Microscale evaluation of the effects of grazing by invertebrates with contrasting feeding modes on river biofilm architecture and composition.

Abstract

River biofilms are a valuable food resource for many invertebrates. In the present study biofilms were cultivated in a rotating annular bioreactor with river water as sole source of inoculum. The resulting biofilms were then presented to starved snails, ostracods, and mayflies as sole food source. The biofilms were then removed and microscopically examined to determine areas that had been grazed. The grazed and ungrazed areas were marked and analyzed for the effects of grazing using confocal laser scanning microscopy and image analyses. Samples were treated with fluorescent probes for nucleic acids to quantify bacterial biomass and fluor-conjugated lectins to quantify exopolymer, and far red autofluorescence was imaged to quantify algal or photosynthetic biomass. Grazing by snails significantly reduced algal biomass (1.1 +/- 0.6 micro m 3 micro m 2 to 0.02 +/- 0.04 micro m 3 micro m 2), exopolymer (5.3 +/- 3.4 micro m 3 micro m 2 to 0.18 +/- 0.18 micro m 3 micro m 2), and biofilm thickness (154 micro m +/- 50 to 11 micro m +/- 5.2; ANOVA, p < or= 0.05). Although bacterial biomass was influenced by grazing snails the impact was not statistically significant (p <or= 0.05). Ostracods had a significant (p <or= 0.05) impact on the algal biomass and exopolymer but not on bacteria. Mayfly grazing resulted in reduction of biofilm thickness to approximately 40 mm and reductions in all biofilm components with little evidence of selectivity. Thus grazing consistently resulted in a significant reduction in autotrophic biomass and exopolymer with a resultant increased importance of bacterial biomass within the grazed regions. Examination of grazed biofilms after a recovery period in the absence of grazing indicated that grazed regions remained predominantly bacterial after 28 days. A comparison of grazing in diclofop methyl and atrazine contaminated (1 ppb and 10 ppb) versus control biofilms indicated no significant influence of the contaminants on grazing patterns.