The Effect of Organism Density on Bioaccumulation of Contaminants from Sediment in Three Aquatic Test Species: A Case for Standardizing to Sediment Organic Carbon

Abstract

Laboratory methods for measuring bioaccumulation of organic contaminants from sediment into aquatic organisms continue to improve, but some aspects are still in need of standardization. From a review of published methods, we noted that the loading density of organisms was determined inconsistently and was primarily based on either sediment volume or total organic carbon (TOC). The rationale mainly expressed for standardizing to TOC was to minimize the depletion of sediment contaminants. However, even when density was standardized to TOC, the relative amount of TOC provided (i.e., ratio of TOC to organism dry weight [dw]) was highly variable. In this study, we examined the effect of organism density (standardized to sediment TOC or volume) on bioaccumulation in three freshwater organisms. The oligochaete Lumbriculus variegatus, mayfly nymph Hexagenia spp., and fathead minnow Pimephales promelas were exposed for 28 days to two field-contaminated sediments that varied in concentration of PCBs and TOC. Densities tested were 50:1 and 27:1 ratios of TOC to organism dw and 140 ml sediment/g wet weight (ww) biomass, yielding low to high organism densities. Bioaccumulation in Hexagenia spp. was significantly higher at the lowest organism density compared with the highest organism density when exposed to site 2 sediment (1.1% TOC) but only with tissue concentrations expressed on a ww basis. Otherwise, there was no significant effect of density on bioaccumulation in organisms exposed to sediments from site 1 (12% TOC) or site 2. Survival of Hexagenia spp. was adversely affected at the highest organism density when the relative amount of TOC was low. The results of this study support the recommendation of standardizing organism density relative to a particular amount of TOC for invertebrate species. A 27:1 ratio of TOC:organism dw was selected as a standard organism density for a new bioaccumulation method because survival, growth, and bioaccumulation were not impacted relative to a 50:1 ratio, and less sediment was required. This density is recommended as an appropriate ratio for sediment bioaccumulation assessments in general.

Sediment Bioaccumulation Test with Lumbriculus variegatus: Effects of Organism Loading

“…1 and Supplemental Data: Table S4). The observation of increasing loadings of organism causing decreasing growth is contrary to the studies of Van Geest et al (2011) and Kukkonen and Landrum (1994) in which there was no relationship between growth and loading of the L. variegatus. This study used a larger set of sediments and a wider range of loading of the organisms.…”

“…In a review of sediment-testing methods (Van Geest et al 2010), other ratios of TOC/Lv have been used with the sediment-bioaccumulation test, and the review suggested that an optimal organisms density might not exist for all sediments. In follow-up work by Van Geest et al (2011), PCB residues from sediment-bioaccumulation tests performed at ratios of 50:1 and 27:1 were not significantly different. Ontario Ministry of the Environment testing protocols have adopted the 27:1 ratio (Van Geest and Watson-Lueng 2013) and all tests are performed using this ratio in their sediment-bioaccumulation tests with L. variegatus.…”

“…An unexpected result of the study by Kukkonen and Landrum (1994) was the observation of an inverse relationship between TOC/Lv ratio and contaminant bioaccumulation, i.e., with increasing ratio, residues in L. variegatus decreased. Other studies have shown the same relationship (Lotufo et al 2000;Van Geest et al 2011), and currently the causes of this apparent behavior are unknown. In a review of sediment-testing methods (Van Geest et al 2010), other ratios of TOC/Lv have been used with the sediment-bioaccumulation test, and the review suggested that an optimal organisms density might not exist for all sediments.…”

Sediment Bioaccumulation Test with Lumbriculus variegatus: Effects of Feeding

“…The standard bioaccumulation testing protocol (USEPA 2000) specifies that the ratio of total OC in the sediment to organism dry weight should be no less than 50:1 in this test; some of the tested sediments did not meet this requirement (Tables S3 and S6). Bioaccumulation testing with L. variegatus at a ratio of 27:1 has been shown to provide the same results as the 50:1 ratio (Van Geest et al 2011). …”

A comparison of the bioaccumulation potential of three freshwater organisms exposed to sediment‐associated contaminants under laboratory conditions

“…Tests were conducted at 23 AE 2 8C, under a 16:8 h light:dark photoperiod with an intensity of 500 to 1,000 lux. Based on previous research evaluating the effect of loading density on organism survival, growth, and bioaccumulation, a ratio of approximately 27:1 TOC to organism dry weight was selected as a standard loading density of organisms for the bioaccumulation method [15].…”