title: Petrogenesis of Pliocene limestones in southern Hawke’s Bay, New Zealand: a contribution to unravelling the sequence stratigraphy and diagenetic pathways of cool-water shelf carbonate facies

authors: Caron, Vincent

abstract: Among carbonate sedimentologists, it is now appreciated that shallow-marine carbonates are not restricted to warm-water tropical settings, but can also be produced on modern cool-water shelves outside tropical latitudes. In the last two decades, many publications have not only advanced our knowledge about modern non-tropical carbonates, but have documented ancient skeletal limestones that are fossil analogues of these modern deposits. While the broad environmental and sedimentary characteristics of non-tropical deposits are becoming well established, the nature, timing, and relative importance of the various diagenetic processes affecting their evolution into limestones are yet to be fully appreciated, a topic addressed in the present study.

In New Zealand, the Plio-Pleistocene (5-1.5 Ma) Te Aute limestones in southern Hawke’s Bay (eastern North Island) are non-tropical in origin, and formed in shoal areas about the margins of a forearc basin seaway adjacent to the convergent Pacific-Australian Plate boundary. Because of the dynamic tectonic setting, which led to narrowing and shallowing of the host basin over time, the studied carbonates exhibit a variety of sedimentary facies (i.e., from pure coquina to mixed carbonate-siliciclastic) and mineralogical signatures (i.e., calcite- to, more rarely, aragonite-dominated), and have undergone varying degrees of burial before being uplifted to their present position. As such, the Te Aute deposits provide a unique opportunity to: (1) decipher the suite of processes that have altered their texture and composition through time; (2) address the questions of what likely regional and more global parameters, namely tectonics and eustasy, may have driven diagenetic pathways, and how this occurred; and (3) more generally advance fundamental knowledge about models of porosity evolution in non-tropical carbonates.

The present study integrates the combined results of detailed field investigations with a variety of analytical techniques, including standard, stained, and cathodoluminescence (CL) microscopic examinations of rock thin-sections, X-ray diffraction (XRD), and elemental (ICP-OES and microprobe) and stable isotope (δ¹⁸O and δ¹³C) geochemistry.

Sedimentologic analysis has identified 14 different facies types that correspond to a spectrum of depositional settings associated with both continent-attached shelves and isolated upthrust-cored antiforms within the forearc seaway. Facies are representative of environments ranging from below storm-wave base, in outer shelf-type locations, to predominantly shallow-water shoreface-type conditions under the influence of strong tidal current flows and storm waves.

A new petrographic approach for studying cool-water carbonate diagenesis has been developed. The method entails recording the high-resolution chronological occurrence of all diagenetic features within samples, and noting their vertical and lateral distribution within sedimentary formations. It is demonstrated that the attributes of pre-compaction diagenetic suites between consecutive sedimentary discontinuities change vertically in relation to the stacking of depositional facies. This original approach, by also revealing marine-cemented condensed horizons and key surfaces otherwise unrecognized in the field, has helped unlock the cryptic sedimentary architecture within the limestones. Thus, the Te Aute deposits appear to be constructed of metre-scale (1-15m) sequences that could be integrated into a sequence stratigraphic scheme, emphasizing that not only the production and deposition, but also the diagenesis of these carbonate deposits, were forced by glacio-eustatically and tectonically-driven processes.

In order to better explain the probable progression of diagenetic environments that moved through Te Aute limestones, concepts that classically underpin the sequence stratigraphic analysis of sedimentary successions have been used to group cement zones, revealed under cathodoluminescent (CL) light, into diagenetic discontinuity-bounded CL sequences. In most Pliocene limestones, the procedure, in combination with geochemical data, has: (1) identified recurring and correlative motifs of pre- and post-compaction CL sequence stratigraphies; (2) enabled regional mapping of CL sequences; (3) highlighted the spatial geometry and timing of emplacement of cementing aquifers; and (4) led to the construction of paleohydrologic models, invoking location of recharge areas and fluid-flow directions. Pre-compaction CL sequences formed locally from marine and more widely from oxidized meteoric (or mixed marine-meteoric) waters after exposure of limestone caps due to relative drops in sea level. Post-compaction burial CL sequences precipitated from reduced fluids recharged from areas of basin margin exposure, while the next CL sequences in the chronological suite are related to the ingress of oxidized meteoric waters as a result of uplift.

