Sediment Retention by a Mediterranean Posidonia oceanica Meadow: The Balance between Deposition and Resuspension

The role of Posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at Fanals Point (NW Mediterranean, Spain) by comparing particle deposition within the meadow and adjacent bare sediment. Small sediment traps were used to measure deposition within and above the meadow and over bare sand. A model, based on measurements of particle deposition at increasing distances from the bottom, was used to partition the total depositional flux between primary (sediment particles deposited for the first time at the measuring site) and resuspended deposition (sediment particles that have been previously deposited at the measuring site). Measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition, and to form a balance of particle transport at the annual time scale. Significant differences in total deposition were found over time, ranging from 1·5 to 500 g DW m−2 d−1, including those between bare and vegetated sediments. The effect of P. oceanica in increasing primary deposition at an annual scale was modest, however, P. oceanica significantly buffered sediment resuspension, which was reduced more than three fold compared to the unvegetated bottom. The annual flux of deposition was dominated by settling of resuspended materials, which represented 85% of the total flux within the meadow, but 95% of the total deposition on bare sand. Thus, seagrass meadows reduce resuspension in the NW Mediterranean littoral, thereby contributing to increased sediment retention and, therefore, reducing erosion in the coastal zone.