Disturbance frequency and carbon supplementation affect trophic dynamics of recovered freshwater biofilms

Background/Question/Methods

Stream biofilms are comprised of a taxonomically diverse consortium of microorganisms inhabiting a complex matrix of organic exudates adhered to submerged substrata. Consisting of both heterotrophic and autotrophic populations, biofilm trophic dynamics are often closely linked to community succession, as colonization time dictates the availability of many resources, including space, access to dissolved resources from the overlying water column, and light. However, because biofilm stability is often a function of external factors, such as increased water velocity and associated scouring by entrained sediments, disturbance events can greatly alter biofilm dynamics, including trophic interactions and successional patterns.

Clay pots colonized with biofilms in a northeast Ohio stream were subjected to low- and high-frequency artificial perturbations and amendments with labile DOC to examine how disturbance affects the trajectory of biofilm trophic dynamics. During a 4-week recovery period, weekly samples were collected and ratios of extracellular enzyme activity (EEA) were used to assess C and N acquisition patterns during the recovery period. Ratios of EEA have been used to evaluate nutrient needs of microbial communities. In addition, bacterial community composition was examined based on terminal restriction length polymorphisms of 16S rRNA genes to examine links between bacterial community composition and function.

Results/Conclusions

The activity of enzymes varied among treatments. Specifically, low (single perturbations) and high-frequency (multiple perturbations over the course of 12 days) disturbances significantly (p<0.05) altered the activity ratio of high labile C (BGLU) to low labile C (BXYL) acquiring enzymes during early biofilm recovery, while the effects of high-frequency disturbances remained constant throughout the study. Comparisons of EEA ratios of BGLU and BXYL to those linked to N acquisition (LAP) showed similar results, although BGLU:LAP ratios were more sensitive to both low- and high-frequency disturbances at later stages in the recovery, indicating a persisting shift in nutrient requirements. In conclusion, this study highlights the complexity of biofilm recovery trajectory and suggests that the resiliency of biofilm trophic dynamics varies with disturbance frequency.