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

Abra Penezić,Slađana Strmečki,Marina Mlakar,Blaženka Gašparović,Vlado Cuculić,Tamara Djakovac

doi:10.3390/w14030349

Abstract

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 O2, 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.

Highlights

Trace metals (TMs), such as Zn, Cu, Co, Fe, Ni, Cd etc., are considered important micronutrients that affect marine productivity and plankton community composition [1].Zn, Cu, Co, Ni, and Cd play a role in enzymatic activities of microorganisms [2,3,4], while some TMs, including Pb and Hg, have been recognized as toxic and may be indicators of anthropogenic activities [5]
To expand the understanding of the biogeochemistry of TMs in this shallow marine environment under riverine influence, we focused on the dissolved fraction of six biologically relevant trace metals—Zn, Cu, Cd, Co, Ni, and Pb—by tracking their monthly distribution in the productive upper 10 m of the water column at two stations of different trophic characteristics, while performing a comprehensive organic matter (OM) characterization
The Po River waters in the western part of the northern Adriatic (NA) are an important source of nutrients and trace metals, especially in spring/summer, when, during the stratification period, fresh waters could spread towards the eastern NA

Summary

Introduction

Trace metals (TMs), such as Zn, Cu, Co, Fe, Ni, Cd etc., are considered important micronutrients that affect marine productivity and plankton community composition [1].Zn, Cu, Co, Ni, and Cd play a role in enzymatic activities of microorganisms [2,3,4], while some TMs, including Pb and Hg, have been recognized as toxic and may be indicators of anthropogenic activities [5]. Trace metals are necessary for the uptake and metabolism of macronutrients: C, N, P and Si [6] are indirectly involved in the global carbon cycle. The distribution, bioavailability and toxicity of TMs are determined by their concentrations, and by their forms, with TMs in the dissolved fraction having a higher potential to become bioavailable. This is highly dependent on various environmental parameters, from pH to the availability of complexing ligands [7]. In its effort to utilize TMs or protect itself from their potentially toxic effects, phytoplankton regulate the distribution, chemical speciation and cycling of TMs through cellular uptake and recycling processes, downward flux of biogenic particles, mediation of redox reactions and biological release of organic ligands [1,8]

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