NUTRIENT AND TRACE-METAL DISTRIBUTIONS OVER A COMPLETE SEMI-DIURNAL TIDAL CYCLE IN THE FORTH ESTUARY, SCOTLAND

The distribution of dissolved trace metals and nutrients has been exam ined during a complete 12.5 hour tidal cycle at an anchor station in t he turbid industrialized Forth estuary. The elemental composition of m ajor (C, Al, Si, Fe, Ti, Ca and Mn) and minor (Cd, Cu, Pb, Zn and Ni) elements in the particulate phase has also been examined. Generally th e mixing behaviour of dissolved components was similar to that observe d during axial surveys of the estuary at high water. At the onset of t he flood tide, however, resuspension of bottom sediment occurred befor e any increase in salinity. This resuspension was accompanied by incre ased concentrations of ammonia and nitrite, and is attributed to a flu shing of porewaters. Further, in contrast to results from axial survey s where no clear pattern is evident, dissolved lead distributions indi cate removal over the salinity gradient sampled (0-20). This is attrib uted to scavenging by the high concentrations of suspended particles ( up to 2000 mg.dm-3) which were a feature of the site. At times of high particle loadings (ebb and flood tides) the composition of suspended particulate matter (SPM), e.g. organic carbon and C/N ratio, approache d that of bulk sediment. When suspended loads were low, e.g. around hi gh water, the resuspended contribution to SPM was low and C/N ratios a pproached the Redfield ratio indicative of fresh or recently degraded phytoplankton. The distribution of SPM loading during the tidal cycle controls total metal concentrations in the particulate phase. Using Ti as a geochemically unreactive reference element for aluminosilicate p hases it is demonstrated that trace-metal distributions are more close ly linked to those of Mn than to Fe. This implies that manganese oxyhy droxides are an important phase for the transport of trace metals. Iro n oxyhydroxides are also likely to be important but high particle conc entrations and the abundance of Fe make potential changes in the Fe/Ti ratio difficult to detect. Using dissolved and particulate trace meta l results, partition coefficients (K(d)) have been calculated. For Mn the lowest K(d) values are associated with the oxygen minimum zone of the estuary, K(d) values for Zn also tend to be low in this zone. Thes e results are linked to the dissolution of oxyhydroxide particulate ph ases in waters undersaturated with respect to oxygen. K(d) values for Cd decrease with increasing salinity; this is attributed to complexati on in the dissolved phase by chloride.