VARIABLE RETENTION OF DIATOMS ON SCREENS DURING SIZE SEPARATIONS

Particles smaller than filter mesh pores can collide and stick to mesh fibers, biasing size separations based only on pore diameter. The rem oval of flocculating diatoms (Chaetoceros gracilis, Nitzschia angulari s) was greater than removal of a nonfloc-forming species (Thalassiosir a weissflogii) even after including the effects of cell size. Over a 5 -d period an average of 1.4% of cells of N. angularis were removed by 230-mum pore-diameter screens; on day 6, 7.5% of cells were removed as cells began to flocculate. The percentage of C. gracilis removed cont inually increased from 0.4 to 28% during the same period, while the re moval of T. weissflogii was constant at 0.19% over a 10-d period. Prio r to the onset of flocculation, sticking coefficients (rate of cell at tachment to mesh fibers/rate striking fibers) were 0.05 for T. weissfl ogii, 0.26 for N. angularis, and 0.73 for C gracilis. Size separations will therefore tend to concentrate more flocculent species of phytopl ankton into size fractions much larger than cell diameters.