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12.22.10: Notes on “Local stratification control of marine productivity in the subtropical North Pacific” by Dave and Lozier (2010)

“1. How is interannual variability in primary productivity in the subtropical North Pacific related to variability in the density stratification of the local water column? How can this relationship be interpreted within the context of a two layer model (i.e., in the upper and lower euphotic zone)?”

“2. How is interannual variability in stratification and productivity in this region impacted by basin‐wide climate variability? What is the spatial (i.e., vertical) and temporal structure of any correlations that do exist?”

The following is an important interpretation of satellite-derived surface chlorophyll a:

“In the upper euphotic zone at Station ALOHA, chl a is elevated during winter months when mixed layers are deeper and stratification is weaker (Figure 2a). This signal, however, is unlikely to result from productivity driven by entrainment of deep nutrients, since the nutricline here lies below the wintertime mixed layer. In fact, seasonal primary productivity is almost completely out of phase with chl a, in that it is elevated during the spring and summer months when stratification is increasing. In the absence of large seasonal inputs of allochthonous nutrients, the wintertime chl a increase has been attributed to phytoplankton photoadaptation and changing photosynthetic efficiency with deeper mixing rather than to an actual increase in biomass [Venrick, 1993; Letelier et al., 1993; Winn et al., 1995]. This dynamic highlights the potential difficulties in using satellite‐ based estimates of chlorophyll concentrations from this region as a proxy for phytoplankton biomass.”

A comment on summer blooms:

“The observed summer productivity maximum is thought to reflect phytoplankton blooms that result from enhanced nitrogen fixation, which increases in response to a synchrony of several conditions which favor the growth of key diazotrophic species: increased light intensity, stronger thermal stratification that further restricts nitrogen supply from depth and a calm, less turbulent air‐sea interface [Karl et al., 1995; Dore et al., 2002; Grabowski et al., 2008; White et al., 2007; Church et al., 2009].”

A comment on spring bloom in the lower euphotic zone:

“summertime nitrate and phosphate concentrations along isopycnals in the lower layer are depleted relative to wintertime concentrations, as a result of increased uptake by phytoplankton during months with greater light intensity (Figure 2f). Productivity and phytoplankton biomass are greater during these months and exhibit a strong springtime increase that is analogous to the surface bloom events observed in light‐limited waters at higher latitudes [Letelier et al., 2004].”

A comment on productivity on the lower euphotic zone:

“Productivity in the lower layer may also be impacted by annual cycles of stratification at the base of the euphotic zone. Since significant nitrogen fixation does not occur at these depths, upward fluxes from the deep nutrient pool are critically important for the maintenance of new production and the replenishment of depleted nitrogen and phosphorous in this layer. The data at Station ALOHA suggest that vertical density gradients may regulate these fluxes over seasonal time scales (Figure 2d). Lower layer stratification in the spring and early summer months is weaker than in the fall and winter and is negatively correlated with FPN‐NO3, ostensibly a measure of new lower layer production driven by deep nitrate (r = −0.82, p < 0.05). The correlation is consistent with the expectation that increased diffusivity due to weaker vertical density gradients would produce a greater upward flux of deep nutrients. In the lower layer, therefore, stratification appears to be related to seasonal productivity variability in a more conventional sense.”

No significant correlation is found between the interannual variability of biological indeces (primary production, chlorophyll a, etc) and the stratification. Various definitions are used for the stratification but to no avail. Furthermore

“[w]e compare productivity, chl a and particulate export to changes in solar irradiance at the surface and at depth, local wind stirring and wind stress curl (which is directly related to Ekman pumping velocities), density values at the base of the euphotic zone (which would be sensitive to the heaving of deep isopycnals on seasonal and longer scales) and nutrient concentrations along isopycnals below the euphotic zone (which would indicate variability in the deep nutrient reservoir). Our analysis, however, finds no clear evidence of a correlative relationship between productivity and any of these properties.”

“Intriguingly, the strong biological and physical coupling observed here on seasonal time scales does not extend to interannual scales.”

