title: Impact of black swan grazing and anthropogenic contaminants on New Zealand seagrass meadows

authors: Dos Santos, Virginie

subjects: Zostera muelleri

abstract: Seagrass is the only marine angiosperm (flowering plant) and decline of seagrass meadows has been reported worldwide. The important ecological and economic values of this marine habitat make it essential to preserve. Seagrass meadows of New Zealand are composed of only one species, Zostera muelleri. They are mainly present in sheltered, intertidal sand flats in harbours. Seagrass decline has been recorded in many New Zealand estuaries during the last century. Grazing by the black swan (Cygnus atratus) and anthropogenic contaminants resulting from catchment development are postulated to contribute to seagrass meadow loss in New Zealand and these factors have been assessed in this study. The aim of this study was to quantify black swan grazing pressure and examine effects of potential anthropogenic contaminants (specifically nutrient enrichment, sedimentation and herbicide residues) on seagrass meadows in New Zealand. The impact of current levels of these anthropogenic contaminants on seagrass resilience to swan grazing was also assessed in situ. To quantify black swan grazing pressure on seagrass meadows and to evaluate the potential relationship between swan grazing pressure and seagrass standing stock, observations and measurements were conducted in Tauranga Harbour where more than a third of the seagrass meadows disappeared between 1959 to 1996. Observations have shown that black swans graze intensively on Zostera muelleri meadows during high tide and were most numerous in the harbour during autumn. Black swan grazing resulted in the formation of circular devegetated patches (0.28 m2), where 92 % of shoots, 25 % of roots and 99 % of rhizomes are removed. The consumption rate was estimated to be 394 g dry weight (DW) swan-1 day-1. Significant seagrass loss on an annual basis was detected at the most swan populated site, which also had the highest swan grazing pressure (19-20 % of biomass consumed annually). This suggests a potential threshold of 19-20 % of annual seagrass biomass removal above which black swan grazing may contribute to substantial damage on seagrass meadows. Applied at the harbour scale, where there is a total of ~29 km2 of seagrass meadows, this corresponds to a swan population (i.e. grazing pressure) > 4630 birds. To identify the potential environmental stressors affecting seagrass condition, two contrasting harbours (Tauranga and Aotea) were surveyed where a range of environmental parameters, including parameters indicative of anthropogenic contamination, and seagrass condition metrics were measured. The survey showed that many of the seagrass condition metrics were positively correlated with sediment porewater ammonium (NH4-N) concentrations suggesting that nitrogen availability regulates seagrass growth. The presence of higher porewater nitrate (NO3-N) concentrations was linked to higher proportions of urban and crop land use in subcatchments, these latter parameters being negatively correlated with light availability during high tide. In spite of this, current levels of nutrient enrichment were apparently not detrimental to seagrass condition with evidence of N-limited plant growth and abundant light availability. Herbicide compounds were detected in seagrass sediment and at consistently higher concentrations in Tauranga harbour, which is the harbour more exposed to developed land use. The seagrass photosynthetic potential was lower in more herbicide contaminated sediments, although there was no evidence of any impact on seagrass biomass or morphometry. In a final experiment, black swan grazing was simulated in situ at four sites in Tauranga Harbour differing in exposure to potential anthropogenic contaminants and environmental characteristics. Grazing was simulated by removing seagrass biomass from patches (0.25 m2) in the meadow equivalent to natural swan removals (see above). Two levels of grazing intensity were applied to these patches; ~100 % of biomass removed (high) and ~ 40% of biomass removed (low). Results showed that the cover and the biomass of the low grazing intensity patches were not significantly different to controls (0 % of biomass removed) during the entire experiment. For the high grazing intensity patches, although the cover was restored after 9 months, biomass recovery was incomplete with only 30 % of total biomass regenerated, after one year. This suggests that swan grazing at high intensity can cause long-lasting damage to seagrass meadows, and that > 3 years is probably needed for a devegetated patch to fully recover from grazing (assuming a constant plant regeneration rate). The resilience response was similar across all sites examined in this harbour, suggesting that the current magnitude and range of environmental variability and potential contaminant levels within the seagrass meadows do not affect the ability of seagrass to regenerate from grazing. In conclusion, from these results, I can not conclude that anthropogenic contaminant or black swan grazing pressure are by themselves responsible for seagrass decline in Tauranga Harbour. However, the study suggests that herbicide residues have the potential to detrimentally affect the seagrass photosynthetic activity as do high grazing pressure by the black swan on the seagrass meadow condition. Overall, this study contributes to the understanding of impacts of black swan grazing pressure combined with potential contaminant effects on seagrass ecosystems. It emphasizes the importance of herbivory in temperate seagrass systems, which it is often underestimated and highlights the seagrass habitat contamination due to coastal development. It provides crucial information that will assist resource management agencies to preserve this valuable natural ecosystem.

