title: Dispersal and remineralisation of biodeposits: Ecosystem impacts of mussel aquaculture

authors: Giles, Hilke

subjects: faeces

abstract: Suspension-feeding bivalves produce biodeposits (faeces and pseudofaeces) that have much higher sinking velocities than their constituent particles. Consequently they cause sedimentation of material that might otherwise not be deposited. The benthic remineralisation of biodeposits increases sediment oxygen demand and nutrient regeneration, thus enhancing the benthic-pelagic coupling of nearshore ecosystems. In New Zealand the mussel Perna canaliculus has a high natural abundance and is also intensively cultured. This thesis examines the dispersal and remineralisation characteristics of mussel P. canaliculus biodeposits and the impacts of sedimentation from a mussel farm in the Firth of Thames on sediment biogeochemistry by combining laboratory, field and modelling studies. 

Dispersal characteristics were examined in the laboratory by measuring sinking velocities and erosion thresholds of biodeposits produced by mussels of a wide size range fed three experimental diets. The results show that biodeposit dispersal is a function of mussel diet and size and thus could differ significantly between locations and seasons. Estimates of dispersal distances based on these results demonstrated that the initial dispersal of biodeposits produced by cultured mussels is not far. Depending on the hydrodynamic conditions, secondary dispersal via resuspension potentially plays a more important role in the dispersal of biodeposits from mussel farms than initial dispersal and almost certainly serves as the major means of transport of biodeposits from natural mussel beds.

Biodeposit mineralisation was studied by incubating coastal sediments with added biodeposits and measuring oxygen and nutrient fluxes as well as sediment characteristics over an 11 d period. Sediment oxygen consumption and ammonium release increased immediately after biodeposit addition and remained elevated compared to control cores without additions for the incubation period. A biodeposit decay rate (0.16 d-1) was calculated by fitting a first-order G model to the observed increase in oxygen consumption. This rate is 1 - 2 orders of magnitude higher than published decay rates of coastal sediments without organic enrichment or plant material. Nutrient fluxes during the incubation period illustrated that biodeposit remineralisation alters the stoichiometry of the nutrients released from the sediments which may potentially be more significant than the changes of the individual fluxes.

To determine the impact of a mussel farm in the Firth of Thames I measured sediment oxygen and nutrient fluxes by deploying benthic chambers, sediment characteristics by collecting sediment cores and sedimentation rates by deploying sediment traps in four seasons. Oxygen consumption and sediment nutrient release rates were generally higher under the farm compared to a reference site, demonstrating the typical response to increased organic input. Unusually low nitrogen release rates measured in summer may indicate enhanced denitrification under the farm. A simple budget demonstrated the importance of benthic nutrient regeneration in maintaining primary production in this region and that mussel culture can lead to a redistribution of nutrients. This study showed that site-specific hydrodynamic and biogeochemical conditions have to be taken into account when planning new mussel farms to prevent excessive modifications of nutrient dynamics.

Results of the laboratory and field studies conducted in this thesis were used to parameterise, calibrate and validate models of mussel biodeposit dispersal and remineralisation. A particle tracking model showed that the maximum initial dispersal of faecal pellets from the mussel farm is approximately 300 m and that pellets can be transported several times this distance via resuspension. The remineralisation model was able to simulate the increased nitrogen fluxes from the sediments well and highlighted the need for thorough calibration and parameterisation of the model. 

This thesis contributed to the current understanding of the ecosystem impacts of mussel culture and provided numerical models and model parameters that will assist in the assessment of mussel culture sustainability and the contribution of mussels to the nutrient cycling in nearshore ecosystems.

text: Dispersal and remineralisation of biodeposits: Ecosystem impacts of mussel aquaculture Suspension-feeding bivalves produce biodeposits (faeces and pseudofaeces) that have much higher sinking velocities than their constituent particles. Consequently they cause sedimentation of material that might otherwise not be deposited. The benthic remineralisation of biodeposits increases sediment oxygen demand and nutrient regeneration, thus enhancing the benthic-pelagic coupling of nearshore ecosystems. In New Zealand the mussel Perna canaliculus has a high natural abundance and is also intensively cultured. This thesis examines the dispersal and remineralisation characteristics of mussel P. canaliculus biodeposits and the impacts of sedimentation from a mussel farm in the Firth of Thames on sediment biogeochemistry by combining laboratory, field and modelling studies. 

Dispersal characteristics were examined in the laboratory by measuring sinking velocities and erosion thresholds of biodeposits produced by mussels of a wide size range fed three experimental diets. The results show that biodeposit dispersal is a function of mussel diet and size and thus could differ significantly between locations and seasons. Estimates of dispersal distances based on these results demonstrated that the initial dispersal of biodeposits produced by cultured mussels is not far. Depending on the hydrodynamic conditions, secondary dispersal via resuspension potentially plays a more important role in the dispersal of biodeposits from mussel farms than initial dispersal and almost certainly serves as the major means of transport of biodeposits from natural mussel beds.

Biodeposit mineralisation was studied by incubating coastal sediments with added biodeposits and measuring oxygen and nutrient fluxes as well as sediment characteristics over an 11 d period. Sediment oxygen consumption and ammonium release increased immediately after biodeposit addition and remained elevated compared to control cores without additions for the incubation period. A biodeposit decay rate (0.16 d-1) was calculated by fitting a first-order G model to the observed increase in oxygen consumption. This rate is 1 - 2 orders of magnitude higher than published decay rates of coastal sediments without organic enrichment or plant material. Nutrient fluxes during the incubation period illustrated that biodeposit remineralisation alters the stoichiometry of the nutrients released from the sediments which may potentially be more significant than the changes of the individual fluxes.

