Seasonality and inter-annual variability in recruitment patterns of temperate encrusting fauna

Sets of triplicate slate panels were immersed at two sites in SW Ireland and at three depths and analysed monthly between 1997 and 2001. The mean number of recruits varied on both seasonal and interannual time-scales, peaking during summer periods. Relative sizes of recruitment peaks varied both between sites and depths. Recruitment on intertidal panels was an order of magnitude lower than on those in the subtidal, even when allowing for immersion period. The mean number of species followed a predictable seasonal pattern, peaking in summer, and varied little on an inter-annual basis. Per cent cover varied less predictably with season and year. Typically just two of three colonist species were responsible for 90% of space occupation, but this also varied with time on a variety of scales; Recruitment of individual taxa showed a variety of patterns both in terms of the timing of initiation or cessation and duration. These included predictable bursts of settlement of similar seasonal timing (e.g. Aetea sp.) and predictable long duration bouts (e.g. Chorizopora brongniartii ) to less predictable bursts (e.g. the sponge Scypha ciliatum ) and low level fluctuations (e.g. Scrupocellaria sp). The variability of the study assemblages in both time and space illustrate the importance of within- and between-site replication and the use of long-term data sets.