Data for: Alloctonous DOM in open sea waters of the Mediterranean Sea: New insights from optical properties
Description
Samples for DOC, CDOM and FDOM analysis were collected in open sea waters of the Med Sea during 3 oceanographic cruises carried out in March/April 2008, August 2010 and November 2011 on board the R/V Urania of the Italian National Research Council (CNR). DOC measurements were carried out with a Shimadzu TOC-VCSN, by high-temperature catalytic oxidation. Samples were acidified with HCl 2N and sparged for 3 min with CO2-free pure air in order to remove inorganic carbon. From 3 to 5 replicate injections were performed until the analytical precision was lower than 1% (±1 μM). A 5-point calibration curve was done by injecting standard solutions of potassium hydrogen phthalate between 20 and 130 μM. At the beginning and at the end of each analytical day, the system blank was measured using Milli-Q water and the functioning of the instrument was checked by comparison of data with DOC Consensus Reference Material (CRM) (Hansell, 2005) (batch#7-2007, batch#10-2010/Lot#5-10, batch#11-2011/Lot#03-11, Consensus values: 41 - 44 µM, measured concentration: 41.9 ± 1.3 µM, standard error = 0.23 µM, n = 39). For further analytical details see Santinelli et al. (2010). CDOM absorbance was measured throughout the UV and visible spectral domains (230-700 nm) with a resolution of 0.5 nm, by using a JASCO Spectrophotometer V-550 and a 10 cm quartz cuvette. Fluorescence Excitation Emission Matrixes (EEMs) were recorded by using the Fluoromax4 spectrofluorometer (model FP770 Horiba) with a 1 x 1 cm quartz cuvette in the range 250-450 nm for the excitation and 300-600 nm for the emission. The EEMs were corrected for instrumental bias and subtracted by the EEM of Milli-Q water measured in the same conditions (blank). The Rayleigh and Raman scatter peaks were removed by using the monotone cubic interpolation (shape-preserving) (Carlson and Fritsch, 1989), since water subtraction did not completely remove their signals (Gonnelli et al., 2016; Margolin et al., 2018). EEMs were normalized to the water Raman signal, dividing the fluorescence by the integrated Raman band of Milli-Q water (ex=350 nm, em=371-428 nm), measured the same day of the analysis (Lawaetz and Stedmon, 2009). The fluorescence intensity is therefore reported as equivalent water Raman Units (R.U.). This standardized method was chosen because it is rapid, simple and suitable for routine measurements. No significant variation was observed in the integral of the Raman peak from repeated measurements during the period of the analysis (< 2%). In order to check the repeatability of our measurements the same sample was analyzed 5 times during a period of 3 months, the results showed that the variation was less than 2 % for all the components.