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Photolytic effect of natural UV on organic metal species and thiol compounds. A cruise was carried out using the Navicula on the Scheldt estuary, and water samples were irradiated with natural UV light, to investigate whether metal complexing ligands were broken down. Controlled experiments were carried out in an incubator in the laboratory using samples that had been stored frozen. The measurements showed (see Figure below) that significant amounts of the copper complexing ligands, and of thiol compounds, were broken down by the UV light.
Potential impact of trace metals on growth of phytoplankton was investigated in enrichment experiments where Fe, Cu or EDTA was added. After 48 hours incubation primary production and chlorophyll-biomass was measured. Bioassays indicated only minor and inconsistent effect by the addition of Cu and Fe. Only in one experiment with Cu, was there a significant inhibition of the primary production at the highest concentration. Enrichment experiment indicates that neither Cu nor Fe as a controlling factor for the growth of phytoplankton in Kongsfjorden in the investigated period. The lack of response by addition of EDTA indicates that trace element in general is not limiting for the growth of phytoplankton in Kongsfjorden in the present periods.
Metal speciation determined in estuarine waters, and effect of UV evaluated; significant breakdown of complexing ligands and thiol compounds when water is subject to UV over periods of a several days.
Effects of UV on metal speciation determined experimentally, in the laboratory and in the field.
Effects of UV on metal speciation on Spitsbergen: a rapid reduction of iron(III) to iron(II) by ambient UV was observed, and the effect of screening of natural light of different wavelengths on this reduction was determined.
It appears that the response of bacterioplankton to UVR-exposed DOM depends on the overall reactivity of the DOM. Originally labile DOM such as extra-cellular release of phytoplankton becomes more refractory for bacterioplankton upon solar irradiation while originally refractory DOM becomes more labile after solar irradiation. Using a specific reactivity index for DOM we were able to clearly establish this reactivity index which is used now also by other scientists as well. The general applicability of this pattern has been demonstrated in freshwater, estuarine and marine systems.
We found that dissolved Mn decreases steeply in the upper estuary due to precipitation by oxidation, but with concentrations lower than those published by Paucot and Wolast (1997).
Dissolved Zn is relatively high compared to Bayens et al (1998a) and comparable to the data from Van den Berg et al. (1984). At low salinity (S=0.3) the dissolved Zn concentration is already high, indicating that the solid sulphides of Zn are oxidised. This is in contrast with previous studies (Duinker et al., 1983; Zwolsman et al., 1997) that indicate that dissolved Zn concentrations are low in the upper estuary due to anoxia and increase with salinity when the system becomes oxidised. In our data a decline in dissolved Zn can be seen from S=15 to higher salinities, coinciding with a decrease in dissolved organic ligands and dissolved organic matter by dilution with seawater.
The ratio of the dissolved concentrations of Zn and Cd is displayed in relation to the salinity (Figure 2). This ratio shows a peak when the sulphide concentration in the water is between the values poising the solubility products of solid Zn and Cd sulphides. Going from anoxic to oxic conditions, Zn sulphides dissolve first, increasing the Zn/Cd ratio. When the Cd sulphides also dissolve, the ratio declines to 150. The ratio of dissolved Zn and Cd thus indicate the oxidation of the system by a peak in the ratio of Zn and Cd. These data show that the Scheldt system is hardly suffering from an anoxic regime since the peak is around our first sampling point, S=0.3.
There is not much recent data on dissolved Fe in the Scheldt. Duinker et al. (1983) presented dissolved Fe concentrations which were much higher, probably due to the higher organic load at that time (oxygen depletion up to S=15!) resulting in higher concentration of organic ligands.
Van den Berg et al. (1987) measured higher Fe concentrations than we did in the middle and lower estuary of 100 nM at S=14.4 and 400 nM at S=9. A decrease of dissolved Fe concentrations with time since the 1980s due to a concomitant decrease in organic matter (and thus in the dissolution of Fe) is the probable explanation. Another explanation might be the liquid-liquid extraction we used.
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