Relative efficiencies of different filters in retaining phytoplankton for pigment and productivity studies

Filtration is a critical step in the measurement of phytoplankton productivity and pigment concentration. It is important to know the retentive characteristics of commonly used filters as this would help in choosing the right filter to use. This study investigated the retention properties of four filter types namely, combusted GF, GF, 0.45 and 0.2 lm filters used in the tropical coastal waters off Mangalore on the Southwest coast of India. Significant differences were observed between the filter types. Combusted GF filters performed as well as the 0.2 lm membrane filters for productivity and pigment studies. Compaction of the borosilicate glass microfibers during combustion of the glass-fiber filters aided in the retention of smaller phytoplankton as observed by higher values of phytoplankton pigments and productivity measurements. Scanning electron microscopic images of combusted and noncombusted GF filters revealed that these had an average pore size of 0.3 and 1 lm, respectively, explaining why the former was as efficient as the 0.2 lm membrane filters in their retention characteristics.