EFFECTS OF CROSS-FLOW FILTRATION ON THE ABSORPTION AND FLUORESCENCE PROPERTIES OF SEAWATER

Colloid spectroscopic properties, contamination artifacts, and instrum ental variability were evaluated during a cross-flow filtration (CFF) intercomparison exercise. Analyses were based on absorbance (at 337 nm ), humic and protein fluorescence (337/420 and 270/320 nm, respectivel y), absorption and fluorescence spectra, log-linearized and seawater-n ormalized absorbance spectra, and fluorescence-to-absorbance ratios. P ermeates and retentates (1000 daltons; 1 kD) from five types of CFF sy stems and two types of seawater samples, coastal surface water (WHOI) and open-ocean deep water (off Hawaii; similar to 600 m deep), were ex amined. Retention of absorbance and humic fluorescence (337/420 nm) va ried by a factor of similar to 3 within system types and by a factor o f similar to 5 between system types. Despite these variations, good ab sorbance and humic fluorescence balances were obtained by most systems for both samples, although Hawaiian seawater appeared to be more pron e to contamination effects. Only two of the 5 CFF system types (Amicon and Desal) showed significant colloid retention. Based on those two s ystems, major differences in absorbance and fluorescence properties we re found for the two water types. For the coastal WHOI samples, simila r to 40% of the original dissolved organic matter (DOM) absorbance sig nal was retained as colloid, in agreement with organic carbon (OC) res ults. However, for the deep Hawaiian samples, only similar to 15% of t he absorbance was retained as colloid, in contrast to an average > 40% for OC. These results indicate that a greater percentage of the total pool of absorbing DOM is lower molecular weight (< 1 kD) in the deep Hawaiian samples compared to the coastal samples. Humic fluorescence d isplayed the opposite trend. The absorbance spectra of the retained co lloids for both water types were significantly different from those of the unfractionated seawaters. Thus, the qualitative nature of the col loid fraction changed as function of both concentration factor during CFF and sample type, a result not obtainable from simple OC mass balan ces. These results indicate that organic matter (OM) concentrated by C FF is different from OM in unfractionated seawater in terms of chemica l composition and spectroscopic properties, i.e. chromophoric DOM is n ot uniformly distributed over the total OM pool in seawater. Major, un expected differences were also found for the fluorescence efficiencies and the slopes (S) of the log-linearized absorption spectra for the t wo seawater types prior to CFF.