Structural and functional responses of lotic biofilm to abrasive sediments and P enrichment: an indoor experimental approach

Abstract

Biofilms are hotspots of metabolism in streams. A common natural and anthropogenic disturbance in lotic systems is the input of nutrient-rich sediments, which may affect the stream bottom and thereby biofilms. Here, we present an indoor experiment with artificial channels to test the effect of phosphorus (P) and abrasive sediment inputs on biofilm structure. The experiment had a factorial design with four treatments: (1) without any addition (treatment C); (2) addition of abrasive sediments (treatment G); (3) phosphorus addition (treatment P); and (4) addition of abrasive sediments and phosphorus (treatment GP). Our results showed that the addition of phosphorus alone (treatment P) increased biofilm biomass (increased both chlorophyll-a and carbon [C] content), while the addition of sediment had a strong negative effect on biofilm biomass and elemental ratios (C:nitrogen [N] and N:P), both when added alone (treatment P) and when added with P addition (GP). The P addition treatments (P alone and GP treatments) resulted in a decrease in alkaline phosphatase activity. At the end of the experiment, we observed a shift in the primary producers: while P treatments were dominated by Cyanobacteria, both the G and C treatments were dominated by diatoms. In particular, the G treatment, where the biofilm was composed of a thin layer of diatoms, exhibited the highest photosynthetic efficiency (highest electron transport rate and photosystem II efficiency). The final balance of the input of abrasive sediments with P enrichment resulted in a young-stage biofilm with a high proportion of extracellular polymeric substances. These results highlight the effects of combined stressors since abrasive sediments even at high P levels can severely affect both the structural and physiological parameters of biofilms.