The contamination and degradation of many sediment environments have been attributed to the global surge of anthropogenic activities near coastlines. Organism-sediment interactions (i.e. bioturbation) are known to alter sediment biogeochemistry and potentially increase the risk of contaminant exposure and toxicity to surrounding ecosystems. Current risk assessment frameworks for contaminated sediments seldom consider the impacts of bioturbation on contaminant exposure explicitly, leading to potentially inaccurate assessments and ineffective management strategies. In this thesis, the impacts of bioturbation on benthic organism exposure and toxicity were assessed. The impacts of bioturbation on: (i) metal bioaccumulation and toxicity in the bivalve Tellina deltoidalis in metal contaminated sediments; and, (ii) toxicity (survival and reproduction) in the amphipod Melita plumulosa in sediments contaminated by primarily metals and a mixture of metals and hydrocarbons are addressed in Chapters 2 and 3, respectively. In both studies, increased bioturbation intensities (presence of the amphipod Victoriopisa australiensis) in metalcontaminated sediments resulted in large differences in metal partitioning between the sediment and overlying water column, although the degree of partitioning varied for different metals. Lower toxicity in both T. deltoidalis (survival) and M. plumulosa (survival and reproduction) bioassays with increased bioturbation was attributed to lower copper exposure. In contrast, for sediments contaminated by both metals and hydrocarbons, high bioturbation intensities enhanced toxicity, and was attributed to increased bioavailability of polycyclic aromatic hydrocarbons (PAHs). Both studies demonstrated that bioturbation modifies the concentrations and forms of metals in the particulate and dissolved phase, altering exposure and toxicity to cohabiting organisms. Greater bioturbation intensities were shown to increase metal exposure (due to disruption of contaminant binding) or decrease metal exposure by providing a source of contaminant sequestration in the water column via sediment resuspension and/or increased efflux of dissolved pore water metals. Therefore, overall sediment toxicity may either be increased or decreased depending on the types of organism, bioturbation intensity and contaminants present, and indicates the need to consider such interactions when assessing the risks contaminated sediments pose to aquatic environments. The fourth chapter of this thesis investigated the influence of bioturbation (absence/presence of T. deltoidalis and V. australiensis) on metal bioavailability and toxicity to M. plumulosa (survival and reproduction) in a metal contaminated sediment with different concentrations of acid volatile sulfides (AVS). Acid volatile sulfides are considered a major metal binding phase in anoxic sediments, and the molar difference between AVS and simultaneously extractable metals (SEM, where SEM=Cd+Cu+Ni+Pb+Zn). SEM-AVS is commonly used by environmental scientists and managers to predict risk of effects of these common metal contaminants in sediments. However, such chemical-extraction methods do not consider how organisms burrowing behaviours may modify metal bioavailability in sulfidic sediments, and this is typically not discussed when evaluating the significance of SEM-AVS results. Bioturbation resulted in lower AVS concentrations through oxidation and increased SEM concentrations. Similar to Chapters 2 and 3, toxicity to reproduction was lower in the more highly bioturbated sediments that contained low AVS and corresponded with lower dissolved copper and zinc concentrations in the overlying water column and tissues of M. plumulosa. In contrast, greater bioturbation resulted in higher toxicity in the sediments that contained high AVS, despite lower dissolved copper and zinc concentrations. These results indicate that the AVS-SEM paradigm cannot be accurately used to predict low risk of toxicity in sediments bioturbated or mechanically reworked during remediation. The addition of bioturbators to degraded sediments can either immobilise contaminant stressors or mobilise and facilitate their removal, potentially facilitating further recruitment of benthic organisms assisting in restoring functioning ecosystems. The fifth chapter of this thesis investigated the tolerance of bioturbating organisms to hypersalinity, another common physicochemical stressor in many degraded coastal ecosystems. In order to utilise bioturbators to assist with hypersaline sediment remediation, salinity tolerance thresholds need to be determined for a variety of benthic organisms. Hypersaline sediments (~400‰ salinity) collected from decommissioned salt ponds were used to delineate the salinity threshold limits for a range of benthic organisms to better understand their role in remediation strategies for removing stressors such as hypersalinity. The results from this study identified the tolerances for a range of benthic organisms to hypersaline sediments and enabled the extrapolation of a range of salinity limits, above which recolonisation by bioturbating organisms may be inhibited. This information is critical for designing monitored natural recovery strategies to restore hypersaline sediments to functioning ecosystems. Collectively, the research in this thesis has illustrated the importance of considering organismsediment interactions within existing sediment quality assessment frameworks and management strategies. Bioturbation by benthic organisms has been shown to alter the biogeochemistry and toxicity of many common contaminants in sediments. These changes can either be positive or negative for the resident biota. Despite these differences, the presence of bioturbation in contaminated sediments may reduce the concentration of sediment contaminants over time, and ultimately lead to sediment recover and rehabilitation.