78
been developed (Youngblut et al. 2018a) as well as tools to model data and aid in
experimental design and interpretation (Youngblut et al. 2018b).
5.8 Biogeochemical Proxies
Geochemical proxies have been used to infer integrative rates of N 2 fixation over
relatively large time and space scales. This complements those estimates derived
from extrapolating limited field measurements using small volumes of seawater
assayed over short time periods.
A correspondence between the elemental composition of plankton of the surface
ocean with the ratios of key nutrients, and in particular nitrate and phosphate (N:P),
in the deep ocean resulting from the remineralization of surface derived organic
matter was first noted by Alfred Redfield (Redfield 1934) and is the average ratio of
C:N:P in plankton of 106:16:1 that is generally referred to as the Redfield Ratio. It
was long assumed that these ratios were relatively invariant in the oceans (Michaels
et al. 2001). However, extensive surveys of ocean nutrients over the last several
decades showed that in fact these ratios ranged substantially in value globally
(Gruber 2008; see also Martiny et al. 2013) (Fig. 8.3a). One consequence of N 2 fixation is an enrichment of N relative to P in organic matter and high sub-surface N:P
ratios are associated with areas of substantial N 2 fixation. Similarly, denitrification
and anammox can decrease the ratio of N to P in major nutrient pools. Thus, regional
imbalances in N 2 fixation and denitrification drive these deviations from the
Redfield Ratio.
N* is a linear expression of the regeneration rate of N and P from organic matter
and in its simplest form is:
N
NO
P O
∗ =
−
−
−
3
4
3
16
(5.6)
N* is usually determined and integrated along isopycnal surfaces (surfaces of
equal density) in sub-surface ocean waters (Gruber and Sarmiento 1997). Where N*
is zero, regeneration is occurring in Redfield proportions. Where N* is less than
zero, it is indicative of NO 3
−
removal relative to PO 4
3−
while N* values greater than
zero indicate relative enrichment of NO 3
−
. Robust basin and global scale estimates
of N 2 fixation and denitrification have been derived based on geochemical gradients
of these indices.
Similarly, a related geochemical parameter is P* and the reciprocal of N* where
P
PO
N O
∗ =
−
−
16
16
4
3
3
−
/
(5.7)
P* has been used to identify areas in surface waters that exhibit excesses of PO 4
3−
relative to NO 3
−
(Deutsch et al. 2007). Gradients of decreasing P* in the surface
5 Measurements of Organism Abundances and Activities
been developed (Youngblut et al. 2018a) as well as tools to model data and aid in
experimental design and interpretation (Youngblut et al. 2018b).
5.8 Biogeochemical Proxies
Geochemical proxies have been used to infer integrative rates of N 2 fixation over
relatively large time and space scales. This complements those estimates derived
from extrapolating limited field measurements using small volumes of seawater
assayed over short time periods.
A correspondence between the elemental composition of plankton of the surface
ocean with the ratios of key nutrients, and in particular nitrate and phosphate (N:P),
in the deep ocean resulting from the remineralization of surface derived organic
matter was first noted by Alfred Redfield (Redfield 1934) and is the average ratio of
C:N:P in plankton of 106:16:1 that is generally referred to as the Redfield Ratio. It
was long assumed that these ratios were relatively invariant in the oceans (Michaels
et al. 2001). However, extensive surveys of ocean nutrients over the last several
decades showed that in fact these ratios ranged substantially in value globally
(Gruber 2008; see also Martiny et al. 2013) (Fig. 8.3a). One consequence of N 2 fixation is an enrichment of N relative to P in organic matter and high sub-surface N:P
ratios are associated with areas of substantial N 2 fixation. Similarly, denitrification
and anammox can decrease the ratio of N to P in major nutrient pools. Thus, regional
imbalances in N 2 fixation and denitrification drive these deviations from the
Redfield Ratio.
N* is a linear expression of the regeneration rate of N and P from organic matter
and in its simplest form is:
N
NO
P O
∗ =
−
−
−
3
4
3
16
(5.6)
N* is usually determined and integrated along isopycnal surfaces (surfaces of
equal density) in sub-surface ocean waters (Gruber and Sarmiento 1997). Where N*
is zero, regeneration is occurring in Redfield proportions. Where N* is less than
zero, it is indicative of NO 3
−
removal relative to PO 4
3−
while N* values greater than
zero indicate relative enrichment of NO 3
−
. Robust basin and global scale estimates
of N 2 fixation and denitrification have been derived based on geochemical gradients
of these indices.
Similarly, a related geochemical parameter is P* and the reciprocal of N* where
P
PO
N O
∗ =
−
−
16
16
4
3
3
−
/
(5.7)
P* has been used to identify areas in surface waters that exhibit excesses of PO 4
3−
relative to NO 3
−
(Deutsch et al. 2007). Gradients of decreasing P* in the surface
5 Measurements of Organism Abundances and Activities