“Forcing from ENSO and PDO has consistently been invoked to explain interannual and decadal scale ecosystem variability at Station ALOHA [Karl et al., 1995, 2001; Dore et al., 2002; Corno et al., 2007; Bidigare et al., 2009]. In the absence of a strong interannual association between stratification and productivity at this location, however, the linkage between local biological variability and climate variability (thought to be mediated by local stratification changes) appears tenuous. Direct comparisons of variability in HOT properties with either of the PDO or ENSO indices do little to strengthen the case for climate linkages. Given the time scales (20–30 years) associated with PDO cycles, the HOT record is probably too short for a robust correlation analysis. Comparisons with the higher‐frequency ENSO signal can be made with greater statistical confidence, but the response of biology at Station ALOHA to a given ENSO state has been demonstrably inconsistent [Dore et al., 2002; Corno et al., 2007]. It may be that the forcing effects from these climate signals are not transmitted strongly to this position, or that, even if these effects are transmitted, complex interactions between the signals originating from the tropics (ENSO) and higher latitudes (PDO) produce an incoherent local expression at this intermediate (∼20°N) location.”

“In the absence of a direct correlation of local stratification and productivity with ENSO or PDO, recent studies have instead treated ecosystem variability at Station ALOHA as a function of the combined variability of the two climate processes. Corno et al. [2007], for example, argue that the records of local stratification, vertical nutrient delivery, productivity and community structure at Station ALOHA from 1988–2004 can be separated into intervals that are each characterized by a distinct pattern of phasing and relative intensity between ENSO and PDO. Similarly, Bidigare et al. [2009] assert that a strong, concurrent transition in both ENSO and PDO during the summer of 1998 forced a transition to a weaker stratification state leading to stronger vertical mixing, greater nutrient fluxes from depth and subsequent changes in plankton assemblages and particulate export at ALOHA. However, while a comparison of productivity rates before and after this 1998 ENSO/PDO shift does suggest a shift to higher productivity (most pronounced at “middle depths” within the euphotic zone), there is no clearly observable change in stratification before and after this transition, either in the surface layer or at the base of the euphotic zone (Figure 4).”

“A comparison with the NPGO, however, reveals an interesting pattern of relatively stronger correlations between this climate process and primary production as well as with specific features of the local hydrography.”

“Subsurface primary productivity at ALOHA is weakly positively correlated with NPGO variability for time lags of 0–50 months. The correlations are maximized for “middle” and lower depths within the euphotic zone (45– 100 m) and for time lags between 12 and 18 months, but are able to explain at most 12.5% of the observed variability (r = 0.35 at 18 months).”

“Whatever the mechanisms that underlie the NPGO productivity correlation, it appears that this linkage is not mediated by local stratification changes.”

“Interestingly, however, a strong, positive correlation is observed between the NPGO index and local subsurface salinity over a similar range of time lags and depths as the NPGO productivity correlations. These correlations extend to the surface but are maximized for “middle lower” euphotic zone (50–100 m and 100–150 m, abruptly weakening in waters >150 m) and for time lags between 12 and 18 months (again similar to the depths and time lags observed for the maximum NPGO productivity correlations). The correlations are able to explain up to 37% of the observed variability at 100–150 m (r = 0.61 at 12 months). Salinity variability at these depths may be linked to the variability in North Pacific Tropical Water (NPTW), which is formed in frontal regions to the north and east of Hawaii characterized by high surface salinity. The subduction and subsequent advection of NPTW to Station ALOHA creates a persistent subsurface salinity maximum between the 24.3 and 24.7 ‘sigma theta’ isopycnals, which lie on average between 100 and 140 m in depth [Suga et al., 2000; Lukas, 2001; Lukas and Santiago‐Mandujano, 2008; Stammer et al., 2008]. Variations in the intensity of the salinity maximum have been suggested to arise from changes in the surface heat and freshwater fluxes and dynamical convergences that produce the high‐salinity signature of the NPTW source regions [Lukas, 2001]. The observed correlation between the NPGO and subsurface salinity at ALOHA, therefore, probably reflects the strong role of this climate process in driving the formation of NPTW and possibly other water masses in the central subtropical gyre. The lag times for which the correlation between NPGO forcing and salinity variability is maximized, 12–18 months, are roughly consistent with time scales required for advection of NPTW from its source region to ALOHA [Lukas and Santiago‐Mandujano, 2008].”

