Tauranga Harbour Research Articles

Modelling extreme water levels using intertidal topography and bathymetry derived from multispectral satellite images

Abstract. Topographic and bathymetric data are essential for accurate predictions of flooding in estuaries because water depth and elevation data are fundamental components of the shallow-water hydrodynamic equations used in models for storm surges and tides. Where lidar or in situ acoustic surveys are unavailable, recent efforts have centred on using satellite-derived bathymetry (SDB) and satellite-derived topography (SDT). This work is aimed at (1) determining the accuracy of SDT and (2) assessing the suitability of the SDT and SDB for extreme water level modelling of estuaries. The SDT was created by extracting the waterline as it tracks over the topography with changing tides. The method was applied to four different estuaries in Aotearoa / New Zealand: Whitianga, Maketū, Ōhiwa and Tauranga harbours. Results show that the waterline method provides similar topography to the lidar with a root-mean-square error equal to 0.2 m, and it is slightly improved when two correction methods are applied to the topography derivations: the removal of statistical bias (0.02 m improvement) and hydrodynamic modelling correction of waterline elevation (0.01 m improvement). The use of SDT in numerical simulations of surge levels was assessed for Tauranga Harbour in eight different simulation scenarios. Each scenario explored different ways of incorporating the SDT to replace the topographic data collected using non-satellite survey methods. In addition, one of these scenarios combined SDT (for intertidal zones) and SDB (for subtidal bathymetry), so only satellite information is used in surge modelling. The latter SDB is derived using the well-known ratio–log method. For Tauranga Harbour, using SDT and SDB in hydrodynamic models does not result in significant differences in predicting high water levels when compared with the scenario modelled using surveyed bathymetry.

An ephemeral sea star (Coscinasterias muricata) wasting event at Tauranga, New Zealand

ABSTRACT During late December 2018 a sea star wasting event occurred at Pilot Bay, Tauranga Harbour, North Island/Te Ika-a-Māui, Aotearoa New Zealand. Forty-seven specimens of the asteroid Coscinasterias muricata Verrill, 1867, with a maximum diameter of 50 - 150 mm, were found in varying states of wasting on and between rocks at low tide over an area of ∼70 m at the NW end of Pilot Bay. The wasting event followed a brief period of heavy rain (97.8 mm during the previous 24 h) and was short-lived, with no affected sea stars observed after three days. The affected C. muricata may have had low resilience to changes in water quality, particularly salinity. However, the cause of this wasting event remains unresolved. These opportunistic data suggest that small scale sea star wasting events of this nature may be easily overlooked due to their ephemeral nature. urn:lsid:zoobank.org:pub:C60C26A8-10D7-436D-B2F2-C547DD84BAD8

Total organic carbon estimation in seagrass beds in Tauranga Harbour, New Zealand using multi-sensors imagery and grey wolf optimization

Estimation of carbon stock in seagrass meadows is in challenges of paucity of assessment and low accuracy of the estimates. In this study, we used a fusion of the synthetic aperture radar (SAR) Sentinel-1 (S-1), the multi-spectral Sentinel-2 (S-2), and coupled this with advanced machine learning (ML) models and meta-heuristic optimization to improve the estimation of total organic carbon (TOC) stock in the Zostera muelleri meadows in Tauranga Harbour, New Zealand. Five scenarios containing combinations of data, ML models (Random Forest, Extreme Gradient Boost, Rotation Forest, CatBoost) and optimization were developed and evaluated for TOC retrieval. Results indicate a fusion of S1, S2 images, a novel ML model CatBoost and the grey wolf optimization algorithm (the CB-GWO model) yielded the best prediction of seagrass TOC (R 2, RMSE were 0.738 and 10.64 Mg C ha−1). Our results provide novel ideas of deriving a low-cost, scalable and reliable estimates of seagrass TOC globally.

