Thermocline denitrification

Interpretation of the N distribution in the Pacific and Indian oceans is more difficult because of the existence of strong denitrification sites adjacent to the places where one expects to find the N2 fixation signals. In the subtropical North Pacific, where N2 fixation is well documented (Karl et al., 1997), the N maximum exists just near the surface and does not extend into the thermocline as is the case in the subtropical North Atlantic. This is likely a result of the eroding effect by waters that obtain extremely low N values in the eastern Pacific and are then advected westwards in the main thermocline (Fig. 1.12). 

Figure 34-4 Water column profiles of nitrate concentration (open symbols) and c5 N itrate (filled symbols) in the Eastern Tropical North Pacific (ETNP, coastal Baja California), Southern Ocean and North Atlantic (Sargasso Sea). The ETNP shows a large increase in c5 N i ate in the thermocline owing to local water column denitrification. The Southern Ocean shows little deviation from the global deep mean c5 N it te except at the surface, where partial NO3 assimilation leaves residual nitrate enriched in N. The North Atlantic profile shows low c5 N itrate in the thermocline owing to the nitrification of locally fixed N. Note that the ETNP profile also includes deep measurements from near Hawaii (diamonds) the smooth transition between samples at distant locations emphasizes the homogeneity of the deep Pacific.

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