Mixed layer depth variability and phytoplankton phenology in the
Mediterranean Sea
H. Lavigne1, F. D’Ortenzio1, M. Ribera d’Alcalà2, H. Claustre1
1. Laboratoire d’Océanographie de Villefranche, France2. Stazione Zoologica A. Dohrn, Naples, Italy
45th Liège Colloquium – May 17th 2013
•It is now well recognized that the phytoplankton phenology is impacted by mixed layer depth (MLD) variability (blooms events are good examples).•However, it is still challenging to observe
and characterize the impact of MLD on phytoplankton (MLD and phytoplankton biomass change rapidly, low availability of the phytoplankton biomass data). •Merging in situ MLD data and ocean color
chlorophyll-a concentration ([Chl]SAT) data represents a way to explore interactions between MLD annual cycle and phytoplankton phenology.
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
Present analysis was performed on the Mediterranean Sea because
Data availability (weak cloud coverage for [Chl]SAT, and important CTD sampling).
contrasting biogeochemical regimes co-exist over the basin.
Present analysis was based on :The generation of concomitant MLD and [Chl]SAT
annual cycles.Spatial averages computed in established
bioregions.Description of MLD and [Chl]SAT cycles based on a
new set of metrics.
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
Data
•SeaWiFS (1998 – July 2007) et MODIS-Aqua (July 2007 - 2010) •Level 3, 8-day , 9km •Standard NASA algorithm (O’Reilly et al.,
2007)
Satellite surface chlorophyll-a concentration ([Chl]SAT)
•Historical database (D’Ortenzio et al. 2005 updated with Coriolis).•72186 profiles of temperature and
salinity•Computation of MLD (criteria in density
difference 0.03 kg m-3)
Mixed Layer Depth (MLD) calculated from in situ CTD measurements.
The data density is not sufficient to
work with a regular mesh grid.
A bioregionalization was used instead.
The geographical framework - Starting point the bioregionalization of the Mediterranea Sea proposed by D’Ortenzio and Ribera d’Alcalà (2009).
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
bloom no bloomintermittent
3 mains kinds of dynamics appeared
Result from a k-means cluster analysis based on the seasonal
cycle of SeaWiFS
chlorophyll- a concentrationSource: D’Ortenzio et Ribera D’alcalà (2009)
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
Med NW - Bloom bioregion
Ionian - No bloom bioregion
Climatological scaleAll MLD and [Chl]SAT observations are
mixed to produce a climatological cycle.
Interannual scaleData are averaged for
each year separately.
spatial
tem
pora
l
Data processing
MLD and [Chl]SAT observations
Generation of concomitant MLD and [Chl]SAT annual cycles at 8-day resolution
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
CHL-MAX: annual maximum of [Chl]SATMLD-MAX: annual maximum of MLDΔINIT: Time lag between the initiation of mixing and the initiation of [Chl]SAT increase (determination of the initiation date: annual median + 5%; Siegel et al., 2002).ΔMAX: Time lag between the date of MLD maxima and the date of [Chl]SAT maxima.
Metrics to describe annual MLD and [Chl]SAT cycles
July year n Juneyear n+1
MLD [Chl]SAT
MLD-MAXCHL-MAX
ΔINIT ΔMAX
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
The climatological scaleMed NW - Bloom
Ionian - No Bloom
MLD-MAX CHL-MAX ΔINIT ΔMAX
185 m 0.99 mg m-3 48 days 48 days
MLD-MAX CHL-MAX ΔINIT ΔMAX
90 m 0.22mg m-3 16 days 8 days
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
The interannual scale : the analysis of annual cycles
•The shape of MLD and [Chl]SAT cycles vary from year to year.•For 4 cycles out of 5, the succession MLD deepening followed by [Chl]SAT increase
and decay is repeated.•Cycle 2006/2007 is anomalous.
•The shape of MLD and [Chl]SAT cycles are fairly similar to the climatology.•The absolute values, especially for MLD and [Chl]SAT peaks, are variable.