Understanding the consequences of sea level rise: the ecological implications of losing intertidal habitat

ABSTRACT Sea level rise (SLR) has been described as one of the greatest potential causes of ecosystem disruption, putting many coastal areas at risk of irreversible changes. However, the loss of intertidal areas from SLR and the associated ecological and social repercussions receive little attention. Within estuaries, extensive intertidal areas harbour a variety of habitats and communities and represent hotspots of ecosystem functions. Any changes to their distribution or extent are likely to have far reaching implications. Here we summarise the ecological consequences of a reduction in intertidal area from increasing SLR, and the implications for people, management and planning. To facilitate this discussion, changes in the occurrence and abundance of two ecologically and culturally important intertidal shellfish species (Austrovenus stutchburyi and Macomona liliana) were modelled under different SLR scenarios for Tauranga Harbour, Aotearoa New Zealand. We highlight how the squeezing of intertidal areas will likely alter the distribution and extent of key habitats and communities, and discuss the implications for coastal food webs, ecosystem functioning and service provision. Pre-emptive planning and adaptive management are needed that incorporate ecological losses in risk assessments and focuses on pro-active solutions to increase resilience to the effects of SLR.

Future Changes in Built Environment Risk to Coastal Flooding, Permanent Inundation and Coastal Erosion Hazards

Sea-level rise will cause erosion of land, deeper and increasingly frequent flooding and will eventually permanently inundate low-elevation land, forcing the adaptation of seaside communities to avoid or reduce risk. To inform adaptation planning, we quantified the effects of incremental relative sea-level rise (RSLR) on exposed land area, number and replacement value of buildings within Tauranga Harbour, New Zealand. The assessment compared three coastal hazards: flooding, permanent inundation and erosion. Increasingly frequent coastal flooding will be the dominant trigger for adaptation in Tauranga. In the absence of adaptation, coastal flooding, recurring at least once every 5 years on average, will overtake erosion as the dominant coastal hazard after about 0.15–0.2 m RSLR, which is likely to occur between the years 2038–2062 in New Zealand and will rapidly escalate in frequency and consequence thereafter. Coastal erosion will remain the dominant hazard for the relatively-few properties on high-elevation coastal cliffs. It will take 0.8 m more RSLR for permanent inundation to reach similar impact thresholds to coastal flooding, in terms of the number and value of buildings exposed. For buildings currently within the mapped 1% annual exceedance probability (AEP) zone, the flooding frequency will transition to 20% AEP within 2–3 decades depending on the RSLR rate, requiring prior adaptive action. We also compared the performance of simple static-planar versus complex dynamic models for assessing coastal flooding exposure. Use of the static-planar model could result in sea level thresholds being reached 15–45 years earlier than planned for in this case. This is compelling evidence to use dynamic models to support adaptation planning.

Detecting Multi-Decadal Changes in Seagrass Cover in Tauranga Harbour, New Zealand, Using Landsat Imagery and Boosting Ensemble Classification Techniques

Seagrass provides a wide range of essential ecosystem services, supports climate change mitigation, and contributes to blue carbon sequestration. This resource, however, is undergoing significant declines across the globe, and there is an urgent need to develop change detection techniques appropriate to the scale of loss and applicable to the complex coastal marine environment. Our work aimed to develop remote-sensing-based techniques for detection of changes between 1990 and 2019 in the area of seagrass meadows in Tauranga Harbour, New Zealand. Four state-of-the-art machine-learning models, Random Forest (RF), Support Vector Machine (SVM), Extreme Gradient Boost (XGB), and CatBoost (CB), were evaluated for classification of seagrass cover (presence/absence) in a Landsat 8 image from 2019, using near-concurrent Ground-Truth Points (GTPs). We then used the most accurate one of these models, CB, with historic Landsat imagery supported by classified aerial photographs for an estimation of change in cover over time. The CB model produced the highest accuracies (precision, recall, F1 scores of 0.94, 0.96, and 0.95 respectively). We were able to use Landsat imagery to document the trajectory and spatial distribution of an approximately 50% reduction in seagrass area from 2237 ha to 1184 ha between the years 1990–2019. Our illustration of change detection of seagrass in Tauranga Harbour suggests that machine-learning techniques, coupled with historic satellite imagery, offers potential for evaluation of historic as well as ongoing seagrass dynamics.

