Sediment dynamics and resuspension processes in a shallow-water Posidonia oceanica meadow

In the study, we describe the sedimentary dynamics in a shallow-water seagrass meadow. We examined the dynamics of vertical sedimentation flux over a Posidonia oceanica meadow during the course of a one-year investigation. Total sedimentary flux was monitored off the northern Latium coast (Italy) in the central Tyrrhenian Sea using sediment traps from October 2015 to December 2016 at three stations established on bare sand, in the P. oceanica meadow, and on a sand patch within the meadow. The traps were deployed during 11 surveys planned on the basis of the weather conditions and seagrass growth phase. The trap content was analysed for the amount of mass trapped and grain-size statistics, useful for describing sedimentary processes during storms. The meteorological data, P. oceanica parameters, and sand patch evolution during the year were analysed to determine their relation to the vertical sedimentation flux. The resuspension process dominated the shallow-water dynamics, but P. oceanica buffers the resuspension, especially during calm sea conditions. The sedimentological tracers showed that when high energy-flow regimes were present, the vegetation near the bottom enhanced the deposition of particles transported from adjacent unvegetated areas. During late Summer, leaf litter deposited on the unvegetated seafloor buffered the seabed sediment particles, preventing resuspension. We also detected the active transport of material from the sand patch to the meadow that was dependent on the energy flow regime, showing that the vegetation plays two roles: preventing resuspension and promoting deposition.