Epixylic biofilm and invertebrate colonization on submerged pine branches in a regulated lowland stream

Epixylic biofilm and invertebrate assemblages on pine branches (Pinus syl- vestris) experimentally submerged in October 2000 were investigated, after an initial colonization period of 3 months, by two-monthly sampling intervals for 13 months in a nutrient-rich sandy lowland stream subjected to flow disturbances caused by infre- quent impoundment openings. After 12 weeks of exposure an epixylic biofilm with 0.58 ± 0.25 µg/cm 2 chlorophyll-a (mean ± 1 SE) and 0.23 ± 0.04 µg/cm 2 ATP, as well as an abundant invertebrate community (14326 ± 2532 Ind/m 2 ; biomass: 974.6 ± 360.1 mg/m 2 ) became established on the branch surfaces. During the subsequent sam- pling dates invertebrate numbers decreased significantly during periods of high dis- charge and simultaneously chlorophyll-a values of epixylic biofilms increased, likely due to reduced feeding by invertebrate grazers. During periods with low discharge fluctuations and mainly low flow conditions, the epixylic biofilm, especially algal growth, was negatively correlated with invertebrate grazer and shredder numbers. After the initial growth period of 12 weeks, ATP values of the epixylic biofilm showed a strongly negative response to sand deposited on the wood surfaces. The present study displays the influence of seasonal discharge fluctuations and sand deposition on the wood surfaces on epixylic biofilms and invertebrate assembla- ges on experimentally submerged pine branches, but also indicate interactions between food sources (algae and fine particulate organic matter) and invertebrates (grazers and collector/gatherers).