Responses of algae with different life histories to temporal and spatial variability of disturbance in subtidal reefs

Colonisation of patches disturbed at different times of the year and at different locations was investigated from November 1995 to August 1997 in subtidal, rocky-shore, algal assemblages (Mediterranean Sea, Italy). A field experiment tested specifically: (1) whether and how the success of turf-forming, encrusting and erect algae are influenced by the timing of disturbance and (2) whether recovery of these species is consistent across space (from about 1 m to 100s of metres). Clearings were made at 8 different times within 1 year, in order to identify possible temporal trends in responses of algae to disturbance. Reproductive traits of the most abundant algae were also investigated. Encrusting, turf-forming and erect algae responded differently to spatial and temporal variation in disturbance, depending on their varying abilities to colonise available space and on spatial and temporal differences in rates of recruitment and growth. Turf-forming algae colonised space by vegetative propagation and quickly regained spatial dominance in patches cleared at all times of the year and at all locations. Conversely, encrusting and erect species occupied space mostly by colonisation of propagules; their recruitment was influenced by the timing and location of disturbance, probably depending on the concomitant availability of bare rock, viable propagules and favourable environmental conditions. A regime of disturbance adverse to recruitment of erect algae was identified as the probable cause of their low abundance in the study area. Overall, results suggested that while recruitment by sexual propagules can be highly variable through space and time, recovery by vegetative propagation can be constant and highly predictable over a range of environmental conditions, and that vegetative reproduction is likely to be fundamental to spatial dominance in a variety of habitats.