title: A coupled hydrodynamic-ecological model of management options for restoration of Lake Ohinewai

authors: Allan, Mathew Grant

abstract: Koi carp (Cyprinus carpio) is regarded as one of the ecologically most destructive invasive freshwater species capable of degradation of shallow aquatic ecosystems. This degradation is largely due to their benthic foraging activity which resuspends sediment and increases nitrogen (N) and phosphorus (P) concentrations, leading to algal blooms. In addition, carp have the potential to excrete large quantities of nutrients into the water column. This report describes a modelling study of a shallow Waikato lake subject to invasive fish removal to investigate how carp contribute to the overall nutrient cycling at an ecosystem scale. The study site was Lake Ohinewai, a relatively small shallow, polymictic, hypertrophic riverine lake located in the northern Waikato region. Evidence of water quality degradation has been documented since the 1980's, before the introduction of carp. Lake Ohinewai has undergone an intensive programme of invasive fish removal and a permanent adult koi carp barrier was installed on the outflow drain of Lake Ohinewai to prevent passage into the lake, but allow carp to exit.

We applied DYRESM-CAEDYM (DYCD), a 1-D water quality model that has been developed at the Centre for Water Research, University of Western Australia. Daily inflows into Lake Ohinewai were derived from INCA, a process-based model. Carp-driven sediment resuspension was estimated using results from experimental ponds stocked with varying densities of carp. Carp-suspended sediment was then used to estimate carp-translocated porewater due to benthic feeding. Koi carp excretion was estimated using an allometric scaling model. Nine catchment and riparian management scenarios were tested with no fish (NF) and with current invasive koi carp removal (F). These scenarios included: simulation of current biomass; pre-removal fish biomass; reduction of external load by 50%; an all native forest catchment; climate change; DOC administered land sub-catchment retirement; enhanced lake riparian margins; stream riparian zones; and creation of a wetland. For all scenarios, specified changes in nutrient concentrations were applied to internal loads (phosphate and ammonium release rates) from the sediment.
Simulations demonstrated that, at a density of 374 kg ha⁻¹ and assuming they are feeding on the benthos, koi carp can contribute 21% of total phosphorous load and 10% of total nitrogen load via nutrient translocation. The major proportion of this load was due to koi carp excretion. While koi carp introduction in Lake Ohinewai has contributed to its current hypertrophic state based on this modelling exercise, evidence of water quality degradation was originally documented prior to koi carp introduction, and has most likely been occurring since the catchment land use was originally modified from its natural state. The simulations suggest that the removal of koi carp decreased Trophic Level Index (TLI4) from 6.45 to 6.28, indicating that koi carp removal alone is not sufficient for a significant restoration of lake water quality. In order to return the system to a stable clear water state, water clarity must improve to a level to permit significant macrophyte re-establishment. However, restoration simulations showed that at a lower TLI4 the removal of koi carp is critical to lake restoration. Simulations estimated that the removal of koi carp decreased TSS concentrations by approximately two-thirds in most of the scenarios. However, in the all-native catchment scenario, removal of koi carp was predicted to increase Secchi depth by c. 2.5 m, and reduce TSS by c. 13 mg l⁻¹.

The scenario simulations within the present study showed that integrated catchment management involving use of stream riparian zones and constructed wetlands would be required in order to restore lake water quality significantly. Using these mitigation techniques a TLI4 target of 5.27 would likely be achieved. This mesotrophic TLI4 may enable the reestablishment of macrophytes.

text: A coupled hydrodynamic-ecological model of management options for restoration of Lake Ohinewai Koi carp (Cyprinus carpio) is regarded as one of the ecologically most destructive invasive freshwater species capable of degradation of shallow aquatic ecosystems. This degradation is largely due to their benthic foraging activity which resuspends sediment and increases nitrogen (N) and phosphorus (P) concentrations, leading to algal blooms. In addition, carp have the potential to excrete large quantities of nutrients into the water column. This report describes a modelling study of a shallow Waikato lake subject to invasive fish removal to investigate how carp contribute to the overall nutrient cycling at an ecosystem scale. The study site was Lake Ohinewai, a relatively small shallow, polymictic, hypertrophic riverine lake located in the northern Waikato region. Evidence of water quality degradation has been documented since the 1980's, before the introduction of carp. Lake Ohinewai has undergone an intensive programme of invasive fish removal and a permanent adult koi carp barrier was installed on the outflow drain of Lake Ohinewai to prevent passage into the lake, but allow carp to exit.