text: Petrogenesis of Pliocene limestones in southern Hawke’s Bay, New Zealand: a contribution to unravelling the sequence stratigraphy and diagenetic pathways of cool-water shelf carbonate facies Among carbonate sedimentologists, it is now appreciated that shallow-marine carbonates are not restricted to warm-water tropical settings, but can also be produced on modern cool-water shelves outside tropical latitudes. In the last two decades, many publications have not only advanced our knowledge about modern non-tropical carbonates, but have documented ancient skeletal limestones that are fossil analogues of these modern deposits. While the broad environmental and sedimentary characteristics of non-tropical deposits are becoming well established, the nature, timing, and relative importance of the various diagenetic processes affecting their evolution into limestones are yet to be fully appreciated, a topic addressed in the present study.

In New Zealand, the Plio-Pleistocene (5-1.5 Ma) Te Aute limestones in southern Hawke’s Bay (eastern North Island) are non-tropical in origin, and formed in shoal areas about the margins of a forearc basin seaway adjacent to the convergent Pacific-Australian Plate boundary. Because of the dynamic tectonic setting, which led to narrowing and shallowing of the host basin over time, the studied carbonates exhibit a variety of sedimentary facies (i.e., from pure coquina to mixed carbonate-siliciclastic) and mineralogical signatures (i.e., calcite- to, more rarely, aragonite-dominated), and have undergone varying degrees of burial before being uplifted to their present position. As such, the Te Aute deposits provide a unique opportunity to: (1) decipher the suite of processes that have altered their texture and composition through time; (2) address the questions of what likely regional and more global parameters, namely tectonics and eustasy, may have driven diagenetic pathways, and how this occurred; and (3) more generally advance fundamental knowledge about models of porosity evolution in non-tropical carbonates.

The present study integrates the combined results of detailed field investigations with a variety of analytical techniques, including standard, stained, and cathodoluminescence (CL) microscopic examinations of rock thin-sections, X-ray diffraction (XRD), and elemental (ICP-OES and microprobe) and stable isotope (δ¹⁸O and δ¹³C) geochemistry.

Sedimentologic analysis has identified 14 different facies types that correspond to a spectrum of depositional settings associated with both continent-attached shelves and isolated upthrust-cored antiforms within the forearc seaway. Facies are representative of environments ranging from below storm-wave base, in outer shelf-type locations, to predominantly shallow-water shoreface-type conditions under the influence of strong tidal current flows and storm waves.

A new petrographic approach for studying cool-water carbonate diagenesis has been developed. The method entails recording the high-resolution chronological occurrence of all diagenetic features within samples, and noting their vertical and lateral distribution within sedimentary formations. It is demonstrated that the attributes of pre-compaction diagenetic suites between consecutive sedimentary discontinuities change vertically in relation to the stacking of depositional facies. This original approach, by also revealing marine-cemented condensed horizons and key surfaces otherwise unrecognized in the field, has helped unlock the cryptic sedimentary architecture within the limestones. Thus, the Te Aute deposits appear to be constructed of metre-scale (1-15m) sequences that could be integrated into a sequence stratigraphic scheme, emphasizing that not only the production and deposition, but also the diagenesis of these carbonate deposits, were forced by glacio-eustatically and tectonically-driven processes.

In order to better explain the probable progression of diagenetic environments that moved through Te Aute limestones, concepts that classically underpin the sequence stratigraphic analysis of sedimentary successions have been used to group cement zones, revealed under cathodoluminescent (CL) light, into diagenetic discontinuity-bounded CL sequences. In most Pliocene limestones, the procedure, in combination with geochemical data, has: (1) identified recurring and correlative motifs of pre- and post-compaction CL sequence stratigraphies; (2) enabled regional mapping of CL sequences; (3) highlighted the spatial geometry and timing of emplacement of cementing aquifers; and (4) led to the construction of paleohydrologic models, invoking location of recharge areas and fluid-flow directions. Pre-compaction CL sequences formed locally from marine and more widely from oxidized meteoric (or mixed marine-meteoric) waters after exposure of limestone caps due to relative drops in sea level. Post-compaction burial CL sequences precipitated from reduced fluids recharged from areas of basin margin exposure, while the next CL sequences in the chronological suite are related to the ingress of oxidized meteoric waters as a result of uplift.

