Responses of turf-forming algae to spatial variations in the deposition of sediments

Responses of a turf-forming, filamentous, algal assemblage to spatial variation in the deposition of sediments were investigated from September 1994 to September 1996 on a rocky subtidal reef south of Livorno (Mediterranean Sea). Large-and small-scale spatial differences in the amount and size of sediment settling on and accumulating in the turf were investigated over 1 yr in relation to meteorological conditions and to patterns of abundance of the turf. A multifactorial manipulation of the deposition of sediment was used to test the hypotheses that (1) different amounts and grain-sizes of settling material affect the cover and biomass of the filamentous algae, (2) any effects of sedimentation are independent of the stage of development of the turf and (3) patterns are consistent at different spatial scales, ranging from a few centimetres to several metres. During the investigation, the regime of sedimentation fluctuated greatly over time and among sites. The distribution of the turf across different sites was, however, not apparently related to large-scale variations in the deposition of sediment. Within sites, accumulation of sediment in the turf was patchy. The results of the experiment indicated that patchiness may affect the local biomass of established turf-forming algae, the abundance of which significantly decreased in quadrats which had higher deposition of sediments. On a scale of centimetres, high accumulation of sediment decreased variability of algal biomass within individual quadrats. The effects of sediment on biomass were also significantly influenced by the stage of development of the turf. No effects were observed on the cover of the filamentous algae. These results suggest that the responses of the turf to variations in the characteristics of the depositional environment may vary with changes in spatial scale and may be dependent on the concomitant action of other physical and biological factors.