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Nutrient and trace metal distributions over a complete semi-diurnal tidal cycle in the Forth estuary, Scotland
Balls, P.W.; Laslett, R.E.; Price, N.B. (1994). Nutrient and trace metal distributions over a complete semi-diurnal tidal cycle in the Forth estuary, Scotland. Neth. J. Sea Res. 33(1): 1-17
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579, more
Peer reviewed article  

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Keyword
    Brackish water

Authors  Top 
  • Balls, P.W.
  • Laslett, R.E.
  • Price, N.B.

Abstract
    The distribution of dissolved trace metals and nutrients has been examined during a complete 12.5 hour tidal cycle at an anchor station in the turbid industrialized Forth estuary. The elemental composition of major (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 the mixing behaviour of dissolved components was similar to that observed during axial surveys of the estuary at high water. At the onset of the flood tide, however, resuspension of bottom sediment occurred before any increase in salinity. This resuspension was accompanied by increased concentrations of ammonia and nitrite, and is attributed to a flushing of porewaters. Further, in contrast to results from axial surveys where no clear pattern is evident, dissolved lead distributions indicate removal over the salinity gradient sampled (0-20). This is attributed 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, approached that of bulk sediment. When suspended loads were low, e.g. around high water, the resuspended contribution to SPM was low and C/N ratios approached 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 phases it is demonstrated that trace-metal distributions are more closely linked to those of Mn than to Fe. This implies that manganese oxyhydroxides are an important phase for the transport of trace metals. Iron oxyhydroxides are also likely to be important but high particle concentrations and the abundance of Fe make potential changes in the Fe/Ti ratio difficult to detect. Using dissolved and particulate trace metal results, partition coefficients (Kd) have been calculated. For Mn the lowest Kd values are associated with the oxygen minimum zone of the estuary, Kd values for Zn also tend to be low in this zone. These results are linked to the dissolution of oxyhydroxide particulate phases in waters undersaturated with respect to oxygen. Kd values for Cd decrease with increasing salinity; this is attributed to complexation in the dissolved phase by chloride.

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