year: 2002

title: Producing near-net shape titanium alloy parts by metal injection moulding

authors: Gnanavinthan, Thavanayagam

subjects: Titanium

abstract: Metal injection moulding (MIM), is a metalworking process for producing complex parts from metal powders. It is often more cost effective to use MIM to make small, precise componentry from expensive materials such as titanium than traditional metal processes such as casting, forming, extrusion, investment casting and machining. The four processing steps in MIM are: formulating homogenous feedstock of metal power and binders, injection moulding, removing the binder (debinding) and sintering. The binder affects feedstock rheology, allowing the required shape to be moulded. It then needs to be removed so the object can be sintered to consolidate into a near-net shaped part. As MIM has many advantages, including low wastage of material and the ability to produce intricate shapes, it is used in sophisticated applications such as biomedical instruments, in the aerospace industry, and in niche applications that use expensive materials such as titanium. 

The objective of this research was to identify binders and processing methods that produced easily moulded titanium-based feedstock and final products with minimal defects. This involves understanding how factors such as binder composition, powder loading, and processing method affect feedstock rheology, homogeneity, debinding and sintering, and to develop a model for investigating feedstock rheology.

Feedstocks were manufactured by mixing different amounts of hydride-dehydride titanium alloy powder (HDH Ti-6Al-4V) with a three-component binder containing different amounts of polyethylene glycol (PEG), polyvinyl butyral (PVB) and stearic acid (SA). Most research on titanium powder metallurgy uses spherical, fine powders with mean diameters (d50) between 0.3 and 19 μm. This research used HDH Ti-6Al-4V, an irregular, coarse powder with a d50 of 52 μm and a d90 of 117 μm. The effect of using different types of mixers for various mixing times was evaluated by measuring mixing torque and thermal analysis. Feedstock homogeneity was evaluated by density measurements and DTA/TGA analysis, and mouldability was evaluated by rheological properties. After mixing and extrusion, binders were removed from the formed (green) part in a two-stage process involving aqueous debinding followed by thermal debinding. The part was then sintered to remove any remaining binder and to create the final product. Morphological characteristics of feedstocks, moulded, debound and sintered parts were assessed using optical and scanning electron microscopes.

Polyethylene glycol, PVB and SA were used as binder components because they are inexpensive and readily available. PEG is environmentally safe, easily extractable in water and reduces feedstock viscosity. The PVB is water-insoluble and provides strength to the formed (green) parts and debound (brown or grey) parts so used as a backbone component. The SA acts as a surfactant and helps wet the powder particles. Thermal characteristics were determined with DTA/TGA and rheological analyses with a capillary rheometer.

The four-stage process developed to manufacture homogeneous material suitable for titanium-MIM (Ti-MIM) involved dry-mixing irregular HDH Ti-6Al-4V powder and binder components in a planetary mixer for 30 min at room temperature, followed by 16 h further mixing at room temperature in a roller mixer rotating at 250 rpm, then melt-mixing at 125 ˚C for 45 min in a roller compounder. The final step involved extruding the processed feedstock in a twin-screw extruder operating with a feed-to-nozzle temperature profile of 125, 140, 135, 135, 140 ˚C.

Viscosity, density and mixing torque data for the PEG-PVB-SA binders indicated that the maximum (so-called critical) powder loading was 65 vol. %. Feedstocks made with 75:20:5 vol. % PEG:PVB:SA and 80:15:5 vol. % PEG:PVB:SA binders and 60 vol. % powder loading, were optimal for Ti64-MIM. This powder loading is higher than reported for spherical, fine powders. The flow behaviour index of the feedstock could be used to identify optimal powder loading and temperatures for injection moulding. A feedstock with low viscosity (<80 Pa-s), low flow activation energy (<30 kJ/mol), high mouldability index (15 – 22), high melt flow rate (up to 860 g/10 min), and high stability was selected for Ti-MIM.
 
