Colloidal Organic Matter and Metal(loid)s in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Large volumes of seawater were sampled in the Gulf of Trieste (northern Adriatic Sea) in order to study the interactions between colloidal organic matter (COM) and metal(loid)s (Me) in coastal waters. COM (> 5 kDa) was isolated by ultrafiltration and characterized using H-1 NMR spectroscopy and elemental C-org. and N-tot. analyses. COM in the gulf represents about one quarter of the dissolved organic carbon (DOC), and according to H-1 NMR analysis, it is composed of polysaccharides (30-45%), lipids (30-55%), proteins and carboxyl-rich alicyclic molecules (CRAM) (15-20%), and humics (< 1%). An accumulation of COM was observed in the late spring-early summer. The polysaccharide and lipid fractions increased up to twofold and the protein fraction decreased, reflected in a higher C-org./N-tot. (28, molar) ratio. Higher concentrations of humics were observed due to local freshwater discharges in spring. COM from the Isonzo/Soca River differed from the marine COM exhibiting higher protein/CRAM and higher humic contents. COM from the Isonzo/Soca mouth at salinities 16-33 was compositionally similar to marine COM. Analysis of Me, performed by ICP-MS and CVAFS (Hg), showed that Hg (nearly 100%), Cu (20%), Cr (10%), and Se (10%) have the highest Me affinity to colloids. Similar to COM, Hg and Cu rapidly increased till summer due to their sequestration in accumulated COM (transfer to particulate phase). The observed Me/C-org. ratios (Co, Cd, Hg < U, Cr, Ni, Mn < As, Zn, Cu, V < Se, Al, Fe) differ somewhat from those of the Irving-Williams series and can be explained by the composition of COM and variable background concentrations of studied Me in the northern Adriatic. Data from the salinity gradient in the metal-contaminated (especially Hg, Pb, Zn) Isonzo/Soca mouth showed flocculation of Al and Ba and desorption of V, Co, As, Se, Cs, U, and Hg, from the riverine particles with increasing ionic strength, while Fe, Mn, Cu, Cr, Ni, Zn, Cd, and Pb did not correlate with salinity.