Seston retention by Whatman GF/C glass-fiber filters

Abstract

The efficiency of Whatman GF/C glass-fiber filters for the retention of seston (dry weight) from North Sea water was tested. Uni-Pore polycarbonate membranes with pore sizes of 0.4, 1 and 5 pm diameter were used as standard 'sieve' fllters, since they have well-defined pore sizes. Using means of the differences of paired filters, it was found that GF/C glass-fiber filters retain seston from North Sea samples as efficiently as 0.4 pm Uni-Pore filters at a seston concentration range of about 1.5 to 15 mg dm-3; this covers most of the German Bight water bodies in summer. Glass-fiber filters retained significantly (P < 0.001) more seston than 1 pm Uni-Pore filters and thus more as their nominal mean retention size of 1.2 p would suggest. Comparison of seston retention of GF/C and Uni-Pore filters by regression analysis revealed that GF/C filters tend to retain relatively more seston as the water becomes clearer and sample volumes greater; this is the case in the western German Bight (Secchi depth about 7 to 9 m, seston concentrations < 2 mg dm-3, sample volumes filtered: 600 to 1000 cm3). This higher retention of GF/C filters is significant (P = 0.05) when compared with 0.4 pm Uni-Pore filters but highly significant (P < 0.001) with 1 pm Uni-Pore filters. This indicates that particles < 1 pm contributes significantly to the seston weight in such open North Sea water It seemed unlikely. however, that adsorbed dissolved organic matter caused a 'seston' weight increase.