Impact of Deep Convection on the Ecosystems in the Northwestern Mediterranean Sea
by P. Conan (1), P. Testor (2), C. Estournel (3), F. D’Ortenzio (4), M. Pujo-Pay(1)and X.Durrieu de Madron(5)
(1) Laboratoire d’Océanographie Microbienne, Observatoire Océanologique de Banyuls, Banyuls-sur-Mer, France
(2) Laboratoire d’Océanographie et du Climat : Expérimentation et Approches Numériques, IPSL Paris, France
(3) Laboratoire d’Aérologie, Université de Toulouse, Toulouse, France
(4) Laboratoire d’Océanographie de Villefranche, Villefranche-sur-Mer, France
(5) Centre d’Etude et de Formation sur les Environnements Méditerranéens, Université de Perpignan Via Domitia, Perpignan, France
Conan, P., P. Testor, C. Estournel, F. D'Ortenzio, M. Pujo-Pay and X. Durrieu de Madron (2018). "Preface to the Special Section: Dense Water Formations in the Northwestern Mediterranean: From the Physical Forcings to the Biogeochemical Consequences." Journal of Geophysical Research: Oceans 123(10): 6983-6995. https://doi.org/10.1029/2018JC014301
Subject to a strong anthropogenic pressure and particularly sensitive to climate change, the Mediterranean Sea is a major place of environmental, social and economic interests. French and Mediterranean biogeochemical oceanographers coordinate their efforts since many years, to better understand its particular hydrodynamics and the effects of key natural and anthropogenic forcings on ecosystems and organisms, from coastal zones to open-ocean.
Both of the western and eastern parts of the Mediterranean Sea are known to be oligotrophic, with areas of low biological productivity which negatively influences higher trophic levels. Only the northwestern Mediterranean Sea (NWM) exhibits a repetitive significant spring phytoplankton bloom (Figure 1, from D’ortenzio and Ribera, 2009), which seems to be linked with large winter deep convection episodes in this area.
In reality, both bloom and deep convection events seem the result and consequence of processes that take place over the previous 6-8 months. In order to investigate more precisely the links between the key bloom episodes to the mechanisms of forcing which occurs in the NWM, a large experiment called DeWEx (DEep Water formation EXperiment, Testor et al., 2018) has been set up during winter 2012-2013 in the framework of MISTRALS-Mermex program. The objective was to reconstruct the physical and biogeochemical "history" of the NWM water bodies and their potential evolution in the climatic change context.
By defining a novative experimental strategy, a complete seasonal cycle has been sampled and each step of deep convection events and settings of the spring bloom have been observed. Thanks to a multiplatform approach, a new reference data set has been collected that was used a benchmark for advancing the modeling of the surface fluxes, convective processes, dense water formation rates and physical-biogeochemical coupling processes.
Figure 1: Spring phytoplankton bloom in the Gulf of Lion (from D’ortenzio et al. 2009)
DATA AND METHOD
Deep convection in the Gulf of Lion was first described by the MEDOC-Group (1970) in three phases:
- the preconditioning of the area by a cyclonic gyre circulation in the whole northwestern Mediterranean Sea producing a doming of isopycnals toward the surface centered at about (42°N, 5°E), exposing a large body of weakly stratified waters to local cooling and evaporation, due to dry and cold Mistral and Tramontane winds blowing over the Gulf of Lion;
- the vertical mixing due to buoyancy loss generated by intense surface cooling and evaporation reaching about 1,000 W m-2for short periods and allowing overturning of the water column; and
- the spreading/restratification phase with newly formed deep waters propagating away from the formation site while stratified waters around invade the deep convection area.
DEWEX cruises have been scheduled to cover the 3 phases of open-sea convection (Kessouri et al., 2018). The sampling strategy for the DEWEX experiment (Figure 2) was based on all in situ potential temperature, salinity, potential density and fluorescence of chl-a profiles, as well as currents and depth-average currents estimates collected thanks to ships, gliders, moorings, profiling floats and surface drifters. It included also four intensive oceanographic cruises (Table 1), leaded between July 2012 and July 2013 in the NWM in completion to MOOSE* and HYMEX* cruises, thus resolving a complete annual cycle.
Figure 2 : Sketch of sampling strategy and examples of trajectory for airborne and oceanic platforms during the Deep Water formation Experiment (DEWEX) in the northwestern Mediterranean. From Conan et al., 2018
R/V Le Suroit
R/V Tethys II
R/V Le Provence
DEWEX - Leg 1
R/V Le Suroit
R/V Tethys II
R/V Le Provence
February, March and May 2013
DEWEX - Leg 2
R/V Le Suroit
MOOSE - GE 2013
R/V Tethys II
R/V Tethys II
Table 1: List of Cruises carried out in the Framework of the DEWEX Experiment
Figure 3: (top) Spatial coverage during the so-called ‘‘preconditioning’’ (1 September to 15 December 2012), ‘‘mixing’’ (15 December 2012 to 31 March 2013) and ‘‘restratification’’ (1 April to 31 May 2013) phases of deep convection. The number of profiles, respectively collected by gliders, Argo profiling floats and R/V is indicated. (bottom) Surface chlorophyll-a concentration retrieved by satellite (L3 MODIS product) and averaged on (left) 1–2 November 2012, (middle) 13–21 February 2013, (right) 12–14 April 2013 that correspond to cloud-free periods during each phase. Adapted from Testor et al., 2018.
