Macrobenthic recovery processes following catastrophic sedimentation on estuarine sandflats

Land use can exacerbate the rate of sediment delivery to estuaries. In particular, for catchments with steep terrain and heavy, sporadic rainfall, changes in land use can increase the risk of catastrophic deposition of terrestrial sediment. One of the key issues in assessing the ecological significance of catastrophic sedimentation events is determining the rate of recovery of the macrobenthic community and understanding how physical and biological processes influence the recovery rate in different locations. We conducted a field experiment over 212 days to assess the impact of terrestrial sediment deposits at six sites on the intertidal sandflats of Whitianga Harbour (New Zealand). Differences in the sedimentary habitat as a result of the deposition of terrestrial sediment lasted for similar to50d, although these effects varied between different sediment properties and between sites. The deposition of terrestrial sediment had an immediate and negative effect on resident macrofauna, although complete defaunation of the experimental plots did not occur. Macrobenthic recovery lagged behind the recovery of the sediment properties. Based on multivariate analysis, three sites never recovered over the duration of the experiment, while based on univariate analysis four sites never recovered. Macrofaunal assemblages living deep (2-15 cm) within the sediment were generally slower to recover than those found in the top 2 cm. A meta-analysis of recovery rates for macrofauna was conducted on information generated in this study and in two similar experiments carried out in other New Zealand estuaries. This analysis revealed a consistent negative relationship between the magnitude of disturbance generated by the terrestrial sediment layer and the recovery of the macrobenthos. Measures of recovery at the community level emphasized the importance of site environmental factors reflecting increased wave disturbance, flow velocity, and the wetting and drying of the deposited terrestrial sediment in speeding recovery. Overall, our results indicate that the long-term effects of catastrophic sediment disturbance are influenced by local hydrodynamic conditions and the composition of resident macrofauna living in sediments adjacent to disturbed areas. Given the long time scale of recovery from these. experiments they indicate the potential for catastrophic sediment deposition to result in broad-scale degradation of estuarine macrobenthic communities.