Density and habitat dependent effects of crab burrows on sediment erodibility

Abstract

Despite biological interactions being highlighted as a key process in determining particle fluxes, relatively few studies have attempted to establish the links between burrow building bioturbators and sediment stability. The mud crab Austrohelice crassa, is a key burrowing species in New Zealand estuaries that has shown context-specific interactions with its environment. Here we use annular flumes to test if sediment stability and erodibility were altered as a function of A. crassa burrow density in two contrasting sediment types: a cohesive sandy-mud and a non-cohesive sand. Three burrow density treatments (n=3) reflecting the natural density range in each sediment type (sand; 0-100m-2, sandy-mud; 0-400m-2), were collected from the field and subjected to sequential increases in water flow velocity. Flow profiles were measured and bed shear stresses were calculated for each treatment. Increasing burrow density reduced the mass of sediment eroded at 0.35ms-1(ME-35, gm-2) in cohesive sandy-mud, while in non-cohesive sand a unimodal pattern was observed, whereby erosion rates were greatest at the lowest burrow density (19m-2). In the cohesive sediment, the linear decrease in erodibility with increasing burrow density was likely affected by the sluicing of fine particulates (silt-clay) from burrows when the tide was out creating both a smoothing and consolidating effect on the sediment surface. A reduction in flow velocity due to the increased presence of surficial pellets and greater trapping of bedload transported material was attributed to the reduction in the mass of sediment eroded in sand at high burrow densities. This study demonstrates that burrow builders influence sediment transport by more than just vertical particle mixing and highlights some of the complexities of small-scale sediment processes. Knowledge of different organism-sediment interactions among sediment types and spatial scales will enhance the accuracy of sediment transport models