The influence of short term water level fluctuations and desiccation stress on periphyton development at a riparian zone of a large regulated river

Periphyton can play an important role for biogeochemical processes in large regulated rivers. The development of benthic algae during low flow periods are especially suitable in braided sections, near bank zones and shallow areas. Light availability, fine sediment accumulation and flow velocity are assumed to be the main factors controlling periphyton growth, while the effect of water level fluctuations in large rivers is often neglected. However, the latter could also have severe impacts on the development of benthic algae. The present study aims to analyse how short term (hourly and daily) water level fluctuations influence the periphyton development and species richness and how desiccation stress affects the specific primary production. An in-situ experiment was conducted for one month on a Danube River bank in the free-flowing stretch of the river east of Vienna. Plates with artificial substrates were exposed at several defined depths along a fluctuating water level gradient. A short term laboratory experiment was also conducted to assess the effect of drying and rewetting on benthic algal production. Our results showed the benthic chlorophyll-a content was 40 to 60 times lower and diatom species were 5 times fewer at a desiccation-affected zone compared with the permanently-immersed zone. However, after a short term flow pulse lasting 3 days, the differences between the two zones were alleviated. In the laboratory experiment, the benthic algal community from two different colonisation stages with different chlorophyll-a content and species composition (days 12 and 21 of development) were capable of efficient photosynthesis for 2 and 5 hours of desiccation, respectively, followed by no measurable photosynthetic activity for an additional hour. Complete recovery of photosynthesis took 3 and 5 hours after rehydration. The results of this study showed that the periphyton on a riparian zone of a large regulated river is a very dynamic community with a high capacity for resilience with regard to the highly fluctuating environmental conditions. Furthermore, short-term desiccation stress should be considered as an important form of disturbance that affects algal communities in altered river systems.