The effects of terrigenous sediment on the behaviour of Macomona liliana (Bivalvia) in permeable sediments: Implications for porewater exchange

摘要

Catastrophic sedimentation events create one off disturbances that can cause mass mortality events in receiving estuaries. In contrast, the deposition of small amounts of terrigenous sediment (TS) is on-going and likely to have long-term consequences for benthic communities who, play important roles in ecosystem functioning. The aim of this thesis was to look at the effects of small amounts of TS on the behaviour of an infaunal deposit-feeding bivalve (Macomona liliana), which may have implications for sediment porewater fluxes and ecosystem functioning. To determine the effect of TS on behaviour, mesocosm experiments in a recirculating flume tank were run. Time lapse imagery was used to capture surface activity, and high resolution pressure sensors were buried within the sediment and captured changes in hydraulic activity. The effect of three TS treatments were tested, low, high and mixed. The low and high treatments were surface additions of TS (0.09 g cm -2 and 0.20 g cm -2, respectively) and represented the initial deposition and build-up of TS. The mixed treatment, mixed 0.20 g cm -2 through the top 2 cm of sediment, to mimic the incorporation of TS into marine sediments over time. Behaviours were observed pre- and post-treatment addition, and were compared to determine the effect of TS on M. liliana behaviour. The surface (top 2 cm) sediment properties were measured to determine the degree of surface modification by the TS treatments.TS significantly modified the properties of the sediment surface and the hydraulic conductivity. These caused subtle, but significant, changes in some of the behaviours observed in M. liliana, and on a whole, behaviour which bioturbated the sediment surface decreased. Two feeding modes were observed at the surface, deposit-feeding and suspension-feeding/ventilation. There was a switch in feeding modes, from deposit-feeding to suspension-feeding/ventilation, in the low and mixed treatments. The duration of suspension-feeding/ventilation was, on average, three times longer than deposit-feeding, across all treatments, therefore a switch in modes altered the temporal patterns of sediment pressurisation. The amplitude of sediment pressurisation during feeding was significantly increased in the mixed treatment (up to 5× higher) as a result of decreased hydraulic conductivity. Additionally, the pressure signal measured during deposit-feeding was twice that of suspension-feeding/ventilation in the mixed treatment. Collectively the short-term changes in M. liliana behaviour are likely to increase sediment stability and the persistence of TS in the environment, potentially causing long-term degradation to habitat and M. liliana condition. As a result of reduced hydraulic conductivity, larger porewater fluxes are likely to be generated in the mixed treatment, and this would be two fold larger during deposit-feeding than in suspension-feeding/ventilation. This paired with the different temporal scales of the two feeding modes, and the observed switches in feeding modes, would have important consequences for geochemical conditions. Rather than short periods of high pressurisation, there would be longer periods of smaller pressurisations, which would increase the duration of sediment oxygenation, but reduce the porewater flux. This is likely to promote more stable geochemical conditions, which when compared to oscillating conditions, has been shown to reduce nutrient recycling and estuarine productivity.