title: Nitrogen Isotope Variation in the Environment:  Implications for Interpretation

authors: Tozer, Wade Colin

subjects: d15N

abstract: Natural abundance of 15N varies greatly and unpredictably within and between environments.  The unpredictable nature of 15N limits the use of N isotope natural abundance (d15N) in tracing the flow and fate of N in environments.  Recent investigations have, however, revealed consistent and repeatable patterns of 15N in some ecosystem components.  These patterns suggest that d15N may yet provide a tool to investigate and illuminate ecosystem N cycling processes.  Identifying and quantifying the sources of isotopic variation must precede any significant advance in the application of this technique, and to this end an assessment of isotopic variation associated with major ecosystem components has been carried out in this thesis.  d15N patterns have been established, hypotheses proposed and tested, and conclusions about the application of the technique are presented.

15N patterns in surface and groundwater were measured in a variety of different land-use catchments in an attempt to identify distinct isotopic 'fingerprints'.  High levels of 15N variation were measured in both stream and groundwaters, resulting in strongly overlapping land-use 'fingerprints'.  Environmental 15N variation in streams and groundwaters was found to be too great to differentiate between land-uses based on d15N alone.

In contrast, the artificially 15N enriched signature of effluent N was used to trace its flow and fate, following irrigation, in a forested catchment.  The effluent d15N signature allowed it to be traced into the major ecosystem components, permitting a first order N budget to be determined for effluent N storage and loss.  N sources with significantly different 15N signatures to that of 'background ecosystem N' can therefore be used to trace the flow and fate of N in ecosystems.

During the course of this work a number of higher and lower order plants were observed to have highly depleted (lt; -8 ) d15N signatures.  Epiphytes and lithophytes, strongly reliant on atmospheric N sources, were consistently depleted in 15N, with signatures as low as -24 , measured in a range of environments.  A similar level of depletion was measured in a wide range of plants growing in early primary succession sites (as low as -22.3 ), which could not be accounted for by any abiotic or biotic factor or significantly depleted N source.  The absence of any measurable driver of depletion suggested a universal fractionating mechanism which acts in a wide range of environments and vegetation types.  Diffusive uptake of atmospheric NH3(g) and the proportional uptake of a supplied N source were two proposed mechanisms that could theoretically account for the level and universal nature of depletion. 

Diffusive uptake of atmospheric NH3(g) was tested as a primary fractionating mechanism in plants.  Strongly N deficient plants were capable of utilising NH3(g) as a nutritional source, but the level of 15N depletion measured in these plants closely approximated that of the inherent NH3(g) d15N signature.  No significant additional fractionation is associated with NH3(g) diffusive uptake.  Diffusive uptake of atmospheric NH3(g) by plants cannot alone account for the level of depletion measured in early primary succession plant communities. 

Proportional uptake of a N source as a primary fractionating mechanism was tested by growing plants in various concentrations and rates of applied N.  Fractionation attributed to the proportional uptake of a supplied N source, as a consequence of P limitation or rapid flow over roots, resulted in a significant level of 15N depletion in plants. The level of depletion attributed to this mechanism was, however, not sufficient to account for the level measured in early primary succession plant communities.

Individual 15N fractionating mechanisms cannot alone explain the level of depletion observed in early primary succession plants, however a combination of fractionating mechanisms can.  Fractionation attributed to the proportional uptake of an already depleted N source, i.e., wet deposited N, largely accounts for the level of depletion measured in early succession plant communities.  This two-step fractionation model can act on both higher and lower plants, independent of ecosystem biotic and abiotic factors.  Additional, and less dramatic fractionations attributed to atmospheric NH3(g) uptake, mycorrhizal associations, internal remobilisation, and taxon-specific N acquisition strategies, will contribute to the level of d15N depletion.  
This thesis presents the first extensive survey of highly depleted d15N signatures in terrestrial vegetation.  Furthermore, thorough testing of theoretically plausible mechanisms has resulted in a full account of the highly depleted d15N signatures measured in a wide range of vegetation types and environments.

text: Nitrogen Isotope Variation in the Environment:  Implications for Interpretation Natural abundance of 15N varies greatly and unpredictably within and between environments.  The unpredictable nature of 15N limits the use of N isotope natural abundance (d15N) in tracing the flow and fate of N in environments.  Recent investigations have, however, revealed consistent and repeatable patterns of 15N in some ecosystem components.  These patterns suggest that d15N may yet provide a tool to investigate and illuminate ecosystem N cycling processes.  Identifying and quantifying the sources of isotopic variation must precede any significant advance in the application of this technique, and to this end an assessment of isotopic variation associated with major ecosystem components has been carried out in this thesis.  d15N patterns have been established, hypotheses proposed and tested, and conclusions about the application of the technique are presented.

