Sediment retention by a mediterranean Posidonia oceanica meadow: The balance between deposition and resuspension

The role of Posidonia oceanica in promoting sediment stability and accretio n was studied in a 15 m deep meadow at Fanals Point (NW Mediterranean, Spai n) by comparing particle deposition within the meadow and adjacent bare sed iment. Small sediment traps were used to measure deposition within and abov e 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 f or the first time at the measuring site) and resuspended deposition (sedime nt particles that have been previously deposited at the measuring site). Me asurements were conducted monthly over a year to establish the magnitude an d seasonality of deposition, and to form a balance of particle transport at the annual time scale. Significant differences in total deposition were fo und over time, ranging from 1.5 to 500 g DW m(-2) d(-1), including those be tween bare and vegetated sediments. The effect of P. oceanica in increasing primary deposition at an annual scale was modest, however, P. oceanica sig nificantly buffered sediment resuspension, which was reduced more than thre e fold compared to the unvegetated bottom. The annual flux of deposition wa s dominated by settling of resuspended materials, which represented 85% of the total flux within the meadow, but 95% of the total deposition on bare s and. Thus, seagrass meadows reduce resuspension in the NW Mediterranean lit toral, thereby contributing to increased sediment retention and, therefore, reducing erosion in the coastal zone. (C) 2001 Academic Press.