We applied DYRESM-CAEDYM (DYCD), a 1-D water quality model that has been developed at the Centre for Water Research, University of Western Australia. Daily inflows into Lake Ohinewai were derived from INCA, a process-based model. Carp-driven sediment resuspension was estimated using results from experimental ponds stocked with varying densities of carp. Carp-suspended sediment was then used to estimate carp-translocated porewater due to benthic feeding. Koi carp excretion was estimated using an allometric scaling model. Nine catchment and riparian management scenarios were tested with no fish (NF) and with current invasive koi carp removal (F). These scenarios included: simulation of current biomass; pre-removal fish biomass; reduction of external load by 50%; an all native forest catchment; climate change; DOC administered land sub-catchment retirement; enhanced lake riparian margins; stream riparian zones; and creation of a wetland. For all scenarios, specified changes in nutrient concentrations were applied to internal loads (phosphate and ammonium release rates) from the sediment.
Simulations demonstrated that, at a density of 374 kg ha⁻¹ and assuming they are feeding on the benthos, koi carp can contribute 21% of total phosphorous load and 10% of total nitrogen load via nutrient translocation. The major proportion of this load was due to koi carp excretion. While koi carp introduction in Lake Ohinewai has contributed to its current hypertrophic state based on this modelling exercise, evidence of water quality degradation was originally documented prior to koi carp introduction, and has most likely been occurring since the catchment land use was originally modified from its natural state. The simulations suggest that the removal of koi carp decreased Trophic Level Index (TLI4) from 6.45 to 6.28, indicating that koi carp removal alone is not sufficient for a significant restoration of lake water quality. In order to return the system to a stable clear water state, water clarity must improve to a level to permit significant macrophyte re-establishment. However, restoration simulations showed that at a lower TLI4 the removal of koi carp is critical to lake restoration. Simulations estimated that the removal of koi carp decreased TSS concentrations by approximately two-thirds in most of the scenarios. However, in the all-native catchment scenario, removal of koi carp was predicted to increase Secchi depth by c. 2.5 m, and reduce TSS by c. 13 mg l⁻¹.

The scenario simulations within the present study showed that integrated catchment management involving use of stream riparian zones and constructed wetlands would be required in order to restore lake water quality significantly. Using these mitigation techniques a TLI4 target of 5.27 would likely be achieved. This mesotrophic TLI4 may enable the reestablishment of macrophytes.

year: 2016

title: Transport and retention of benthic marine invertebrates in the Southern Tauranga Basin

authors: Monahan, Bradley John

subjects: Dispersal

abstract: Dispersal is the process in which individuals are transferred between populations, and is essential for population sustainability and longevity. In estuarine soft sediment communities, dispersal often occurs as the interaction between the individual and tidal flows. Many coastal benthic marine invertebrate populations have been considered open because of the dispersal potential of a pelagic larval phase, with populations maintained by an external supply of recruits. However, for estuarine populations, recent evidence has suggested that populations may be more closed because of local hydrodynamic conditions causing larval retention within the source estuary. In this thesis, field observations and numerical simulations were used to investigate the effect of estuary morphology on the transport and retention of benthic invertebrate larvae in Tauranga Harbour, a large (218 km2), shallow (mean depth = 2.8 m) meso-tidal estuary. Previous studies indicate that residence times extracted from hydrodynamic modelling vary throughout the harbour, due to residual circulation patterns caused by complex channel morphology. Field observations indicate increased abundance and differing community composition of invertebrate larvae in the upper harbour compared to the lower. Tidal circulation is asymmetrical, with transport through a seaward channel during flood tides, and in a landward channel during ebb, suggesting that larval retention in the upper harbour may provide a source of larvae for the lower harbour. Larval transport pathways were evaluated by a coupled hydrodynamic-transport model and confirmed those observed in the field. Modelled larvae, released under different stages of spring-neap tidal cycle in the upper harbour, were retained for up to 16 d whereas retention for larvae released in other harbour locations was much more variable and depended on the timing of release and direction of initial transport. Results of this study highlight the impact of complex channel morphology (particularly at a scale of 10s to 100s of meters) on the large scale circulation and subsequent transport and retention of benthic invertebrate larvae. Understanding these influences on the transportation of benthic invertebrate larvae and the identification of retentive regions, will aid future population management or conservation efforts

