Fouling in natural flows: Cylinders and panels as collectors of particles and barnacle larvae

Fouling of cylinders and panels by particulates and barnacles was studied off of a floating dock system in an estuarine area with semidiurnal tides. On a 5 min scale, over a 14 h interval, Acoustic Doppler Velocimetry (ADV) showed time-dependent variation in speed of mean flow but not in direction. On a 1 s scale, relevant to larval responses, ADV data revealed complex velocity structure. Six settlement experiments, each consisting of 5 blocks of six treatments were conducted. In each treatment there were five cylinders with outer diameters of 0.8, 1.5, 6.5, 11.6 and 31.7 cm and a 30 cm wide by 0.7 cm thick panel. Treatments were varied systematically within blocks and mounted so that 50 cm of each cylinder and panel was exposed. When barnacle fouling occupied 1 to 10% of the space on a 1.5 cm diameter cylinder, experiments were scored for particulate and barnacle fouling. Particulate fouling followed the expectations of deposition of long sticky mucus strands. Some larger diameter cylinders were more efficient collectors of barnacles than smaller diameter cylinders, but the relationship between number of barnacles and cylinder size was variable. The 30 cm wide panels collected barnacles as effectively as 6.5 cm diameter cylinders. In general, cylinders facing perpendicular and oblique downstream to bulk flow, collected the largest numbers of barnacles. Patterns of fouling reflect delivery of particles and larvae as well as their interactions with complex flow and surface geometry. Rocking of the floating docks may explain deviations of settlement from expectations based on mean flows.