Seabed sediments and foraminifera over the Lido Inlet: Comparison between 1983 and 2006 distribution patterns

Abstract

Extensive sampling in the Lagoon and Gulf of Venice during the 1980s provided material to establish ‘base’ conditions for subsequent studies of distributions of benthic foraminiferal biotopes and sediments. In 1983, the marine area around the Lido Inlet was characterised by two predominant sedimentary provinces: (a) Sandy Marine Province, along the shore, of very fine sand (3.25–3.50 Φ) and (b) Residual Sand Marine Province, along the beach area south of the inlet, of fine sand (2.00–2.75 Φ). Similar Residual Sand Lagoon (2.00–2.75 Φ) and Sandy Lagoon Provinces (3.50–3.75 Φ) predominated near the inlet within the Lagoon. Within these different sediment provinces, the foraminiferal assemblages were essentially uniform in the marine and lagoon area around the Lido Inlet. In 2006, sampling was repeated at 23 of the 1983 sites using the same methodology, and identical laboratory processing. Whilst the foraminiferal assemblages of the two sampling periods are very similar, the sediment grain size data differ with the 2006 samples having an overall decrease in the predominant sand sizes. Thus the primary mode in the 1983 samples ranges from 2.00 to 4.50 Φ, whereas for the 2006 samples the primary mode is always between 3.50 and 3.75 Φ. The high similarity in the foraminiferal assemblages of the two sampling periods strongly suggests that the physical and chemical characteristics of the water masses, within the sampled area, have not greatly changed during the intervening 23 years. The high level of grain sorting and the presence of a very restricted grain size in the 2006 samples (between 3 and 4 Φ) suggest, for the bottom sediment at the Lido Inlet, an origin due to human intervention, which has been particularly extensive since 2003.