text: Transport and retention of benthic marine invertebrates in the Southern Tauranga Basin Dispersal is the process in which individuals are transferred between populations, and is essential for population sustainability and longevity. In estuarine soft sediment communities, dispersal often occurs as the interaction between the individual and tidal flows. Many coastal benthic marine invertebrate populations have been considered open because of the dispersal potential of a pelagic larval phase, with populations maintained by an external supply of recruits. However, for estuarine populations, recent evidence has suggested that populations may be more closed because of local hydrodynamic conditions causing larval retention within the source estuary. In this thesis, field observations and numerical simulations were used to investigate the effect of estuary morphology on the transport and retention of benthic invertebrate larvae in Tauranga Harbour, a large (218 km2), shallow (mean depth = 2.8 m) meso-tidal estuary. Previous studies indicate that residence times extracted from hydrodynamic modelling vary throughout the harbour, due to residual circulation patterns caused by complex channel morphology. Field observations indicate increased abundance and differing community composition of invertebrate larvae in the upper harbour compared to the lower. Tidal circulation is asymmetrical, with transport through a seaward channel during flood tides, and in a landward channel during ebb, suggesting that larval retention in the upper harbour may provide a source of larvae for the lower harbour. Larval transport pathways were evaluated by a coupled hydrodynamic-transport model and confirmed those observed in the field. Modelled larvae, released under different stages of spring-neap tidal cycle in the upper harbour, were retained for up to 16 d whereas retention for larvae released in other harbour locations was much more variable and depended on the timing of release and direction of initial transport. Results of this study highlight the impact of complex channel morphology (particularly at a scale of 10s to 100s of meters) on the large scale circulation and subsequent transport and retention of benthic invertebrate larvae. Understanding these influences on the transportation of benthic invertebrate larvae and the identification of retentive regions, will aid future population management or conservation efforts

year: 2018

title: He whare mātātuhi Tūhoe:  A Tūhoe re-visioning of English language literary texts

authors: Salsano, Marama

subjects: Māori writers

abstract: This thesis urges more Māori to undertake iwi-based readings of English language literary texts. I have chosen to privilege my father’s iwi, Ngāi Tūhoe, and to examine what Tūhoetana means to me and how it might thus enrich my reading of texts. In our ancestral tongue, Ngāi Tūhoe have a rich and pronounced history of storytelling, literary composition, analysis and critique. Unfortunately, this richness is woefully underappreciated within the English language literary landscape. Yet, as the ranks of Māori writers swell, along with the diversity of literary forms that Māori create, so too must Māori – and more specifically iwi – literary scholarship flourish, for if we do not critique the texts that are written about, of, and on behalf of us, who will? Throughout this thesis, I use the terms ‘iwi literary scholars’ and ‘iwi literary scholarship’ to consciously and repeatedly remind the reader of the rich, literary legacies residing within iwi archives; I also hope to highlight the exciting and endless possiblitites for the English language literary landscape to be enriched by readings that are founded within these archives – including the histories, everyday experiences and understandings from our tīpuna. 

