| Abstract |
A total of 28 chemical variables were measured on water samples taken on an approximately longitudinal section of the Firth of Clyde in August 1989, November 1989, March 1990, July 1990 and March 1991. The data were integrated in a study of the overall processes acting on these variables. The sampling section crossed two mixing zones, i.e. the Clyde Estuary Plume (30 < S < 32) and the Clyde Sea (32 < S < 34), with mixing times of 4-25 days and 60-150 days respectively. The interpretation of the distributional data was complicated in July 1990 by the penetration of an ''older'' water mass originating from one of the five sealochs bordering the Firth of Clyde. Heterogeneous reactions within the Clyde Estuary Plume generally modified the export fluxes of dissolved trace metals to the Clyde Sea in the order: Fe, Pb > Mn, Co > Zn, Cd, Cu, Ni. In the deep layers of the sealochs, dissolved Pb was scavenged by newly formed hydrous Mn oxides following Mn(II) diffusion from the underlying sediments. Resuspension of bottom sediments was a permanent feature of the inner Firth, although it did not significantly affect the solid-solution partitioning of trace metals. Organic matter decomposition promoted a tight inverse relationship between O-2-% and pCO(2) in the deep layers (especially in November 1989 and July 1990), as did photosynthetic activity in the surface layers in July 1990. The latter process also manifested itself in the surface depletion of dissolved PO4, NO3, Si, Sigma CO2, Fe, Zn and Cd. A ratio P:N:Si:C of 1:9:6:68 was obtained in relation to the uptake of these elements by phytoplankton. Gas exchange at the air-sea interface - coupled with turbulent vertical mixing - was the main process controlling O-2 and CO2 concentrations in the surface layers in March 1990 and 1991. At wind speeds below 8 m s(-1), i.e. in the absence of bubble-induced gas exchange, O-2 reached saturation in the outer Firth but pCO(2) remained above its atmospheric equilibrium value of 350 ppm. At wind speeds above 12 m s(-1) (March 1990) pCO(2) was close to 350 ppm while O-2 became supersaturated. Water composition should be examined not only in relation to that anticipated from instantaneous mixing between end members but also in relation to the transit times of water masses. An application of this approach suggests that the inner Firth, situated at the triple junction Clyde Estuary-Sealoch System-outer Firth, is the area where the rates of heterogeneous reactions are fastest. |
