The context-specific roles of a bioturbating crab (Austrohelice crassa) on ecosystem functioning
type: CreativeWork
description: 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, Austro…
abstract: 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, Austro…
author: Needham, Hazel Rosemary
keywords: Bioturbation, Ecosystem function
license: https://researchcommons.waikato.ac.nz/bitstreams/c447fcb7-4135-4799-84da-e7335b7694db/download
date published: 2011
date modified: 2026-07-24T05:57:21Z
provider: University Waikato
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Waikato