Solvent debinding green parts made of 60 vol. % Ti64 powders and PEG-PVB-SA binders at 35 °C was affected by binder composition. Between 4.67 – 5.20 h, PEG removal changed from dissolution-controlled to diffusion-controlled. Increasing debinding temperature to 55 °C decreased the time at which the transition occurred. Debinding time affected PEG removal, porosity and reaction depth to a similar extent, indicating these parameters are inter-related. The empirical relationship describing solvent debinding kinetics of Ti-MIM parts showed PEG removal (weight loss %) was linearly related to the nth root of debinding time (n√t). This model can be used to predict optimum solvent debinding temperature and time. 

The maximum relative density, tensile strength and the oxygen content of as-sintered parts were 95%, 455 MPa and 1.58 wt. % respectively. Fracture surfaces exhibited transgranular fracture. The poor mechanical properties are mainly due to porosity and high oxygen content.

New rheological models to predict feedstock flow properties, relative viscosity and critical powder loading were developed and validated. An improved model for predicting relative viscosity and critical powder loading from powder loading, surface area, shear rate, and flow behaviour index is proposed.

The Einstein relationship for relative viscosity of dilute suspensions was modified and used for MIM feedstocks, which have a much higher powder loading.
 
Recommendations for further work are given.

text: Producing near-net shape titanium alloy parts by metal injection moulding Metal injection moulding (MIM), is a metalworking process for producing complex parts from metal powders. It is often more cost effective to use MIM to make small, precise componentry from expensive materials such as titanium than traditional metal processes such as casting, forming, extrusion, investment casting and machining. The four processing steps in MIM are: formulating homogenous feedstock of metal power and binders, injection moulding, removing the binder (debinding) and sintering. The binder affects feedstock rheology, allowing the required shape to be moulded. It then needs to be removed so the object can be sintered to consolidate into a near-net shaped part. As MIM has many advantages, including low wastage of material and the ability to produce intricate shapes, it is used in sophisticated applications such as biomedical instruments, in the aerospace industry, and in niche applications that use expensive materials such as titanium. 

The objective of this research was to identify binders and processing methods that produced easily moulded titanium-based feedstock and final products with minimal defects. This involves understanding how factors such as binder composition, powder loading, and processing method affect feedstock rheology, homogeneity, debinding and sintering, and to develop a model for investigating feedstock rheology.

Feedstocks were manufactured by mixing different amounts of hydride-dehydride titanium alloy powder (HDH Ti-6Al-4V) with a three-component binder containing different amounts of polyethylene glycol (PEG), polyvinyl butyral (PVB) and stearic acid (SA). Most research on titanium powder metallurgy uses spherical, fine powders with mean diameters (d50) between 0.3 and 19 μm. This research used HDH Ti-6Al-4V, an irregular, coarse powder with a d50 of 52 μm and a d90 of 117 μm. The effect of using different types of mixers for various mixing times was evaluated by measuring mixing torque and thermal analysis. Feedstock homogeneity was evaluated by density measurements and DTA/TGA analysis, and mouldability was evaluated by rheological properties. After mixing and extrusion, binders were removed from the formed (green) part in a two-stage process involving aqueous debinding followed by thermal debinding. The part was then sintered to remove any remaining binder and to create the final product. Morphological characteristics of feedstocks, moulded, debound and sintered parts were assessed using optical and scanning electron microscopes.

Polyethylene glycol, PVB and SA were used as binder components because they are inexpensive and readily available. PEG is environmentally safe, easily extractable in water and reduces feedstock viscosity. The PVB is water-insoluble and provides strength to the formed (green) parts and debound (brown or grey) parts so used as a backbone component. The SA acts as a surfactant and helps wet the powder particles. Thermal characteristics were determined with DTA/TGA and rheological analyses with a capillary rheometer.

