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Organism to ecosystem responses to copper contaminated sediments in model freshwater ecosystems
thesisposted on 2022-03-28, 14:02 authored by Stephanie Gardham
Most knowledge of the effects of metal contamination is derived from laboratory studies investigating molecular, biochemical, physiological or individual organism responses. The applicability of this knowledge to the effects of metals at higher levels of biological organisation (population, community and ecosystem) in the natural environment has been questioned for some time, but the challenge remains. Further, in studies that have explored the effects of metals at higher levels of biological organisation, exposure pathways to the metal in question have often been unrealistic. Copper is a classic example; the majority of studies exploring the effects of copper analysed responses at low levels of biological organisation and those that explored responses at higher level of biological organisation performed experiments with dissolved exposures in the overlying waters. However, in the natural environment sediments have acted as a sink of historical contamination and dissolved concentrations of copper are now generally present in the low μg/L range. Thus, exposure pathways of copper to biota in the natural environment are likely to be very different to those that were present in previous high level studies. The main aim of this research was to identify relationships between the effects of copper at low and high levels of biological organisation under a realistic exposure scenario. Twenty artificial semi-field ecosystems (mesocosms) were created with environmentally relevant copper spiked sediments. At the organism level, the effects of copper on the snail Physa acuta and the shrimp Paratya australiensis were assessed via in situ exposures. The development of the invertebrate community within the mesocosms was monitored by traditional optical techniques, allowing an assessment of population and community effects of copper. In addition, changes over time of the eukaryote community composition were monitored by environmental DNA (eDNA) metabarcoding. Finally, the effects of copper on ecosystem function were assessed by measuring primary production and organic matter decomposition within the mesocosms. Identifying the relationships between levels of biological organisation proved to be most useful when unexpected responses, contrary to known direct effects of copper, were observed. Increased decomposition of organic matter on the sediment surface with increasing copper concentrations was linked to an associated increase in periphyton cover; both of these responses were unexpected based on previous studies that explored the direct effects of copper on organic matter decomposition and periphyton biomass. However, the increase in periphtyon cover could be explained by a decrease in grazing pressure in the high copper treatments compared to the control. Snails, particularly Physa acuta, were abundant in the control and low copper treatments, but had low abundances in the high copper treatments. This was probably a direct effect of copper, as the in situ exposures demonstrated an inhibition of growth (i.e. fitness) of individuals of Physa acuta in the high copper treatments. Beyond the main aim, this research demonstrates the importance of performing an environmentally realistic exposure scenario in manipulative studies. It provides ecologically relevant data, which will aid the further development of guideline threshold concentrations for copper. The study also contains a valuable comparison of community level analysis by traditional techniques of optical analysis and eDNA metabarcoding. The latter technique proved to be more sensitive because it incorporated a much greater proportion of the community present, from the micro- to macro- scale. Perhaps most importantly, the study demonstrates the variety of assessments that can be performed in the pursuit of understanding ecosystem health and the value of each type of assessment. It demonstrates that by using multiple lines of evidence, which assess the effects of a contaminant across levels of biological complexity, a comprehensive understanding of the interactions that drive the biotic response within an ecosystem can be gained.