A total of 119 ship days and 500 CTD stations have been operated during one year of the study. Long-term observations have been performed with autonomous instruments deployed in the area:
- 30 gliders missions, representing 13,000 profiles collected over the year
- and 8 ARGO bio floats, representing 1,500 T/S profiles collected over the year
Lastly, a coupled physical-biogeochemistry numerical model has been set up both to allow real-time monitoring of conditions and to improve modelling parameters using the in-situ data collected.
Thanks to a multiplatform approach enriched with new autonomous platform technologies (gliders and Argo profiling floats in particular), the DEWEX experiment provided a unique data set collected during one year in the Gulf of Lion. It has led to a better description of the underlying hydrodynamic processes at work before, during and after deep ocean convection events in the NWM Sea, and its link with an important spring phytoplankton bloom in this area.
- Even if convection takes place at small spatial scales (a few kilometers) and at high frequency (typically a day), the resulting spring high productivity stimulates a significant biological response in the upper trophic levels. The interplay between the large scales, mesoscales, sub-mesoscales and convective scales has been particularly investigated. Small-scale circulation features have been highlighted and play a key role on deep convection and subsequent bloom.
- For the first time, not only qualitative but also quantitative aspects concerning deep convection has been obtained thanks to this experiment, like estimates of mass and energy fluxes over a period of a year in the deep convection area and deep water formation rates.
- Significant insights has been made, particularly with regard to the biodiversity of microbial organisms involved in the spring bloom and the export fluxes of matter to the deep layers linked to biological activities (Leblanc et al., 2018)
- Thanks to this unique data set, a coupled hydrodynamic-biogeochemistry model has been calibrated and validated. Simulations on interannual variations in deep water formations and planktonic ecosystem response have been successful. The numerical model gives a good reproduction of winter mixing, water column restratification, as well as the nutrient stock in the surface layer and the structure and productivity of the phytoplankton ecosystem.
Finally, the DEWEX experiment motivates to develop the same multiplatform/multiscale strategy for other areas/processes. It also underlines the need for national and european collaborations to achieve such an experiment which allows a great step forward knowledge improvements.
- Conan P., Testor P., Estournel C., D'Ortenzio F., Pujo-Pay M., Durrieu De Madron X. (2018). Preface to the Special Section: Dense water formations in the North Western Mediterranean: from the physical forcings to the biogeochemical consequences. JGR, 123(10), 6983-6995. https://doi.org/10.1029/2018JC014301
- D’Ortenzio, F., and Ribera d’Alcalà, M. (2009). On the trophic regimes of the Mediterranean Sea: a satellite analysis. Biogeosciences, 6(2), 139–148. https://doi.org/10.5194/bg-6-139-2009
- Kessouri, F., Ulses, C., Estournel, C., Marsaleix, P., D'Ortenzio, F., Severin, T., Taillandier, V., and Conan, P. (2018) : Vertical Mixing Effects on Phytoplankton Dynamics and Organic Carbon Export in the Western Mediterranean Sea, Journal of Geophysical Research: Oceans, 123, 1647-1669. https://doi.org/10.1002/2016JC012669
- Leblanc, K., Quéguiner, B., Diaz, F., Cornet, V., Michel-Rodriguez, M., Durrieu de Madron, X., Bowler, C., Malviya, S., Thyssen, M., Grégori, G., Rembauville, M., Grosso, O., Poulain, J., de Vargas, C., Pujo-Pay, M., and Conan, P.: Nanoplanktonic diatoms are globally overlooked but play a role in spring blooms and carbon export, Nature Communications, 9, 953, 2018. https://doi.org/10.1038/s41467-018-03376-9
- MERMEX group (2011). Marine ecosystems’ responses to climatic and anthropogenic forcings in the Mediterranean. Progress in Oceanography, 91(2), 97–166.
- Mayot, N., D’Ortenzio, F., Ribera d’Alcalà, M., Lavigne, H., & Claustre, H. (2016). Interannual variability of the Mediterranean trophic regimes from ocean color satellites. Biogeosciences, 13(6), 1901–1917. https://doi.org/10.5194/bg-13-1901-2016
- MEDOC-Group, T. (1970). Observation of formation of deep water in the Mediterranean Sea, 1969. Nature, 225, 1037–1040. https://doi.org/ 10.1038/2271037a0
- Testor, P., Bosse, A., Houpert, L., Margirier, F., Mortier, L., Legoff, H., Conan, P. (2018). Multiscale observations of deep convection in the northwestern Mediterranean Sea during winter 2012–2013 using multiple platforms. JGR Oceans, 123. https://doi.org/10.1002/2016JC012671