Contaminating metals were removed from the buffer by overnight equilibration with 100 μM MnO2 (van den Berg 1982) and then filtered (0.02 μm inorganic membrane, Whatman); 100 μL of this buffer in 10 mL seawater gave a pH of 8.2. Reference humic and fulvic acid used for calibrations were Suwannee River HA (International Humic Substances Society