Abstract. The Gulf of Lion shelf (GoL, NW Mediterranean) is one of the most
productive areas in the Mediterranean Sea. A 3D coupled
hydrodynamic–biogeochemical model is used to study the mechanisms that drive
the particulate organic carbon (POC) dynamics over the shelf. A set of
observations, including temporal series from a coastal station, remote
sensing of surface chlorophyll a, and a glider deployment, is used to
validate the distribution of physical and biogeochemical variables from the
model. The model reproduces the time and spatial evolution of
temperature, chlorophyll a, and nitrate concentrations well and shows a clear
annual cycle of gross primary production and respiration. We estimate an
annual net primary production of ∼ 200 × 104 t C yr−1 at the scale of the shelf. The primary production is marked by a
coast-slope increase with maximal values in the eastern region. Our results
show that the primary production is favoured by the inputs of nutrients
imported from offshore waters, representing 3 and 15 times the inputs of the
Rhône in terms of nitrate and phosphate. In addition, the empirical
orthogonal function (EOF) decomposition
highlights the role of solar radiation anomalies and continental winds that
favour upwellings, and inputs of the Rhône River, in annual changes in
the net primary production. Annual POC deposition (27 × 104 t C yr−1) represents 13 % of the net primary production. The delivery
of terrestrial POC favours the deposition in front of the Rhône mouth, and
the mean cyclonic circulation increases the deposition between 30 and 50 m
depth from the Rhône prodelta to the west. Mechanisms responsible for
POC export (24 × 104 t C yr−1) to the open sea are
discussed. The export off the shelf in the western part, from the Cap de
Creus to the Lacaze-Duthiers canyon, represents 37 % of the total POC
export. Maximum values are obtained during shelf dense water cascading
events and marine winds. Considering surface waters only, the POC is mainly
exported in the eastern part of the shelf through shelf waters and Rhône
inputs, which spread to the Northern Current during favourable continental
wind conditions. The GoL shelf appears as an autotrophic ecosystem with a
positive net ecosystem production and as a source of POC for the adjacent NW
Mediterranean basin. The undergoing and future increase in temperature and
stratification induced by climate change could impact the trophic status of
the GoL shelf and the carbon export towards the deep basin. It is crucial to
develop models to predict and assess these future evolutions.
Badlands as a hot spot of petrogenic contribution to riverine particulate organic carbon to the Gulf of Lion (NW Mediterranean Sea)