photoacclimation effect is determined from PAR, K d , and MLD. The residual
Chl:C variability is due to nutrient effects and is linearly proportional to NPP
variability.
An important aspect of characterizing photoacclimation in the mixed layer (i.e.,
the portion of the water column sampled from space) is identifying the light level
to which a natural phytoplankton community is acclimated. The uppermost
reaches of the surface ocean are a turbulent, well mixed environment where a
given phytoplankton population may circulate through a typical mixed layer of
50 m thickness [10 times per day (D’Asaro 2003). Laboratory studies indicate
that the light-dependent signal for chlorophyll synthesis is keyed to the redox state
of the plastoquinone (PQ) pool between photosystem II and photosystem I (Escoubas et al. 1995). An oxidized PQ pool signals for chlorophyll synthesis, while a
reduced pool indicates that adequate chlorophyll (i.e., light harvesting capacity)
exists. PQ pool reduction occurs at all saturating light levels, thus regulation of
pigment synthesis is essentially an on–off switch (i.e., once light exceeds saturation, the signal for chlorophyll synthesis is off and further increases have no
additional impact). Acclimation in such a system is thus best characterized as a
function of the median light level within the mixed layer, rather than the average
light level with is impacted by light levels in excess of saturation. The remaining
issue is spatially- and temporally characterizing global MLD. Unfortunately, MLD
is not a property directly retrieved by remote sensing. Consequently, we currently
must rely on model or model-data assimilation schemes to generate the necessary
MLD fields (e.g., Clancy and Sadler 1992). Notably, significant uncertainty
remains in these MLD products, particularly at high latitudes (e.g., Southern
Ocean), reducing this uncertainty will make a significant contribution toward
advancing NPP assessments.
An additional issue regarding characterization of phytoplankton photoacclimation is distinction between the ‘physiological’ mixed layer depth from ‘physical’ mixed layer depth calculated from water column density or temperature
properties. For much of the year and many parts of the ocean, there may be little
difference between the ‘physiological’ and ‘physical’ mixed layers, but under
certain, important conditions significant differences may exist. For example, the
onset of winter-spring stratification in temperate and high-latitude seas can be
rapid and non-monotonic. During this period, photoacclimation of the phytoplankton community will be responding to changes in the mixed layer light
environment on timescales of order *1–2 weeks. Photoacclimation timescales are
also dependent on the direction the light changes, taking longer when I g is
decreasing than when it is increasing. At timescales significantly \1 week, it is
likely that passage of brief mixing events (associated with meteorological fronts)
will not be registered by the phytoplankton, whereas the physical mixed layer
depth may be significantly perturbed for a day or so, then return to its previous
position. A practical approach to estimate a physiological MLD may exist through
the use of dissolved O 2 profiles. Castro-Morales and Kaiser (2012) demonstrated
the utility of this approach over a limited geographic region and found significant
differences from traditional hydrographically determined mixed layers.
8 Oceanic Net Primary Production
217
Chl:C variability is due to nutrient effects and is linearly proportional to NPP
variability.
An important aspect of characterizing photoacclimation in the mixed layer (i.e.,
the portion of the water column sampled from space) is identifying the light level
to which a natural phytoplankton community is acclimated. The uppermost
reaches of the surface ocean are a turbulent, well mixed environment where a
given phytoplankton population may circulate through a typical mixed layer of
50 m thickness [10 times per day (D’Asaro 2003). Laboratory studies indicate
that the light-dependent signal for chlorophyll synthesis is keyed to the redox state
of the plastoquinone (PQ) pool between photosystem II and photosystem I (Escoubas et al. 1995). An oxidized PQ pool signals for chlorophyll synthesis, while a
reduced pool indicates that adequate chlorophyll (i.e., light harvesting capacity)
exists. PQ pool reduction occurs at all saturating light levels, thus regulation of
pigment synthesis is essentially an on–off switch (i.e., once light exceeds saturation, the signal for chlorophyll synthesis is off and further increases have no
additional impact). Acclimation in such a system is thus best characterized as a
function of the median light level within the mixed layer, rather than the average
light level with is impacted by light levels in excess of saturation. The remaining
issue is spatially- and temporally characterizing global MLD. Unfortunately, MLD
is not a property directly retrieved by remote sensing. Consequently, we currently
must rely on model or model-data assimilation schemes to generate the necessary
MLD fields (e.g., Clancy and Sadler 1992). Notably, significant uncertainty
remains in these MLD products, particularly at high latitudes (e.g., Southern
Ocean), reducing this uncertainty will make a significant contribution toward
advancing NPP assessments.
An additional issue regarding characterization of phytoplankton photoacclimation is distinction between the ‘physiological’ mixed layer depth from ‘physical’ mixed layer depth calculated from water column density or temperature
properties. For much of the year and many parts of the ocean, there may be little
difference between the ‘physiological’ and ‘physical’ mixed layers, but under
certain, important conditions significant differences may exist. For example, the
onset of winter-spring stratification in temperate and high-latitude seas can be
rapid and non-monotonic. During this period, photoacclimation of the phytoplankton community will be responding to changes in the mixed layer light
environment on timescales of order *1–2 weeks. Photoacclimation timescales are
also dependent on the direction the light changes, taking longer when I g is
decreasing than when it is increasing. At timescales significantly \1 week, it is
likely that passage of brief mixing events (associated with meteorological fronts)
will not be registered by the phytoplankton, whereas the physical mixed layer
depth may be significantly perturbed for a day or so, then return to its previous
position. A practical approach to estimate a physiological MLD may exist through
the use of dissolved O 2 profiles. Castro-Morales and Kaiser (2012) demonstrated
the utility of this approach over a limited geographic region and found significant
differences from traditional hydrographically determined mixed layers.
8 Oceanic Net Primary Production
217