Quantitative measures of sedimentation in an estuarine system and its relationship with intertidal soft-sediment infauna

Increased sedimentation from changes in land use in coastal areas is a potentially important impact of human urbanisation. The potential impact of sedimentation on benthic infauna was quantitatively investigated in the Okura estuary, which is at the northern fringes of urban development in Auckland, New Zealand. A structured mensurative sampling programme, measuring benthic infauna and various environmental variables, is described. Of the variance in macrofaunal assemblages from 15 sites throughout the Okura estuary on 6 sampling occasions, 70% was explained by environmental variables. Proportions of ambient sediment grain-sizes, depositional categories from previous models, the amount and characteristics of trapped sediments, organic content, changes in bed height and distance from the mouth of the estuary were all useful in explaining variation in macrofaunal assemblages. Levels of sedimentation recorded in this study were sub-catastrophic (<3 cm of deposition d(-1)), corresponding to natural fluctuations in sedimentation. Bivalves generally had a negative relationship with sedimentation, while certain burrowing crabs and polychaetes were more abundant in high-deposition environments. The total amount and the grain-size characteristics of trapped sediments explained a significant proportion of the variation in soft-sediment assemblages, over and above the variation explained by ambient sediment variables. Thus, sedimentation appears to be an important structuring force in these intertidal estuarine macrobenthic assemblages.