8.6.2 Nutrient Effects
Short-term perturbations in macronutrient availability can result in a brief period of
unbalanced growth where phytoplankton assimilation efficiencies are reduced.
However, under the steady state conditions found across most of the ocean, phytoplankton are well acclimated to their nutrient environment. Under such conditions,
cellular chlorophyll levels are adjusted in direct proportion to nutrient availability
and apparent assimilation efficiencies can be as high as under nutrient replete growth.
Thus, the long-held assumption that nutrient stress is associated with inefficient
photosynthesis is largely incorrect. This conclusion is supported by laboratory work
demonstrating highly tuned photosynthetic light harvesting capacities optimized to
macronutrient-defined growth rates (Laws and Bannister 1980).
While the aforementioned considerations suggest that assessing nutrient status
may not be as critical once thought for assessing global ocean NPP, this conclusion
may not be valid for conditions of iron stress. Iron concentrations are vanishingly
low over much of the open ocean, and phytoplankton have evolved a variety of
strategies for optimizing iron economy (Behrenfeld and Milligan 2013). These
adjustments, however, can have a significant impact on apparent assimilation
efficiencies, particularly under conditions where macronutrients are replete. Greater
than one-third of the ocean surface area has conditions of low iron and high macronutrients (the so-called High-Nutrient, Low-Chlorophyll (HNLC) regions).
Ironically, recent studies have shown that phytoplankton under HNLC conditions
actually over-express chlorophyll synthesis relative to growth (Behrenfeld and
Milligan 2013; Schrader et al. 2011; Behrenfeld et al. 2006b). This excess Chl does
not contribute to photosynthesis (it is functionally ‘‘decoupled’’ from the photosystems), but is registered in satellite Chl retrievals. The pool of ‘dysfunctional’ Chl
may account for[40 % of the total Chl in HNLC waters (Behrenfeld et al. 2006b;
Schrader et al. 2011) and must be accounted for when assessing assimilation efficiencies. Behrenfeld et al. (2006b) estimated the magnitude of error in satellite NPP
introduced by this bias in Chl over the Equatorial Pacific Ocean. The authors
exploited field measurements of fluorescence signatures linked to functional and
dysfunctional Chl and concluded that annual NPP for the region (using the VGPM
and CbPM) may need to be revised downward by *15 %.
Recent advances in our understanding of solar-stimulated chlorophyll fluorescence measured from satellite (i.e., MODIS, MERIS) may provide an avenue for
correcting satellite Chl and NPP fields for Fe-stress effects globally. Similar to the
field diagnostics of Fe-stress employed by Behrenfeld et al. (2006b), satellite Chl
fluorescence registers the imprint of Fe-stress (Behrenfeld et al. 2009; Westberry
et al. 2013). Due to the pool of dysfunctional Chl described above and to shifts in
photosystem stoichiometry occurring under iron stress (Behrenfeld and Milligan
2013), higher intrinsic fluorescence yields are observed in iron-stressed ocean region
under the high light conditions of satellite fluorescence measurements. As a demonstration of this link between Fe-stress and satellite Chl fluorescence, Westberry
et al. (2013) showed that purposeful addition of Fe to natural phytoplankton
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T. K. Westberry and M. J. Behrenfeld
Short-term perturbations in macronutrient availability can result in a brief period of
unbalanced growth where phytoplankton assimilation efficiencies are reduced.
However, under the steady state conditions found across most of the ocean, phytoplankton are well acclimated to their nutrient environment. Under such conditions,
cellular chlorophyll levels are adjusted in direct proportion to nutrient availability
and apparent assimilation efficiencies can be as high as under nutrient replete growth.
Thus, the long-held assumption that nutrient stress is associated with inefficient
photosynthesis is largely incorrect. This conclusion is supported by laboratory work
demonstrating highly tuned photosynthetic light harvesting capacities optimized to
macronutrient-defined growth rates (Laws and Bannister 1980).
While the aforementioned considerations suggest that assessing nutrient status
may not be as critical once thought for assessing global ocean NPP, this conclusion
may not be valid for conditions of iron stress. Iron concentrations are vanishingly
low over much of the open ocean, and phytoplankton have evolved a variety of
strategies for optimizing iron economy (Behrenfeld and Milligan 2013). These
adjustments, however, can have a significant impact on apparent assimilation
efficiencies, particularly under conditions where macronutrients are replete. Greater
than one-third of the ocean surface area has conditions of low iron and high macronutrients (the so-called High-Nutrient, Low-Chlorophyll (HNLC) regions).
Ironically, recent studies have shown that phytoplankton under HNLC conditions
actually over-express chlorophyll synthesis relative to growth (Behrenfeld and
Milligan 2013; Schrader et al. 2011; Behrenfeld et al. 2006b). This excess Chl does
not contribute to photosynthesis (it is functionally ‘‘decoupled’’ from the photosystems), but is registered in satellite Chl retrievals. The pool of ‘dysfunctional’ Chl
may account for[40 % of the total Chl in HNLC waters (Behrenfeld et al. 2006b;
Schrader et al. 2011) and must be accounted for when assessing assimilation efficiencies. Behrenfeld et al. (2006b) estimated the magnitude of error in satellite NPP
introduced by this bias in Chl over the Equatorial Pacific Ocean. The authors
exploited field measurements of fluorescence signatures linked to functional and
dysfunctional Chl and concluded that annual NPP for the region (using the VGPM
and CbPM) may need to be revised downward by *15 %.
Recent advances in our understanding of solar-stimulated chlorophyll fluorescence measured from satellite (i.e., MODIS, MERIS) may provide an avenue for
correcting satellite Chl and NPP fields for Fe-stress effects globally. Similar to the
field diagnostics of Fe-stress employed by Behrenfeld et al. (2006b), satellite Chl
fluorescence registers the imprint of Fe-stress (Behrenfeld et al. 2009; Westberry
et al. 2013). Due to the pool of dysfunctional Chl described above and to shifts in
photosystem stoichiometry occurring under iron stress (Behrenfeld and Milligan
2013), higher intrinsic fluorescence yields are observed in iron-stressed ocean region
under the high light conditions of satellite fluorescence measurements. As a demonstration of this link between Fe-stress and satellite Chl fluorescence, Westberry
et al. (2013) showed that purposeful addition of Fe to natural phytoplankton
218
T. K. Westberry and M. J. Behrenfeld