Med NW - Bloom
Ionian – No Bloom
bioregion MLD-MAX CHL-MAX ΔINIT ΔMAX
Med NW - Bloom 368m[119 – 524]
1.44 mg m-3
[0.84 – 1.72]14 days[0 – 40]
31 days[(-16) – (+72)]
Ionian - No Bloom 108m[76 – 158]
0.25 mg m-3
[0.19 – 0.30]17 days
[(-8) – (+32)]11 days
[(-56) – (+88)]
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
The interannual scale: the analysis of metrics
•The metric MLD-MAX is highly variable, by comparison to CHL-MAX.• MLD-MAX and CHL-MAX are both higher in the
“Bloom” bioregion than in the “No Bloom” bioregion. •ΔINIT is relatively stable and similar for the “Bloom”
and “No Bloom” bioregions.•ΔMAX is more variable, especially for the “No Bloom”
bioregion. •ΔMAX is higher for the “Bloom” than for the “No
Bloom” bioregion.
DATA & METHODS RESULTS DISCUSSION CONCLUSIONINTRODUCTION
Summary
Metrics are a powerful tool to identify patterns in the MLD and [Chl]SAT cycles.
These patterns are relatively consistent between interannual and climatological analyses.
Metrics analysis confirmed that in the Mediterranean Sea stronger biomass accumulation matches with areas where winter MLDs are the deepest.
Metrics analysis revealed temporal differences between main MLD and [Chl]SAT events (measured with ΔINIT and ΔMAX). c ΔINIT ΔMAX
BLOOM ~30 days ~30 days
NO BLOOM ~30 days ~0 days
How we can explain the ΔMAX difference and the ΔINIT of 30 days?
DATA & METHODS DISCUSSION CONCLUSIONINTRODUCTION RESULTS
Do light and nutrient availability can explain the ΔINIT and ΔMAX values in the « Bloom » and « No Bloom » bioregions? PrNUT PrLIGHT
Probability that the MLD is deeper than the nitracline depth.
Empirical estimation (MLD and nitracline datasets are confronted)
Nitracline data set: Nitracline = isoline 1µM
Calculated from a dataset of 5318 nitrates profiles (MEDAR, SESAME
projects).
Probability that the MLD is above the critical depth (Dcr, Sverdrup
1953).Empirical estimation (MLD
dataset is compared to a climatological estimation of the
critical depth, calculation method Siegel et al. 2002)
SeaWiFS climatology
1.3 mol photon m-2 d-1
DATA & METHODS DISCUSSION CONCLUSIONINTRODUCTION RESULTS
Med NW - Bloom Ionian – No Bloom
Do light and nutrient availability can explain the ΔINIT and ΔMAX values in the « Bloom » and « No Bloom » bioregions?
Med NW - Bloom Ionian – No Bloom
P rNU
TP rL
IGHT
DATA & METHODS CONCLUSIONINTRODUCTION RESULTS DISCUSSION
ΔINIT ♦ BLOOM : ~ 30 days♦ NO BLOOM : ~ 30 days
ΔMAX ♦ BLOOM: ~ 30 days
♦ NO BLOOM : ~ 0 days
Hypothesis:[Chl]SAT increase only when the MLD is below the nitracline (November)
Hypothesis:Episodically, a deficit of light could limit the growth during winter.
Hypothesis:Light is always available and irregular nutrients supplies by mixing sustain phytoplankton growth during winter.
• Metrics are a powerful tool to identify phenological patterns and characterize the influence of the mixed layer.
• The relevance of metrics ΔINIT and ΔMAX was emphasized.
• In the Mediterranean Sea, we proposed some hypotheses to explain their behaviors. (Lavigne et al. JGR, in revision)
• The proposed phenological metrics could be particularly adapted for profiling floats observations.
DATA & METHODS CONCLUSIONINTRODUCTION RESULTS DISCUSSION
Conclusions and Perspectives
Thanks to Fabrizio D’Ortenzio (my supervisor), Loïc Houpert (CEFREM, Perpignan FRANCE) and Rosario Lavezza (SZN, Napoli, Italy) for their help on CTD and nitrate data.
Thank you to for your attention
Contact: [email protected]
Method Lavigne et al., 2012