An ecological survey of maritime rockpools at il-Qaliet (Malta)

Abstract

A study on the ecology of seven rockpools at il-Qaliet, a locality on the North-Eastern coast of Malta has been undertaken to determine the nature of abiotic and biotic components at work in this ecosystem. This was done by carrying out detailed investigations on six selected rockpools during the first two months of the wet season, and by mapping the vegetation of pools with a macrophytic community in the dry season. All the rockpools studied are situated on lower coralline limestone terrain less than 15m away from the shore-line, with some being relatively large (>7m in length). A layer of bottom sediment which varies in colour and texture is present in all rockpools. Most of the sediment is derived from surrounding terrain and organic matter from biological activity within the pool. Fluctuations in water depth are closely related to climatic conditions but presence of macrophytic plants in the pool greatly increases loss of water as a consequence of evapotranspiration. Electrical conductivity, and hence salinity was generally higher than that of inland pools but less than expected, by virtue of their position so close to the shore. Only the pool closest to the shore showed brackish characteristics on a few occasions. Hydrogen ion concentration was generally low with the corresponding pH values obtained being slightly alkaline and similar to those obtained in previous studies for pools on limestone substrata. Nitrate ion concentration was generally undetectable but on two occasions a level of 4mgl was recorded. This is the highest nitrate concentration ever recorded for lentic water bodies of this type in the Maltese Islands. Several pool inhabitants are r-strategists that rapidly colonize such water bodies. Rockpool fauna is dominated by microcrustaceans and most heterotrophs are either herbivores or detrivores. Few carnivores colonize the rockpools. The most important discovery regarding fauna is that of the species Branchipus visnyai Kertesz, a fairy shrimp that was only recorded twice before in the Maltese Islands, in smaller numbers. At il-Qaliet, B. visnyai was found to coexist with the similar and more widely distributed Branchipus schaefferi Fisher. Ostracods predominate in abundance, while two Cladoceran species represent new records for these pools. Absent crustacean groups which are typical of such pools include the Copepoda and the Notostraca (of which the species Triops cancriformis Bose was last recorded at this locality in the early 1980's). Other groups represented are the Collembola (springtails), Diptera and Ephemeroptera. The pool flora is dominated by macrophytes and the :filamentous alga Oedogonium. Faunal species richness of the pool communities was found to be significantly correlated with vegetational diversity in the pools. Comparison tests between pools based on general features and fauna! communities showed a generally high similarity between all pools.