To determine the impact of a mussel farm in the Firth of Thames I measured sediment oxygen and nutrient fluxes by deploying benthic chambers, sediment characteristics by collecting sediment cores and sedimentation rates by deploying sediment traps in four seasons. Oxygen consumption and sediment nutrient release rates were generally higher under the farm compared to a reference site, demonstrating the typical response to increased organic input. Unusually low nitrogen release rates measured in summer may indicate enhanced denitrification under the farm. A simple budget demonstrated the importance of benthic nutrient regeneration in maintaining primary production in this region and that mussel culture can lead to a redistribution of nutrients. This study showed that site-specific hydrodynamic and biogeochemical conditions have to be taken into account when planning new mussel farms to prevent excessive modifications of nutrient dynamics.

Results of the laboratory and field studies conducted in this thesis were used to parameterise, calibrate and validate models of mussel biodeposit dispersal and remineralisation. A particle tracking model showed that the maximum initial dispersal of faecal pellets from the mussel farm is approximately 300 m and that pellets can be transported several times this distance via resuspension. The remineralisation model was able to simulate the increased nitrogen fluxes from the sediments well and highlighted the need for thorough calibration and parameterisation of the model. 

This thesis contributed to the current understanding of the ecosystem impacts of mussel culture and provided numerical models and model parameters that will assist in the assessment of mussel culture sustainability and the contribution of mussels to the nutrient cycling in nearshore ecosystems.

year: 2006

title: Comparison of the telegraph and hyperdiffusion approximations in cosmic-ray transport

authors: Litvinenko, Yuri E.

abstract: The telegraph equation and its generalizations have been repeatedly considered in the models of diffusive cosmic-ray transport. Yet the telegraph model has well-known limitations, and analytical arguments suggest that a hyperdiffusion model should serve as a more accurate alternative to the telegraph model, especially on the timescale of a few scattering times. We present a detailed side-by-side comparison of an evolving particle density profile, predicted by the telegraph and hyperdiffusion models in the context of a simple but physically meaningful initial-value problem, compare the predictions with the solution based on the Fokker–Planck equation, and discuss the applicability of the telegraph and hyperdiffusion approximations to the description of strongly anisotropic particle distributions.

text: Comparison of the telegraph and hyperdiffusion approximations in cosmic-ray transport The telegraph equation and its generalizations have been repeatedly considered in the models of diffusive cosmic-ray transport. Yet the telegraph model has well-known limitations, and analytical arguments suggest that a hyperdiffusion model should serve as a more accurate alternative to the telegraph model, especially on the timescale of a few scattering times. We present a detailed side-by-side comparison of an evolving particle density profile, predicted by the telegraph and hyperdiffusion models in the context of a simple but physically meaningful initial-value problem, compare the predictions with the solution based on the Fokker–Planck equation, and discuss the applicability of the telegraph and hyperdiffusion approximations to the description of strongly anisotropic particle distributions.

year: 2016

title: Te tuhirau kōrero

authors: Corbett, Mahaki

subjects: Reo Māori

abstract: Ko tō tātou reo, he reo tapu, he reo kōrero, he reo pohewa, he reo reiuru, he reo rauriki.  E rā, he piringa tē taea te whakangaro atu, nā te mea ko te reo Māori te manawatanga mai i a koe e tūhono ai koe ki ngā mea o mua, ki ngā mea o muri, ki ngā mea o nāianei (J. Peri, kōrero ā-waha, Hune 3, 2013). Heoi anō, i ngā rā nei, kua whānau hou mai te reo Māori pā tuhi-ā-waea ki te ao mārama me te aha kua whakarerekē a ia i te āhuatanga o tō tātou reo. Ko te kaupapa o tēnei tuhinga, he whakatakoto kupu mai i te ngākau aroha me te wete hoki i te taumata whakaaro e whakahākiri nei ki tēnei mangainga o te ao hangarau. Waihoki, kua nahanaha i ahau ngā wāhanga, ā, e whitu ēnei kua tuhia ki te kupu waiwaiā, kia whakapāho-ā-rorohiko atu rānei i te whia maruāpō mō te ātaahuatanga o te pā tuhi-ā-waea. Nō reira rā, koiā te taonga whakapiripiri e koroa nei e te ringa, purutia kia tata, whakamaua kia tina!

text: Te tuhirau kōrero Ko tō tātou reo, he reo tapu, he reo kōrero, he reo pohewa, he reo reiuru, he reo rauriki.  E rā, he piringa tē taea te whakangaro atu, nā te mea ko te reo Māori te manawatanga mai i a koe e tūhono ai koe ki ngā mea o mua, ki ngā mea o muri, ki ngā mea o nāianei (J. Peri, kōrero ā-waha, Hune 3, 2013). Heoi anō, i ngā rā nei, kua whānau hou mai te reo Māori pā tuhi-ā-waea ki te ao mārama me te aha kua whakarerekē a ia i te āhuatanga o tō tātou reo. Ko te kaupapa o tēnei tuhinga, he whakatakoto kupu mai i te ngākau aroha me te wete hoki i te taumata whakaaro e whakahākiri nei ki tēnei mangainga o te ao hangarau. Waihoki, kua nahanaha i ahau ngā wāhanga, ā, e whitu ēnei kua tuhia ki te kupu waiwaiā, kia whakapāho-ā-rorohiko atu rānei i te whia maruāpō mō te ātaahuatanga o te pā tuhi-ā-waea. Nō reira rā, koiā te taonga whakapiripiri e koroa nei e te ringa, purutia kia tata, whakamaua kia tina!

year: 2017