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Deep Mixing in the Southern Ocean and the Spring Bloom as seen by SOCCOM profiling floats.

The spring bloom is well underway in the Southern Ocean.  SOCCOM float 9095 at 50 degrees South shows the event quite nicely.  The following figure (Fig. 1) has observations of temperature, salinity, pH at in situ temperature and pressure (on the total proton scale), oxygen, nitrate, backscatter (a measure of particle abundance), and chlorophyll that have been reported by 9095 since it was deployed on the P16S cruise in April.  I’ve superimposed the Mixed layer Depth (MLD) on most of the plots as a black line.  The MLD is (here) defined as the depth where the density change from the surface value is 0.01 kg/m^3.

The MLD seen by 9095 had deepened to 360 m in late September.  The sharp transitions in the values of pH, nitrate, chlorophyll, backscatter and oxygen at depths above and below the MLD show how well mixed the water had become as the upper ocean was homogenized.  In the first week of October, the mixed layer shoaled to about 20 m and phytoplankton biomass, as shown by the backscatter signal began to increase rapidly, as did phytoplankton chlorophyll.  Nitrate concentrations in the upper 50 to 100 m have dropped about 3 µmol/kg at the time this blog was written and dissolved inorganic carbon (computed from pH and an estimate of alkalinity) has dropped about 20 µmol/kg.

Click here for a larger image: http://www.mbari.org/chemsensor/images/9095SpringBloom.jpg

When did the bloom actually begin?  That’s an interesting question.  Was it around Oct. 1, when the MLD rose from below 300 m to around 20 m?  Or was it earlier?  This is a question that has stimulated a lot of discussion, of late.  The Sverdrup Critical Depth (SCD) Hypothesis (Sverdrup, 1953), posits that a bloom cannot begin until the MLD shoals above the Critical Depth.  “A ‘critical depth’ is defined as the bottom of a layer in which the total production of organic matter by the phytoplankton community—from this depth to the surface—is equal to its destruction by respiration”  (Fischer et al., 2014).  If the MLD is deeper than the critical depth, then phytoplankton would be mixed so deep that respiration would exceed primary production and biomass could not accumulate.  The Critical Depth depends on the amount of light integrated over the day (less in winter than summer), and it might range from 20 to 80 m in the open ocean around 50 degrees latitude.  That is much shallower than the MLD until early October.  The data in Fig. 1 seem to make the case: biomass didn’t accumulate until the MLD shoaled.  The SCD, which has been a central fixture of ocean science for 60 years, is proven.

But wait, it’s not so simple.  Behrenfeld (2010) and Boss and Behrenfeld (2010) have argued that the bloom started much earlier.  And their point is well taken.  The phytoplankton population was growing  all winter.  But as the mixed layer deepened from 80 m in April to >300 m in September, the expanding population of phytoplankton was continually being diluted with deep, plankton-free water.  Look at the backscatter signal in Fig. 1, which is a proxy for the amount of phytoplankton biomass.  The values stay relatively constant from May through September, but the mixed layer is deepening so the total amount of phytoplankton biomass is increasing.   There must be net phytoplankton growth!!  We can test that idea by integrating (adding up) the backscatter signal or chlorophyll signal from the surface to the MLD on each float profile.  Those values are estimates of the total amount of phytoplankton produced in the system.  I’ve first converted the back scatter signal (units of /m /sr – sr is steradian, the squared radian, an SI unit) to an estimate of the Particulate Organic Carbon in the system (POC [µmol/L] = {backscatter – 0.00005} x 2 x 3.14 x 1.1 x 13000 /12).

Fig. 2 shows the time course of the logarithm (base 10) of the integrated POC signal.  The integrated stock of POC was increasing steadily from May through September.  A linear trend of the logarithm means there is a constant growth rate during this period (see Behrenfeld).  So Sverdrup was wrong!!!  The bloom really started back in May while the MLD was increasing.  The SCD Hypothesis, which has been a central fixture of ocean science for 60 years, is disproven!!!  

