Subtidal fouling assemblages usually consist of short-lived organisms. Colonisation sequences on manmade structures may thus be greatly aVected by the temporal and spatial variability of propagule supply. This study explores the inXuence of seasonality on succession in the macrofouling assemblage of a Patagonian harbour (Argentina, Southwest Atlantic). Replicated artiWcial substrata were suspended horizontally and sampled at quarterly intervals during 1 year. The inXuence of seasonality on 1-yearold assemblages was further analysed using additional sets of replicated panels submersed at diVerent seasons and collected 1 year later. Upper surfaces were always dominated by ephemeral algae, while lower surfaces exhibited high coverage of Wlter-feeding invertebrates. Regardless of submersion length, species richness was signiWcantly higher on lower than on upper surfaces. A signiWcant interaction between orientation and submersion length was found for the Shannon diversity index, meaning that temporal changes in diversity depended on substratum orientation.On the lower surfaces, diversity reached a maximum after 9 months and then declined, mainly due to extensive dislodgment of two species of ascidians. On algal-dominated upper surfaces, diVerences in structure of annual assemblages were due to seasonal changes in the abundance of ephemeral algae. This study shows that constancy or variability of 1-year-old assemblages whose development began at diVerent seasons depended greatly on the life history of the organisms that settled and managed to persist on both surfaces, which in turn depended on substratum orientation.

The recruitment of subtidal macrofouling organisms was studied in the Patagonian harbour of Comodoro Rivadavia (Argentina, 45851 0 35 00 S 67827 0 23 00 W). Changes in coverage and density were analysed in the central 100 cm 2 of upper and lower surfaces which were replaced monthly from January to December 2004. The fouling assemblage consisted of algae, spirorbid polychaetes, compound ascidians, hydrozoans, bryozoans and egg masses spawned by the small fish Helcogrammoides cunninghami. Monthly changes in richness and diversity of taxa on upper and lower surfaces were significantly correlated with sea surface temperature. Fouling assemblage structure differed significantly between upper and lower surfaces. Filamentous algae were dominant on upper surfaces, while filter-feeding invertebrates were more abundant on the lower surfaces of the experimental panels. The density of the spirorbid Romanchella scoresbyi was two orders of magnitude higher on lower than on upper surfaces. Its recruitment began in late winter, reaching maximum values in spring.

Predatory control of community structure, through consumption of herbivores and subsequent positive effects on the biomass or productivity of primary producers is common in marine ecosystems. We designed an experiment to analyse the effect of large-sized predators on the development of the macrofouling assemblage of a harbour. Predation was manipulated by randomly assigning artificial substrata to three treatments: (1) exclosure cages preventing access of large predators, (2) cage controls, i.e. cages with lateral walls but without roofs, allowing predators free access, (3) uncovered panels. Substrata were submersed horizontally and upper surfaces sampled at quarterly intervals over a 1-year period.No differences in species richness were observed among treatments but diversity was lower within exclosure cages than on cage controls/uncovered panels. In sessile assemblages, structure differed between treatments: cage controls/uncovered panels were dominated by algae and associated mesoherbivores, which were scarce/absent within cages. Exposure to predators also affected the structure of mobile assemblages. The sea urchin Pseudechinus magellanicus (Philippi) occurred within exclosure cages but was never found on exposed panels. We conclude that large predators exert a significant effect on assemblage structure and diversity, indirectly promoting the development of filamentous algae and mesoherbivores by controlling the abundance of browsers.