The importance of dietary exposure in the determination of bioavailability of sediment-sorbed organic compounds to benthic macroinvertebrates.

Hydrophobic organic compounds released into the environment have the potential to persist within aquatic sediments for many decades. The hydrophobic nature of such compounds results in sediment contamination that is orders of magnitude greater than the concentrations within the overlying water column However, simple chemical measures of bulk amounts of organic contaminants within sediment cannot accurately predict uptake and assimilation of sediment contaminants into aquatic biota. In particular, there is uncertainty related to the potential for uptake of compounds that are sorbed to sediment particles. It has been suggested that the sole fraction of a contaminant that is available for uptake is the proportion found as aqueous solute within the interstitial spaces (termed “pore water”) of sediment. This thesis tests the hypothesis that uptake of particle-sorbed contaminants is a viable assimilation route: the relative importance of which may vary in response to both chemical partitioning and organism biology. In addressing this hypothesis, four species of benthic macroinvertebrate: Chironomus riparius larvae, Lumbriculus variegatus, Asellus aquaticus and Gammarus pulex, were exposed to a compound sorbed strongly to sediment particles and insoluble in water. [14C]-Dioctadecyldimethylammoniumchloride (DODMAC) was used as the model compound for these exposures in both artificial and field-collected sediments. Tissue loadings measured within whole organisms were verified by autoradiography of histological tissue sections and comparison of tissue loadings in feeding and non-feeding individuals as well as moulted and non-moulted individuals. In response to measured variation in DODMAC uptake, organism feeding selectivity, biotransformation, the proportion of lipid and water within tissues and gut retention time were assessed as potential mechanisms of uptake variation. In addition, literature-derived factors were also evaluated as potential mechanisms. Subsequently, the variation in proportional contribution of aqueous and particulate uptake routes was examined in three polyaromatic hydrocarbon compounds (pyrene, fluorene and naphthalene). Novel apparatus was used to measure the proportion of uptake due to pore water relative to uptake in whole sediment. The influence of compound Kow upon route of uptake in G. pulex was measured using pyrene, fluorene and naphthalene. Additionally, the effect of the presence of gills upon naphthalene uptake was assessed using the four species previously exposed to DODMAC. All four species achieved genuine assimilation of DODMAC. The dietary assimilation of this compound varied between the species and between artificial and field-collected sediment. Chironomus riparius consistently accumulated higher tissue loadings of DODMAC than the remaining species. The variation in DODMAC assimilation could not be explained by the experimentally-measured factors. Instead, the potential importance of gut-fluid surfactancy, solubilisation of sediment particles and periodic increases in lipid assimilation was highlighted through existing literature. Polyaromatic hydrocarbon compounds were found to be assimilated from both aqueous and particulate sources. Uptake via sediment ingestion assumed greater importance as compound KoW increased. Conversely, the presence of gills appeared to increase the proportion of aqueous assimilation. Sediment-sorbed compounds were concluded to be bioavailable to benthic macroinvertebrates via dietary uptake. Such bioavailability appears to vary amongst species. Further research into gut-fluid properties and changes in tissue morphology in relation to contaminant assimilation was prompted. For compounds present in both particulate and aqueous solute fractions, the proportion of dietary uptake increases with K««. Conversely, ventilatory appendages can increase the proportion of aqueous uptake. These conclusions support the general hypothesis addressed in this thesis.