Do factors influencing recruitment ultimately determine the distribution and abundance of encrusting algae on seasonal tropical shores?

Abstract

Similar to many low-latitude shores, encrusting algae are the dominant space-occupying organisms on moderately exposed rocky shores in Hong Kong. A multifactorial experiment assessed the roles of herbivory, season, tidal height and substratum inclination on the initial recruitment of encrusting algae to artificial surfaces. Herbivores reduced recruit densities at certain times of the year and tidal heights, but did not prevent settlement. In contrast, the relative timing of recruitment and the prevailing environmental conditions greatly affected recruit success. No species recruited at any time to the high shore, and species recruited as high as the mid shore only during cooler months. Species that recruited during the cooler months (e.g. Hapalospongidion gelatinosum and coralline crusts) could colonise the mid shore, while Ralfsia expanse, which recruited primarily during the hot season, was restricted to the low shore and subtidal. Recruitment also varied with substratum inclination; cool-season recruits were found in greater densities on horizontal surfaces, whereas hot-season recruits exhibited no difference between horizontal and vertical plates. Seasonal availability of free space may influence the life-history strategies of encrusting algae. R. expanse recruited during the hot season to the low shore, when free space was available following the annual die-back of algae. In contrast, cool-season recruits (e.g. H. gelatinosum) have less free space to colonise and as a result become established in greater densities higher on the shore, resulting in increased physical stress and mortality during the following hot season. The timing of recruitment relative to spatial and temporal variation in the physical environment, therefore, greatly affects the potential distribution of mature encrusting algal populations on Hong Kong shores.