A.W. Morris

Trace Metals in the Central and Southern North Sea

Measurements of both dissolved and particulate trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) were made on samples collected simultaneously in the central and southern North Sea (below 56°N), during four cruises covering the seasonal cycle. Analyses of trace metals in both phases were undertaken using similar analytical protocols, and the data subjected to rigorous quality controls. Clean techniques were used throughout sampling and analysis. Trace metal concentrations ranged from those characteristic of waters entering the shelf region from the North Atlantic Ocean to values which in some cases were two orders of magnitude higher. Marked increases in concentrations were often associated with waters directly influenced by riverwaters, but the results showed that the distributions could be influenced by processes other than fluvial inputs. The redox-sensitive metals, Fe and Mn, were influenced at some sites, including those overlying areas of fine-grained sediments, by benthic inputs associated with seasonal changes in oxidation-reduction conditions in the benthic interfacial zone. The behaviour of metals which are not redox-sensitive (Cd, Zn) in some cases paralleled those of Fe and Mn, suggesting an association with Mn-or Fe-rich solid phases, or with organic matter. However, seasonal variations in concentrations of Cd and Zn were not directly linked to the cycle of biological utilization and regeneration of nutrients, suggesting that their geochemistry is more influenced by boundary inputs. Resuspension of Pb-rich sediments, long-range transport of Pb-rich suspended particulate matter (SPM), and enhanced, though diffuse, atmospheric inputs of lead were considered to have contributed to the elevated concentrations of particulate Pb (mass metal/mass sediment) in the water column during winter. Estimates of partitioning (K D s) were derived from the dissolved and particulate data for each of the metals over the seasonal cycle. For the more particle-reactive metals (Co, Fe, Mn, Pb), transport in the suspended particulate phase is a significant, sometimes dominant factor. In contrast, for Cd, Cu and Zn, transport in solution will dominate under almost all conditions in the southern North Sea. Furthermore, the values of K D , for a particular metal, varied widely, partly in response to seasonal differences in the relative contributions of lithogenous and biogenous particles to the particles in suspension. Only for Cu and Zn during the winter could the partitioning be described by a narrow range of K D values covering the observed concentrations of SPM. The data from this study give unique information concerning the contemporaneous distribution of dissolved and particulate trace metals in the southers North Sea and they provide a baseline against which the effects of reductions of inputs into the North Sea may be more accurately assessed. Viewed in combination, the dissolved and particulate data have yielded new insights into mechanisms affecting trace metal cycling and transport, essential in the development of coupled hydrodynamic-geochemical models.

The solid-solution partitioning of trace metals in the southern North Sea—in situ radiochemical experiments

Abstract An in situ experimental technique using radioisotopes for studying trace metal behaviour in coastal waters has been developed and results from the southern North Sea presented. Isotopes109Cd,137Cs and65Zn were adsorbed rapidly (within 25 h) by particles from the Humber and Thames estuarine plumes. The reversibility of this sorption was assessed by a series of leaching experiments and decreased in the order109Cd (removed by seawater) >65Zn (removed by 1M HCl) 137Cs (40% resistant to 1M HCl). In contrast,54Mn was deposited onto plume particles by slow oxidative precipitation which proceeded linearly. However, the protracted nature of this reaction meant that particle affinity (as quantified by a solid-solution distribution coefficient, KD) after 5 days was significantly greater than other isotopes studied; thus, 54Mn (5 × 104) >65Zn (5 × 103) >109Cd,137Cs (5 × 102). Moreover, 5-day KDs for109Cd and65Zn were enhanced by plankton compared with plume samples, whereas a137Cs association with plankton was not detectable. These trends were corroborated by KDs derived from ambient dissolved and digested particulate metal concentrations as determined by atomic absorption methods. Ambient KDs were, however, consistently an order of magnitude or more greater than their radiochemical counterparts because of an overwhelming detrital (unavailable) component of particulate metals inherent in their calculation. Accordingly, results from the radiochemical approach are favoured for incorporation into incipient shelf sea pollutant dispersal models and for assessing the potential for accumulation in the aquatic foodchain.

