Phytoplankton community structure along saline and trophic state gradients in urban clay-pit ponds (Austria)

We quantified interactive effects of human activities and ionic composition on algal community patterns. The work was done in ten urban clay-pit ponds within the vicinity of Vienna (Austria), characterized by gradients of clay-rich geological deposits and different trophic states caused by multiple sources of anthropogenic stress like nutrient input, swimming or shoreline destruction. A total number of 397 algal taxa were found with some halotolerant species occurring in high abundance like the diatom taxa Entomoneis paludosa, Chaetoceros muelleri, Navicula halophila or the rare Gyrosigma parkerii. Halophil and halotolerant taxa were insensitive to eutrophication. Although the ponds showed on average only moderately elevated electric conductivities (1833 +/- 580 (SD) mu S cm(-1)), taxa number were negatively related to ion concentration. SO(4)(2-) and HCO(3)(-) were the most important anions, with SO(4)(-2) dominating at most sites. Ca(2+) was the major cation in six ponds; four more had Ca(2+) and Na(+) as co-dominant cations. The Viennese sites experienced higher nutrient loadings than the ponds located in Lower Austria. Cluster analysis revealed eight significant groups which represented both spatial and temporal differences among individual ponds. For these groups 32 indicator species were defined. With only two exceptions no halophil nor halotolerant taxa were found as indicator species because these were highly abundant in all investigated ponds. Canonical correspondence analysis showed a strong spatial separation of the ponds with 20 % of the species variation being explained by environmental variables. Variance partitioning revealed the composite variables eutrophication as a function of anthropogenic pressure and ion concentration of equal importance for species variability data.