in both the sinking flux and in plankton tows col-
lected in a 200 µm mesh net between the surface and 200 m are much higher
than Redfield ratios would predict. Benitez-Nelson [5] report POC:PON ratios
in plankton tows between 2001 and 2002 of 5.6
±
0.5 and N:P ratios of 53
±
9 indicating a P deficit rather than an N deficit in the particulate organic
matter. Sinking particulate matter collected in sediment traps is also depleted
in phosphorus relative to both C and N. We do not have N:P data for phyto-