Trace metals at two sites in the southern North Sea: Results from a sediment resuspension study

Abstract Particulate and dissolved trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) have been determined following intensive sampling in January and May 1989 at two sites in the southern North Sea. Site A (52° 39.5′N, 003° 40.0′E) had sandy sediments, a permanently well-mixed water column and salinities >34.5, associated with waters from the English Channel. Particulate trace metal concentrations were higher in January than in May. The concentrations in the dissolved phase were unchanged between the two periods, except for dissolved Mn which had low concentrations in January, as a result of uptake onto particles. Atmospheric inputs to surface waters were evident in January, mainly for particulate Pb. Tidally induced resuspension processes produced temporary changes in the particulate and dissolved trace metal concentrations of the bottom waters for Mn, Pb and Zn. The variation in particulate trace metal concentrations, in May, fitted a two-component particle mixing model. It is argued that a significant source of dissolved metals was from the remineralisation of particulate material held within biological fluff at the sea bed. Site B (54° 35.0′ N, 004° 50.0′ E) had sediments of muddy sand, a seasonally stratified water column and it was influenced mainly by Central North Sea Water, with salinities >34.5. Particulate and dissolved trace metal concentrations did not vary between the cruises, only particulate Pb had higher concentrations in January. The tidal resuspension of particulate metals was negligible and, in May, only the concentrations of dissolved Pb and Zn increased at higher current velocities. The results identify pathways and particle–water interactions that affect trace metal transport in the North Sea.

Arsenic in the Thames Plume, UK

Abstract Dissolved and pariculate arsenic species have been determined in the Thames Estuary and Plume. During February 1989 only dissolved inorganic arsenic was detected in the estuary (mean concentration 97.8 ± 5.7 nM) and the plume (mean concentration 43.7 ± 12.0 nM). In July 1990, the mean concentration of dissolved inorganic arsenic in the plume was 28.2 ± 14.0 nM and about 8% of the total dissolved arsenic concentrations comprised the methylated species monomethylarsenic (mean 0.52 ± 0.22 nM) and dimethylarsenic (mean 1.85 ± 1.70 nM). Concentrations of arsenic in suspended particulate matter were 89 ± 39 nmol g−1 in February and 61 ± 12 nmol g−1 in July. Partition coefficients (KDs) for arsenic were about 103 litre kg−1 and were an order of magnitude lower than those for the Humber Plume. Fluxes of dissolved inorganic arsenic form the estuary based on zero salinity end member concentrations were 22 kg d−1 in February and 7 kg d−1 in July. The concentrations of dissolved inorganic arsenic in sediment porewaters were five times higher than those in the water column indicating diffusional fluxes from the sediments amounting to 1.2–2.6 kg d−l over the whole plume. The results were synthesised as an annual arsenic budget for the Thames Plume, which showed that cycling of arsenic by phytoplankton was the dominant process.

Arsenic species in the Humber Plume, U.K.

Dissolved inorganic arsenic (DIA), monomethylarsenic (MMA) and dimethylarsenic (DMA) have been determined in the water column and sediment porewaters of the Humber Plume, during December 1988, May 1990 and August 1990. In winter 1988 only DIA was present, whereas 2 and 18% of total arsenic was in the methylated forms in spring and summer 1990, respectively. Partition coefficients ( Kd) for suspended particles and DIA were in the range 2–25 × 103 1 kg−1 and were compatible with values obtained in the English Channel. Concentrations of DIA, MMA and DMA in porewaters from bottom sediments of the plume were 5–20 times greater than in the overlying water column. Estimates of porewater diffusional fluxes for DIA were in the range 2–30 μmol m−2 a−1, whereas for MMA and DMA they were 30–110 nmol m−2 a−1. A two-dimensional hydrodynamic model was used to determine the fluxes of arsenic species in the Humber Plume. The DIA fluxes across the plume boundaries were approximately two orders of magnitude greater than the estuarine flux of DIA and decreased from 104 kg day−1 in winter to 5 × 103 kg day−1 in spring and summer. Mass balance calculations for the arsenic species in the plume showed that there were no net fluxes of DIA and MMA during the periods of investigation and for DMA in spring. DMA was exported to the North Sea in summer. Diffusional fluxes of DIA from sediment porewaters increased the concentrations of DIA by about 10% during the spring and summer. Arsenic assimilation by phytoplankton accounted for the decrease in DIA concentrations between winter and spring and phytoplankton degradation for the appearance of methylated species in summer. These processes are discussed in terms of their relevance to the improvements of arsenic ecosystem models for the North Sea.

