Coupled roles of bottom-up and top-down effects throughout stream microbial biofilm succession

The importance of bottom-up conditions and top-down grazer effects throughout aquatic biofilm succession is difficult to differentiate in natural ecosystems. We investigated this relationship in a coupled field and laboratory study. Biofilm communities were grown in a natural stream under ambient, modified flow (turbulent), dark, and modified flow + dark conditions for 7, 14, 21, and 28 days and then subjected to no grazing, mayfly (Maccafertium sp.), snail (Goniobasis sp.), and snail + mayfly grazing treatments in the laboratory. Communities grown under modified flow had significantly greater biomass after 14 days compared to the other conditions. After subjected to grazers, communities at 21 days under modified flow were preferred showing the greatest decrease in biomass; while communities at 7 days had the least. These patterns suggest preferential grazing of older biofilms that was influenced by modified flow. Grazing was the most significant factor structuring biofilm community profiles, and the only significant differences of taxa were between the grazing treatments and the no grazing control. Taken together, these results suggest timing differences where bottom-up abiotic factors of light and flow are more influential during initial stream biofilm succession and top-down effects by invertebrate grazing become more influential as biofilms mature.