Sediment analysis evidences two different depositional phenomena influencing seagrass distribution in the Gulf of Oristano (Sardinia, western Mediterranean)

Sediment grain size and total organic carbon (TOC) content in sediments, coupled with image processing analysis, were investigated to assess natural us. anthropogenic distribution patterns of Posidonia oceanica seagrass meadow, in the proximity of a river mouth and a port embankment in the Gulf of Oristano (Sardinia, Italy). Two unvegetated areas (similar to 9 km(2) each) were detected by meadow mapping: (i) in front of the river mouth, and (ii) close to the port. Both unvegetated areas were characterised by a high silt+clay content in the bottom sediments, allowing us to identify the deposition of fine sediments as a factor limiting P. oceanica distribution. A marked variation within the depth of sediment grain size was recorded close to the port, where a 5-cm layer with silt+clay > 50% overlapped a sediment with silt+clay < 20%, suggesting a recent change in depositional processes. Grain-size fractionation was analysed using the silt: clay ratio, which indicated high-energy conditions in the area close to the port. Differently high TOC concentrations were found in front of the river mouth. These data allowed us to identify two different types of sedimentary process impacts on P. oceanica distribution: (i) a natural limitation because of the river inflow and (ii) an anthropogenic impact due to fine-sediment deposition following the dredging and building of the port.