THE EFFECT OF PH, DISSOLVED HUMIC SUBSTANCES, AND IONIC COMPOSITION ON THE TRANSFER OF IRON AND PHOSPHATE TO PARTICULATE SIZE FRACTIONS IN EPILIMNETIC LAKE WATER

Transformations of dissolved iron and phosphate to particulate material (>0.2 mu m) were investigated by adding (FeCl3)-Fe-55 and (PO43-)-P-32 to pH adjusted epilimnetic lake water samples of contrasting ionic strengths and dissolved humic substances (DHS) concentrations. The distributions of these tracers between particulate size fractions in samples without appreciable DHS after 24 h of dark incubation varied markedly over the pH range 4-8. The effect of pH on the movement of Fe-55 and P-32 to particulate size fractions in clear-water samples depended on the ionic strength. Over the same pH range, the presence of DHS suppressed the movement of Fe-55 and P-32 to particulate size fractions. There was little difference between low and high ionic strength DHS-rich samples. The capability of DHS to maintain added Fe-55 and P-32 in solution (<0.2 mu m) was confirmed by simultaneous measurements of precipitation losses.