The Effect of Acidification on Abiotic Interactions of Dissolved Humic Substances, Iron and Phosphate in Epilimnetic Water from the Humex Lake SkjervatjernSeparation of Molecular-Size Classes of Aquatic Humic Substances Using Ultrafiltration and DialysisApplicability and Reliability of Gel-Filtration to Study Aquatic Humic Substances Revisited - the Effects of Ph on Molecular-Size DistributionsEffects of Dissolved Humic Substances on the Speciation of Iron and Phosphate at Different Ph and Ionic-Strength