Effect of sediment spatial variance and collection method on cladoceran toxicity and indigenous microbial activity determinations

Quantitative assessments of sediment quality frequently are based on data derived from sediments collected by grab sampling. Often only one sediment or one composited sediment is collected at a sample site, thereby preventing any determination of spatial variance. The significance of spatial variance and collection method on cladoceran toxicity and indigenous microbial activities was determined at a creosote contaminated site in the Little Scioto River in Marion County, Ohio. Replicate sediments were collected on two occasions from a horizontal sample grid (110 m2 and 1.4 m2) using an Ekman dredge and hand core. Sediment particle size fractions did not vary significantly between replicate grab samples. Survival of Daphnia magna and Ceriodaphnia dubia in 48-h solid phase exposures varied, however, from 0 to 100% horizontally and vertically at the test site. Percent survival and β-glucosidase activity decreased in sediment sections below a 4 to 5 cm depth. Dredged sediments produced less toxicity to the daphnids than did core collected samples. The coefficient of variance was high (>40%) for both survival and hydrolase activities (alkaline phosphatase, β-galactosidase and β-glucosidase) between horizontal sediment replicates collected by either dredge or core. Subsample variance, however, was small (<20%). These results highlight the important role that sediment heterogeneity and collection method have on sediment quality assessments.