Turbulence and the Spring Phytoplankton Bloom

Turbulence and the Spring Phytoplankton Bloom Raffaele Ferrari Earth, Atmospheric and Planetary Sciences, MIT Collaborators: Sophia Merrifield and John Taylor Toronto, February 2, 2012

Phytoplankton Bloom Phytoplankton blooms can grow explosively over a few days or weeks The image shows a bloom that formed east of New Zealand October 11, 2009 NASA image by Robert Simmon and Jesse Allen based on MODIS data

Phytoplankton Bloom Phytoplankton blooms can grow explosively over a few days or weeks The image shows a bloom that formed east of New Zealand October 25, 2009 NASA image by Robert Simmon and Jesse Allen based on MODIS data

Phytoplankton Bloom Phytoplankton account for nearly half of the global primary production (45-50 Gt C/year, Longhurst et al. 1995) Large phytoplankton blooms occur in the spring at high latitudes, particularly in the North Atlantic. SeaWiFs Animation

Importance of Blooms Fisheries past, present, future Seabirds Marine Mammals Carbon dioxide & Climate

Recipe for a bloom

Recipe for a bloom Light penetrates down to 40m Phytoplankton live in mixed layer 0 50 100 Mixed Layer Nutrients are stored below top 100m P across Pacific Depth (m) 150 200 250 300 Ocean Interior!132!130!128!126!124!122 Longitude 24.5 25 25.5 26 26.5 Potential Density (kg/m 3 )

Outline What triggers a spring bloom? 1. Classical theory for North Atlantic Spring Bloom 2. New theory for North Atlantic Spring Bloom 3. Test of new theory with remote sensing data 4. Test new theory with in-situ data

Classical Theory of Phytoplankton Blooms

Classical theory Traditional Description 1. In winter, surface cooling leads to convection and deep mixed layers. 2. Nutrients are abundant, but growth is limited by low light exposure. 3. In the spring, the mixed layer depths are shallower on average. 4. Abundant nutrients and light lead to rapid phytoplankton growth. Winter Spring Surface cooling Euphotic layer Mixed layer Mixed layer Euphotic layer

Phytoplankton Model 0-10 m µ(z) P t =(µ(z) m) P + z T P z -20-30 Growth rate depends only on light available for photosynthesis Depth (m) -40-50 Mixed layer depth, H µ = µ 0 e z/h l Mortality is constant -60 Mixed layer turbulence mixes plankton in the vertical -70-80 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Day -1 Under what conditions does plankton grow in the mixed layer?

Sverdrup s Critical Depth 0 µ(z) P t =(µ(z) m) P + z T P z -10 m Strong turbulence: -20 mixing is faster than growth/decay -30 ML depth controls whether bloom occurs Depth (m) -40-50 Mixed layer depth, H H apple H c µ 0 m h l Winter: H>H c (Sverdrup, 1953) light limited -60 Spring: H<H c bloom -70-80 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Day -1

Spring Bloom Dale et al. 1999, Sarsia 84:419-435 Weather Station M (66ºN, 2ºE) Stratification Winter: Deep mixed layers Nigh nutrients Low Chlorophyll Summer: Shallow mixed layers Low nutrients High Chlorophyll Chlorophyll Nutrient

Spring Bloom Timing Dale et al. 1999, Sarsia 84:419-435 Weather Station M (66ºN, 2ºE) Stratification The onset of the spring bloom precedes re-stratification! Chlorophyll

Huisman s Critical Turbulence 0 µ(z) P t =(µ(z) m) P + z T P z -10 m Weak turbulence: -20 mixing is slower than growth/decay -30 turbulence controls whether bloom occurs Depth (m) -40-50 Mixed layer depth, H apple T apple apple c µ2 0 Winter: m h2 l T > c (Taylor & Ferrari, 2011) light limited -60 Spring: T < c bloom -70-80 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Day -1

New Theory for North Atlantic Spring Bloom

Critical Buoyancy Flux Convective scaling (Deardorff 1972): T H 4/3 B 1/3 0 B 0 3 T /H 4 Critical buoyancy flux: B0 B c 3 c/h 4 B c h 2 l µ 2 0 m 3, H 4 H

Critical Heat Flux New Theory Bloom starts when heat flux becomes positive 300 B c h6 l µ6 0 m 3 H 4 Q c = c P 0 g B c Mixed layer depth H (m) 250 200 150 100 Growing Decaying s * = - 0.75 s * = - 0.667 s * = - 0.5 s * = 0 50 s * = 0.5 s * = 1 s * = 2 s * = 4 0-10 -6-10 -4-10 -2 Q (W/m 2-10 0-10 2 ) 0 Heat flux

Numerical Simulations Q 0 Density Plankton 100m Critical depth Hc 200m 200m z = H N 2 =9 10 5 s 2 0 P 0 Start with a deep mixed layer H>H c Drive convection with a uniform surface heat flux Q 0 < 0 Grid resolution O(1m): (N x,n y,n z )= (192, 192, 100) Biological parameters: µ 0 =1.0day 1, m =0.1day 1, h l =7m

Plankton Concentration Q 0 = 1W/m 2 Top View z y x Bottom View z y x P/P 0 0.93 0.97 1.01 1.05 1.09 1.13

Spindown Experiment Q 0 (W/m 2 ) 0-50 -100 Surface heat flux 1 2 3 4 5 6 7 Time (days) 50 45 40 Analytical growth rate s*= - 0.75 0 <P>/P Plankton concentration (color), Isopycnals (white) 0 1.5 35 30 s * = - 0.667-20 H * =H /h l 25 20 s * = - 0.5 Depth (m) -40-60 -80 Critical depth 1 15 10 s*= 0 s*= 0.5 5 s * = 1 s*= 2 s*= 4 0 10 0 10 1 10 2 10 3 10 4 10 5 k * =k/(m h 2 l ) -100 1 2 3 4 5 6 7 Time (days) 0.5 The onset of the bloom immediately follows change in sign of surface heat flux

Testing New Theory with Remote Sensing Data

Bloom Timing Heatflux [W m 2 ] 400 300 200 100 0 100 200 300 Subpolar gyre (45-61ºN, 10-50ºW) 10 1 10 0 10 1 Chlorophyll [mg chl m 3 ] 400 J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J 10 2 400 300 Subtropical gyre (20-40ºN, 10-50ºW) Heatflux [W m 2 ] 200 100 0 100 200 300 10 1 Chlorophyll [mg chl m 3 ] 400 J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J 10 2 Month NCEP heat flux averaged over 8 days SeaWIFS Cholorphyll data averaged over 8 days

Conclusions The onset of the North Atlantic Spring bloom coincides with the change in sign of the surface heat flux. The change in heat flux is a better indicator of bloom initiation than the critical depth. Deep mixed layers and weak turbulence can lead to large depth-integrated biomass. Locally the bloom first starts at submesoscale fronts, where turbulence is suppressed.