title: Speciation and Dynamics of Phosphorus in Relation to Lake Restoration Methods

authors: Özkundakci, Deniz

subjects: Lake Okaro

abstract: Lake Okaro is a small, warm monomictic lake in the central North Island of New Zealand. It has remained highly eutrophic despite an intensive catchment and in-lake restoration program which commenced in 2003. The program has included the implementation of a constructed wetland, riparian protection, an alum application and application of a modified zeolite mineral (Z2G1) to reduce internal nutrient loading. This study examines water column and sediment nutrient dynamics; focusing on phosphorus (P) and the ecosystem response to lake restoration designed to reduce levels of P.
Trends in P concentrations in Lake Okaro were linked to the restoration efforts over a six-year period (2002-08) including the period shortly before the restoration program. Over the entire study period, the annual average total phosphorus (TP) concentration in the lake decreased by 56 %. Two predictive models, which derive the annual average P concentration in the water column based on external P loading, generally underestimated the measured TP concentrations in the water column due to internal P loading. Of all restoration methods, the application of Z2G1 produced the most effective reduction in water column TP concentrations. However the lake trophic state showed high resilience to reduced internal P loading even though the combined effect of all restoration procedures resulted in significantly decreased TP concentrations in the lake.
The sources and sinks of nutrients in the hypolimnion of Lake Okaro were investigated using field measurements in a comprehensive nutrient budget model in order to determine changes in sediment nutrient fluxes resulting from a whole lake sediment capping trial using Z2G1. Sediment nutrient fluxes in the hypolimnion were estimated as the residual term in the nutrient budget model that accounted for mineralisation of organic nutrients, nutrient uptake by phytoplankton, nitrification, adsorption or desorption of P from inorganic particulate material in the water column, and diffusion of dissolved nutrients at the thermocline. The model indicated that during a period of seasonal stratification in 2007-08 up to 60% of hypolimnetic phosphate fluxes and 50% of ammonium fluxes were derived from bottom sediments. Diffusion across the thermocline, adsorption/desorption of phosphate to suspended solids, and nitrification were of relatively minor importance (less than or equal to 9%) to the total fluxes. Any reduction in sediment nutrient release by Z2G1 was small compared with both the total sediment nutrient flux and the sum of other hypolimnetic fluxes.
Sediment and settling seston organic P composition was determined using 31P nuclear magnetic resonance (NMR). Settling seston and sediment samples were analysed during winter and summer, representing, respectively, a mixing period when the water column was well oxygenated and a stratified period when the hypolimnion was anoxic. The bottom sediments and settling seston contained orthophosphate, orthophosphate mono- and diesters, pyrophosphates, polyphosphates, and phosphonates with organic P content exceeding 60% of the total extracted P occasionally. Phosphorus content in settling seston increased 2.5-fold in winter, with a marked increase in orthophosphate content. The 31P NMR analyses revealed the presence of several potentially bioavailable P compounds, which may be recycled from the sediment to the water column. An  apparent half-life‟ value was used to quantify the time scales on which these compounds are degraded within the sediment and likely being recycled to the overlying water column. Relatively long half-life values, ranging from 8 to 23 years, indicate that this recycling could potentially reduce the efficacy and longevity of in-lake restoration procedures that have been applied to Lake Okaro.
A one-dimensional process based ecosystem model (DYRESM-CAEDYM) was used to simulate the potential effect on water quality of Lake Okaro of separate and combined reductions in external and internal loads of nitrogen (N) and P. The model was calibrated against field data for a two-year period and validated over two separate one-year periods including a year immediately following a Z2G1 application and a year when there was an extraordinary algal bloom from an invasive, highly buoyant, N-fixing cyanobacterium, Anabaena planktonica. The model simulations reproduced the scale of phosphate and ammonium concentrations at 14 m depth, corresponding to the deeper region of the hypolimnion, both before and after the application of Z2G1, with no adjustment of parameters, suggesting that there was little effect of the Z2G1, at least within the uncertainties of the model runs. The model simulations were less successful in reproducing the Anabaena planktonica bloom. This was attributed to a lake of flexibility in the conceptualisation and calibration of the model, which meant that it could not encompass this invasive species. In the model scenarios with reduced nutrient loading, the trophic status of Lake Okaro, given quantitatively by the Trophic Level Index (TLI), decreased to a greater extent with a given fractional reduction of the internal load than a reduction of the external load. The control of both N and P was shown in simulations to be more effective in reducing phytoplankton biomass than for N or P alone, tending to affirm an N+P control paradigm.
Undesirable shifts in zooplankton and phytoplankton species composition due to the application of Z2G1 were investigated by comparing the plankton community structure before and after the Z2G1 application. No significant differences in species composition were found at the depths investigated (surface and 9 m). However, further analyses showed statistically significant differences between seasons, indicating that seasonal variations in plankton composition far outweighed changes that occurred as a result of the Z2G1 application.
In this study, field measurements and numerical modelling provided a comprehensive assessment methodology of testing the response of Lake Okaro to reduced nutrient loading, with a focus on P dynamics. Although nutrient loads to Lake Okaro were reduced using catchment and in-lake restoration methods, further substantial and prolonged reduction in both N and P loading appear to be required to decrease phytoplankton biomass and the trophic state of the lake. This study highlights the need for investigating water column P content and sediment P composition for evaluating the potential magnitude of internal loading, and emphasises the importance of using process based numerical modelling as a decision support tool for lake management.

