Delineating Sources and Estimating Cadmium Bioaccumulation and Susceptibility Differences among Aquatic Insects.

Abstract

Human activities such as mining and smelting have altered the distribution of trace metals in the environment, resulting in trace metal pollution of aquatic ecosystems worldwide. Trace metal contamination has been shown to adversely affect benthic macroinvertebrate community structure by influencing species diversity, density and abundance. Aquatic macroinvertebrates play integral roles in stream ecosystem function and are widely used in biomonitoring programs as indicators of stream integrity. Relative to marine and other lentic invertebrates, lotic insects are grossly underrepresented in ecotoxicological studies. A major challenge in ecotoxicology lies in generating data under experimental conditions that are relevant to understanding contaminant effects in nature. Biodynamic modeling is one laboratory-based approach for understanding trace metal bioaccumulation in a number of aquatic invertebrates. This approach combines species-specific physiological traits associated with metal bioaccumulation from both aqueous and dietary sources to make predictions of metal- and species-specific bioaccumulation that compare well relative to field-based measurements. This thesis represents the first application of biodynamic modeling to stoneflies, a dominant insect group in North American streams. A second goal of these comparative studies was to determine how much variability, or conversely, predictability there is in some of the physiological processes related to bioaccumulation. Because insects as a group are so species rich, a comprehensive testing regime is unfeasible. These comparative studies were designed to determine if closely related taxa share physiological characteristics such as cadmium uptake rate from solution, elimination rate, and subcellular compartmentalization of accumulated metal. Biodynamic models were generated for seven predatory stonefly (Plecoptera) species representing the families Perlidae (5) and Perlodidae (2). Each taxon was exposed to cadmium independently via diet and via solution. Species varied approximately 2.6 fold in predicted steady state Cd concentrations. It has become increasingly evident that diet plays a major role in the accumulation of metals in aquatic invertebrates and the importance of diet is consistent in stoneflies. Diet was the predominant source of accumulated Cd in five of the seven species and averaged 53.2 ± 9.6 % and 90.2 ± 3.7% of the total Cd accumulated in perlids and perlodids, respectively. These results are relevant to water quality criteria that are often based on toxicity tests involving aqueous exposures only. Differences in Cd bioaccumulation between the two families were largely driven by differences in dissolved uptake rates, which were considerably slower in perlodids than in perlids. Cadmium uptake rates also varied within families, suggesting that family level generalizations about metal accumulation may be erroneous. We further examined the subcellular compartmentalization of cadmium accumulated from dissolved and dietary exposures. Predicted steady state concentrations were modified to only consider Cd accumulated in potentially susceptible subcellular compartments. These values ranged 5.3 fold. This variability is discussed within a phylogenetic context as well as its implications for bioassessment.