Studies on chemical, physical and biological conditions in Swedish rockpool ecosystems

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Abstract
Although rockpools are highly dynamic and changing systems which cannot be strictly classified, it has been found convenient to recognize five main types: permanent salt-water pools, ephemeral salt-water pools, brackish-water pools, permanent freshwater pools and ephemeral freshwater pools. The physical and chemical conditions of the rockpools are very variable. Salinity, temperature, oxygen and pH have especially been considered in this investigation. These parameters are not only regulated by the ambient conditions of the climate and the sea water, but also by such biological conditions as population density and algal growth, which particularly control oxygen and pH conditions. The salinity has formerly been considered to be the main regulating parameter in rockpools, but along the Baltic coast salinity fluctuations are mostly moderate in large and medium-sized pools. In these pools the temperature is considered to be of greatest importance for the rockpool organisms; diel fluctuations of 10°C, annual extremes of 35°C and below 0°C and ice may occur. The oxygen and pH variations are often large. Diel amplitudes of 300% oxygen saturation and 2 pH units, from pH 8 to pH 10, are commonly recorded. These environmental conditions give significant faunistic features, individual for each pool. Only few, very tolerant organisms occur in the pools. Among the algae, Enteromorpha intestinalis is dominant in many types of pools. It serves both as shelter, substratum and food for many rockpool organisms, e.g., Nais elinguis, Nitocra spinipes, Heterocypris salinus, Gammarus duebeni, and Hyadesia fusca. These permanent members of the rockpool ecosystems and a few occasional guests like chironomid larvae, corixids, water beetles, the three-spined stickleback, and a few other migrating species from the nearby littoral zone constitute the food webs of the pools. These are short and simple due to the low number of carnivores. The diversity index is reduced in rockpools due to, e.g., the severe abiotic conditions and a strong interspecific competition caused by the restricted number of niches available. Besides, the rockpool ecosystems are young, never growing very old. The land elevation changes the rockpool communities towards a terrestrial climax community. This fact tends to support the low species diversity of the rockpool ecosystems. The rockpool organisms are well adapted to the variable conditions. A high biotic potential and environmental resistance, including short life cycles, rapid development, parthenogenetic development, resistant eggs and possibility for some animals to migrate from the pools when conditions become too severe promote occurrence of rockpool organisms in their characteristic environment.