Observing mixed layer depth, nitrate and chlorophyll concentrations in the NW Mediterranean: a combined satellite and NO3 profiling floats experiment (2014)

F. D’Ortenzio (1,2)*, H. Lavigne (1,2), F. Besson (1,2), H. Claustre (1,2), L. Coppola (1,2), N. Garcia (3), A. Laës-Huon (4), S. Le Reste (4), D. Malardé (5), C. Migon (1,2), P. Morin (6), L. Mortier (1,7), A. Poteau (1,2), L. Prieur (1,2), P. Raimbault (3) and P. Testor (1,7)

 

(1) : Sorbonne Universités, UPC Universités Paris 06, UMR 7093, LOV, Observatoire Océanologique, Villefranche-sur-Mer, France
(2) : CNRS, UMR 7093, LOV, Observatoire Océanologique, Villefranche-sur-mer, France
(3) : MIO, CNRS- Université de la Méditerranée, Marseille, France
(4) : Recherches et Développements Technologiques - Electronique, Informatique et Mesures in-situ, RDT-EIM, IFREMER, Plouzané, France
(5) : NKE Instrumentation, rue Gutenberg, Hennebont, France
(6) : IPEV, Z.I. Pointe du diable, Plouzané, France
(7) : Sorbonne Universités (UPMC, Université Paris 06)-CNRS-IRD-MNHN, LOCEAN, Paris, France
* Corresponding author : dortenzio@obs-vlfr.fr 
Full paper: D’Ortenzio F., H. Lavigne, F. Besson, H. Claustre, L. Coppola, N. Garcia, A. Laës-Huon, S. Le Reste, D. Malardé, C. Migon, P. Morin, L. Mortier, A. Poteau, L. Prieur, P. Raimbault and P. Testor, 2014, Observing mixed layer depth, nitrates and chlorophyll concentrations in the North Western Mediterranean: a combined satellite and NO3 profiling floats experiment, Geophysical Research Letters, 41, 6443–6451, doi:10.1002/2014GL061020, 2014.

Data & Method

Two profiling floats, equipped with nitrate concentration sensors were deployed in the north-western Mediterranean from summer 2012 to summer 2013 (d’Ortenzio et al., 2014). Satellite ocean colour data were extracted to evaluate surface chlorophyll concentration at float locations. Time series of mixed layer depths and nitrate and chlorophyll concentrations were analyzed to characterize the interplay between the physical-chemical and biological dynamics in the area (Figure 1). 

 

Results

Deep convection (mixed layer depth > 1000 m) was observed in January–February, although high-nitrate surface concentrations could be already observed in December. Chlorophyll increase is observed since December, although high values were observed only in March. The early nitrate availability in subsurface layers, which is likely due to the permanent cyclonic circulation of the area, appears to drive the bloom onset. The additional nitrate supply associated with the deep convection events, although strengthening the overall nitrate uptake, seems to be decoupled from the December increase of chlorophyll.

 Figure 1: Time series and metrics of the PRONUTS and satellite observations for the (a and b) ISUS and for the (c and d) SUNA. In Figures 1a and 1c, grey diamonds represent MLD, red diamonds NO3mld, yellow points Ze, and green diamonds CHL. The blue marks on the upper x axis of Figures 1a and 1c indicate the date of the satellite images drawn in Figure 2. In Figures 1b and 1d, vertical short colored lines indicate the dates of parameter increasing (red for NO3, dark green for CHL × MLD, light green for CHL, and grey for MLD); circles indicate date of absolute maximum (same color code); and black lines indicate the dates of the first and the last occurrences of the MLD = Ze condition. Triangles indicate the date of negative-to-positive Qtot inversion. See Table S2 in the supporting information for the list of the metric values and of the method to compute them.

A new study from Xing et al., 2014 has demonstrated the ability of bio-Argo floats to measure and better understand the seasonal variability of 3 bio-optical properties (Chla, bbp(532) and cp(660)), and their inter-correlations. 

An another study published by Mignot et al, 2014 addresses the seasonal phytoplankton dynamics in the euphotic layer and explore its dependence on light regime dynamics. The bio-optical mechanisms and their relationship to light regimes that are revealed by the time series appear to be generic and potentially characteristic of all of the areas where a deep chlorophyll maximum forms, which is 50% of the open ocean.

 

References
  • Mignot, A., Claustre, H., Uitz, J., Poteau, A., D'Ortenzio, F. and X. Xing : “Understanding theseasonal dynamics of phytoplankton biomass and DCM in oligotrophic environments: a Bio-Argo float investigation”.Global Biogeochemical Cycles, doi: 10.1002/2013GB004781, 2014.
  • Xing, X., Claustre, H., Uitz, J., Mignot, A., Poteau, A. and H. Wang : ” Seasonal Evolutions of Bio-optical Properties and Their Inter-relationships Observed by Bio-Argo Floats in the Sub-polar North Atlantic” , Journal of Geophysical Research - Ocean, 119, doi:10.1002/2014JC010189, 2014.