A comparison between glass fiber and membrane filters for the estimation of phytoplankton POC and DOC production

We tested the performance of 2 types of glass fiber filters (GF/F: 0.7 mu m ; GF/C: 1.2 mu m) and 2 membrane filters (PC0.2: polycarbonate 0.2 mu m; CE 0.22: mixed cellulose esters 0.22 mu m) in estimating chlorophyll a and pri mary production with the C-14 technique. Four experiments were carried out with water samples from the NW Mediterranean, the NE Atlantic and the Antar ctic Ocean. The first experiment compared measurements of particulate organ ic carbon (POC) production whereas the other 3 also considered total (TOC) and dissolved (DOC) carbon fixation. No significant differences among filte rs were found regarding chlorophyll a retention but large discrepancies exi sted in the amount of labelled organic carbon retained in all the experimen ts. Both types of glass fiber filters, especially GF/F, yielded higher valu es of apparent pO(14)C recovery than the membrane filters. The GF/F-derived POC production rates were up to twice the PC0.2-derived rates and 63% high er than CE0.22-derived ones. Accordingly, the estimated rates of phytoplank tonic DOC production were higher with the membrane filters in comparison to the GF/F ones. This discrepancy was attributed to a high (DOC)-C-14 adsorp tion to the glass fibers of GF filters. Due to uncertainties in the magnitu de of this process in other samples, we conclude that GF filters are not su itable when particulate primary production must be measured without interfe rence of released dissolved products. and that membrane filters should be u sed instead.