Biofilm-induced drag in natural seawater

The mechanisms underlying the drag produced by soft biofilms are complex due to the interaction between the flow and the biological communities. To fully understand this phenomenon, hydrodynamic studies need to be paired with biological analysis involving the characteristics of what is present on the drag-producing surfaces. Here, the drag production and biological characterization of flat inert PVC plates exposed to natural seawater for up to 14 weeks are analyzed. At successive growth stages, biofilm samples were collected and drag forces were measured using a turbulent channel flow. Although biofilm age dominated drag penalty, this varied widely for plates grown under nominally equal conditions, ranging from 20% to 60% for 14-week biofilms tested at 3 m s^-1. While bacterial community composition within biofilms varied significantly with both age and shear disturbance frequency, biofilm age was the primary driver of community structure. Communities were dominated by Planctomycetia, Bacteroidia, and Alphaproteobacteria, with shifts in relative abundance over time. Genus-level patterns indicated successional changes and revealed that taxa within the class Alphaproteobacteria may be resilient to multiple shear disturbances. Overall, biofouling composition may be identified as a contributing factor to the wide variance in drag measurements, although other aspects, such as fouling arrangement and volume, can also produce drag variations.