text: Impact of black swan grazing and anthropogenic contaminants on New Zealand seagrass meadows Seagrass is the only marine angiosperm (flowering plant) and decline of seagrass meadows has been reported worldwide. The important ecological and economic values of this marine habitat make it essential to preserve. Seagrass meadows of New Zealand are composed of only one species, Zostera muelleri. They are mainly present in sheltered, intertidal sand flats in harbours. Seagrass decline has been recorded in many New Zealand estuaries during the last century. Grazing by the black swan (Cygnus atratus) and anthropogenic contaminants resulting from catchment development are postulated to contribute to seagrass meadow loss in New Zealand and these factors have been assessed in this study. The aim of this study was to quantify black swan grazing pressure and examine effects of potential anthropogenic contaminants (specifically nutrient enrichment, sedimentation and herbicide residues) on seagrass meadows in New Zealand. The impact of current levels of these anthropogenic contaminants on seagrass resilience to swan grazing was also assessed in situ. To quantify black swan grazing pressure on seagrass meadows and to evaluate the potential relationship between swan grazing pressure and seagrass standing stock, observations and measurements were conducted in Tauranga Harbour where more than a third of the seagrass meadows disappeared between 1959 to 1996. Observations have shown that black swans graze intensively on Zostera muelleri meadows during high tide and were most numerous in the harbour during autumn. Black swan grazing resulted in the formation of circular devegetated patches (0.28 m2), where 92 % of shoots, 25 % of roots and 99 % of rhizomes are removed. The consumption rate was estimated to be 394 g dry weight (DW) swan-1 day-1. Significant seagrass loss on an annual basis was detected at the most swan populated site, which also had the highest swan grazing pressure (19-20 % of biomass consumed annually). This suggests a potential threshold of 19-20 % of annual seagrass biomass removal above which black swan grazing may contribute to substantial damage on seagrass meadows. Applied at the harbour scale, where there is a total of ~29 km2 of seagrass meadows, this corresponds to a swan population (i.e. grazing pressure) > 4630 birds. To identify the potential environmental stressors affecting seagrass condition, two contrasting harbours (Tauranga and Aotea) were surveyed where a range of environmental parameters, including parameters indicative of anthropogenic contamination, and seagrass condition metrics were measured. The survey showed that many of the seagrass condition metrics were positively correlated with sediment porewater ammonium (NH4-N) concentrations suggesting that nitrogen availability regulates seagrass growth. The presence of higher porewater nitrate (NO3-N) concentrations was linked to higher proportions of urban and crop land use in subcatchments, these latter parameters being negatively correlated with light availability during high tide. In spite of this, current levels of nutrient enrichment were apparently not detrimental to seagrass condition with evidence of N-limited plant growth and abundant light availability. Herbicide compounds were detected in seagrass sediment and at consistently higher concentrations in Tauranga harbour, which is the harbour more exposed to developed land use. The seagrass photosynthetic potential was lower in more herbicide contaminated sediments, although there was no evidence of any impact on seagrass biomass or morphometry. In a final experiment, black swan grazing was simulated in situ at four sites in Tauranga Harbour differing in exposure to potential anthropogenic contaminants and environmental characteristics. Grazing was simulated by removing seagrass biomass from patches (0.25 m2) in the meadow equivalent to natural swan removals (see above). Two levels of grazing intensity were applied to these patches; ~100 % of biomass removed (high) and ~ 40% of biomass removed (low). Results showed that the cover and the biomass of the low grazing intensity patches were not significantly different to controls (0 % of biomass removed) during the entire experiment. For the high grazing intensity patches, although the cover was restored after 9 months, biomass recovery was incomplete with only 30 % of total biomass regenerated, after one year. This suggests that swan grazing at high intensity can cause long-lasting damage to seagrass meadows, and that > 3 years is probably needed for a devegetated patch to fully recover from grazing (assuming a constant plant regeneration rate). The resilience response was similar across all sites examined in this harbour, suggesting that the current magnitude and range of environmental variability and potential contaminant levels within the seagrass meadows do not affect the ability of seagrass to regenerate from grazing. In conclusion, from these results, I can not conclude that anthropogenic contaminant or black swan grazing pressure are by themselves responsible for seagrass decline in Tauranga Harbour. However, the study suggests that herbicide residues have the potential to detrimentally affect the seagrass photosynthetic activity as do high grazing pressure by the black swan on the seagrass meadow condition. Overall, this study contributes to the understanding of impacts of black swan grazing pressure combined with potential contaminant effects on seagrass ecosystems. It emphasizes the importance of herbivory in temperate seagrass systems, which it is often underestimated and highlights the seagrass habitat contamination due to coastal development. It provides crucial information that will assist resource management agencies to preserve this valuable natural ecosystem.

