Dissolved Trace Metals and Organic Matter Distribution in the Northern Adriatic, an Increasingly Oligotrophic Shallow Sea

We report a monthly distribution of Zn, Cd, Pb, Cu, Ni and Co, biologically relevant trace metals (TMs), within one year, in the productive surface layer at two stations with different trophic characters in the northern Adriatic (NA). The TM data was accompanied by a multivariable dataset, including dissolved organic carbon (DOC), surface-active organic substances (SAS), nitrogen-containing polymeric organic material (N-POM), nutrients, pH, dissolved O-2, chlorophyll a (Chl a), seawater temperature, and the Po River discharge rate. At the eu- to mesotrophic station 108, the concentrations of dissolved TMs were 5-116 nM for Zn, 0.04-0.18 nM for Cd, 0.05-0.63 nM for Pb, 3-17 nM for Cu, 4-11 nM for Ni, and 0.2-1.2 nM for Co, while at the oligotrophic station 107, they were 6-224 nM for Zn, 0.03-0.16 nM for Cd, 0.05-1.25 nM for Pb, 3-17 nM for Cu, 4-19 nM for Ni, and 0.1-0.7 nM for Co. The characterization of organic matter (OM) in conjunction with the analysis of correlations with TMs indicated that the OM-TMs interactions differed between the two stations; namely, the freshly produced OM detected at station 108 was probably involved in the complexation of Cu, Co, Cd, and Ni, whereas at station 107, such complexation processes were not present. Accumulation of DOC was observed during the summer months at both stations. Our results present a significant contribution to biogeochemical studies in the NA by focusing on the complexity of TM-OM interactions, which is a prerequisite for interpretation of their responses to local and global changes.