Abstract
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.
Highlights
1.1 The importance of continental margins in the organic carbon dynamicsContinental margins are of particular interest concerning the input, production, deposition, and export to the deep open ocean of particulate organic carbon (POC) (Bauer and Druffel, 1998; Liu et al, 2010)
Model results highlight the high net primary production (NPP) occurring in the Gulf of Lion shelf (GoL) shelf (198.9 × 104 t C yr−1), with maximum values in spring and in the outer shelf, in particular in the eastern region
The interannual variability of the NPP at the Gulf of Lion scale is especially low (SD = 4 %), and monthly interannual anomalies in NPP are mainly explained by changes in the intensity of solar radiation
Summary
Continental margins are of particular interest concerning the input, production, deposition, and export to the deep open ocean of particulate organic carbon (POC) (Bauer and Druffel, 1998; Liu et al, 2010). These buffer regions often show high biological productivity, induced by solar radiation and nutrient availability from river inputs and coastal upwellings (Legendre, 1990; Dagg et al, 2004; Lohrenz et al, 2008). The input of terrigenous POC and this high productivity make these coastal zones areas of high organic matter deposition (Gao and Wang, 2008; Dagg et al, 2008). The understanding of the input, deposition, and export of POC is essential to estimate the carbon dynamics of coastal areas at a world scale
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