The role of large macrofauna in mediating sediment erodibility across sedimentary habitats

Abstract

Large macrofauna influence sediment erodibility via their activity and presence in the sediment. This article explores how the depletion of large animals from macrobenthic communities influences sediment erodibility across a natural grain size gradient of poorly sorted habitats within an estuary. We sampled seven sites in Mahurangi Harbour, New Zealand, to investigate how heterogeneity within and among these habitats influences sediment stability. We depleted the large macrofauna in the sediment to determine how changes in community structure will affect ecosystem functioning in the context of sediment stability. A core-based erosion device (EROMES) was used to measure three different parameters associated with sediment resuspension potential: surface erosion threshold (τc; N m−2); erosion rate (ER; g m−2 s−1); and the subsurface erosion constant (me; g N−1 s−1). Multiple regression models were developed for each parameter to identify important drivers of change. Sediment grain size, as a proxy for habitat type, explained 53% of the total variation in both the early surface erosion measures, τc and ER. Once the surface layers had been eroded, me was best defined by a site-specific combination of biological, chemical, and physical variables that explained 40% of the subsurface erosion. Our results demonstrate that at all sites reducing the abundance of large animals within the macrofaunal community contributed to substantial impacts on erosion and thus ecosystem functioning associated with sediment composition and water clarity.