Seasonal Dynamics of Dissolved Trace Metals in the Scheldt Estuary: Relationship with Redox Conditions and Phytoplankton Activity |
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Authors: | M A Chaudry J J G Zwolsman |
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Institution: | (1) National Institute for Marine and Coastal Management (RIKZ), P. O. Box 8039, 4330 EA Middelburg, The Netherlands;(2) Kiwa Water Research, P.O. Box 1072, 3430 BB Nieuwegein, The Netherlands |
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Abstract: | Seasonal dynamics of dissolved trace metals (Cd, Co, Cu, Ni and Zn) and its relationship with redox conditions and phytoplankton
activity has been studied in the Scheldt estuary, during nine surveys carried out between May 1995 and June 1996. Seasonal
profiles of dissolved trace metals and general estuarine water quality variables are compared, to identify the geochemical
and biological processes responsible for the observed trace metal distributions. In keeping with previous studies, the behavior
of dissolved Cd, Cu, and Zn can be explained by the presence of anoxic headwaters and the restoration of dissolved oxygen
within the estuary. In the river water, the concentration of dissolved Cu and Zn is generally low, except during winter when
dissolved oxygen is present in the water column, although highly undersaturated. Mobilization of particle-bound Cd, Cu, and
Zn occurs as dissolved oxygen increases with increasing salinity, possibly because of oxidation of metal sulfides in the suspended
matter. The geochemistry of dissolved Co is also related to the redox conditions but in an opposite way. Dissolved Co is mobilized
in the anoxic upper estuary, along with the reduction in Mn (hydro) oxides, and subsequently coprecipitated with Mn (hydro)
oxides when dissolved oxygen is restored. Conservative behavior is observed for dissolved Ni within the estuary. In the middle
estuary, Cd and Zn are readsorbed during phytoplankton blooms, as suggested by the low concentrations of these metals during
the most productive periods in spring and early summer. The removal may be caused by direct biological uptake and/or increased
adsorption to suspended matter because of the pH increase associated with algae blooms. In the lower estuary, chemical gradients
are much weaker and dilution with seawater is the dominant process. |
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