15N patterns in surface and groundwater were measured in a variety of different land-use catchments in an attempt to identify distinct isotopic 'fingerprints'.  High levels of 15N variation were measured in both stream and groundwaters, resulting in strongly overlapping land-use 'fingerprints'.  Environmental 15N variation in streams and groundwaters was found to be too great to differentiate between land-uses based on d15N alone.

In contrast, the artificially 15N enriched signature of effluent N was used to trace its flow and fate, following irrigation, in a forested catchment.  The effluent d15N signature allowed it to be traced into the major ecosystem components, permitting a first order N budget to be determined for effluent N storage and loss.  N sources with significantly different 15N signatures to that of 'background ecosystem N' can therefore be used to trace the flow and fate of N in ecosystems.

During the course of this work a number of higher and lower order plants were observed to have highly depleted (lt; -8 ) d15N signatures.  Epiphytes and lithophytes, strongly reliant on atmospheric N sources, were consistently depleted in 15N, with signatures as low as -24 , measured in a range of environments.  A similar level of depletion was measured in a wide range of plants growing in early primary succession sites (as low as -22.3 ), which could not be accounted for by any abiotic or biotic factor or significantly depleted N source.  The absence of any measurable driver of depletion suggested a universal fractionating mechanism which acts in a wide range of environments and vegetation types.  Diffusive uptake of atmospheric NH3(g) and the proportional uptake of a supplied N source were two proposed mechanisms that could theoretically account for the level and universal nature of depletion. 

Diffusive uptake of atmospheric NH3(g) was tested as a primary fractionating mechanism in plants.  Strongly N deficient plants were capable of utilising NH3(g) as a nutritional source, but the level of 15N depletion measured in these plants closely approximated that of the inherent NH3(g) d15N signature.  No significant additional fractionation is associated with NH3(g) diffusive uptake.  Diffusive uptake of atmospheric NH3(g) by plants cannot alone account for the level of depletion measured in early primary succession plant communities. 

Proportional uptake of a N source as a primary fractionating mechanism was tested by growing plants in various concentrations and rates of applied N.  Fractionation attributed to the proportional uptake of a supplied N source, as a consequence of P limitation or rapid flow over roots, resulted in a significant level of 15N depletion in plants. The level of depletion attributed to this mechanism was, however, not sufficient to account for the level measured in early primary succession plant communities.

Individual 15N fractionating mechanisms cannot alone explain the level of depletion observed in early primary succession plants, however a combination of fractionating mechanisms can.  Fractionation attributed to the proportional uptake of an already depleted N source, i.e., wet deposited N, largely accounts for the level of depletion measured in early succession plant communities.  This two-step fractionation model can act on both higher and lower plants, independent of ecosystem biotic and abiotic factors.  Additional, and less dramatic fractionations attributed to atmospheric NH3(g) uptake, mycorrhizal associations, internal remobilisation, and taxon-specific N acquisition strategies, will contribute to the level of d15N depletion.  
This thesis presents the first extensive survey of highly depleted d15N signatures in terrestrial vegetation.  Furthermore, thorough testing of theoretically plausible mechanisms has resulted in a full account of the highly depleted d15N signatures measured in a wide range of vegetation types and environments.

year: 2006

title: Small-scale turbulence and its influence on forest-scale morphodynamics within a coastal mangrove forest

authors: Norris, Benjamin

subjects: mangroves

abstract: Mangroves are coastal wetland ecosystems consisting of salt-tolerant trees and shrubs that inhabit the upper intertidal zone of estuaries, river banks and barrier islands. These trees grow in the intersection between the coastal ocean and land, and form a barrier against hazards such as waves, coastal flooding and erosion for many densely populated areas that often lack hard coastal defense structures. By damping tidal currents and waves, mangroves also facilitate sedimentation, and may contribute to coastal stability in the face of rising sea levels. The valuable ecosystem services provided by mangroves are the result of characteristic bio-physical feedbacks between the mangrove vegetation, hydrodynamics and sediment dynamics within intertidal zones (Chapter 1). This thesis focuses on understanding these bio-physical feedbacks, specifically: (i) how mangrove root density affects tidal current and wave dissipation, (ii) how mangrove root structures modify the flow field, and (iii) how mangrove roots facilitate sediment transport within forested areas. 

