8.6 Future Research Directions
As detailed herein, significant progress has been made in quantifying NPP from
global satellite data and applying these estimates to ecological questions, yet
opportunities abound for making major improvements. Such improvements may
take a variety of forms, including greater accuracy of NPP retrievals when compared to field measurements, development of new field metrics for validation,
exploitation of advances in future satellite design capabilities, and more sophisticated physiological formulations in NPP models. Some of these improvements
are relatively straightforward and may entail advances in engineering (e.g., higher
spectral/spatial resolution on future ocean color sensors), while others will be more
challenging (treatment of phytoplankton physiology). In this final section, we
attempt a forward-looking view at potential avenues for refining global NPP
assessments, particularly with respect to advancing characterization of physiological attributes.
8.6.1 Photoacclimation
Global surface ocean chlorophyll concentrations vary by roughly 3 orders of
magnitude. Physiological changes in intracellular chlorophyll from varying light
and nutrient conditions can span over 1.5 orders of magnitude, with the light effect
alone (i.e., photoacclimation) contributing up to a factor of 10 variability (Falkowski and Laroche 1991). While variability in chlorophyll concentration due to
changes in biomass or nutrient availability is positively correlated with changes in
NPP, changes in chlorophyll due to photoacclimation are inversely correlated with
NPP. In other words, all else being constant, an increase in daily light exposure
results in a decrease in chlorophyll and an increase in assimilation efficiency.
Given the magnitude of the photoacclimation response, it is somewhat surprising
therefore that this physiological property is routinely ignored in all but a few
marine NPP models. Even an imperfect assessment of photoacclimation could
significantly improve NPP predictions and does not require a highly sophisticated
model to effectuate. For example, the simple wavelength- and depth-integrated
VGPM could be applied to satellite chlorophyll fields that are first corrected for
photoacclimation. For this approach, global data on incident PAR, diffuse attenuation (K d ), and mixed layer depths (MLD) are needed to calculate I g . Next, a
laboratory-based relationship between I g and cellular chlorophyll content can be
employed to normalize satellite chlorophyll data to a uniform photoacclimation
state and then these data applied in the VGPM with a constant value for P
b
opt . In
essence, this strategy is equivalent to the approach of the Carbon-based Production
Model (CbPM) of Westberry et al. (2008). The CbPM distinguishes Chl variability
into that due to biomass changes and intracellular pigmentation. The latter
property is then divided into light- and nutrient-dependent terms, where the
216
T. K. Westberry and M. J. Behrenfeld
As detailed herein, significant progress has been made in quantifying NPP from
global satellite data and applying these estimates to ecological questions, yet
opportunities abound for making major improvements. Such improvements may
take a variety of forms, including greater accuracy of NPP retrievals when compared to field measurements, development of new field metrics for validation,
exploitation of advances in future satellite design capabilities, and more sophisticated physiological formulations in NPP models. Some of these improvements
are relatively straightforward and may entail advances in engineering (e.g., higher
spectral/spatial resolution on future ocean color sensors), while others will be more
challenging (treatment of phytoplankton physiology). In this final section, we
attempt a forward-looking view at potential avenues for refining global NPP
assessments, particularly with respect to advancing characterization of physiological attributes.
8.6.1 Photoacclimation
Global surface ocean chlorophyll concentrations vary by roughly 3 orders of
magnitude. Physiological changes in intracellular chlorophyll from varying light
and nutrient conditions can span over 1.5 orders of magnitude, with the light effect
alone (i.e., photoacclimation) contributing up to a factor of 10 variability (Falkowski and Laroche 1991). While variability in chlorophyll concentration due to
changes in biomass or nutrient availability is positively correlated with changes in
NPP, changes in chlorophyll due to photoacclimation are inversely correlated with
NPP. In other words, all else being constant, an increase in daily light exposure
results in a decrease in chlorophyll and an increase in assimilation efficiency.
Given the magnitude of the photoacclimation response, it is somewhat surprising
therefore that this physiological property is routinely ignored in all but a few
marine NPP models. Even an imperfect assessment of photoacclimation could
significantly improve NPP predictions and does not require a highly sophisticated
model to effectuate. For example, the simple wavelength- and depth-integrated
VGPM could be applied to satellite chlorophyll fields that are first corrected for
photoacclimation. For this approach, global data on incident PAR, diffuse attenuation (K d ), and mixed layer depths (MLD) are needed to calculate I g . Next, a
laboratory-based relationship between I g and cellular chlorophyll content can be
employed to normalize satellite chlorophyll data to a uniform photoacclimation
state and then these data applied in the VGPM with a constant value for P
b
opt . In
essence, this strategy is equivalent to the approach of the Carbon-based Production
Model (CbPM) of Westberry et al. (2008). The CbPM distinguishes Chl variability
into that due to biomass changes and intracellular pigmentation. The latter
property is then divided into light- and nutrient-dependent terms, where the
216
T. K. Westberry and M. J. Behrenfeld