UNDERESTIMATION OF PARTICULATE NITROGEN CONCENTRATIONS IN OPEN-OCEAN BY THE USE OF GF F FILTERS/

GF/F filters are commonly used in oceanography for the determination o f particulate matter, in order to estimate biomass or element fluxes. Here we compare the collection efficiency of GF/F filters to that of t he rigid Anopore Anodisc filters of 0.2 mu m porosity. Results obtaine d within the water column, from a north-south transect in the equatori al Pacific, show that particulate nitrogen concentrations determined w ith GF/F filters are lower (by as much as 50%) than with Anopore filte rs. The higher retention capacity of the latter indicates that a non-n egligible amount of particles would not be retain ed by GF/F filters. This quantity increases in oligotrophic waters, because of the increas ed relative abundance of picoplanktonic and bacterial organisms. The d ecreased disparity between the two types of filter at the deep chlorop hyll maximum equally shows the variable proportion of these small part icles in the water. We conclude that Anopore filters are better suited for particulate matter sampling (and resulting matter budgets) in the open ocean than are GF/F filters.