Distributions and fluxes of contaminant metals in the North Sea: comparisons between field measurements and model simulations using NOSTRADAMUS

Abstract

Environmental context. The North Sea, which is of significant ecological, economic and recreational value to NW Europe, has for many years received enhanced inputs of contaminant metals arising from human activity around its shores. Fluxes of copper, nickel, zinc, chromium and other constituents throughout the southern North Sea were estimated using a numerical model in order to identify the main sources and sinks for these metals. Comparison of model output with independent and other data showed good agreement in general. The results indicate that models of the kind developed here can be used to provide useful information on contaminant metal transport in coastal waters. Abstract. The steady-state 2-D vertically integrated numerical transport model NOSTRADAMUS has been used to simulate concentrations, distributions and fluxes of dissolved and particulate Cu, Cr, Ni and Zn in the southern North Sea between 51 and 56°N. The model results for the metals, and also for salinity and suspended sediments, were compared with field measurements from the Natural Environment Research Council (NERC) North Sea Project (NSP). The model generated realistic concentrations of all constituents, and in many cases reproduced the major features of the distributions, i.e. higher concentrations in the coastal zones and lower values in the central region. The sensitivity of the model results to variations in forcing data was extensively tested. In most cases, the default data provided the best results (quantified by residual sums of squares scores) despite no formal model calibration having been undertaken. Simulated fluxes indicated that metal inputs across the open sea boundaries were significant (Cu, 51%; Cr, 36%; Ni, 57%; Zn, 32%), although for Cu, Ni and Zn, river inputs were also important (13–15%). Metal transfers associated with particle settling and resuspension were significant (23–55%), and resuspended sediments were a minor net source, relative to particle settling, of Cu, Ni and Zn to the water column over the simulated year. In contrast, the resuspension of sediment from the seabed appeared to be a major source of Cr to the water column, a feature not explained at present. The major export from the North Sea in the model was through the northern boundary adjacent to the coast of Denmark. The fraction lost was in the range 61%, for Zn, to 90%, for Cr. Model results were compared with available independent data. The comparisons showed that the simulated constituent transfers approximated reasonably well with contemporary understanding of metal, and other constituent, fluxes in this region. These outcomes serve to emphasise the underlying capabilities of the model, and it can be concluded that NOSTRADAMUS provides the basis for heuristic studies of contaminant metals in the southern North Sea.