Posidonia oceanica banquette removal: Sedimentological, geomorphological and ecological implications

Posidonia oceanica Delile (L.) is the most widespread seagrass species of the Mediterranean sea. On the sandy shores, cast litters of this seagrass are deposited as wedge structures from few centimeters to several meters thick defined "banquette". Banquettes are removed from beaches mainly for aesthetic reasons. The aim of this work is to evaluate the potential impact of banquette removal from beaches of the Sardinia island from the sedimentological, ecological and geomorphological points of view. Data on banquette management were collected by means of a questionnaire given to the coastal municipalities and private companies in charge of beach-cleaning operations during 2004. P. oceanica banquette removal resulted in a widespread practice applied on 44 beaches (out of 116). Total removed amount for in 2004 was estimated at 106,180 m3. Heavy machines were generally used to remove banquettes. The relationships between the morphology and composition of banquettes and the beach exposure to the dominant waves were analyzed to evaluate the potential impacts of banquette removal on beach geomorphology. GPS survey techniques and banquette sampling were carried out on three beaches characterized by different exposure to dominant waves. Banquettes showed a higher volume, thickness and cross-shore length on exposed beaches, whereas narrower litter deposits were found on the sheltered beach. On exposed beaches, banquettes were deposited in beach sections characterized by changes in elevation. These changes in elevation were mainly due to the deposition and erosion of sediments and secondly to the deposition and or erosion of leaf litter. On the sheltered beach, the variability of beach morphology was low and was restricted to sectors where the banquettes were located. In addition, banquettes represent a temporary sink of biogenic elements for the seagrass ecosystems, and their removal causes a permanent loss of C, N and P. In our study the highest value found of N and P losses were respectively 2.3-5.4% and 0.6-1.2% of the annual plant requirement. Furthermore the removed material were disposed on a unauthorized area, leading to a potential impact on soils. This study highlights that (i) banquette removal could have a strong impact on the beach morphology, particularly on the beachface of exposed beaches, modifying the berm - bar exchange of sediment and seagrass litter; (ii) the sediment removed during the removal operation is permanently lost from the beach system and (iii) a depletion of nutrient and biomass for the coastal ecosystem may occur.