text: Speciation and Dynamics of Phosphorus in Relation to Lake Restoration Methods Lake Okaro is a small, warm monomictic lake in the central North Island of New Zealand. It has remained highly eutrophic despite an intensive catchment and in-lake restoration program which commenced in 2003. The program has included the implementation of a constructed wetland, riparian protection, an alum application and application of a modified zeolite mineral (Z2G1) to reduce internal nutrient loading. This study examines water column and sediment nutrient dynamics; focusing on phosphorus (P) and the ecosystem response to lake restoration designed to reduce levels of P.
Trends in P concentrations in Lake Okaro were linked to the restoration efforts over a six-year period (2002-08) including the period shortly before the restoration program. Over the entire study period, the annual average total phosphorus (TP) concentration in the lake decreased by 56 %. Two predictive models, which derive the annual average P concentration in the water column based on external P loading, generally underestimated the measured TP concentrations in the water column due to internal P loading. Of all restoration methods, the application of Z2G1 produced the most effective reduction in water column TP concentrations. However the lake trophic state showed high resilience to reduced internal P loading even though the combined effect of all restoration procedures resulted in significantly decreased TP concentrations in the lake.
The sources and sinks of nutrients in the hypolimnion of Lake Okaro were investigated using field measurements in a comprehensive nutrient budget model in order to determine changes in sediment nutrient fluxes resulting from a whole lake sediment capping trial using Z2G1. Sediment nutrient fluxes in the hypolimnion were estimated as the residual term in the nutrient budget model that accounted for mineralisation of organic nutrients, nutrient uptake by phytoplankton, nitrification, adsorption or desorption of P from inorganic particulate material in the water column, and diffusion of dissolved nutrients at the thermocline. The model indicated that during a period of seasonal stratification in 2007-08 up to 60% of hypolimnetic phosphate fluxes and 50% of ammonium fluxes were derived from bottom sediments. Diffusion across the thermocline, adsorption/desorption of phosphate to suspended solids, and nitrification were of relatively minor importance (less than or equal to 9%) to the total fluxes. Any reduction in sediment nutrient release by Z2G1 was small compared with both the total sediment nutrient flux and the sum of other hypolimnetic fluxes.
Sediment and settling seston organic P composition was determined using 31P nuclear magnetic resonance (NMR). Settling seston and sediment samples were analysed during winter and summer, representing, respectively, a mixing period when the water column was well oxygenated and a stratified period when the hypolimnion was anoxic. The bottom sediments and settling seston contained orthophosphate, orthophosphate mono- and diesters, pyrophosphates, polyphosphates, and phosphonates with organic P content exceeding 60% of the total extracted P occasionally. Phosphorus content in settling seston increased 2.5-fold in winter, with a marked increase in orthophosphate content. The 31P NMR analyses revealed the presence of several potentially bioavailable P compounds, which may be recycled from the sediment to the water column. An  apparent half-life‟ value was used to quantify the time scales on which these compounds are degraded within the sediment and likely being recycled to the overlying water column. Relatively long half-life values, ranging from 8 to 23 years, indicate that this recycling could potentially reduce the efficacy and longevity of in-lake restoration procedures that have been applied to Lake Okaro.
A one-dimensional process based ecosystem model (DYRESM-CAEDYM) was used to simulate the potential effect on water quality of Lake Okaro of separate and combined reductions in external and internal loads of nitrogen (N) and P. The model was calibrated against field data for a two-year period and validated over two separate one-year periods including a year immediately following a Z2G1 application and a year when there was an extraordinary algal bloom from an invasive, highly buoyant, N-fixing cyanobacterium, Anabaena planktonica. The model simulations reproduced the scale of phosphate and ammonium concentrations at 14 m depth, corresponding to the deeper region of the hypolimnion, both before and after the application of Z2G1, with no adjustment of parameters, suggesting that there was little effect of the Z2G1, at least within the uncertainties of the model runs. The model simulations were less successful in reproducing the Anabaena planktonica bloom. This was attributed to a lake of flexibility in the conceptualisation and calibration of the model, which meant that it could not encompass this invasive species. In the model scenarios with reduced nutrient loading, the trophic status of Lake Okaro, given quantitatively by the Trophic Level Index (TLI), decreased to a greater extent with a given fractional reduction of the internal load than a reduction of the external load. The control of both N and P was shown in simulations to be more effective in reducing phytoplankton biomass than for N or P alone, tending to affirm an N+P control paradigm.
Undesirable shifts in zooplankton and phytoplankton species composition due to the application of Z2G1 were investigated by comparing the plankton community structure before and after the Z2G1 application. No significant differences in species composition were found at the depths investigated (surface and 9 m). However, further analyses showed statistically significant differences between seasons, indicating that seasonal variations in plankton composition far outweighed changes that occurred as a result of the Z2G1 application.
In this study, field measurements and numerical modelling provided a comprehensive assessment methodology of testing the response of Lake Okaro to reduced nutrient loading, with a focus on P dynamics. Although nutrient loads to Lake Okaro were reduced using catchment and in-lake restoration methods, further substantial and prolonged reduction in both N and P loading appear to be required to decrease phytoplankton biomass and the trophic state of the lake. This study highlights the need for investigating water column P content and sediment P composition for evaluating the potential magnitude of internal loading, and emphasises the importance of using process based numerical modelling as a decision support tool for lake management.

