Early development of algal assemblages under different regimes of physical and biotic factors on a seasonal tropical rocky shore

Abstract

The roles of physical factors and herbivory in structuring the early development of algal assemblages were investigated on a seasonal, tropical rocky shore in Hong Kong. Physical stress was manipulated by irrigating areas with filtered seawater, whilst herbivore access was controlled using fences. Algal colonisation was monitored at 2 tidal heights (high- and lowshore), once in the cool and dry season and once in the hot and wet season. Dry areas were only colonised in the lowshore and, even when herbivores were excluded, dry highshore areas never developed a macroalgal cover. Algal assemblages did however, develop in highshore irrigated areas where, in the presence of herbivores, encrusting algae (e.g. Hapalospongidion gelatinosum and coralline crusts) developed. In areas excluding herbivores, ephemeral erect algae (e.g. Ulva spp.) developed. All lowshore treatments developed algal assemblages. In the hot season, after an initial phase of erect algae, encrusting algae developed where herbivores had access, whereas, in the cool season, erect Corallina spp. dominated these treatments. In herbivore exclusions, erect ephemeral algae developed in the hot season with a sparse cover of encrusting algae (Ralfsia expansa and coralline crusts) whilst, in the cool season, a cover of Corallina spp. and erect ephemeral algae became abundant. When irrigation was terminated, erect algal cover decreased dramatically in each season in the highshore, although encrusting algae persisted at reduced cover. In lowshore areas, algal cover decreased marginally when irrigation was terminated and only during the hot season. The timing (i.e. hot or cool season) of liberation of rock space was, therefore. important for the development of algal assemblages. In lowshore areas, assemblages dominated by encrusting algae typical of rocky shores in Hong Kong developed only in the presence of herbivores and only during the hot season. During the cool season, Corallina spp. became dominant, while exclusion of herbivores (during either season) resulted in the development of ephemeral, erect algal assemblages.