Assessing the microbial bioavailability and degradation rate constants of dissolved organic matter by fluorescence spectroscopy in the coastal upwelling system of the Ría de Vigo

The time course of colored dissolved organic matter (CDOM) absorption and fluorescence were monitored during 50 to 70 days of laboratory incubations with water collected in the coastal upwelling system of the Ría de Vigo (NW Iberian Peninsula) under contrasting hydrographic conditions. CDOM fluorescence at peak-T (Ex/Em, 280/350 nm), characteristic of protein-like materials, decayed at a 1st order degradation rate constant ( k T) of 0.28 ± 0.13 day − 1 (average ± SD). k T covaried ( R 2 = 0.86, p < 0.0002) with the degradation rate of the bulk DOC ( k DOC), but the protein-like materials degraded 72 ± 23% faster than DOC. Therefore, this study confirms that the CDOM fluorescence at peak-T can be used as a proxy to a DOM fraction significantly more labile than the bulk bioavailable DOC. In parallel with the decay of DOC and protein-like fluorescence, an increase in CDOM fluorescence at peak-M (Ex/Em, 320/410 nm) during the course of the incubations verified the production of marine humic-like substances as a by-product of the microbial metabolism. CDOM fluorescence at peak-M built-up at a production rate ( k M) of 0.06 ± 0.01 day − 1 (average ± SD) in the Ría de Vigo. Furthermore, the slope of the linear regression between k DOC and k M ( R 2 = 0.64, p < 0.001) revealed that the formation of marine humic-like substances occurred at about one fifth of the rate of net DOC consumption.