Comparison of the biological effects of humic materials under acidified conditions

The acute and chronic effects of three types of humic material were studied using Daphnia magna at pH 5, 6, and 7. At neutral pH humic material had no biological effect. At pH 6 and 5 maximum survival and population growth rate occurred at intermediate levels of humus material (10 mg C/L, 300 mg Pt/L). Lowest survival and population growth rate occurred at pH 5 in either the absence of humus material or at concentrations exceeding 10 mg C/L (300 mg Pt/L). A second order polynomial was used to fit the dose response curve at pH 5 for both Aldrich humic acid and a natural bog water. Aldrich had greater biological activity. It is suggested that the biological behavior of aquatic humus conforms to second order rate phenomena controlled by high levels of free metals ions at low humus concentrations and direct toxic action of the dissolved organics at low pH due to increased lipophilicity.