355
Micronutrients in the Oceans
the smallest quantity relative to the requirement for growth of an organism will be the
limiting factor), it is clear that the P:N is limiting compared to carbon (the actual P:C ratio
is ~1000). Typical values of N:P in the North Atlantic and Pacific Oceans are shown in
Figure 8.18. The ratios of the waters in the Atlantic vary from 16:1 at the surface to 15:1
in deep waters. The deep waters in the Pacific are also close to 15:1, but the waters in the
O 2 minimum layer are as low as 14:1. This is due to the conversion of NO 3
– to N 2 O in these
low- oxygen waters as discussed previously.
The higher values of the N and P in the North Atlantic may be due to nitrogen fixation.
The values of N and P from the WOCE studies on some cruises in the Atlantic (A16), Pacific
(P16), and Indian Oceans (I8) are shown in Figure 8.19. The least- square values of the ratios
45
10
0
5
10
15
20
TDN: TDP
25
30
35
40
45
15
20
25
30
Latitude (deg N)
35
40
0
1
2
3
4
N (µM)
5
6
7
8
Subtropical
Gyre
Subtropical
Transition
Subarctic
Frontal
Equatorial
Transition
0.0
0.1
0.2
0.3
0.4
P (µM)
0.5
TOP
PO 4
0.6
0.7
0.8
TON
DIN
Figure 8.17
The distribution of TOP, TON, and TOC in North Pacific waters. TDP = total dissolved phosphorus. (Adapted
from Abell et al., J. Mar. Res. 58, 203–222, 2000.)