Structurally, this thesis is divided into three broad strands. Chapters 1-3 focus on whakawhanaunatana and background information to my research. Because this thesis also represents the singular piece of writing that I wish I had read prior to enrolling in the thesis paper for this Master’s degree, my academic hīkoi – frustrations and all – is a necessary component and is covered in chapter 2. Chapters 4-6 then examine three different types of literary texts created by wāhine. These texts include the short story “Birth Rights” by Ngāi Tahu science fiction and fantasy writer, J.C. Hart (2010), the Instagram post Te Kuharere tapes by Ngāti Kahungungu and Ngāti Tuwharetoa songwriter and storyteller @tekahureremoa (2019) – also known as the performer Ladyfruit – and the young adult novel Dark Souls by Ngāti Wai, Ngāti Manuhiri, and Ngāti Whatua writer, Paula Morris (2011). While 21st century texts published on platforms such as Instagram may not sit easily within a traditional English literary curriculum, and indeed the novelty of these specific forms may well prove fleeting, the ‘everydayness’ of online texts yields rewarding analysis opportunities and is currently amiss within mainstream English literary studies. Each of these analysis chapters contains a focused introduction and conclusion, since chapter 7, the concluding chapter of this thesis, is a letter to future iwi literary scholars. 

Woven throughout this rangahau is a call for iwi scholars to consciously: engage with the literary traditions of their own iwi; identify the colonial thoughts and motives that they bring to the reading of literary texts; bring their tīpuna to the reading, but let the text itself breathe life into the critique and; consider the genres and types of texts that they critique. Ultimately, this thesis considers how engaging with the literary legacies of our tīpuna might broaden the English language literary landscape and thus enhance the body of iwi literary scholarship in Oceania and beyond.

text: He whare mātātuhi Tūhoe:  A Tūhoe re-visioning of English language literary texts This thesis urges more Māori to undertake iwi-based readings of English language literary texts. I have chosen to privilege my father’s iwi, Ngāi Tūhoe, and to examine what Tūhoetana means to me and how it might thus enrich my reading of texts. In our ancestral tongue, Ngāi Tūhoe have a rich and pronounced history of storytelling, literary composition, analysis and critique. Unfortunately, this richness is woefully underappreciated within the English language literary landscape. Yet, as the ranks of Māori writers swell, along with the diversity of literary forms that Māori create, so too must Māori – and more specifically iwi – literary scholarship flourish, for if we do not critique the texts that are written about, of, and on behalf of us, who will? Throughout this thesis, I use the terms ‘iwi literary scholars’ and ‘iwi literary scholarship’ to consciously and repeatedly remind the reader of the rich, literary legacies residing within iwi archives; I also hope to highlight the exciting and endless possiblitites for the English language literary landscape to be enriched by readings that are founded within these archives – including the histories, everyday experiences and understandings from our tīpuna. 

Structurally, this thesis is divided into three broad strands. Chapters 1-3 focus on whakawhanaunatana and background information to my research. Because this thesis also represents the singular piece of writing that I wish I had read prior to enrolling in the thesis paper for this Master’s degree, my academic hīkoi – frustrations and all – is a necessary component and is covered in chapter 2. Chapters 4-6 then examine three different types of literary texts created by wāhine. These texts include the short story “Birth Rights” by Ngāi Tahu science fiction and fantasy writer, J.C. Hart (2010), the Instagram post Te Kuharere tapes by Ngāti Kahungungu and Ngāti Tuwharetoa songwriter and storyteller @tekahureremoa (2019) – also known as the performer Ladyfruit – and the young adult novel Dark Souls by Ngāti Wai, Ngāti Manuhiri, and Ngāti Whatua writer, Paula Morris (2011). While 21st century texts published on platforms such as Instagram may not sit easily within a traditional English literary curriculum, and indeed the novelty of these specific forms may well prove fleeting, the ‘everydayness’ of online texts yields rewarding analysis opportunities and is currently amiss within mainstream English literary studies. Each of these analysis chapters contains a focused introduction and conclusion, since chapter 7, the concluding chapter of this thesis, is a letter to future iwi literary scholars. 

Woven throughout this rangahau is a call for iwi scholars to consciously: engage with the literary traditions of their own iwi; identify the colonial thoughts and motives that they bring to the reading of literary texts; bring their tīpuna to the reading, but let the text itself breathe life into the critique and; consider the genres and types of texts that they critique. Ultimately, this thesis considers how engaging with the literary legacies of our tīpuna might broaden the English language literary landscape and thus enhance the body of iwi literary scholarship in Oceania and beyond.

year: 2021