|dc.description.abstract||Intertidal ecosystems contribute a significant portion of the world’s ecosystem goods and services. Recently, an increasing amount of focus has been generated surrounding the central role of sediment mixing (bioturbation) in estuarine ecosystems. Bioturbating macrofauna such as the burrowing mud crab austrohelice crassa are found in many New Zealand ecosystems, and have been shown to be key ecosystem engineers in these environments. Burrow building extends the sediment-water interface, creating changes in solute and particle fluxes through modification of near-bed flows. Burrows and holes have been poorly studied compared with their above-ground counterparts and the exact manner in which burrows affect small-scale water movement at the sediment water interface is yet to be understood.
This project used measurements of fine scale flows surrounding austrohelice crassa burrows in situ as well as in a laboratory setting. Experiments used artificial burrows in a unidirectional flume to explore the impact that crab burrow orientation has on flow. Measurements of flow velocities were taken and it was found that flow extended further into the burrow when the burrow was aligned downstream with the flow rather than at right angles or aligned upstream to the flow.
Subsequently, the impact of burrow density on near-bed flows was quantified in the laboratory using different density arrays of artificial burrows. Measurements of velocity were used to calculate turbulent kinetic energy (TKE). It was found that with increasing burrow density there was a split in flow regimes between high and low flow speeds. At low flow speeds, the TKE increased to a peak followed by a decline, owing to the development of skimming flow.
Field experiments examined flows around arrays of artificial and natural burrows including a control, a sparse, and a dense artificial burrow array (0, 40, and 74 burrows/m2, respectively), as well as a control, a sparse, and a dense array of natural burrows (0, 30, and 62 burrows/m2, respectively). Measurements taken within these arrays found that for the artificial burrow array, TKE appeared to increase to a peak for the sparse array, and decreased again for the dense array. This relationship is similar to that found in the burrow array laboratory experiment. For the natural burrow array, TKE increased with burrow density. For all field experiments combined, a split in flow regimes between high and low flow speeds emerges, similar to the split found in the laboratory experiments.
Sediment trapping within the artificial burrows was measured for over one tidal cycle. It was found that the dense array trapped a greater amount of sediment (1.95 g/L) compared with the sparse array (1.66 g/L). Overall, it appeared that the sediment caught within the burrows had a greater proportion of silt when compared with the local surficial sediment, indicating the possibility of ecosystem engineering. Volumes trapped of this magnitude have considerable potential to alter the morphology of tidal flats. Overall, these results offer intriguing insight into the role of burrowing fauna in affecting flows and sediment fluxes on intertidal sandflats.||