Response of Tropical African Macroinvertebrates with Varying Tolerances to Different Levels of Nitrate and Phosphate

Acute toxicity test was performed to determine the sensitivity of Neorpela spio, Baetis harrisoni, and Tubifex spp. to nitrates (NO3-N) and phosphates (PO4-P) with different concentrations after 96 hours of exposure time. The observed lethal effects and/or mortality increased with concentration and exposure time among tested species of different sensitivities. The results demonstrated that both nitrate and phosphate are toxic to the three studied organisms under the test conditions, with Neorpela spio displaying the highest acute effect in water with nitrate and phosphate compared with Baetis harrisoni and Tubifex spp. The 100% cumulative mortality was experienced at 3.2 mg NO3-N/L and 2.4 mg PO4-P/L for N. spio, 5.6 mg NO3-N/L and 4.8 mg PO4-P/L for B. harrisoni, and 128 mg NO3-N/L and 24 mg PO4-P/L for T. spp. However, N. spio and B. harrisoni showed high mortality at the Tanzanian nitrate recommended lower and maximum limits of 10 and 75 mg NO3-N/L, respectively, for drinking water and significant mortality at the recommended limits of nitrite (20 mg NO3-N/L) and phosphorus (6 mg PO4-P/L) concentrations for municipal and industrial wastewaters. Therefore, there is a need for these Tanzanian recommended nitrate ranges for drinking water of 10 to 75 mg NO3-N/L and 20 mg NO3-N/L and 6 mg PO4-P/L for municipal and industrial wastewaters to be refined for the betterment of protecting both human health and riverine organisms.