122
Nutrient limitation is often done directly by experimentally determining the response
of communities to additions of different combinations of nutrients (Mills et  al.
2004; Moisander et al. 2011; Halm et al. 2012). The expression of key transport
enzymes have also been used to diagnose P or Fe limitation (Rouco et  al. 2016;
Webb et al. 2007, see Chap. 6).
Advances in methods, including molecular methods, to measure and model N, P
and Fe limitation have helped to predict some general geographic patterns of nutrient limitation of N 2 fixation in the oceans (Moore et al. 2013; Sohm et al. 2011a;
Tang et al. 2019; Wang et al. 2019) (Fig. 7.4).
50°N
25°N
25°S
60°E
120°E
180°W
120°W
60°W
50°S
0°
0°
b
a
125
25
5
1
2
4
8
60° N
30° N
30° S
60° S
60° E 120° E 180°120° W 60° W 0°
0°
Replete
Temperature
Iron
Phosphorus
Fe-limited
γ Fe / γ P
P-limited
0°
Fig. 7.4 Nutrient limitation regimes of surface diazotrophs in the oceans. (a) From Weber and
Deutsch (2014), with permission; (b) From Wang et al. (2019), with permission
7 Biogeography of N 2 Fixation in the Surface Ocean