Effects of sample storage on a copper-spiked freshwater sediment

Abstract

Abstract Freshwater sediment from an Oregon lake was spiked with copper in the laboratory to study the effect of storage on copper toxicity. Peat moss was added to enhance the carbon content of a portion of the sediment. One-half of the sediment samples were stored at 5°C while the other half were frozen (−20°C). At intervals up to 25 weeks, samples were brought to 20°C and 48-h toxicity tests were conducted in 1-liter beakers (200 ml sediment, 800 ml water) using the cladoceran Daphnia magna Straus. Copper released into the water from the sediment stored at 5°C increased over the first 8 weeks of storage, then decreased. Freezing the sediment attenuated the release of total and soluble copper into the overlying water, resulting in low or no toxicity to daphnids. Based on total copper concentrations in the overlying test water, LC50s ranged from 7 to 44 μg/L in sediments without peat and from 52 to 96 μg/L in sediments with added peat. Soluble copper LC50s for-the same test waters ranged from 3 to 18 and from 25 to 68 μg/L, respectively. No LC50s could be calculated from the frozen sediment tests because of low mortality. Freezing is not recommended as a storage procedure because of its undependability in preserving sediment integrity for toxicity tests.