Contrasting biogeochemical behaviours of trace metals in European shelf seas
Shelf seas are considered an important source of trace metals (TMs) to the open ocean. However, the processes controlling distributions of dissolved TMs (DTMs) in shelf seas, their seasonal variability are not well constrained. Our results present the first seasonal study of the DTMs, bioessential cadmium (DCd), zinc (DZn), nickel (DNi) copper (DCu), cobalt (DCo), aluminium (DAl), potentially toxic lead (DPb) and their contrasting biogeochemical behaviours in the Celtic Sea continental system. Strong correlations (r2 = 0.91 – 0.97, n = 454) between nutrient-like metals (DCd, DZn, DCu, DNi, DCo) and macronutrients (phosphate (PO43-) and silicic acid (SiO44-)) were observed over all seasons (autumn, spring and summer) indicating that biological uptake in the euphotic zone and remineralization of sinking phytoplankton debris in deeper waters form key controls on their distributions. Surprisingly, DAl also correlated with PO43- (r2 = 0.80) and SiO44- (r2 = 0.85) in the upper waters (< 1500 m depth) and mimicked the DZn distributions. Contrasting biogeochemical behaviour of DTMs were observed across the continental shelf. Observations from the Celtic Sea region suggest the influence of distinct water masses, phytoplankton community structure and potentially metal specific biogeochemical processes, on DTMs distributions on a seasonal time-scale.
Beside the natural sources of DTMs, anthropogenic activities have resulted in enhanced DTMs emissions to the environment over the past centuries. Lead has a strong anthropogenic signal as a result of the combustion of leaded fuel and coal. Presented is the first combined DPb, labile Pb (LpPb) and particulate Pb (PPb) distributions from the Celtic Sea since the phasing out of leaded fuel in Europe. Concentrations of DPb in surface waters have decreased by 4-fold over the last four decades. Nevertheless, a distinct anthropogenic Pb signal was observed from the Mediterranean Sea at intermediate depths, showing that Pb can transported over long distances (>2500 km). Our results highlight the requirements of continuing efforts in regulating Pb emissions but also of detailed seasonal TMs studies in coastal systems in future.