Linking Mangrove Root Density and Turbulent Dissipation
The link between mangrove root density and turbulent dissipation was explored in a coastal mangrove forest that is exposed to an energetic wave environment. Measurements of turbulent kinetic energy (TKE) dissipation were collected over millimeter to centimeter scales within clusters of mangrove pneumatophore roots (‘canopies’) spanning regions from the unvegetated mudflat to the densely vegetated forest. High-resolution root geometries were reconstructed using a newly developed photogrammetric method. The frontal area density of the vegetation (a) was compared to the TKE dissipation rate estimates at the same height above bed. Mangrove tree (and hence, root) density was greatest along a narrow band between the mudflat and forest: the forest ‘fringe’. Temporally variable turbulence was maximum in the fringe and was often elevated in the forest relative to the unvegetated mudflat.  The largest dissipation rates (4.5 x 10⁻³ W kg⁻¹) were measured as breaking waves propagated over root canopies in very shallow water. Dissipation rates were reduced, but often remained intense (e.g., between 10⁻⁵ – 10⁻⁴ W kg⁻¹) under non-breaking waves at the fringe, likely indicating turbulent generation in pneumatophore wakes. Turbulence was positively correlated with root density and wave height and was negatively correlated with water depth. Substrate grain size distributions in the fringe were larger (sandier) than those offshore and onshore, suggesting intense turbulence may winnow fine-grained sediments from the fringe.

Observations of Turbulence in Mangrove Root Canopies
High-resolution velocity measurements were collected within and above two dense canopies of mangrove pneumatophore roots in a wave-exposed mangrove forest. In both canopies, root density decreased steadily with height above bed owing to the variability in root heights and the tapered shape of the roots. Within the canopies, we consider turbulence within three zones: near the bed above the wave boundary layer, around the mean canopy height, and above the canopy. The near-bed turbulence was particularly intense (up to 6.5 x 10⁻⁴ W kg⁻¹), likely owing to oscillatory wave-driven currents flowing past dense vegetation. Near the bed and around the mean canopy height, peaks in horizontal velocity power spectra at frequencies corresponding to Strouhal numbers of ~0.2 may indicate Von Kármán wake shedding in the lee of the pneumatophores. Furthermore, a recirculation zone was observed immediately behind a cluster of pneumatophores at intermediate heights. These coherent flow structures were associated with zones of enhanced Reynolds stresses (up to 5.3 x 10⁻³ m² s⁻²), and eddy viscosities (up to 1.9 x 10⁻³ m² s⁻¹). Large near-bed stresses were associated with near-bed drag coefficients that are up to an order of magnitude larger than those expected in the absence of vegetation. Observed eddy viscosities are consistent with theoretical expectations, derived from scaling arguments using a standard mixing-length model. These results suggest that pneumatophore roots can contribute greatly to turbulent mixing (e.g., eddy viscosities were on average O(10⁻⁴ – 10⁻³ m² s⁻¹), and therefore may enhance the sediment entrainment occurring in mangrove forest fringes. 

Understanding the Hydrodynamic and Morphodynamic Feedbacks in Mangroves 
Field experiments were carried out within a wave-exposed coastal mangrove forest to quantify the change in bed level throughout a tidal cycle using high-resolution velocity and bed level measurements collected in situ. Experiments spanned the unvegetated mudflat, mangrove forest fringe and forest, where data were collected during single tidal cycles (flood-ebb) during a two-week period. Cross-wavelet transforms of the velocity and bed level measurements were often highly correlated (≥ 90% squared coherence) over a range of frequencies, spanning those corresponding to short-period wind and swell waves to long-period infragravity waves. Bed level change events associated with short-period waves were more frequent in the mudflat and fringe, while those associated with long-period waves were much more frequent in the forest. Net bed level changes over single events were nearly normally distributed, indicating similar numbers of events resulting in accretion and erosion, regardless of the across-shore position. Still, bed level change events exceeding the observed net tidal elevation change occurred infrequently. This pattern suggests that the net tidal elevation change within the mangrove forest must be related to the frequency of these infrequent events. Large bed level changes were often associated with high bed shear stresses (0.3 – 1.5 N m⁻²) and turbulence of O(10⁻³ W kg⁻¹), particularly on the mudflat and in the forest fringe. Across all experiments, the mudflat and forest site experienced net accretion (4.5 and 6.8 mm, respectively) while the fringe experienced net erosion (-9.5 mm). This work suggests a dynamic role for waves in mangroves: short-period waves stir sediment on the mudflat and forest fringe, while infragravity waves help advect entrained sediment inside the forest.