year: 2011

title: Temperature dependence of plant photosynthesis and respiration using the Macromolecular Rate Theory (MMRT)

authors: Darwin Leonel, Moreno Echeverry

subjects: photosynthesis

abstract: This thesis quantifies the temperature dependence of key traits that determine leaf carbon balance in C3 plants, using Sunflower as a model system, with emphasis on the photorespiratory CO2 compensation point (Γ*), the rate of CO2 release in the light (DL), the maximal carboxylation capacity (Vcmax), the maximal electron transport capacity (Jmax), and the components of dark respiration (Rdark). The central objective was to identify temperature dependence models for each trait that were both parsimonious and physiologically interpretable.
The research combined controlled leaf gas exchange across wide ranges of temperature and CO2 with comparative model fitting. I evaluated constant-Q10 relationships, simple and peaked Arrhenius formulations, and macro molecular rate theory (MMRT), with and without an explicit high temperature deactivation term. Γ* increased approximately exponentially across the measurement range from 4 to 42 °C, and a constant-Q10 model best captured this behaviour. The temperature dependence of DL was well represented by MMRT. For the photosynthetic capacities, I used an MMRT formulation that used inflection-point temperature (Tinf) as parameters and included explicit deactivation terms. That approach captured the curvature at cool to moderate temperatures and the decline at supra-optimal temperatures, with Jmax exhibiting a cooler inflection and a cooler optimum than Vcmax. For Rdark and its components, an MMRT anchored at the observed inflection temperature and fitted without a deactivation term captured the smooth warm side curvature expected from progressive changes in mitochondrial coupling and accelerated substrate use with increasing temperature.
This thesis provides a unified framework for selecting temperature dependence models for leaf gas exchange traits. It introduces DL as the light linked composite flux of CO2 release, rather than a proxy of dark respiration alone, and it presents a published ii
refinement for estimating Γ* and DL using the Laisk method based on gas-exchange measurements at low light and low intercellular CO2 concentrations. The refined methodology explicitly used photosynthetic theory in our parameter estimation and reduced systematic bias in estimating Γ* and DL. These advances improve the parameterisation of photosynthesis models and strengthens the transfer to crop and land- surface modelling under variable thermal environments.
The thesis is organised in seven chapters. Chapter 3 has been published, and when it is cited elsewhere in the thesis it is referenced as Moreno-Echeverry et al., 2026.
Chapter 1. General introduction. It sets the global climate and carbon cycle context, frames leaf carbon balance as the net outcome of photosynthesis and respiration, and explains why accurate parameterisation of Γ*, DL, Vcmax and Jmax matters for models. It defines temperature as a primary driver of gas-exchange rates, outlines the limit of Arrhenius and constant-Q10 functions, introduces MMRT as a thermodynamic alternative, and states the thesis aims.
Chapter 2. Background. It reviews photosynthesis and respiration at the leaf scale, outlines gas exchange principles, describes the FvCB model and presents methods used to estimate Γ* and DL. It summarises temperature response models, including the constant- Q10 function, two variants of the Arrhenius equation and MMRT. The chapter also identifies the knowledge gaps that motivate the specific work described in the thesis.
Chapter 3. Estimating Γ* and DL using the Laisk method combined with photosynthetic theory. It examines the classical Laisk approach, identifies sources of bias arising from linearisation of inherently curvilinear Anet-Cc responses, and formalises estimation of Γ* and DL within the FvCB framework. It uses simulations and leaf gas exchange measurements to test performance and uncertainty of the modified approach and compares it with the performance and uncertainty of the original Laisk approach. This chapter is based on the published methodological study.
iii

