Sediment deposition and movement over a turf assemblage in a shallow rocky coastal area of the Ligurian Sea

Abstract

The depositional environment over a turf assemblage of the rocky shore south of Livorno was studied from April 1992 to September 1993 by means of sediment traps positioned 50 and 200 cm above the turf surface. Changes in environmental conditions and in the intensity of terrestrial erosion and wind-wave resuspension of bottom sediments played a major role in controlling both the magnitude of sediment deposition and the nature of sedimented material, while limited responses to seasonal patterns of sedimentation of phytoplanktonic material were found. Mean rates of sediment deposition fluctuated between 1.6 and 52.4 g m-2 d-1, and the amount of trapped material decreased with distance from bottom, reflecting intense sediment resuspension and movement over the algal turf. The material sedimenting was mostly composed of small-size (<200 μm) inorganic particles, and the organic material was characterized by the presence of highly degraded detritus. Besides faecal pellets few recognizable structures were found in the traps, such as charcoal fragments which probably originated from extensive burning of the coastal vegetation. Considerable variations in the intensity of the depositional environment over the algal turf were evident: the maxima sediment deposition and movement occurred in autumn and were related to heavy rainfall and wind-wave action, while in the summer the algal turf was generally free from sediment smother and scour. The ecological significance of these quantitative and qualitative temporal fluctuations is discussed, and the possible long-term impact of fire deforestation and increased soil erosion on turf structure and composition is hypothesized.