Effects of cross-flow filtration on the absorption and fluorescence properties of seawater

Colloid spectroscopic properties, contamination artifacts, and instrumental 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 , respectively), absorption and fluorescence spectra, log-linearized and seawater-normalized absorbance spectra, and fluorescence-to-absorbance ratios. Permeates and retentates (1000 daltons; 1 kD) from five types of CFF systems and two types of seawater samples, coastal surface water (WHOI) and open-ocean deep water (off Hawaii; ~ 600 m deep), were examined. Retention of absorbance and humic fluorescence ( 337 420 nm ) varied by a factor of ~ 3 within system types and by a factor of ~ 5 between system types. Despite these variations, good absorbance and humic fluorescence balances were obtained by most systems for both samples, although Hawaiian seawater appeared to be more prone to contamination effects. Only two of the 5 CFF system types (Amicon and Desal) showed significant colloid retention. Based on those two systems, major differences in absorbance and fluorescence properties were found for the two water types. For the coastal WHOI samples, ~ 40% of the original dissolved organic matter (DOM) absorbance signal was retained as colloid, in agreement with organic carbon (OC) results. However, for the deep Hawaiian samples, only ~ 15% of the 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 displayed the opposite trend. The absorbance spectra of the retained colloids for both water types were significantly different from those of the unfractionated seawaters. Thus, the qualitative nature of the colloid fraction changed as function of both concentration factor during CFF and sample type, a result not obtainable from simple OC mass balances. These results indicate that organic matter (OM) concentrated by CFF is different from OM in unfractionated seawater in terms of chemical composition and spectroscopic properties, i.e. chromophoric DOM is not uniformly distributed over the total OM pool in seawater. Major, unexpected differences were also found for the fluorescence efficiencies and the slopes ( S) of the log-linearized absorption spectra for the two seawater types pr