An important question arising from the aforementioned studies was what the
underlying basis is for the inverse relationship between chlorophyll and SST
anomalies. The magnitude of the SST anomalies is far too small to be directly
responsible for the observed chlorophyll responses. Instead, and as suggested by
Behrenfeld et al. (2006a), changes in SST are likely functioning as a proxy for
altered surface mixing depths, where shallower mixing is accompanied by
decreasing chlorophyll. Two mechanisms likely contribute to the link between
surface chlorophyll concentrations to mixing depth: an impact on vertical nutrient
transport from depth and changes in the average light level experienced by surface
phytoplankton. To distinguish which of these two factors dominate, Behrenfeld
et al. (2008) separated chlorophyll variability in permanently stratified ocean into
that due to biomass changes and that due to intracellular chlorophyll (Chl:carbon)
changes. Their study showed that, over most of the SeaWiFS record, chlorophyll
variability was largely due to physiological changes in Chl:carbon and that most of
this variability was attributable to changes in the upper ocean light environment,
not nutrients. These findings imply that significant variations in chlorophyll
detected in the satellite record are likely not linked to parallel changes in NPP.
However, most contemporary NPP models are not equipped to make this distinction (see Sect. 8.6.1). More recently, Siegel et al. (2013) extended the analysis
of physiological variability to all global ocean regions and additionally showed
that apparent anomalies in chlorophyll may be, in part, traceable instead to variations in colored dissolved organic material. Taken together, these studies once
again emphasize that careful attention must be given to physiological attributes if
global ocean NPP and its temporal variability are to be accurate evaluated.
As global, synoptic estimates of NPP have advanced, opportunities have arisen for
investigating relationships between NPP and more derived, yet critical, carbon cycle
parameters. For example, the fraction of NPP delivered from the surface ocean to
depth is a critical quantity of interest when addressing the ocean’s role in carbon
sequestration. This export production is traditionally measured in the field using
sediment traps that collect and preserve sinking material, but these measurements
have longstanding caveats (Buesseler et al. 2007 and references therein) and are
extremely sparse in both space and time. Satellite observations of NPP, when combined with ecosystem model results and field measurements have provided simple,
yet powerful empirical parameterizations that allow globally resolved fields of export
production (Laws et al. 2000; Dunne et al. 2005). Current estimates of export production are *10 Pg C year
-1 globally and its spatial distribution gives us insight into
ecosystem functioning. For example, Laws et al. (2000) showed that the Atlantic and
Pacific Oceans contribute equally to total export (*4.3 Pg year
-1 each), despite their
two-fold differences in size (*75 9 10
6 km
2 and *160 9 10
6 km
2 , respectively).
Recently, Westberry et al. (2012) applied satellite NPP estimates to field-derived
photosynthesis-respiration relationships to characterize global ocean respiration
rates and net community production. These net community production rates set an
upper constraint on export production for comparison with alternative approaches.
Validation of these advanced, satellite-based carbon cycle parameters has yet to be
carried out and should be an active research area in the future (see following section).
8 Oceanic Net Primary Production
215
underlying basis is for the inverse relationship between chlorophyll and SST
anomalies. The magnitude of the SST anomalies is far too small to be directly
responsible for the observed chlorophyll responses. Instead, and as suggested by
Behrenfeld et al. (2006a), changes in SST are likely functioning as a proxy for
altered surface mixing depths, where shallower mixing is accompanied by
decreasing chlorophyll. Two mechanisms likely contribute to the link between
surface chlorophyll concentrations to mixing depth: an impact on vertical nutrient
transport from depth and changes in the average light level experienced by surface
phytoplankton. To distinguish which of these two factors dominate, Behrenfeld
et al. (2008) separated chlorophyll variability in permanently stratified ocean into
that due to biomass changes and that due to intracellular chlorophyll (Chl:carbon)
changes. Their study showed that, over most of the SeaWiFS record, chlorophyll
variability was largely due to physiological changes in Chl:carbon and that most of
this variability was attributable to changes in the upper ocean light environment,
not nutrients. These findings imply that significant variations in chlorophyll
detected in the satellite record are likely not linked to parallel changes in NPP.
However, most contemporary NPP models are not equipped to make this distinction (see Sect. 8.6.1). More recently, Siegel et al. (2013) extended the analysis
of physiological variability to all global ocean regions and additionally showed
that apparent anomalies in chlorophyll may be, in part, traceable instead to variations in colored dissolved organic material. Taken together, these studies once
again emphasize that careful attention must be given to physiological attributes if
global ocean NPP and its temporal variability are to be accurate evaluated.
As global, synoptic estimates of NPP have advanced, opportunities have arisen for
investigating relationships between NPP and more derived, yet critical, carbon cycle
parameters. For example, the fraction of NPP delivered from the surface ocean to
depth is a critical quantity of interest when addressing the ocean’s role in carbon
sequestration. This export production is traditionally measured in the field using
sediment traps that collect and preserve sinking material, but these measurements
have longstanding caveats (Buesseler et al. 2007 and references therein) and are
extremely sparse in both space and time. Satellite observations of NPP, when combined with ecosystem model results and field measurements have provided simple,
yet powerful empirical parameterizations that allow globally resolved fields of export
production (Laws et al. 2000; Dunne et al. 2005). Current estimates of export production are *10 Pg C year
-1 globally and its spatial distribution gives us insight into
ecosystem functioning. For example, Laws et al. (2000) showed that the Atlantic and
Pacific Oceans contribute equally to total export (*4.3 Pg year
-1 each), despite their
two-fold differences in size (*75 9 10
6 km
2 and *160 9 10
6 km
2 , respectively).
Recently, Westberry et al. (2012) applied satellite NPP estimates to field-derived
photosynthesis-respiration relationships to characterize global ocean respiration
rates and net community production. These net community production rates set an
upper constraint on export production for comparison with alternative approaches.
Validation of these advanced, satellite-based carbon cycle parameters has yet to be
carried out and should be an active research area in the future (see following section).
8 Oceanic Net Primary Production
215