year: 2010

title: The effect of deferment length on perennial ryegrass (Lolium perenne) in established grazing pasture and under controlled conditions

authors: North, Elizabeth

subjects: Deferred

abstract: Perennial ryegrass is the dominant sown grass species in New Zealand, however its persistence in pasture is an issue for hill country farms. Deferring the grazing of a pasture over the reproductive cycle of ryegrass may influence changes in the above and below ground biomass of pasture plants and offers a low input and sustainable method for farmers to manage dry matter yields and quality of feed supply.
The aim of this study was to investigate the above and below ground changes in biomass and tiller production of perennial ryegrass under different lengths of deferment, after which standard rotational grazing or simulated grazing was resumed. Two trials were set up to investigate this. A field study was undertaken in an established ryegrass/clover-based pasture with three grazing treatments. Standard rotational grazing already in use on the field site (Def x0), a period of deferment over late spring/summer from November to February (Def x14) and a longer period of deferment from late spring to autumn (Def x27). After deferment, standard grazing was resumed. Above and belowground biomass, tiller densities, nutritive value, pasture composition and ground cover were measured. A glasshouse-based study investigated the effects on perennial ryegrass of different lengths of delayed defoliation and cutting treatments applied to simulate grazing. The control was cut when 2.5 to 3 new leaves per tiller had grown since the last cutting (Def x0); cutting of other treatments was delayed for an extra 4 (Def x4), 8 (Def x8) or 12 weeks (Def x12). After the delayed cutting, plants in each treatment were cut whenever there were 2.5 to 3 new leaves present per tiller. Both trials were affected by an extra-long period of rest between grazing or cutting treatments from the end of February to late April/early May due to Covid-19. This acted as an additional period of deferment for all glasshouse treatments and for Def x0 and Def x14 in the field.
The main results of the field study were that deferring resulted in higher tiller densities in winter and more accumulated dry matter before the deferred was opened for grazing than the rotational grazed control. There was also less bare ground compared with the control during summer. Nutritive value was lower during deferred periods but returned to the same value as the grazed control after standard rotational grazing resumed. There were no differences between any of the deferred treatments in root biomass. The percentage of ryegrass in total dry matter or ryegrass ground cover did not differ significantly between treatments.
In the glasshouse study, delayed defoliation increased leaf biomass, live tiller number and reproductive tiller number. Treatments with longer periods of delayed defoliation reach higher peaks in above ground dry matter and reproductive tiller number. There was a temporary increase in root biomass at depth for treatments with delayed defoliation in January. After standard defoliation resumed, Def x12 had a higher percentage of dead tillers and lower biomass by the end of the trial in July. There were no differences between treatments for live tiller number or root biomass once standard defoliation resumed.
The results suggest that a period of deferred grazing on pasture may improve dry matter yield and tiller densities of ryegrass after normal grazing resumes. The longer deferred treatment accumulated more dry matter in autumn, however it is not certain how the longer grazing rest between February and May affected potential differences between the shorter and longer deferred period. Results from the glasshouse saw no lasting benefit to ryegrass once standard defoliation resumed. Deferment length had no significant effect on belowground biomass. It is concluded that while deferred grazing may offer some benefits to a pasture, the persistence of ryegrass will not be improved by deferring grazing if other variables such as heat, moisture stress or timing of grazing are limiting potential growth. However, due to the unforeseen consequences of the Covid-19 lockdown, further research is recommended.