Chapter 4. Temperature dependence of Γ* and DL. It quantifies how Γ* and DL vary with temperature over the range from 4 to 42 °C. It compares the constant-Q10, simple and peaked Arrhenius, and MMRT, with and without explicit high temperature deactivation term. It reports model selection, parameter estimates and presents relevant diagnostics.
Chapter 5. Temperature dependence of Vcmax and Jmax. The chapter derives these parameters from Anet-Ci curves. It compares the Arrhenius formulations with MMRT, evaluates curvature, inflection and optimum temperatures, and contrast the thermal responses of carboxylation and electron transport.
Chapter 6. Temperature dependence of the components of leaf CO2 release in the dark. It separates and analyses the different components of dark respiration, post illumination burst, light enhanced dark respiration and dark respiration in steady-state across temperature and integrates metabolite profiling across the light-dark transition to relate CO2 fluxes to substrate availability. It tests the Arrhenius and MMRT formulations and identifies the model that best capture warm side curvature without deactivation terms.
Chapter 7. Conclusions and perspectives. It synthesises the finding across chapters, and temperature response models, discusses implications for photosynthesis and respiration modelling, and outlines priorities for future research.

text: Temperature dependence of plant photosynthesis and respiration using the Macromolecular Rate Theory (MMRT) This thesis quantifies the temperature dependence of key traits that determine leaf carbon balance in C3 plants, using Sunflower as a model system, with emphasis on the photorespiratory CO2 compensation point (Γ*), the rate of CO2 release in the light (DL), the maximal carboxylation capacity (Vcmax), the maximal electron transport capacity (Jmax), and the components of dark respiration (Rdark). The central objective was to identify temperature dependence models for each trait that were both parsimonious and physiologically interpretable.
The research combined controlled leaf gas exchange across wide ranges of temperature and CO2 with comparative model fitting. I evaluated constant-Q10 relationships, simple and peaked Arrhenius formulations, and macro molecular rate theory (MMRT), with and without an explicit high temperature deactivation term. Γ* increased approximately exponentially across the measurement range from 4 to 42 °C, and a constant-Q10 model best captured this behaviour. The temperature dependence of DL was well represented by MMRT. For the photosynthetic capacities, I used an MMRT formulation that used inflection-point temperature (Tinf) as parameters and included explicit deactivation terms. That approach captured the curvature at cool to moderate temperatures and the decline at supra-optimal temperatures, with Jmax exhibiting a cooler inflection and a cooler optimum than Vcmax. For Rdark and its components, an MMRT anchored at the observed inflection temperature and fitted without a deactivation term captured the smooth warm side curvature expected from progressive changes in mitochondrial coupling and accelerated substrate use with increasing temperature.
This thesis provides a unified framework for selecting temperature dependence models for leaf gas exchange traits. It introduces DL as the light linked composite flux of CO2 release, rather than a proxy of dark respiration alone, and it presents a published ii
refinement for estimating Γ* and DL using the Laisk method based on gas-exchange measurements at low light and low intercellular CO2 concentrations. The refined methodology explicitly used photosynthetic theory in our parameter estimation and reduced systematic bias in estimating Γ* and DL. These advances improve the parameterisation of photosynthesis models and strengthens the transfer to crop and land- surface modelling under variable thermal environments.
The thesis is organised in seven chapters. Chapter 3 has been published, and when it is cited elsewhere in the thesis it is referenced as Moreno-Echeverry et al., 2026.
Chapter 1. General introduction. It sets the global climate and carbon cycle context, frames leaf carbon balance as the net outcome of photosynthesis and respiration, and explains why accurate parameterisation of Γ*, DL, Vcmax and Jmax matters for models. It defines temperature as a primary driver of gas-exchange rates, outlines the limit of Arrhenius and constant-Q10 functions, introduces MMRT as a thermodynamic alternative, and states the thesis aims.
Chapter 2. Background. It reviews photosynthesis and respiration at the leaf scale, outlines gas exchange principles, describes the FvCB model and presents methods used to estimate Γ* and DL. It summarises temperature response models, including the constant- Q10 function, two variants of the Arrhenius equation and MMRT. The chapter also identifies the knowledge gaps that motivate the specific work described in the thesis.
Chapter 3. Estimating Γ* and DL using the Laisk method combined with photosynthetic theory. It examines the classical Laisk approach, identifies sources of bias arising from linearisation of inherently curvilinear Anet-Cc responses, and formalises estimation of Γ* and DL within the FvCB framework. It uses simulations and leaf gas exchange measurements to test performance and uncertainty of the modified approach and compares it with the performance and uncertainty of the original Laisk approach. This chapter is based on the published methodological study.
iii

