Chemical Characterization of Suspended Particulate Organic Matter by Pyrolysis-Gas Chromatography Coupled with Mass Spectrometry and High Performance Liquid Chromatography in the Bottom Nepheloid Layer of the Rhône Delta

Abstract

During the DYPOL-6 Mission in the Rhône delta the use of a metallic frame provided water samples at three levels of the underlying water column, 2, 1, and 0·5 m above the sea bottom. These sampling levels allowed determination of the characteristics of suspended organic material at the deeper layers of the bottom nepheloid layer and, eventually, their relations with the overlying suspended particulate matter.The results obtained by the measurement of the suspended particulate organic matter, the elemental analysis, the analysis of the main classes of organic compounds by pyrolysis-gas chromatography coupled with mass spectrometry and the determination of phenolic compounds by high performance liquid chromatography (HPLC) emphasized the functional characteristics of the Rhône deltaic system.Near the bottom, the lower water levels of the benthic nepheloid layer showed some obvious biogeochemical gradients related to the location of the sampling station: the amount of pyrolysis-derived phenolic compounds, aromatic hydrocarbons and carbohydrates in the suspended material was high at the stations directly exposed to input of the Rhône River. The stations from the river mouth to offshore areas showed the influence of the Rhône inflow. But sometimes, according to the river regime and the current orientation, the influence of the Liguro-provencal current oriented East-West was noticeable even near the river mouth. In the eastern region of the surveyed area, the stations submitted to this current indicated some specific characteristics: the nepheloid layer, concentrated 1 m above the bottom, had a high particulate organic carbon content and a relatively high amount of phenolic compounds which indicated a possible seagrass (Posidonia oceanica) origin. At the more seaward stations, although the influence of the river input was always detectable, the suspended particulate matter was less abundant and essentially supplied by the local biogenic input: the lower water levels were enriched in compounds yielding acetonitrile upon pyrolysis, i.e. nitrogen-containing compounds.If the proximity of the river source and regime influenced the suspended particulate matter distribution and the geochemical characteristics of organic matter, the evolution through time of the bottom turbid flow from one station to another also had a great influence.Near the bottom, the terrestrial character is shown in the shoreward stations by high contents of pyrolysis-derived aromatic hydrocarbons and phenols as well as by lignin-derived phenolic compounds analysed by HPLC. In contrast, a marine character appeared in the seaward stations, emphasized by high values of pyrolysis-derived compounds indicative of carbohydrates and nitrogenous compounds as well as by phenolic compounds representative of phytoplankton production. However the upper water layers of the bottom nepheloid were generally more influenced by terrestrial input.Some trends emphasizing a microstratification of the water layers in the deeper levels of the bottom nepheloid, related to the existence of very thin bottom currents, or, maybe, to resuspension processes resulting from the bottom proximity, were shown by the geochemical characteristics of the three water levels analysed.