The ecology of freshwater rockpools in Malta

Abstract

A study of the ecology of freshwater rockpools in Malta has been undertaken in order to determine the nature of the interactions between the biotic and abiotic factors of the pool environment. This was done by carrying out detailed investigations on five selected pools in two localities. and through a survey of 19 other pools situated throughout the Maltese Islands. Rockpools generally occur on garigue terrain on outcrops of Upper and Lower Coralline Limestones. Pools seldom exceed Sm in length and are for the greater part less than a metre in length. Several pools possess a layer of bottom sediment comprising soil from the surrounding terrain and organic matter derived from biological activity within the pool. Electrical conductivity, and hence salinity in local inland pools tends to be low. Pools that are situated closer to the coast are characterized by higher conductivity, while pools sited less than 20 metres from the coast are brackish. The only exception to the latter phenomenon is Ghadira s-Safra. which was found to contain water of low conductivity, although a halophytic flora is present. Hydrogen ion concentration in poolwater is low, with a pH that is consequently alkaline . Values recorded ranged from 7.56 to 9.65 pH units. The high pH values are due to the presence of a sediment layer. Turbidity in the pools selected for detailed investigation was negligible. No stratification of electrical conductivity, pH and temperature has been found. and this is related to the low turbidity of the poolwater. The rockpool fauna is dominated by aquatic crustaceans: branchiopods, ostracods and copepods. Other groups represented are the Turbellaria, Insecta and Anura (mainly as tadpoles). The pool flora comprises filamentous algae, macroalgae (Characeae and Palmellaceae) and aquatic angiosperms. Interspecific competition is reduced due to the brevity of the aquatic phase, although some species may undergo niche diversification to allow coexistence with superior sympatric competitors. Several pool inhabitants are r-strategists (or fugitive species) and colonize the aquatic environment very rapidly. Faunal diversity is greater at the start of the wet season. shortly after filling of the pool relative to the end of the wet season, when ostracods have been found to predominate in terms of number. Most pool heterotrophs are detritivores and/or herbivores, and few different carnivores colonize rockpools. Observed adaptations of pool inhabitants to the ephemeral nature of their habitat include wide tolerances of salinity, temperature and desiccation.