Chapter 4. Temperature dependence of Γ* and DL. It quantifies how Γ* and DL vary with temperature over the range from 4 to 42 °C. It compares the constant-Q10, simple and peaked Arrhenius, and MMRT, with and without explicit high temperature deactivation term. It reports model selection, parameter estimates and presents relevant diagnostics.
Chapter 5. Temperature dependence of Vcmax and Jmax. The chapter derives these parameters from Anet-Ci curves. It compares the Arrhenius formulations with MMRT, evaluates curvature, inflection and optimum temperatures, and contrast the thermal responses of carboxylation and electron transport.
Chapter 6. Temperature dependence of the components of leaf CO2 release in the dark. It separates and analyses the different components of dark respiration, post illumination burst, light enhanced dark respiration and dark respiration in steady-state across temperature and integrates metabolite profiling across the light-dark transition to relate CO2 fluxes to substrate availability. It tests the Arrhenius and MMRT formulations and identifies the model that best capture warm side curvature without deactivation terms.
Chapter 7. Conclusions and perspectives. It synthesises the finding across chapters, and temperature response models, discusses implications for photosynthesis and respiration modelling, and outlines priorities for future research.

year: 2026-03-12

title: Arnold Manaaki Wilson: Te Awakaunua

authors: Te Awekotuku, Ngahuia

subjects: Arnold Manaaki Wilson

abstract: Arnold Manaaki Wilson was born in 1928, in Ruatoki, a community which nestles beneath the misty Taiarahia hills, following the curves of the Ohinemataroa river valley – known to others as of his tuhoe people. They know him there as Te Wakaunua, after a provocative late 19th century political visionary. From Such radical Tuhoe ideas fused with the sculptural genius of his father, a renowned carver of the art-making Ngati Tarawhai of Te Arawa, Arnold Wilson emerged.

text: Arnold Manaaki Wilson: Te Awakaunua Arnold Manaaki Wilson was born in 1928, in Ruatoki, a community which nestles beneath the misty Taiarahia hills, following the curves of the Ohinemataroa river valley – known to others as of his tuhoe people. They know him there as Te Wakaunua, after a provocative late 19th century political visionary. From Such radical Tuhoe ideas fused with the sculptural genius of his father, a renowned carver of the art-making Ngati Tarawhai of Te Arawa, Arnold Wilson emerged.

year: 2008