Biogeochemistry of Organic Matter at the Freshwater/Seawater Interface in the Rhone Delta, Mediterranean Sea, France

Abstract

Production, degradation and transport of organic matter have been studied since 1984 at the river/sea interface in the Rhône delta, a Mediterranean deltaic environment, slightly influenced by tide. Several stations were seasonally occupied in the river and off the river mouth of Grand Rhône, and samples of water, suspended particulate material and sediment were collected for the analysis of organic markers of natural inputs of organic matter from various origins (chlorophyll-derived pigments and fatty acids). A rather unique situation occurred in February 1985, characterized by the following facts: (1) the presence of low salinity and high suspended load (8 mg −1 ) in surface waters; (2) whereas chlorophyll a concentrations encountered in winter off the river are usually low (<0.5 μg 1 −1 ) high chlorophyll a (1.6 μg 1 −1 ) and phaeopigment concentrations (4.2 μg 1 −1 ) occurred at 10 meters depth, where the suspended load was 1.55 mg 1 −1 ; (3) the presence of a spreading nepheloid layer off the river mouth, several meters above the sediment, with a suspended load of ≈ 3 mg 1 −1 , was clearly identified at the 100 m depth station, several kilometers from the river mouth. A detailed analysis of particulate fatty acids revealed at one station a planktonic layer of 10 m, characterized by a high concentration of total fatty acids (7172 μg g −1 of suspended material), 15 times more than for surface waters and 5 times more than for underlying waters, and an abundance of unsaturated compounds. This planktonic imprint co-occurs with a high bacterial presence characterized by the presence of n-C 18:1Δ11 , iso and anteiso C 15 and C 17 markers. Surface waters show low imprints from both terrestrial higher plants and from algae, but high bacterial imprints. Low values of unsaturated/saturated fatty acid ratios give evidence of decreased biological activity in deep waters. The fatty acid compositions of the nepheloid layer and surface sediments were distinctly different, the latter characterized by a significant accumulation of terrigenous material together with high bacterial imprints. This suggests that the organic matter present in the nepheloid layer is not influenced by either re-suspension of surface sediment or by the sedimentation of overlying particulate material.