Colonisation, succession and seasonal dynamics on a Maltese rocky shore

Abstract

The processes of colonisation and succession were studied on cleared patches on a Maltese rocky shore. Two transects on the same shore were also sampled in order to study seasonal changes in the shore assemblages. The sequences observed in the mediolittoral and supralittoral fit the accepted general sequence usually observed on rocky shores around the world, and were similar in qualitative features to results obtained by other workers in the Mediterranean and other seas. The most obvious difference was the lack of ephemeral chlorophytes during the early stages of succession. The rate of succession in both the mediolittoral and infralittoral cleared patches was at the low end of the reported range from temperate and tropical shores. The different time scales for colonisation in the three zones, the rate of change, and the patterns of colonisation and succession within the patches, all indicate that colonisation is effected primarily by water-borne propagules, and is not so dependent on vegetative growth from the surrounding algae, even in such small patches (430-700 cm2). However, for the sedentary animal species - limpets and chitons in the mediolittoral zone, and anemones in the infralittoral zone, lateral migration was an important means of colonisation. Hierarchical cluster analysis and ordination by non-metric MDS of the data resulted in similar groups. For the supralittoral patches, the main factor differentiating between groups was the changing density of Littorina neritoides. For the mediolittoral and infralittoral patches, the stages of succession were generally as deduced from the raw data, although the later stages were often not separated in the statistical analyses. Location, seasonal changes, and time from clearing were the main factors separating the patches. An important factor was aerial exposure. particularly during January to March, caused by a combination of several days of good weather and lower than usual sea level. This was observed in successive years, and affected the organisms of the mediolittoral and upper infralittoral zones, with bleaching and some death of algae (predominantly Rhodophyta such as lithothamnoids, L. papillosa and Ceramium).