The four-stage process developed to manufacture homogeneous material suitable for titanium-MIM (Ti-MIM) involved dry-mixing irregular HDH Ti-6Al-4V powder and binder components in a planetary mixer for 30 min at room temperature, followed by 16 h further mixing at room temperature in a roller mixer rotating at 250 rpm, then melt-mixing at 125 ˚C for 45 min in a roller compounder. The final step involved extruding the processed feedstock in a twin-screw extruder operating with a feed-to-nozzle temperature profile of 125, 140, 135, 135, 140 ˚C.

Viscosity, density and mixing torque data for the PEG-PVB-SA binders indicated that the maximum (so-called critical) powder loading was 65 vol. %. Feedstocks made with 75:20:5 vol. % PEG:PVB:SA and 80:15:5 vol. % PEG:PVB:SA binders and 60 vol. % powder loading, were optimal for Ti64-MIM. This powder loading is higher than reported for spherical, fine powders. The flow behaviour index of the feedstock could be used to identify optimal powder loading and temperatures for injection moulding. A feedstock with low viscosity (<80 Pa-s), low flow activation energy (<30 kJ/mol), high mouldability index (15 – 22), high melt flow rate (up to 860 g/10 min), and high stability was selected for Ti-MIM.
 
Solvent debinding green parts made of 60 vol. % Ti64 powders and PEG-PVB-SA binders at 35 °C was affected by binder composition. Between 4.67 – 5.20 h, PEG removal changed from dissolution-controlled to diffusion-controlled. Increasing debinding temperature to 55 °C decreased the time at which the transition occurred. Debinding time affected PEG removal, porosity and reaction depth to a similar extent, indicating these parameters are inter-related. The empirical relationship describing solvent debinding kinetics of Ti-MIM parts showed PEG removal (weight loss %) was linearly related to the nth root of debinding time (n√t). This model can be used to predict optimum solvent debinding temperature and time. 

The maximum relative density, tensile strength and the oxygen content of as-sintered parts were 95%, 455 MPa and 1.58 wt. % respectively. Fracture surfaces exhibited transgranular fracture. The poor mechanical properties are mainly due to porosity and high oxygen content.

New rheological models to predict feedstock flow properties, relative viscosity and critical powder loading were developed and validated. An improved model for predicting relative viscosity and critical powder loading from powder loading, surface area, shear rate, and flow behaviour index is proposed.

The Einstein relationship for relative viscosity of dilute suspensions was modified and used for MIM feedstocks, which have a much higher powder loading.
 
Recommendations for further work are given.

year: 2016

title: Ecological Survey of Tauranga Harbour

authors: Ellis, Joanne I.

