Distribution and Transport of Chemical Constituents in the Clyde Estuary

Five transects of the Clyde Estuary were carried out between August 1989 and March 1991, always during the last 2 h of the flood tide. The following water-column properties were examined: dissolved, leachable particulate and detrital particulate trace elements (Fe, Mn, Zn, Pb, Cd, Co, Cu, Ni), pH, alkalinity, salinity, temperature, dissolved oxygen, dissolved and particulate organic carbon, suspended particulate matter and dissolved nutrients (PO4, NH3 + NH4, NO3, NO2, Si). The distribution patterns for both dissolved metals and nutrients reflected their quasi-conservative behaviour in the lower estuary, but were more difficult to interpret in the upper estuary due to the multiplicity of inputs. The suspended particulate material (SPM) collected in the lower estuary at 3 m was depleted in leachable Fe, Zn, Cd, Co, Cu, Ni, and enriched in leachable Mn and Pb relative to the upper estuary. For Mn, the degree of enrichment increased towards the mouth of the estuary, and also with decreasing ʹeffective freshwater inputʹ, indicating that the oxidation of Mn(II) proceeds outside the estuary (Firth of Clyde) and to an extent dependent on the residence time of water in that coastal sea area. The quasi-conservative behaviour of most dissolved and particulate constituents within the estuary results from a combination of short residence time (3-7 days) and low SPM (2-10 mg l-1). This behaviour makes it possible to estimate their net seaward fluxes before these become modified by the processes of coagulation (Fe), sorption (Zn, Pb, Co) and oxidative precipitation (Mn) operating in the Firth of Clyde. The seaward fluxes of NH4 + NH3, NO2, PO4, dissolved Mn, particulate Fe, Zn and Cu are approximately constant throughout the year. Other dissolved and particulate metal fluxes are best represented by an increasing function of freshwater discharge, although the large residual variability attests to the greater complexity of input functions for trace metals.