The use of urban clay-pit ponds for human recreation: assessment of impacts on water quality and phytoplankton assemblages

Artificially created ponds in urban areas may be important biodiversity refugia and may provide recreational services for populations. In order to obtain information on the seasonal development of the environmental conditions, water quality was determined in ten clay-pit ponds situated in the Austrian capital, Vienna. These ponds show high electrical conductivity (up to 3,000 mu S cm (-aEuro parts per thousand 1)), indicating elevated levels of salinity, which can be attributed to the geological setting of the underground. Furthermore, the ponds experience a gradient from low to high human pressure resulting from recreational activities (swimming, fishing, urbanisation of the pond boundaries). Results obtained from multivariate statistics methods suggest that ponds were mainly structured by salinity and by algal biomass, which can be attributed to resource supply related with eutrophication. According to their water chemistry, the ponds were classified as meso- to hypereutrophic. Stoichiometric N/P ratios suggest that phytoplankton productivity in hypereutrophic ponds is nitrogen limited, whilst algae in ponds with lower trophic levels experience growth imitation by phosphorus depletion. We eventually related environmental conditions to algal species occurrences and developed a model for algal assemblages indicating the particular trophic state at different seasons.