CONTRASTING EXPERIMENTAL MANIPULATIONS OF THE ACIDITY OF CATCHMENT DRAINAGE WATER - EFFECTS ON INTERACTIONS BETWEEN HUMIC SUBSTANCES, IRON,AND PHOSPHATE

Abiotic interactions occurring between dissolved humic substances, iro n, and phosphate were investigated by addition of radioisotope tracers ((FeCl3)-Fe-55, and PO43-) to surface water samples collected from ca tchments subjected to contrasting pH manipulations. Increases from pH 4 to circumneutral pH due to catchment liming resulted in increases in the proportions of added ionic Fe-55 and P-32 recovered in higher mol ecular size fractions following gel chromatography. Changes in ionic c omposition due to catchment liming did not result in marked changes in redistributions of added Fe-55 and P-32. Comparatively small differen ces between the pH values and ionic compositions of the control and ar tificially acidified subcatchments of a humic lake did not result insu bstantial changes in the redistributions of added Fe-55 and P-32.