Effect of riparian vegetation on diatom assemblages in headwater streams under different land uses.

Differences in the structure of diatom assemblages in headwaters with contrasting shading conditions and different land use in the buffer zone and upper catchment were studied in order to evaluate the influence of the lack of riparian vegetation on the biofilm. The objective was to ascertain whether a riparian buffer can mitigate the negative influence of human induced disturbance and pollution on diatom assemblages in headwaters. Four streams were selected in order to maximize the differences in the land cover and minimize other environmental gradients. Multivariate statistics, different comparative and permutation tests and correlations were applied to compare the diatom assemblages, the Specific Polluosensitivity Index (IPS) and the diatom ecological guilds (low profile, high profile and motile) among the sites studied and to evaluate their responses to disturbances. The analysis showed that low profile diatoms typically dominated in forested headwaters with limited resources, whilst assemblages at impacted sites showed a wider range of growth forms. In unimpacted streams, the diatom assemblages were influenced by temperature, pH, conductivity and calcium, as usually reported for oligotrophic streams with high natural disturbance due to fast current and shading. In both shaded and unshaded impacted streams, the importance of nutrients and land use disturbance, especially urbanization, prevailed. This trend was also reflected by the IPS index that showed consistently lower values at impacted sites, correlating most significantly with nutrients. The diatom species composition as well as diatom guilds at impacted sites were similar, regardless of the presence or absence of riparian vegetation, and were significantly influenced by seasonal changes. Our results indicate that diatoms react sensitively to alterations of the water environment in headwaters, induced by anthropogenic activities, and these impacts are not buffered by an intact riparian zone. Diatoms closely reflected land use practices in the upper catchment regardless of the buffer zone status.