Distribution, discharge and disturbance : new insights into faunal spring ecology

Abstract

Springs are unique ecosystems at the interface between groundwater and surface water and are said to provide relatively stable abiotic conditions for the species that live there. Despite numerous studies on spring ecology there are still substantial gaps in our knowledge about the functioning of spring ecosystems. Important hypotheses concerning the abiotic stability and the longitudinal extent of springs first described by August Thienemann have never been questioned. Therefore, the goal of this thesis was to revise the dogmas of spring research and to give answers to unresolved questions concerning the composition and distribution of spring species assemblages. A widespread survey on the chemical, physical and substrate parameters of springs and their influence on the macroinvertebrate assemblages was the first step of the thesis. A nonmetric multidimensional scaling and an analysis of similarities did not reveal a grouping of the springs, neither with the faunistic data nor with the physicochemical and substrate data. However, it was possible to identify that discharge of the springs was the factor that significantly influenced the composition of macroinvertebrate assemblages. The traditional ecomorphological spring typology of August Thienemann is not applicable to the remaining small springs found in the Swiss Jura Mountains that we investigated. We therefore propose to regard springs as existing on a continuous scale of ecomorphological appearance. In consequence the development of a new spring typology based on faunistic parameters and not on the ecomorphological features of springs was the next important goal of the thesis. We took faunistic data collected over three years from springs in our main research area, the Roseren valley. An analysis of similarities and the SIMPER-procedure revealed a grouping of the springs based on the faunistic data, especially on the functional feeding groups and key species characteristic for those springs. Considering that springs occur on a continuous scale we have a continuum of springs from a lenitic to a lotic environment characterized by typical species assemblages. Our first general survey has shown gaps in our knowledge about the zonation of upstream sections of headwaters. The longitudinal extent and boundary of springs are still discussed in spring research and there are still unresolved questions. We examined the longitudinal zonation of the macroinvertebrate assemblages and the substrate composition in the first 100 meters of three headwater catchments. We could significantly differentiate the springhead section from the springbrook section independent of the substrate composition. A significant change in the composition of the macroinvertebrate assemblages occurred just five meters from the source. In accordance with our temperature measurements and many other studies, we support the hypothesis that temperature stability is one of the most important factors for the distribution of spring species. The springhead as the actual spring can be differentiated from the springbrook and should be considered as a distinct ecosystem in future research. Springs are said to exhibit stable abiotic conditions in comparison to lower sections of headwaters. Disturbance events have traditionally not seemed to be relevant for explaining the composition of macroinvertebrate assemblages in springs, and it is still a controversial topic in spring research. Because we identified discharge as an important factor for explaining the composition of macroinvertebrate assemblages in springs we consider discharge variability as a disturbance factor highly relevant for understanding spring ecosystems. To verify this hypothesis a scientifically reliable and easy to handle measurement of the discharge is necessary. We applied the gypsum dissolution method developed for oceanic habitats to springs. During a one year period gypsum spheres were exposed in five natural springs in the Swiss Tabular Jura 22 times for six day-periods on average. After a rigorous calibration in the laboratory the calculation of the discharge in the springs was possible. In addition, seasonal faunistic samples had been conducted to investigate the impact of the discharge variability on the spring species diversity. Our results show that there is a relationship between discharge variability and species diversity, a clear correlation as described in the intermediate disturbance hypothesis was not detected. The gypsum dissolution method is an adequate method for measuring the discharge of springs in short-term and mid-term periods. This is of importance for future research, when discharge variability will increase due to climatic change. With this thesis we are now able to give answers to some of the fundamental questions in spring ecology and provide new insight into the functioning of springs. Springs have to be considered as distinct ecosystems with a longitudinal extent restricted to only a few meters from the source and inhabited by characteristic macroinvertebrate assemblages. They are adapted to the thermally stable environment and their composition is influenced by the discharge regime of the springs. Given predicted global climate change effects, the monitoring of the discharge variability could be an important indicator, and a focus of future research.