revised CAFE NPP rates can be used in conjunction with recent laboratory findings that show remarkable consistency between rates of gross primary production
(GPP) and its use by phytoplankton for cellular growth and maintenance. Halsey
et al. (2010) showed that constant fractions of Chl-normalized GPP were allocated
to light-dependent respiration (15 %), nitrogen and sulfur reduction (10 %), synthesis of short-lived carbon products not reflected in NPP measurements (45 %),
and NPP (30 %). These relationships were valid across the entire range of growth
rates experienced by the phytoplankton (from 0.1 to 1.2 d
-1 ). Thus, oceanic
estimates of GPP may be in the range 150–170 Pg C year
-1 , of which *70 Pg is
fixed carbon, but which is not measured or estimated as NPP. These realizations, if
true at the global scale, require careful reconsideration of energy and matter flow
through marine ecosystems.
8.6.6 Beyond NPP
While NPP is an essential attribute of all surface ocean ecosystems, fully understanding ocean ecological interactions, biogeochemistry, and change necessitates
assessments of many additional properties. Gross primary production, autotrophic
and community respiration, net community production, export production and
other intermediate rate measurements each convey different information about an
ecosystem. Currently, it is unclear what governs the relationships between these
rates or whether universal relationships exist between properties. Halsey et al.
(2010) recently reported remarkable stability in the ratio of Chl-specific gross and
net primary production rates. Similar results are not generally observed in the field,
although the contribution of field methodological issues to this observed variability
is not well constrained. The comparable global values of NPP reported for ocean
and terrestrial systems (e.g., Field et al. 1998; Behrenfeld et al. 2001; Friend et al.
2009) certainly do not exist at the level of GPP, as terrestrial plants have a much
lower ratio of photosynthetic to respiratory tissue. However, the extent of this
difference will not be clear until ocean GPP assessments can be made. Currently, a
wide range of ratios between gross and net primary production have been reported
in the literature (Luz and Barkan 2009; Quay et al. 2010; Marra 2009).
Additional work is also needed in understanding ecosystem balances between
phytoplankton growth and loss rates. In this case, satellite data may be extremely
useful. Currently approaches allow for the calculation of phytoplankton NPP and
assessment of C phyto . As indicated by equation (8.1), the ratio of NPP: C phyto yields
an estimate of l. Continuous time series of C phyto also allow direct assessment of
net population growth rates (r) through calculation of the rate of change in C phyto
between any two observational time points. As r = l – l, it is now possible to
investigate regional relationships between phytoplankton growth and loss
dynamics and relate these interactions to environmental forcings. An example of
this type of analysis is provided by Behrenfeld (2010), where controls on North
Atlantic vernal phytoplankton blooms were investigated and related to mixed layer
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T. K. Westberry and M. J. Behrenfeld
(GPP) and its use by phytoplankton for cellular growth and maintenance. Halsey
et al. (2010) showed that constant fractions of Chl-normalized GPP were allocated
to light-dependent respiration (15 %), nitrogen and sulfur reduction (10 %), synthesis of short-lived carbon products not reflected in NPP measurements (45 %),
and NPP (30 %). These relationships were valid across the entire range of growth
rates experienced by the phytoplankton (from 0.1 to 1.2 d
-1 ). Thus, oceanic
estimates of GPP may be in the range 150–170 Pg C year
-1 , of which *70 Pg is
fixed carbon, but which is not measured or estimated as NPP. These realizations, if
true at the global scale, require careful reconsideration of energy and matter flow
through marine ecosystems.
8.6.6 Beyond NPP
While NPP is an essential attribute of all surface ocean ecosystems, fully understanding ocean ecological interactions, biogeochemistry, and change necessitates
assessments of many additional properties. Gross primary production, autotrophic
and community respiration, net community production, export production and
other intermediate rate measurements each convey different information about an
ecosystem. Currently, it is unclear what governs the relationships between these
rates or whether universal relationships exist between properties. Halsey et al.
(2010) recently reported remarkable stability in the ratio of Chl-specific gross and
net primary production rates. Similar results are not generally observed in the field,
although the contribution of field methodological issues to this observed variability
is not well constrained. The comparable global values of NPP reported for ocean
and terrestrial systems (e.g., Field et al. 1998; Behrenfeld et al. 2001; Friend et al.
2009) certainly do not exist at the level of GPP, as terrestrial plants have a much
lower ratio of photosynthetic to respiratory tissue. However, the extent of this
difference will not be clear until ocean GPP assessments can be made. Currently, a
wide range of ratios between gross and net primary production have been reported
in the literature (Luz and Barkan 2009; Quay et al. 2010; Marra 2009).
Additional work is also needed in understanding ecosystem balances between
phytoplankton growth and loss rates. In this case, satellite data may be extremely
useful. Currently approaches allow for the calculation of phytoplankton NPP and
assessment of C phyto . As indicated by equation (8.1), the ratio of NPP: C phyto yields
an estimate of l. Continuous time series of C phyto also allow direct assessment of
net population growth rates (r) through calculation of the rate of change in C phyto
between any two observational time points. As r = l – l, it is now possible to
investigate regional relationships between phytoplankton growth and loss
dynamics and relate these interactions to environmental forcings. An example of
this type of analysis is provided by Behrenfeld (2010), where controls on North
Atlantic vernal phytoplankton blooms were investigated and related to mixed layer
224
T. K. Westberry and M. J. Behrenfeld