“Despite the similar spatial and temporal structure of the NPGO salinity and NPGO productivity correlations, it is unclear how subsurface salinity variability or NPTW variability can be mechanistically linked to changes in productivity and specifically to nitrogen fixation. One possible linkage may be that the nutrient content of NPTW varies concurrently with observed salinity changes, and that NPTW variability also produces variability in the local advective flux of nutrients transported by this water mass. An interesting future analysis would be to examine how “upstream” property changes in the source regions of the NPTW are related to both the NPGO and local productivity variability at ALOHA.”

The link between interannual change in productivity at station ALOHA and interannual variability in the North Pacific Tropical Water is consistent with my recent interpretation that short events of deep water intrusion into the surface layer are due to the passage of mesoscale features advected southward from the Subtropical Front where there are generated. It would be interesting to test that mechanisnistic link by studying the interannual variability of the generation of these features. One element, however, is not consistent with the present paper: it is the lag (12-18 months) observed in the paper that is much longer the time (1-2 months) it takes for the features to propagate southward. Maybe the lag of 12-18 months is mostly between the NPGO index and the generation of the mesoscale features. See also Lukas and Santiago‐Mandujano (2008) for the time scale (12-18 months) needed for the advection of NPTW between their origin and station ALOHA.

“Inputs of allochthonous nutrients are required for the maintenance of new production, yet the mechanisms that supply these nutrients to surface photosynthesizers in the subtropical North Pacific are still not well understood. Episodic variability in new production at ALOHA has previously been suggested to result from nutrient injections caused by meteorological upwelling events, eddies and planetary waves [Letelier et al., 2000; Sakamoto et al., 2004; Fong et al., 2008; Mahaffey et al., 2008; Nicholson et al., 2008; Rii et al., 2008]. Consideration of this type of stochastic nutrient loading may be essential, since the continuous upward nutrient fluxes derived from observed and modeled openocean turbulent diffusivities have historically been considered insufficient to support observed rates of new production in the subtropical gyres [Lewis et al., 1986; Ledwell et al., 1993; McGillicuddy et al., 1998]. However, the regular, monthly HOT sampling program is not ideally suited for detecting episodic variability in stratification, nutrient fluxes and productivity [Karl et al., 2003]. Higher‐frequency observation programs using moored sensors have been implemented at the HALE ALOHA site, but these have not been long enough to support robust interannual and decadal assessments.”

Read:

Letelier, R. M., D. M. Karl, M. R. Abbott, P. Flament, M. Freilich, R. Lukas, and T. Strub (2000), Role of late winter mesoscale events in the biogeochemical variability of the upper water column of the North Pacific Subtropical Gyre, J. Geophys. Res., 105(C12), 28,723–28,739, doi:10.1029/1999JC000306.

Letelier, R. M., D. M. Karl, M. R. Abbott, and R. R. Bidigare (2004), Light driven seasonal patterns of chlorophyll and nitrate in the lower euphotic zone of the North Pacific Subtropical Gyre, Limnol. Oceanogr., 49(2), 508–519, doi:10.4319/lo.2004.49.2.0508.

Lukas, R., and F. Santiago‐Mandujano (2008), Interannual to interdecadal salinity variations observed near Hawaii: Local and remote forcing by surface freshwater fluxes, Oceanography, 21(1), 46–55.

Nicholson, D., S. Emerson, and C. C. Eriksen (2008), Net community production in the deep euphotic zone of the subtropical North Pacific gyre from glider surveys, Limnol. Oceanogr., 53(5), 2226–2236.

Palter, J. B., M. S. Lozier, and R. T. Barber (2005), The effect of advection on the nutrient reservoir in the North Atlantic Subtropical Gyre, Nature, 437(7059), 687–692, doi:10.1038/nature03969.