The effects of compounded perturbations on stream biofilm recovery trajectory

Background/Question/Methods

Stream biofilms serve as the nexus for a host of vital ecosystem services, such as carbon fixation and storage, and nutrient cycling. Yet, as a sessile community in a transient system, biofilms are highly susceptible to removal due to disturbances affecting stream flow. As landscape modifications (e.g. urbanization) and increasingly erratic weather patterns converge to create flashier rain events, the potential for biofilm removal via disruptive stream flows increases. Of particular concern is how frequent disturbances occurring concurrently with community recovery (i.e. compounded perturbations) affect biofilm recovery trajectory, and whether the structural and functional aspects of the biofilm community are restored post-perturbation. The purpose of this study is to examine how biofilm communities recover after compounded perturbations, such as those expected under future climate scenarios/land use changes.

Biofilm-colonized clay pots were subjected to artificial high-frequency perturbations, with half of the pots receiving labile C amendments in an attempt to elucidate bacterial-algal interactions. During a 4-week recovery period, weekly samples were collected and bacterial and algal community structure was examined using terminal restriction fragment length polymorphisms and visual identification, respectively. Leucine uptake, chlorophyll a, ash-free dry mass (AFDM), and extracellular enzyme activity were used as proxies for biofilm function. 

Results/Conclusions

Initial leucine data suggests that C additions were significant (p ≤ 0.05) to early biofilm recovery, but that the effects of disturbance persisted into biofilm maturation. However, neither disturbance nor C additions significantly affected AFDM, suggesting biofilm biomass persisted throughout the study. Similarly, extracellular enzyme activity appeared to remain consistent across treatments, with the exception of the second week of recovery, as both C and disturbance treatments saw a significant (p ≤ 0.05) increase in enzymatic activity for C, N, and P scavenging enzymes. In conclusion, this study highlights the complexity of recovery dynamics in biofilms and suggests that certain aspects of biofilm functionality may remain altered post-recovery.