The context-specific roles of a bioturbating crab (Austrohelice crassa) on ecosystem functioning
Bioturbating macrofauna can have major effects on their physical, biological and biogeochemical surroundings, altering ecosystem functioning. Austrohelice crassa (herein Austrohelice) is a burrow building estuarine crab endemic to New Zealand. The abundant and widespread nature of this species infers that its effects on sediment processes are likely to be significant. This thesis explores Austrohelice’s impact on ecosystem functioning quantifying both density and habitat induced differences in sediment reworking rates, solute and particle fluxes. The underpinning mechanisms by which changes are mediated are also examined. I hypothesised that organism behaviour, sediment type and interactions between both factors have the potential to mediate changes to ecosystem functioning. Sediment reworking rates were calculated from four parameters: burrow and crab density, burrow morphology, burrow permanency and burrow maintenance, measured across a sedimentary gradient. Burrows were over 18 times more stable in mud than sand equating to over an order of magnitude reduction in sediment reworking rates, shifting the primary bioturbational role from burrow builder in mud to sediment mixer (bulldozer) in sand. Burrow decay rates, combined with differences in burrow and crab densities, were primarily responsible for changes in reworking rates among sediment types. An in situ density manipulation experiment was conducted in a non-cohesive sand and a cohesive muddy-sand to test the hypothesis that the functional plasticity of Austrohelice among sediment types would be reflected in measures of solute exchange; a proxy for ecosystem functioning. In both habitats, Austrohelice regulated nutrient cycling, creating strong density driven effects on solute exchanges. Increasing crab density enhanced sediment O₂ demand and the flux of NH₄⁺ from the sediment, indicating much of the response was physiologically driven. Despite lowering microphyte standing stock through deposit feeding, Austrohelice also increased benthic primary production per unit of chlorophyll a. Important context-specific differences were also revealed, most notably for NH₄⁺ fluxes, which were higher where burrows and their associated microbial communities were most stable. Laboratory based flume experiments were conducted to test if increasing burrow density amplified sediment erodibility and if the different reworking rates (and hence functionality) between sediment types, would affect sediment stability. Context-specific effects on particle fluxes associated with burrow density were observed among sediment types. Increasing burrow density reduced erodibility in cohesive mud, whereas in non-cohesive sand erosion rates were unimodal, being greatest at low burrow densities. Increased trapping of bedload material alongside a reduction in flow velocity due to surficial pellets was attributed to the reduction in the mass of sediment eroded in sand at high burrow densities. In mud, the linear decrease in erodibility associated with increased burrow density was attributed to crab activity at low tide whereby high concentrations of fine particles (silt-clay) are sluiced from burrows, creating both a smoothing and consolidating effect on the sediment surface. This thesis highlights the value of assessing organism characteristics and behaviour alongside organism density to identify the mechanisms which govern ecosystem level processes among habitats. Integration of such information in to functional group studies and sediment dynamic models will broaden conceptual frameworks and avoid oversimplification of highly complex organism-sediment interactions.
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