abstract: This report summarises the results of biological and physical data collected from a broad scale intertidal survey of Tauranga Harbour conducted between December 2011 and February 2012. The survey was designed to understand more fully the role of various anthropogenic stressors on the ecology of the harbour. The research was conducted as part of the Manaaki Taha Moana (MTM) programme. The wider research project aims to restore and enhance coastal ecosystems and their services of importance to iwi/hapū, by working with iwi to improve knowledge of these ecosystems and the degradation processes that affect them.
In this report we assess the health of macrofaunal benthic communities (bottom-dwelling animals) as well as trends in sediments, nutrients and contaminants. The results indicate that the sites identified as most impacted were generally located in the upper reaches of estuaries in some of the locations least exposed to wind, waves and currents. In addition, the biological community composition characterising sites with different sediment textures, nutrient and contaminant loadings were found to vary. Sediments within Tauranga Harbour were predominantly sandy with the percentage of mud within a similar range as measured for other New Zealand estuaries. The exceptions included Te Puna Estuary and Apata Estuary, which experience higher rates of sedimentation.
Heavy metal contamination in sediments is often highly correlated with the percentage of mud content due to the adherence of chemicals to fine sediments and/or organic content. It is, therefore, not surprising that heavy metal concentrations were also highest in the depositional inner areas of the harbour, such as Te Puna Estuary. The heavy metal contaminant levels within Tauranga were well below relevant guideline thresholds and lower than concentrations measured in many other estuaries in New Zealand and overseas. Although the three metals recorded were found to be highly correlated, zinc levels tended to be closer to guideline thresholds for possible biological effects.
Sediment nutrient concentrations in the harbour tended to decline with distance from the inner harbour and associated rivers. Te Puna Estuary showed comparatively high nitrogen and phosphorus loadings. Comparison of sediment nutrient concentrations with other New Zealand estuaries indicates that the Tauranga Harbour sits within a range typical for slightly to moderately enriched estuaries. Although total phosphorus was low compared with other estuaries, total N:P ratios suggest Tauranga Harbour is still limited by nitrogen.
We developed a BHM using statistical ordination techniques to identify key stressors affecting the ‘health’ of macrofaunal communities. Sediments, nutrients and heavy metals were identified as key ‘stressors’, i.e. variables affecting the ecology of the harbour. Therefore, three multivariate models were developed based on the variability in community composition using canonical analysis of principal coordinates (CAP). The ecological assemblages generally reflected gradients of stress or pollution very well. However, the CAP models for sedimentation and contamination performed best. In general, the multivariate models were found to be more sensitive to changing ecological health than simple univariate measures (abundance, species diversity, evenness and Shannon-Wiener diversity). This finding has also been reported in the literature where univariate measures based on abundance and diversity were only able to detect significant differences between the most and least disturbed sites, but were not able to differentiate between smaller relative changes in ecological health. Hence univariate measures were less sensitive to smaller degradative changes in community composition. For Tauranga Harbour, ordination models based on community composition appear to be a more sensitive measure of ‘health’ along an ecological gradient and should enable long term degradative change from multiple disturbances to be assessed. This BHM approach can be used as a management or monitoring tool where sites are repeatedly sampled over time and tracked to determine whether the communities are moving towards a more healthy or unhealthy state.
The key species at ‘healthy’ and ‘impacted’ sites as determined from the CAP models were also identified. Species at ‘impacted’ sites can be considered to be tolerant to the stressor (i.e. sediment, nutrients or contaminants), while species with high abundances at only ‘healthy’ sites are sensitive to increasing stressors. We developed species response models for 20 taxa. Although the type of response differed by taxa and stressor, variation in the abundance of most of the taxa modelled was most likely to be better predicted by sedimentation. Unimodal responses were almost always observed in response to nutrients, while declines or skewed unimodal responses were most often observed in response to sedimentation and metals.
The results from this study are consistent with models of macrofaunal species occurrence with respect to sediment mud content developed across a range of New Zealand estuaries by Thrush et al. (2003). Within this report we extend this analysis by also developing models of macrofaunal species occurrence with respect to nutrient and contaminants loadings. Ultimately such statistical models provide a tool to forecast the distribution and abundance of species associated with habitat changes in sediments, nutrients and metals.
In conclusion, Tauranga Harbour is a predominantly sandy harbour with slight to moderate enrichment and low levels of heavy metal contaminants. Sites identified as most impacted by elevated sediments, heavy metal contaminants and nutrients were generally located in the upper reaches of estuaries in some of the least exposed locations. To some extent, this reflects the natural progression of an estuary from land to sea; however, the rates of accumulation of sediments and nutrients have been accelerated as a result of anthropogenic land-based activities. Sediments and contaminants were found to explain the largest variance in benthic communities. Species response models suggest that taxa were either sensitive to elevated sediments, nutrients loading or contamination at all levels, or sensitive to these stressors beyond a critical point.

