Antagonistic and synergistic effects on a stream periphyton community under the influence of pulsed flow velocity increase and nutrient enrichment.

Aquatic ecosystems are generally affected by multiple stressors, and therefore, analysing single stressor responses is not appropriate to evaluate the whole range of effects on these ecosystems. We assessed the interaction effects of two strong stressors (higher flow velocity due to e.g. daily hydropeaking) and nutrient enrichment for an oligotrophic stream periphyton community. As periphyton has a rapid reproduction rate and very short life cycles, it can therefore be expected to reflect short-term impacts and sudden changes/disturbances in the environment. We measured biomass development, algal group distribution and photosynthesis efficiency during a time period of 33days in an experimental flume setting in Lunz am See (Austria). We conducted the experiment with two treatments (no hydropeaking and hydropeaking) and three nutrient enrichments (nitrate, phosphate and nitrate+phosphate enrichment) and control (no nutrient addition). The results showed a significant lower biomass development in the hydropeaking treatment (HP), compared to the no-hydropeaking treatment (NHP) in a later successional stage (day 33). Nutrient subsidy effects were not observed, because the biomass development (chlorophyll-a) of periphyton was highly diminished through the pulsed flow velocity increase. Also a negative synergistic interaction (more negative than predicted additively) was observed. Our study confirmed for periphyton communities that for different algal groups and functional guilds the same multiple stressor combination can be detrimental for one species group (e.g. chlorophyta) while beneficial for another (e.g. diatoms). We conclude for multiple stressor studies to consider the successional stage and community composition, when estimating the interaction effects of these stressors. Copyright Â