Surface and Underwater Autonomous Vehicles Observe Winter Convection and Biogeochemical Impact in the Ligurian Sea

During the 2019-2020 Saildrone Atlantic to Mediterranean (ATL2MED) mission, the Laboratoire d’Océanographie de Villefranche-sur-Mer (LOV) led the Nice-Calvi Line Inter-Comparison. This sub-mission sent the two ATL2MED saildrones and two gliders on several round trips between France and Corsica, providing an opportunity to cross the current and observe what will later spread into the Gulf of Lion (a wide bay reaching from Toulon to Catalonia) and the northwestern basin.

The goal of the sub-mission is to increase the spatial coverage of critical ocean convection and bloom event in the Ligurian Sea, including oxygen, partial pressure of carbon dioxide (pCO2) and pH, and chlorophyll, as well as to demonstrate the capacity of saildrones as a tool for studying CO2 uptake and acidification.

The Ligurian Sea is a basin in the Western Mediterranean between Liguria (the Italian Riviera) and the island of Corsica. The Nice-Calvi line, a 100-nautical mile (185-kilometer) passage between France and Corsica, is a critical section in the Ligurian Sea where the northern current (moving from east to west) transports water masses and particles from the Eastern Mediterranean. The Mediterranean carbonate system is unique because of the intrinsic characteristics of this basin—warm water, high salinity, and high total alkalinity associated with a permanent and rapid thermohaline circulation loop (imagine an underwater conveyor belt of fast-moving water). Consequently, the Mediterranean Sea is usually considered a CO2 sink, however, existing fixed observing stations do not provide the wide spatial coverage required to fully understand the dynamics of air-sea carbon exchange in the region.

‍

The DYFAMED time series (a French acronym for Atmospheric Flux Dynamics in the Mediterranean), part of the Mediterranean Ocean Observing System for the Environment (MOOSE) since 2010, is a scientific area of the Ligurian Sea made up of two buoys and one fixed mooring. The BOUSSOLE buoy is used for marine optical measurements and calibration and validation of ocean color remote sensing products (satellites), the ODAS Cote d’Azur buoy is part of the Météo-France network and provides meteorological data and oceanic surface measurements (temperature and salinity) down to 250 meters (820 feet), and the European Multidisciplinary Seafloor and Water Column Observatory (EMSO) ERIC deep mooring includes sensors for oceanographic measurements and sediment traps for measuring particle flux.

The DYFAMED site was used as a waypoint and as a reference site for the saildrones and gliders; the autonomous vehicles passed through the DYFAMED area approximately two miles from the buoys. The two saildrones provided continuous surface measurements, while the glider surveyed the water column down to 1,000 meters (3,281 feet). 

The resulting data set from the Nice-Calvi Line Inter-Comparison study will allow researchers to investigate the spatial extension of the spring phytoplankton bloom, its influence on pCO2 uptake, and its impact on oxygen production in surface waters.

“Saildrones will provide new insight on carbon uptake and pH monitoring in surface waters where atmospheric CO2 and air-sea exchanges are the main drivers. Since the start of the DYFAMED time series, only fixed CO2 measurements have been done, and we lack spatial variability on CO2 uptake,” said Laurent Coppola, a research scientist at LOV and principal investigator of the sub-mission. “Both the saildrones and the glider will provide essential oceanic and climate variables that are key to observing and understanding the impact of climate change on the oceanic environment.”

Midway along the Nice-Calvi line, the influence of winds and the cyclonic circulation of the water creates an intense vertical mixing and supply of nutrients in the surface layer, triggering large phytoplankton blooms in spring, while also exporting organic matter and oxygen towards the deep layers. This phenomenon, called “spring bloom,” usually takes place in April.

The size of the zone impacted by convections (mixed patches of water) strongly influences the intensity of the resulting spring bloom and the depth of convections plays a key role in the bloom diversity. The variability of winter convective mixing leads to interannual variability in the intensity, timing, and spatial extent of phytoplankton spring blooms. Due to the strong winter convection occurring in winter, the Ligurian Sea is emerging as a “high bloom” bioregion. However, the complex spatial distribution of the mixed patch and its submesoscale instabilities make the size and form of this bioregion very difficult to observe. 

“Increasing CO2 and decreasing pH of seawater can selectively affect the photosynthetic activity and growth of marine phytoplankton. On the other hand, the production and composition of phytoplankton have an impact on the absorption of atmospheric CO2. Few studies have explored the relationships between phytoplankton dynamics and sea surface pCO2, particularly in the complex system of bioregions such as the Ligurian Sea where observations have been based on fixed time series (e.g. DYFAMED) and satellite ocean color data,” explained Coppola. “The saildrone-glider combination will allow us to develop and improve and more accurate 3D vision of this process.”

As long-range, durable, wind and solar-powered vehicles equipped with instruments to collect data above and below the sea surface, saildrones play a unique role in the Global Ocean Observing System (GOOS). While satellites provide a general look at the sea surface, weather, and ocean topography, moored buoys, Argo floats, and gliders provide data deep into the water column. Saildrones are a bridge between those two groups of technologies, an itinerant platform providing high-resolution in situ observations that can be adapted to specific use cases.

ATL2MED is a collaboration between Saildrone and more than two dozen oceanographic research institutions from seven countries to address a variety of scientific objectives. The saildrones will sail next into the Tyrrhenian Sea off the west coast of Italy where they will continue to study carbon near fixed-point observatories and biogeochemical variability between fixed stations, as well as performing a sub-mission near the Aeolian Islands to investigate natural CO2 seeps due to volcanic activity.