The ProvBio floats in the Mediterranean Sea

Hervé Claustre (1) , Fabrizio D'Ortenzio (1)  

   

 

(1) : Laboratoire d'Océanographie de Villefranche, France

More information: Oceanographic Autonomous Observations project

 

Context

The Mediterranean is an endangered Sea (Gibelin et al., 2003), because the strong anthropogenic pressure on its coasts and its relative small size could accentuate the impact of the global warming. The Mediterranean ecosystems, and all the related economical activities (i.e. fisheries, tourism etc) could be then strongly perturbed by the climatic changes. For these reasons, in 2008, the Mediterranean was selected as a pilot site to test the new generation of biogeochemical profiling floats, which are developed in the framework of the French Project "PABO" (ANR Plateformes Autonomes et Biogéochimie Océanique, PI Hervé Claustre).

Data

The new PROVBIO floats are based on the widely used PROVOR-CTS3 model, which was specifically modified adding two fluorimeters (calibrated for Chlorophyll and Colored Disolved Organic Matter (CDOM) concentrations), an irradiance sensor (measuring in water incident light at three different wavelengths), transmittance and backscattering meters (which give information on the stock and size of organic particles present in the water). Moreover, to support the increased amount of information to be transmitted, the original ARGOS antenna of the PROVOR was replaced by a more performing IRIDIUM system, which, additionally, allows a two-way communication with the floats (i.e. users on land have the possibility to modify internal parameters of the floats, as, for instance, the temporal frequency of cycling or the vertical resolution).

 

Results

As shown in figure 1, two PROVBIOs were deployed in 2008, in two ecological contrasting regions of the Mediterranean. The first float (NW_B02, WMO 6900677) was initially deployed the 1st May 2008, in the Nord Western Mediterranean Sea, close to the French long term in situ optical mooring BOUSSOLE (approximately 8°E, 43.5°N). Float was programmed to a cycle frequency of 1 day. After 15 days, the cycling frequency was changed to 5 days, and it was kept constant until its end of life. This float has made 148 cycles, corresponding to a life-time of 684 days.

Figure 1. Geographical locations and trajectories of NW_B02 (left) and LW_B06 (right). Deployment (red point), profiles (red points) and last position (green point) - Courtesy : OAO/VLFR 2012

The second float (LW_B06, WMO 6900679) was deployed the 27th, june 2008 in the Levantine Basin, on the site of Eratosthenes seamount (approximately 32.5°E, 33.7°N) during the French cruise "BOUM" (P.I. Thierry Moutin). The deployment zone was selected because a permanent anticyclonic structure is recurrently observed there (Malanotte et al., 1999). The rationale was to "entrap" the float in the density structure, in order to obtain repeated profiles of the same oceanic area (or water mass).

The LV_B06 float was programmed similarly to the NW_B02 : after an initial phase at 1 day cycling frequency (from 06/27/08 to 06/30/08), the float strategy was modified to perform a 5-days cycling. This float has made 186 cycles, corresponding to a life-time of 924 days.

Figure 2 shows the temporal evolution of the potential density and chlorophyll concentration fields obtained from the NW_B02 PROVBIO float. The summer stratification of the water column, both in 2008 and 2009, impacted on the vertical distribution of the chlorophyll concentration, which was then characterized by a Deep Chlorophyll Maximum (DCM) at about 50-70 meters depth in 2008, and almost 100 meters during summer 2009. The beginning of the fall in each September, with the associated atmospheric cooling and the more intense wind mixing, induced a progressive deepening of the mixed layer depth, with a consequent de-stratification of the water column. As the winter advances, DCM was progressively destroyed, as chlorophyll concentration was redistributed uniformly in the mixed layer, by the intense mixing.

 

 Figure 2 : (left) potential density and (right) chlorophyll concentration function of pressure (dbar), estimated along the NW_B02 float trajectory. Courtesy : OAO/VLFR

Dynamical conditions of the LW_06 float are relevantly different (see Figures 3 and 4). The PROVBIO, as initially expected, remained trapped during its first 100 cycles, in the cyclonic structure related to the Eratosthenes seamount, and it profiled the same area for most of the summer and of the fall 2008. The summer stratification was more important than in the Western Mediterranean basin, and winter conditions induced mixed layer depth deepening reaching its maximum depth (400 meters) in winter 2009.

 Figure 3 : (left) potential temperature and (right) potential density, function of pressure (dbar), estimated along the LV_B06 float trajectory. Courtesy : OAO/VLFR

Consequently, a DCM was permanently observed at about 100 meters depth, although the characteristics of the vertical profiles of chlorophyll concentration changed from summer to winter. The absolute values of chlorophyll concentration, always observed in the DCM, tended to decrease with time, passing from 1-2 micron g/l in summer 2008 to 0.5-0.7 micron g/l in fall 2008. The deepening of the mixed layer during winter 2009 induces an uniform vertical redistribution of the chlorophyll concentration around 0.2 micron g/l in the mixed-layer.

Figure 4 : (left) Chlorophyll concentration and (right) CDOM between 0-400 dbar only, estimated along the LV_B06 float trajectory. Courtesy : OAO/VLFR

 

Conclusion

Finally, the firsts two Mediterranean PROVBIO's produced, during almost two years, an invaluable data set on the physical-biological interaction, in two contrasting ecological regions of the basin. Considering the quality of the data, the ratio cost/benefits for this first PROVBIO experiment is extremely favorable.

 

References
  • Claustre et al. (2010), “Bio-optical profiling floats as new observational tools for biogeochemical and ecosystem studies”. Proceedings of the OceanObs09 : Sustained Ocean Observations and Information for Society » Conference, Venice, Italy, 21-25 September, Hall, J., Harrison D.E., Stammer, D., Eds, ESA Pubblication WPP-306, 2010
  • D'Ortenzio, F. and the PABIM group (2010). «White book on Oceanic Autonomous Platforms for biogeochemical studies : instrumentation and measure » -Version 1.3,February 2010 
  • Gibelin, A. L., and M. Déqué (2003), Anthropogenic climate change over the Mediterranean region simulated by a global variable resolution model, Climate Dynamics, 20 (4), 327–339, doi:10.1007/s00 382-002-0277-1.
  • Le Reste, S., André, X., Claustre, H., D'Ortenzio, F. & Poteau, A. (2008). First Succes of ProvBio floats. In : Coriolis letter #5, 6-8.
  • Malanotte-Rizzoli, P., B. Manca, M. d’Alcalà, A. Theocharis, S. Brenner, G. Boudillon, and E. Ozsoy (1999), The Eastern Mediterranean in the 80s and in the 90s: the big transition in the intermediate and deep circulations, Dynamics of atmosphere and oceans, 29, 365–395.