text: Small-scale turbulence and its influence on forest-scale morphodynamics within a coastal mangrove forest Mangroves are coastal wetland ecosystems consisting of salt-tolerant trees and shrubs that inhabit the upper intertidal zone of estuaries, river banks and barrier islands. These trees grow in the intersection between the coastal ocean and land, and form a barrier against hazards such as waves, coastal flooding and erosion for many densely populated areas that often lack hard coastal defense structures. By damping tidal currents and waves, mangroves also facilitate sedimentation, and may contribute to coastal stability in the face of rising sea levels. The valuable ecosystem services provided by mangroves are the result of characteristic bio-physical feedbacks between the mangrove vegetation, hydrodynamics and sediment dynamics within intertidal zones (Chapter 1). This thesis focuses on understanding these bio-physical feedbacks, specifically: (i) how mangrove root density affects tidal current and wave dissipation, (ii) how mangrove root structures modify the flow field, and (iii) how mangrove roots facilitate sediment transport within forested areas. 

Linking Mangrove Root Density and Turbulent Dissipation
The link between mangrove root density and turbulent dissipation was explored in a coastal mangrove forest that is exposed to an energetic wave environment. Measurements of turbulent kinetic energy (TKE) dissipation were collected over millimeter to centimeter scales within clusters of mangrove pneumatophore roots (‘canopies’) spanning regions from the unvegetated mudflat to the densely vegetated forest. High-resolution root geometries were reconstructed using a newly developed photogrammetric method. The frontal area density of the vegetation (a) was compared to the TKE dissipation rate estimates at the same height above bed. Mangrove tree (and hence, root) density was greatest along a narrow band between the mudflat and forest: the forest ‘fringe’. Temporally variable turbulence was maximum in the fringe and was often elevated in the forest relative to the unvegetated mudflat.  The largest dissipation rates (4.5 x 10⁻³ W kg⁻¹) were measured as breaking waves propagated over root canopies in very shallow water. Dissipation rates were reduced, but often remained intense (e.g., between 10⁻⁵ – 10⁻⁴ W kg⁻¹) under non-breaking waves at the fringe, likely indicating turbulent generation in pneumatophore wakes. Turbulence was positively correlated with root density and wave height and was negatively correlated with water depth. Substrate grain size distributions in the fringe were larger (sandier) than those offshore and onshore, suggesting intense turbulence may winnow fine-grained sediments from the fringe.

Observations of Turbulence in Mangrove Root Canopies
High-resolution velocity measurements were collected within and above two dense canopies of mangrove pneumatophore roots in a wave-exposed mangrove forest. In both canopies, root density decreased steadily with height above bed owing to the variability in root heights and the tapered shape of the roots. Within the canopies, we consider turbulence within three zones: near the bed above the wave boundary layer, around the mean canopy height, and above the canopy. The near-bed turbulence was particularly intense (up to 6.5 x 10⁻⁴ W kg⁻¹), likely owing to oscillatory wave-driven currents flowing past dense vegetation. Near the bed and around the mean canopy height, peaks in horizontal velocity power spectra at frequencies corresponding to Strouhal numbers of ~0.2 may indicate Von Kármán wake shedding in the lee of the pneumatophores. Furthermore, a recirculation zone was observed immediately behind a cluster of pneumatophores at intermediate heights. These coherent flow structures were associated with zones of enhanced Reynolds stresses (up to 5.3 x 10⁻³ m² s⁻²), and eddy viscosities (up to 1.9 x 10⁻³ m² s⁻¹). Large near-bed stresses were associated with near-bed drag coefficients that are up to an order of magnitude larger than those expected in the absence of vegetation. Observed eddy viscosities are consistent with theoretical expectations, derived from scaling arguments using a standard mixing-length model. These results suggest that pneumatophore roots can contribute greatly to turbulent mixing (e.g., eddy viscosities were on average O(10⁻⁴ – 10⁻³ m² s⁻¹), and therefore may enhance the sediment entrainment occurring in mangrove forest fringes. 