Distributions and fluxes of non-detrital particulate Fe, Mn, Cu, Zn in the Humber coastal zone, U.K.

Abstract Samples of suspended particulate matter (SPM) were collected at an anchor station at the mouth of the Humber Estuary and in the Humber-Wash coastal zone (U.K.) during the winter (1988), spring (1990) and summer (1990). Non-detrital particulate trace metals (Fe, Mn, Cu and Zn) were determined in the SPM following digestion with 1M HCl. Concentrations of non-detrital particulate trace metals were higher at the anchor station than in the coastal waters, suggesting dilution of metal-polluted estuarine SPM with natural SPM originating from erosion of the adjacent coastline. The distribution of non-detrital Fe and Zn in the coastal waters could be explained in terms of the Humber Estuary being a primary source, whereas for non-detrital Mn and Cu, the estuarine source was less significant because of their participation in biogeochemical processes. A hydrodynamic model of the Humber-Wash region was used to quantify the fluxes of dissolved and non-detrital trace metals into and out of the Humber Plume. The calculations showed that 0–10% of Fe, 5–70% of Mn, 40–100% of Cu and 30–100% of Zn were transported in the dissolved phase. Interpretation of the mass balances showed that the additional source of non-detrital particulate trace metals was sediment resuspension, whereas important sources of dissolved trace metals were porewater infusions and, for Mn, photochemically induced dissolution of particulate Mn. The trace metal sinks were the sedimentation of particulate matter and adsorption from solution. The flux estimates were used to evaluate the likely impact of dissolved and non-detrital trace metals on the wider North Sea.

Zn and Cu removal kinetics in Estuarine waters

The time-dependent removal of dissolved Zn and Cu in the presence of suspended particles has been studied in the laboratory using conditions closely resembling the aquatic environment encountered in the Tamar Estuary. The metals were removed from both river water and low salinity waters and the uptake profiles were of an exponential form. The removal profiles were interpreted in terms of a reversible, heterogeneous, pseudo-first order reaction, for which the forward and reverse rate constants were evaluated. The forward rate constants for Zn were salinity dependent and varied between 0.08 h −1 and 0.61 h −1 . In contrast, the rate constants for Cu ranged from 0.10 h −1 to 0.75 h −1 and in freshwater they were dependent on the suspended particle concentration. The reaction response times, τ resp (defined as the time for the attainment of 63% of a new equilibrium), were 1 to 10 h for Zn and 1 to 8 h for Cu. These values were compared with calculated flushing times of the Tamar Estuary and a new interpretation of field observations of the non-conservative behaviour of dissolved Zn and Cu in the Tamar Estuary was evolved.

BED STRESS INDUCED SEDIMENT RESUSPENSION (SERE 88/89)

Abstract This paper sets the scene for the rest of the papers in the volume by summarising the basis for a study of sediment resuspension and its impacts. The study concentrated on two contrasting sites in the southern North Sea, one with strong tidal currents and a sandy seabed where the water column remained well-mixed throughout the year, the other deeper, with weaker tidal currents and a muddy sea bed which stratified in summer.

The field measurement of sediment transport parameters in estuaries

Abstract Field measurements have been made in the upper reaches of a partially mixed estuary in order to evolve a system of measurements that would provide insight into the parameters associated with the transport of the cohesive and granular sediment mixtures found in the turbidity maximum reaches of estuaries. Measurements were made with electromagnetic current meters, siltmeters, an Owen tube and an in situ particle size analyser. The data were analysed in order to determine relationships between particulate concentrations, fall velocity, suspension density, turbulent mean velocity and bed shear stress. The usefulness of these initial results and of the experimental techniques are discussed and some ideas for further work are presented.