Vorticella spp. are the species of protozoa (cilliate) exist in rivers as well as reservoirs , often attache at stalks in a patch. well, is it count by individual or path? (How many is the patch?)
As we used to do for protozoans, eggs, and other parasite larval forms, you can concentrate your suspensions by centrifugation (1200rpm / 5min) and proceed to individual counting in a 20µl volume, the counted number (n) will be extrapolated to the total volume by multiplying by a factor F (F = total volume of the concentrate / 20), so then N = n*F.