Understanding the Hydrodynamic and Morphodynamic Feedbacks in Mangroves 
Field experiments were carried out within a wave-exposed coastal mangrove forest to quantify the change in bed level throughout a tidal cycle using high-resolution velocity and bed level measurements collected in situ. Experiments spanned the unvegetated mudflat, mangrove forest fringe and forest, where data were collected during single tidal cycles (flood-ebb) during a two-week period. Cross-wavelet transforms of the velocity and bed level measurements were often highly correlated (≥ 90% squared coherence) over a range of frequencies, spanning those corresponding to short-period wind and swell waves to long-period infragravity waves. Bed level change events associated with short-period waves were more frequent in the mudflat and fringe, while those associated with long-period waves were much more frequent in the forest. Net bed level changes over single events were nearly normally distributed, indicating similar numbers of events resulting in accretion and erosion, regardless of the across-shore position. Still, bed level change events exceeding the observed net tidal elevation change occurred infrequently. This pattern suggests that the net tidal elevation change within the mangrove forest must be related to the frequency of these infrequent events. Large bed level changes were often associated with high bed shear stresses (0.3 – 1.5 N m⁻²) and turbulence of O(10⁻³ W kg⁻¹), particularly on the mudflat and in the forest fringe. Across all experiments, the mudflat and forest site experienced net accretion (4.5 and 6.8 mm, respectively) while the fringe experienced net erosion (-9.5 mm). This work suggests a dynamic role for waves in mangroves: short-period waves stir sediment on the mudflat and forest fringe, while infragravity waves help advect entrained sediment inside the forest.

year: 2019

title: Haumanu Hauora: Strengthening health institution responsiveness to climate change

authors: Masters-Awatere, Bridgette

abstract: The “Haumanu Hauora: Strengthening health institution responsiveness to climate change” project (Haumanu Hauora) is funded by the Deep South National Science Challenge. Our research team has 
been working to determine how health institutions (for example DHBs) develop Māori responsive policy as a preparatory step in anticipation of climate change impacts on Māori health. The project 
has involved several components, one of which involved speaking with tāngata whenua and DHB staff from the Bay of Plenty, Lakes and Waikato DHB regions about their experiences of climate 
change and involvement with health services. Another component was an environmental scan of available website information on the 17 other DHBs to ascertain the extent of policy that speaks to 
both the Treaty of Waitangi and climate change. This report presents an overview of the findings. 

This report begins with some context setting. We provide a summary of our literature reviews before a brief outline of the establishment of DHBs, which are the “health institution” of focus for 
this research. As the primary health provider and funders around the country, DHBs have been engaged as the site through which to understand the Māori health and climate change policy needs. 
Although DHBs were disestablished at the conclusion of our project, there are still lessons to learn from the ‘old’ health system. We present themes from tāngata whenua and rangatahi interviews as 
a way to foreground policy development information provided by DHB staff.

text: Haumanu Hauora: Strengthening health institution responsiveness to climate change The “Haumanu Hauora: Strengthening health institution responsiveness to climate change” project (Haumanu Hauora) is funded by the Deep South National Science Challenge. Our research team has 
been working to determine how health institutions (for example DHBs) develop Māori responsive policy as a preparatory step in anticipation of climate change impacts on Māori health. The project 
has involved several components, one of which involved speaking with tāngata whenua and DHB staff from the Bay of Plenty, Lakes and Waikato DHB regions about their experiences of climate 
change and involvement with health services. Another component was an environmental scan of available website information on the 17 other DHBs to ascertain the extent of policy that speaks to 
both the Treaty of Waitangi and climate change. This report presents an overview of the findings. 

This report begins with some context setting. We provide a summary of our literature reviews before a brief outline of the establishment of DHBs, which are the “health institution” of focus for 
this research. As the primary health provider and funders around the country, DHBs have been engaged as the site through which to understand the Māori health and climate change policy needs. 
Although DHBs were disestablished at the conclusion of our project, there are still lessons to learn from the ‘old’ health system. We present themes from tāngata whenua and rangatahi interviews as 
a way to foreground policy development information provided by DHB staff.

year: 2022-07-12