Dissolved trace elements and nutrients in the North Sea-a current baseline

Abstract

Primary production is an important driver of marine carbon storage. Besides the major nutrient elements nitrogen, phosphorus, and silicon, primary production also depends on the availability of nutrient-type metals (e.g., Cu, Fe, Mo) and the absence of toxicologically relevant metals (e.g., Ni, Pb). Especially in coastal oceans, carbon storage and export to the open ocean is highly variable and influenced by anthropogenic eutrophication and pollution. To model future changes in coastal carbon storage processes, a solid baseline of nutrient and metal concentrations is crucial. The North Sea is an important shelf sea, influenced by riverine, atmospheric, Baltic Sea, and North Atlantic inputs. We measured the concentrations of dissolved nutrients (NH₄⁺, NO₃^-, PO₄^3-, and SiO₄^4-) and 26 metals in 337 water samples from various depths within the entire North Sea and Skagerrak. A principal component analysis enabled us to categorize the analytes into three groups according to their predominant behavior: tracers for seawater (e.g., Mo, U, V), recycling (e.g., NO₃^-, PO₄^3-, SiO₄^4-), and riverine or anthropogenic input (e.g., Ni, Cu, Gd). The results further indicate an increasing P-limitation and increasing anthropogenic gadolinium input into the German Bight.

Keywords: Carbon storage; Gadolinium anomaly; ICP-MS; Primary production; Skagerrak; Trace metals.

Fig. 1

Map of sampling area, currents, and sampling stations in the North Sea. Currents are NwAC (Norwegian Atlantic Current), NCC (Norwegian Coastal Current), STC (Southern Trench Current), SJC (Southern Jutland Current), NJC (Northern Jutland Current), and BC (Baltic Current). Currents are taken from Brückner and Mackensen (2006). Station numbers are given in Fig. S1

Fig. 2

Boxplot of salinity (a) and concentrations of Mo (b), NO₃⁻ (c), Cu (d), Gd (e), NH₄⁺ (f) in the North Sea (total) and six regions of the North Sea that were grouped according to their location, see text for more information. The three boxplots per region indicate bottom (dark gray), intermediate depth (medium gray), and surface (light gray) samples. Boxplots of all investigated parameters are shown in Fig. S38

Fig. 3

Factor loadings of the analyzed variables for the four rotated components. Note that only loadings larger than 0.5 are shown. The loadings for all analytes are given in Table S4

Fig. 4

Surface concentrations and depth profiles of salinity (a), NO₃⁻ (b), and Cu (c) across two south–north and two east–west transects. The northernmost transect is east–west 1 and the westernmost transect is south–north 1. x indicates that concentrations were below the LOD, faint points indicate that concentrations were between LOD and LOQ

Fig. 5

Correlation of inorganic nitrogen with PO₄³⁻ for AL557 (circle) and HE586 (triangle) (a) and percentage of Gd_anth versus salinity for the bottom (circle), middle (triangle), and surface (square) samples (b). The diagonal line (a) indicates the N-to-P ratio of 16 (n/n). Pale points indicate the samples with Gd anomalies < 2 (b)