text: The effect of deferment length on perennial ryegrass (Lolium perenne) in established grazing pasture and under controlled conditions Perennial ryegrass is the dominant sown grass species in New Zealand, however its persistence in pasture is an issue for hill country farms. Deferring the grazing of a pasture over the reproductive cycle of ryegrass may influence changes in the above and below ground biomass of pasture plants and offers a low input and sustainable method for farmers to manage dry matter yields and quality of feed supply.
The aim of this study was to investigate the above and below ground changes in biomass and tiller production of perennial ryegrass under different lengths of deferment, after which standard rotational grazing or simulated grazing was resumed. Two trials were set up to investigate this. A field study was undertaken in an established ryegrass/clover-based pasture with three grazing treatments. Standard rotational grazing already in use on the field site (Def x0), a period of deferment over late spring/summer from November to February (Def x14) and a longer period of deferment from late spring to autumn (Def x27). After deferment, standard grazing was resumed. Above and belowground biomass, tiller densities, nutritive value, pasture composition and ground cover were measured. A glasshouse-based study investigated the effects on perennial ryegrass of different lengths of delayed defoliation and cutting treatments applied to simulate grazing. The control was cut when 2.5 to 3 new leaves per tiller had grown since the last cutting (Def x0); cutting of other treatments was delayed for an extra 4 (Def x4), 8 (Def x8) or 12 weeks (Def x12). After the delayed cutting, plants in each treatment were cut whenever there were 2.5 to 3 new leaves present per tiller. Both trials were affected by an extra-long period of rest between grazing or cutting treatments from the end of February to late April/early May due to Covid-19. This acted as an additional period of deferment for all glasshouse treatments and for Def x0 and Def x14 in the field.
The main results of the field study were that deferring resulted in higher tiller densities in winter and more accumulated dry matter before the deferred was opened for grazing than the rotational grazed control. There was also less bare ground compared with the control during summer. Nutritive value was lower during deferred periods but returned to the same value as the grazed control after standard rotational grazing resumed. There were no differences between any of the deferred treatments in root biomass. The percentage of ryegrass in total dry matter or ryegrass ground cover did not differ significantly between treatments.
In the glasshouse study, delayed defoliation increased leaf biomass, live tiller number and reproductive tiller number. Treatments with longer periods of delayed defoliation reach higher peaks in above ground dry matter and reproductive tiller number. There was a temporary increase in root biomass at depth for treatments with delayed defoliation in January. After standard defoliation resumed, Def x12 had a higher percentage of dead tillers and lower biomass by the end of the trial in July. There were no differences between treatments for live tiller number or root biomass once standard defoliation resumed.
