Using Austrovenus stutchburyi as a bioindicator to investigate the impact of terrestrial sediment

Abstract

Changes in land use and modification to coastal ecosystem due to human development have resulted in elevated terrestrial sediment inputs to estuaries and coastal marine environments. This leads to two main environmental problems: the deposition of terrestrial sediment and elevated suspended sediment concentrations (SSC) in the shallow water that pose a threat to the marine animals. The aim of this thesis is to test the use of Austrovenus stutchburyi as a bioindicator of sediment impact on the coastal region. I investigate the impact of terrestrial sediment on the physiology and behaviour of cockles to see if changes indicate a critical response threshold. I mainly focus on the effects of thin sediment deposits (≤ 2 cm) and suspended sediments because small amounts of terrestrial sediment deposition are common in NZ. Although these effects may be subtle they could have consequences for benthic communities under longterm disturbance. A second objective of this thesis is to investigate whether the physiological and behavioural biomarkers used in this study are good to assess the impact of terrestrial sediment. Even thin deposits of terrestrial sediment had adverse effects on the condition and clearance rate of Austrovenus. Clearance rate decreased dramatically when the thickness of terrestrial sediment deposit was higher than 8 mm after 10 days exposure. Condition demonstrated clear and consistent decreasing trend with increasing deposition of terrestrial sediment, with the sharpest decrease being observed at the end of experiment (55 days). Suspended sediment had a much stronger effect on the clearance rate and condition. The clearance rate decreased above a threshold between concentrations of 100 and 200 mg/L. Burrowing behaviour showed no clear response to the sedimentation compared to other biomarkers, which indicates that burrowing time, although an easy and quick measure, is not a good biomarker. However, condition, biodeposit production and clearance rate are useful, especially with clearance rate being extremely sensitive to suspended sediment. Cumulative effects of deposits & suspended sediment were apparent, especially when cockles exposed to high deposit thickness were exposed subsequently to high suspended sediment concentration. A shifted threshold to suspended sediment was observed in clearance rate from 100-200 mg/L to 300 mg/L SSC, which indicated a history effect. My study shows that suspended sediments have adverse effect on the physiology and behaviour of cockles even at concentration as low as 100-200 mg/L. The effect of fine deposits is also obvious even when the thickness is lower than 1 cm. Furthermore, long-term deposition by terrestrial sediment can result in habitat change. The potential combined effects of deposits & suspended sediment are also strong. My study can provide meaningful information to contribute to the investigation of the impact of terrestrial sediment and help predict the subtle impact of terrestrial sediment on the ecosystem at an early stage.