Acute toxicity of Cd in stream invertebrates in relation to pH and test design

The acute toxicity of Cd to three stream invertebrates (Baetis rhodani Pict., Leptophlebia marginata (L.) and Pisidium sp.) was tested at pH 5 and 7 simultaneously in static (ST) and flow through (FT) systems. In the static design, the animals were kept individually in small boxes containing aerated stream water. In the flow through system, the three species were kept together in circular aquaria simulating stream ecosystems with patches of gravel and recirculating aerated stream water. The nominal Cd concentrations ranged from 0 to 5 mg 1^–1 during an exposure period of 120 h.The graphically obtained LC₅₀ (120 h) values for B. rhodani were: pH 7: 2.3 mg 1^–1 (ST), 2.5 mg 1^–1 (FT) and pH 5: 3 mg 1^–1 (ST), 1 mg 1^–1 (FT). For L. marginata the following values were found: pH 7: > 5 mg 1^–1 (ST), 4.4 mg 1^–1 (FT) and pH 5: > 5 mg 1^–1 (ST), 3.6 mg 1^–1 (FT). Pisidium sp. showed a 100% survival in all Cd- and pH-treatments. From thee values it can be concluded that L. marginata is more tolerant than B. rhodani and that especially both species tolerated Cd better in the ST system than in the FT system, especially at pH 5.At neutral pH the Cd-concentrations in the animals were higher than at low pH, probably due to surface adsorption of Cd onto the body. B. rhodani tended to molt more at low than at neutral pH.