The results suggest that a period of deferred grazing on pasture may improve dry matter yield and tiller densities of ryegrass after normal grazing resumes. The longer deferred treatment accumulated more dry matter in autumn, however it is not certain how the longer grazing rest between February and May affected potential differences between the shorter and longer deferred period. Results from the glasshouse saw no lasting benefit to ryegrass once standard defoliation resumed. Deferment length had no significant effect on belowground biomass. It is concluded that while deferred grazing may offer some benefits to a pasture, the persistence of ryegrass will not be improved by deferring grazing if other variables such as heat, moisture stress or timing of grazing are limiting potential growth. However, due to the unforeseen consequences of the Covid-19 lockdown, further research is recommended.

year: 2020

title: Supervision of Maori doctoral students: A descriptive report

authors: McKinley, Elizabeth

subjects: Maori education

abstract: This report follows up a previous paper that outlined the goals and plans of a research project that focused on both theoretical and cultural questions regarding the supervisory process for Māori doctoral students (McKinley, Grant, Middleton, Irwin, & Williams, 2007). The major goal of the project is to enhance understanding of the teaching and learning process of supervision for students and supervisors, particularly around issues of culture that arise in research methodologies and practices. This paper reports on the completed project by providing further operational background, design features, the nature of the student and supervisor samples and a summary of interview findings. The results show that there are indeed distinctive issues arising within the supervision of Māori doctoral students. Some of these are to do with both pleasures and challenges found in the supervision relationship, while others relate to the kinds of projects the students undertake. Many projects for example, push at the disciplinary boundaries of Western knowledge and are often rooted in a political desire to enhance the everyday lives of Māori. Yet others are connected to identity formation processes that concern many Māori during their years as doctoral students. A central message for supervisors from this work is that the supervision of Māori doctoral students may require unfamiliar forms of engagement but that these are likely to be deeply rewarding in many different ways.

text: Supervision of Maori doctoral students: A descriptive report This report follows up a previous paper that outlined the goals and plans of a research project that focused on both theoretical and cultural questions regarding the supervisory process for Māori doctoral students (McKinley, Grant, Middleton, Irwin, & Williams, 2007). The major goal of the project is to enhance understanding of the teaching and learning process of supervision for students and supervisors, particularly around issues of culture that arise in research methodologies and practices. This paper reports on the completed project by providing further operational background, design features, the nature of the student and supervisor samples and a summary of interview findings. The results show that there are indeed distinctive issues arising within the supervision of Māori doctoral students. Some of these are to do with both pleasures and challenges found in the supervision relationship, while others relate to the kinds of projects the students undertake. Many projects for example, push at the disciplinary boundaries of Western knowledge and are often rooted in a political desire to enhance the everyday lives of Māori. Yet others are connected to identity formation processes that concern many Māori during their years as doctoral students. A central message for supervisors from this work is that the supervision of Māori doctoral students may require unfamiliar forms of engagement but that these are likely to be deeply rewarding in many different ways.

year: 2009