Propose a synoptic and integrating vision of these interactions
The ultimate goal of this axis is to implement digital models that provide a synoptic and integrating view of the marine ecosystem and the dispersion of contaminants along the Tunisian and Mediterranean coasts. The objective is also to produce typical scenarios in order for better understanding and highlight the interactions that may exist between contaminants and marine ecosystems on a small, medium and large scale and according to a watershed-coast-wide gradient. The questions that will be addressed under this objective are :
- What is the dispersion of contaminants according to physical forcings (atmosphere, hydrodynamics, sedimentary dynamics)?
- What is the impact of extreme events (storms, floods, etc.) on the dispersion of contaminants?
- How long does the contamination last according to the hydrodynamic and hydro-sedimentary circulation?
- What is the impact of contamination and physical forcing on water quality?
- What is the impact of contamination on the distribution of trophic communities, particularly planktonic? Does it constitute a chemical barrier?
- How does the transfer of contaminants within the food web occur?
- What are the structural and functional changes induced by contamination, and how does contamination modify the overall functioning of the ecosystem?
To answer its various questions, we will adopt a modeling approach that will couple hydrodynamics and biology and biogeochemistry. This will be achieved in several steps necessary to build this coupled model.
Three-dimensional hydrodynamic models
Realistic three-dimensional hydrodynamic models of the littoral and coastal zone of the Tunisian coasts and of the coast-offshore exchanges will be implemented. It will be essential to reproduce the seasonal and inter-annual variability of this circulation. Thus, these models will take into account all the forcing influencing the modeled circulation. Depending on the scale of the phenomena described and the geographical position, these forcings may be: stratification, wind, tide, heat flow and swell. To refine the results at different scales, it will be important to provide for the possibility of nesting these different models.
In addition, we will use the results from previous and future campaigns to validate our models and improve our description of the environment. To improve our understanding of the phenomena, the large-scale model will use satellite data, which are those offering the longest time series. This will make it possible to understand the seasonal and possibly inter-annual variability of certain parameters such as surface temperature, sea surface elevation, etc.
The hydrodynamic circulation generated by extreme events such as storms will be studied by a stochastic and semi-parametric approach which, from these grid data, makes it possible to simulate storms, i.e. events more intense than those present in the data ( Chailan et al., 2017). This work will be carried out in partnership with the Institut Montpellierain Alexander Grothendiek and with the INRA Biostatistics and Spatial Processes research unit in Avignon.
Coupled biogeochemical model
The implementation of a biogeochemical model will take both the dynamic processes of circulation and the bases of biological activity. The impacts of atmospheric deposition of Saharan dust must also be particularly in the Gulf of Gabes, which borders the North African coast, widely considered to be the largest source of dust on Earth. Once conceptualized and built, this model will be coupled to the three-dimensional hydrodynamic model. Like what was done in the Gulf of Lion, the available data and products derived from satellite images (chlorophyll) will be used, among other things, for the calibration/validation of these coupled models.
Trophic transfer models
Trophic transfer models will be developed in pollution hotspots and in reference areas. This approach will make it possible to highlight the main trophic transfer pathways in environments strongly impacted by chemical and organic pollution, then to include the processes of transfer and accumulation of contaminants in key biota and geochemical compartments. An interest will be focused on plankton, especially ultra plankton, to understand how these communities interact with contaminants and how they contribute to the bioaccumulation of contaminants in food webs. These trophic models will also be coupled with three-dimensional biogeochemical models
Transportation models
The development of living particle transport models (eg ICHTYOP) considering the different forcing (temperature, salinity, contaminant, hydrodynamics, turbulence, etc.), but also the behavior of organisms (vertical migration, swimming, etc.) will make it possible to assess the impact of contaminants on the distribution of planktonic communities. A hydro-sedimentary model along the Tunisian coast will be developed to analyze the transformation and dispersion of contaminants by the sediment. This hydro-sedimentary model can be based on the work carried out by HSM and INSTM within the framework of the RYSCMED program aimed at characterizing the continental contributions (from a qualitative and quantitative views) of the Medjerba, the main Tunisian river whose mouth is located in the Gulf of Tunis while integrating the possible contributions of contaminants via groundwater which can represent a significant source of chemical pollutants.
In addition to the integrative aspect, this modeling work will allow :
- Help with campaign planning and analysis of results by offering the possibility of having a synoptic view of the data
- To carry out scenarios on the effects of development or climate change to predict the risks of contamination and assess the state of health of the ecosystem. These scenarios
- To propose appropriate ecological indicators for each ecosystem studied and effective (resilience, organization, maturity, stability, recycling, transfer efficiency, etc.) As well as the scenarios (see above), these indicators can be proposed as tools of decision support for managers in connection with objective 3 of the LMI
The objectives of this line of research are very ambitious. All these activities will not be able to be carried out on all the littoral and the Tunisian coasts on the scale of phase 2 of the LMI. The success criterion will be to develop and couple the models, then to reach a quality criterion that can allow them to be disseminated within the various Tunisian institutes and make them Tunisian reference models. For this, we will focus our modeling work mainly on the Gulf of Gabès and the lagoon of Bizerte.