But wait, it’s not so simple.  Peter Franks (2014) argues that the concept of the mixed layer and plankton mixing, and their exposure to light is a bit more complicated than the ideas posed by Behrenfeld and Boss and Behrenfeld.  Mixing in this region of the Southern Ocean occurs primarily due to convective heat loss.  Surface water gets so cold, it becomes denser than the water below and sinks downward, mixing the water column.  This is a very efficient mechanism for mixing in the ocean.  More efficient than wind.  Compare the depth of mixing seen by float 9095 (360 m) to that seen by float 7663 during Hurricane Gonzalo (90 m), which was discussed in the last SOCCOM blog post.  Mixing extends to 360 m around float 9095 not because of the strong winds in the Southern Ocean (the roaring 40’s, the furious 50’s, and the screaming 60’s), but because of the loss of so much heat during winter (convective mixing).  For an explanation, see Franks’ paper.

Peter makes the point that this convective mixing, while very efficient, is also easily stopped with just a bit of heat input.  Which is how the MLD can rise from >300 m to near the surface in just a few days.  And once convection stops, things can grow.  So you really have to distinguish between the mixed layer depth and mixing.  The ocean may have a deep mixed layer, but if it is not mixing, phytoplankton can grow.  That seems apparent in the data in Fig. 1.  If you squint a bit, you’ll see that when the mixed layer rises during July and August, after just a bit of heat input, there are increases in backscatter and chlorophyll above the mixed layer.  And pH increases too, presumably because phytoplankton are consuming CO2 and protons are removed as the equilibrium H+  +  HCO3- -> CO2 + H2O shifts to the right.  I’ve used a MLD defined by 0.01 kg/m^3 density change to emphasize these periods when the MLD shoals, rather than the conventional definition of 0.125 kg/m^3.  An even smaller limit for the MLD (0.003 kg/m^3) would show the MLD going to near the surface periodically in winter.

So maybe Sverdrup and Behrenfeld and Boss are both right.  The phytoplankton don’t accumulate (bloom) until there is a mixing layer (sensu Franks) that is shallower than the critical depth.  The MLD may be deeper, but there is no mixing through most of the MLD.  This allows phytoplankton to grow and biomass to accumulate.  Subsequent turbulence homogenizes the population down to the MLD.  And Behrenfeld and Boss are right that phytoplankton are accumulating over winter while the mixed layer (but not mixing) are deeper than the critical layer.   

PS:  How do you get this data?  All of the SOCCOM data is available on the web at the SOCCOMViz web page:  http://soccom.princeton.edu/soccomviz.php.  You can plot the data for any float on SOCCOMViz or you can download the whole file by selecting the link for “Float List and Link to Complete ASCII Data Files”.  Once there, pick the QC’d files – these are files where the data has been quality controlled.  If you want automated access with Matlab, or Python…. it’s a bit more complicated because the Princeton web site really points to a web site at MBARI (http://www.mbari.org/SOCCOM) and the PU site is hiding a redirect.  For software access the URL to all the 9095 data is

http://www.mbari.org/lobo/Data/FloatVizData/QC/9095SoOcnQC.txt

Click that link and a tab delimited, ASCII text file will download.  That file is formatted as an ODV (Ocean Data View) spreadsheet.  ODV is freeware designed for viewing oceanographic data and it works very well.  Find it at http://ODV.awi.de.  Once you download the text file, it can be dragged on to the ODV icon on your desktop.  And all the plots and data transformations shown here were created in ODV.  That’s a topic for a later post.

References

  • Behrenfeld, M. J. 2010. Abandoning Sverdrup’s critical depth hypothesis on phytoplankton blooms. Ecology, 91: 977–989.
  • Boss, E., and Behrenfeld, M. 2010. In situ evaluation of the initiation of the North Atlantic phytoplankton bloom. Geophysical Research Letters, 37: L18603. doi:10.1029/2010GL044174.
  • Fischer, A. D., Moberg, E. A., Alexander, H., Brownlee, E. F., Hunter-Cevera, K. R., Pitz, K. J., Rosengard, S. Z., et al. 2014. Sixty years of Sverdrup: a retrospective of progress in the study of phytoplankton blooms. Oceanography, 27: 222–235.
  • Franks, P. J. S. Has Sverdrup’s critical depth hypothesis been tested? Mixed layers vs. turbulent layers. – ICES Journal of Marine Science, doi: 10.1093/icesjms/fsu175.
  • Sverdrup, H. U. 1953. On conditions for the vernal blooming of phytoplankton. Journal du Conseil International pour l’Exploration de la Mer, 18: 287–295.

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