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  • Final Report Summary - SEAQUEST (Meso and Sub-mesoscale Physico-biogeochemical Dynamics in a Coastal NW Mediterranean Sea: Quantifying and Understanding Ecosystem Structure and Transport)

Final Report Summary - SEAQUEST (Meso and Sub-mesoscale Physico-biogeochemical Dynamics in a Coastal NW Mediterranean Sea: Quantifying and Understanding Ecosystem Structure and Transport)

Shelf and coastal seas are at the interface between continents (impacted by human activities) and the open ocean (the main regulator of our planet’s climate and its biogeochemical cycles). They are regions of exceptionally high biological productivity and biogeochemical cycling and play a crucial role in earth system functioning. On a global scale, coastal and shelf seas only account for just 8% of the ocean surface area but for up to 30% of oceanic primary production, 80% of the organic matter burial, 90% of the sedimentary mineralization, and 50% of the deposition of calcium carbonate. In addition, they provide a wide range of ecosystem services such as food and energy supply, natural resources as well as cultural services (tourism).
It has been widely recognised, however, that shelf and coastal seas are subject to increasing pressure from human activities with detrimental effects on the ecosystem which include habitat loss and degradation, pollution, climate change, overexploitation of fish stocks and natural hazards. At the same time, ocean general circulation models struggle to represent the processes and scales relevant to coastal seas and satellite estimates of primary production are invariably contaminated by the non-biotic influence on optical properties. In order to study these important oceanic regions we therefore need in situ observations in combination with a localised (i.e., high resolution 3D regional) modelling approach, paying particular attention to the accurate implementation of the high resolution bathymetry and the forcing at the open boundaries.

Within this context, SeaQUEST aimed to study the horizontal transport and cross-shelf exchange processes including their effect on the biogeochemistry in the coastal zone around the Gulf of Lion in the NW Mediterranean at meso- and sub-mesoscales. Our particular focus was on the Northern Current (NC), a slope current that passes along the continental slope off the Gulf of Lion, where it bounds and controls shelf circulation thus representing the dominant circulation feature. The NC can act as a physical barrier to cross-shelf transport in the Gulf of Lion and thus be of great importance to the local biogeochemistry and plankton dynamics. The project was hosted at the Mediterranean Institute of Oceanography (MIO) which is part of the University of Aix-Marseille (AMU) and we employed two main approaches to address the above questions:
(1) the first approach involved high resolution 3D realistic modelling of the French Mediterranean coast using the MARS3D/ECO3M coupled hydrodynamic-ecosystem model;
(2) the second approach involved in situ observations, both from dedicated cruises as well as using observations from satellites and HF radar.

As part of (1) we successfully upgraded the then existent model version of MARS3D (v8) to the latest version (v10) and adapted the model to the local bathymetry and boundary conditions of the study area. The model was run both in a hind casting and operational mode, thus providing a multi-annual time series to study the physical processes that control horizontal transport and their effect on the biogeochemistry, as well as near real-time forecasts of the physics and biogeochemistry during the period of the dedicated cruise (cf. Fig 1).

As part of (2), SeaQUEST participated and co-funded the OSCAHR cruise in November 2015 (http://www.mio.univ-amu.fr/OSCAHR/). In addition, we organised a dedicated SeaQUEST cruise in April 2016 for which we set up a website at http://mio.pytheas.univ-amu.fr/SEAQUEST/ where the cruise progress, including near real-time forecasts from the meteorological and ocean biogeochemical models, as well as observations from the remote sensing (satellites) and radar data were made available as soon as they became available, allowing project participants on land to track our progress at sea in near real-time.

While we were able to collect a large range of data using a variety of instruments (cf. Fig 2), the French Military refused us sampling permissions for about 90% of the intended sampling area. Although we had sought these permissions months in advance we were only notified 2 days ahead of the cruise which meant that the cruise strategy had to be redesigned at the very last minute and the entire Northern Current was essentially off limits to us. This in turn meant that we could not study the cross-shelf transport processes or the role of the NC as a barrier to this transport as planned. Nevertheless, we were allowed to sample the coastal and off-shore areas which provided data from 2 out of the 3 different ecosystems that typically exist in the area.

At present, about 85% of this in situ data have been analysed and preliminary results show how a strong Mistral event that occurred about halfway through the cruise led to a temporary weakening and near shut-down of the NC, followed by its re-establishment once the Mistral event had subsided. Further analyses, in particular in combination with the results from the modelling effort, will help to further elucidate the underlying dynamics and quantify the impact this had on the level of cross-shore transport and identify any impacts on the ecosystem.

Once all data has been analysed, SeaQUEST will have provided new insights into the ecosystem functioning and biogeochemical-physical interactions in an important and highly productive coastal area in Europe and the results will be of great relevance to many others working in this area including private stakeholders and political decision makers (e.g., for the creation of new marine protected areas, the management of spills, the mitigation of harmful algal blooms, and fisheries management).

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UNIVERSITE D'AIX MARSEILLE
France
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