text: Ecological Survey of Tauranga Harbour This report summarises the results of biological and physical data collected from a broad scale intertidal survey of Tauranga Harbour conducted between December 2011 and February 2012. The survey was designed to understand more fully the role of various anthropogenic stressors on the ecology of the harbour. The research was conducted as part of the Manaaki Taha Moana (MTM) programme. The wider research project aims to restore and enhance coastal ecosystems and their services of importance to iwi/hapū, by working with iwi to improve knowledge of these ecosystems and the degradation processes that affect them.
In this report we assess the health of macrofaunal benthic communities (bottom-dwelling animals) as well as trends in sediments, nutrients and contaminants. The results indicate that the sites identified as most impacted were generally located in the upper reaches of estuaries in some of the locations least exposed to wind, waves and currents. In addition, the biological community composition characterising sites with different sediment textures, nutrient and contaminant loadings were found to vary. Sediments within Tauranga Harbour were predominantly sandy with the percentage of mud within a similar range as measured for other New Zealand estuaries. The exceptions included Te Puna Estuary and Apata Estuary, which experience higher rates of sedimentation.
Heavy metal contamination in sediments is often highly correlated with the percentage of mud content due to the adherence of chemicals to fine sediments and/or organic content. It is, therefore, not surprising that heavy metal concentrations were also highest in the depositional inner areas of the harbour, such as Te Puna Estuary. The heavy metal contaminant levels within Tauranga were well below relevant guideline thresholds and lower than concentrations measured in many other estuaries in New Zealand and overseas. Although the three metals recorded were found to be highly correlated, zinc levels tended to be closer to guideline thresholds for possible biological effects.
Sediment nutrient concentrations in the harbour tended to decline with distance from the inner harbour and associated rivers. Te Puna Estuary showed comparatively high nitrogen and phosphorus loadings. Comparison of sediment nutrient concentrations with other New Zealand estuaries indicates that the Tauranga Harbour sits within a range typical for slightly to moderately enriched estuaries. Although total phosphorus was low compared with other estuaries, total N:P ratios suggest Tauranga Harbour is still limited by nitrogen.
We developed a BHM using statistical ordination techniques to identify key stressors affecting the ‘health’ of macrofaunal communities. Sediments, nutrients and heavy metals were identified as key ‘stressors’, i.e. variables affecting the ecology of the harbour. Therefore, three multivariate models were developed based on the variability in community composition using canonical analysis of principal coordinates (CAP). The ecological assemblages generally reflected gradients of stress or pollution very well. However, the CAP models for sedimentation and contamination performed best. In general, the multivariate models were found to be more sensitive to changing ecological health than simple univariate measures (abundance, species diversity, evenness and Shannon-Wiener diversity). This finding has also been reported in the literature where univariate measures based on abundance and diversity were only able to detect significant differences between the most and least disturbed sites, but were not able to differentiate between smaller relative changes in ecological health. Hence univariate measures were less sensitive to smaller degradative changes in community composition. For Tauranga Harbour, ordination models based on community composition appear to be a more sensitive measure of ‘health’ along an ecological gradient and should enable long term degradative change from multiple disturbances to be assessed. This BHM approach can be used as a management or monitoring tool where sites are repeatedly sampled over time and tracked to determine whether the communities are moving towards a more healthy or unhealthy state.
The key species at ‘healthy’ and ‘impacted’ sites as determined from the CAP models were also identified. Species at ‘impacted’ sites can be considered to be tolerant to the stressor (i.e. sediment, nutrients or contaminants), while species with high abundances at only ‘healthy’ sites are sensitive to increasing stressors. We developed species response models for 20 taxa. Although the type of response differed by taxa and stressor, variation in the abundance of most of the taxa modelled was most likely to be better predicted by sedimentation. Unimodal responses were almost always observed in response to nutrients, while declines or skewed unimodal responses were most often observed in response to sedimentation and metals.
The results from this study are consistent with models of macrofaunal species occurrence with respect to sediment mud content developed across a range of New Zealand estuaries by Thrush et al. (2003). Within this report we extend this analysis by also developing models of macrofaunal species occurrence with respect to nutrient and contaminants loadings. Ultimately such statistical models provide a tool to forecast the distribution and abundance of species associated with habitat changes in sediments, nutrients and metals.
In conclusion, Tauranga Harbour is a predominantly sandy harbour with slight to moderate enrichment and low levels of heavy metal contaminants. Sites identified as most impacted by elevated sediments, heavy metal contaminants and nutrients were generally located in the upper reaches of estuaries in some of the least exposed locations. To some extent, this reflects the natural progression of an estuary from land to sea; however, the rates of accumulation of sediments and nutrients have been accelerated as a result of anthropogenic land-based activities. Sediments and contaminants were found to explain the largest variance in benthic communities. Species response models suggest that taxa were either sensitive to elevated sediments, nutrients loading or contamination at all levels, or sensitive to these stressors beyond a critical point.

year: 2017