Turbulence and the Spring Phytoplankton Bloom

Transcription

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

2 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

3 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

4 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

5 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

6 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. Prochlorococcus Coccolithophores Diatoms Dinoflagellates

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

8 Recipe for a bloom

9 Recipe for a bloom

10 Recipe for a bloom Light penetrates down to 40m Phytoplankton live in mixed layer Mixed Layer Nutrients are stored below top 100m P across Pacific Depth (m) Ocean Interior!132!130!128!126!124!122 Longitude Potential Density (kg/m 3 )

11 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

12 Classical Theory of Phytoplankton Blooms

13 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

14 Phytoplankton Model 0-10 m µ(z) P t =(µ(z) m) P + z T P z Growth rate depends only on light available for photosynthesis Depth (m) Mixed layer depth, H µ = µ 0 e z/h l Mortality is constant -60 Mixed layer turbulence mixes plankton in the vertical Day -1 Under what conditions does plankton grow in the mixed layer?

15 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) 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 Day -1

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

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

18 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) 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 Day -1

19 New Theory for North Atlantic Spring Bloom

20 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

21 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

22 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) Growing Decaying s * = s * = s * = s * = 0 50 s * = 0.5 s * = 1 s * = 2 s * = Q (W/m ) 0 Heat flux

23 Numerical Simulations Q 0 Density Plankton 100m Critical depth Hc 200m 200m z = H N 2 = 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

24 Plankton Concentration Q 0 = 1W/m 2 Top View z y x Bottom View z y x P/P

25 Spindown Experiment Q 0 (W/m 2 ) Surface heat flux Time (days) Analytical growth rate s*= <P>/P Plankton concentration (color), Isopycnals (white) s * = H * =H /h l s * = Depth (m) Critical depth s*= 0 s*= s * = 1 s*= 2 s*= k * =k/(m h 2 l ) Time (days) 0.5 The onset of the bloom immediately follows change in sign of surface heat flux

26 Testing New Theory with Remote Sensing Data

27 Bloom Timing Heatflux [W m 2 ] Subpolar gyre (45-61ºN, 10-50ºW) 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 Subtropical gyre (20-40ºN, 10-50ºW) Heatflux [W m 2 ] 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

28 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.