He waka eke noa/we are all in the same boat: A framework for co-governance from aotearoa New Zealand

Societal values are crucial for the prioritisation of activities within the marine environment and are therefore fundamental to successfully implementing Ecosystem Based Management (EBM) in New Zealand (NZ). Documenting Social Values (Mauri Moana) is a research project within the Sustainable Seas National Science Challenge (NSC) that is exploring how societal values for the marine environment like intrinsic and recreational along with Māori values are integrated into EBM.Using a case study approach, we explore how Māori values are catered for within planning approaches and institutions for Tauranga Moana (Tauranga Harbour). Through our earlier work in the Sustainable Seas NSC, we developed a framework (He Waka Taurua) for a NZ marine management system that was informed by EBM and mātauranga Māori (Māori knowledge). The current study further elaborates on the components that form a proposed NZ marine management system based on the Tauranga Moana case study. This framework provides a useful approach for informing how co-governance institutions can manage the different values and aspirations of Māori, and local communities with government institutions to support healthy oceans and healthy people.

Description of two new species of Dysidea (Porifera, Demospongiae, Dictyoceratida, Dysideidae) from Tauranga Harbour, Bay of Plenty, New Zealand.

Differentiation of species within the genus Dysidea Johnston, 1842 (Order Dictyoceratida Minchin, 1900, Family Dysideidae Gray, 1867) is extremely difficult as they lack spicules which are strongly diagnostic in other Demospongiae, and their primary and secondary fibres and the mesh that they form, may be irregular in shape and thickness, thus difficult to measure for comparisons. Here we review species of Dysidea known from the New Zealand Exclusive Economic Zone (EEZ), validating five species: Dysidea cristagalli Bergquist, 1961a, from the Hauraki Gulf; D. hirciniformis (Carter, 1885a) sensu Dendy (1924), from North Cape; D. navicularis Lendenfeld, 1888, from Port Lyttleton on the east coast of the South Island; D. ramsayi (Lendenfeld, 1888) from the Chatham Islands; D. spiculivora Dendy, 1924, from Cape Maria Van Diemen and the Three Kings Islands to the north of New Zealand. Dysidea fragilis (Montagu, 1818) sensu Bergquist (1961b), from Mernoo Bank on Chatham Rise, is now considered to be invalid, and D. elegans (Nardo, 1847) sensu Brøndsted (1927), from the Coromandel Peninsula, is considered unrecognisable. Several partially characterised species have also been cited in the literature. Two new species from Tauranga Harbour, on the northeast coast of the North Island, Dysidea tuapokere sp. nov. and D. teawanui sp. nov., are described. These descriptions are based on fresh material and in situ photography, facilitating clear, informative descriptions, that will enable ease of identification of these species in the future.

Shrinkage test of concrete: Methodology and variation of strain

Evaluation of shrinkage strain in long concrete structures such as prestressed concrete box girder bridges is very important for the design of such structures. A wrong evaluation may contribute to loss of prestressing force, excessive deflections, cambers and cracking of the structure. It may also affect the durability and serviceability of the structure. The value of shrinkage strain is required during design, far before the construction process. However, realistic evaluation is still a difficult problem because shrinkage strain is influenced by the concrete composition and environmental factors such as humidity and temperature. Therefore, to gain the best evaluation, an experimental program was conducted to measure the shrinkage strain of concrete versus concrete age. Several concrete specimens were collected during pouring of several box girder segments of Tauranga Harbour Link, New Zealand. Variation of shrinkage strain versus concrete age for almost two years of data collection was plotted.

Porewater nutrient enrichment alters benthic-pelagic coupling on intertidal sandflats

With the intensification of urban development, agriculture, and forestry, many coastal waters are becoming enriched with nitrogen (N). Long term, excess terrestrial N accumulates in the sediment porewaters impacting benthic fauna and microphytobenthic productivity, altering the complex bio-geophysical interactions that occur on the seafloor. These interactions may influence sediment erodibility and resuspension rates on the seafloor, which have wider ecosystem consequences through changes in water column turbidity, the fluxes of organic matter and sediment porewater nutrients into the water column, and the redistribution of deposited terrestrial materials (i.e. pollutants and sediment). We conducted a field experiment to explore how bio-geophysical interactions change in the context of porewater N enrichment. The experiment enriched plots (4 m2) on an intertidal sand flat in Tauranga Harbour, New Zealand, with three concentrations of N fertiliser (0, 150 and 600 g N m−2; n = 6). After 10 weeks, we collected intact sediments from the plots, and in the laboratory, subjected these sediments to sequential increases in water flow velocity (between 0.05 and 0.4 m s−1) using annular flumes (bed area = 0.17 m−2). We calculated sediment erosion rates and thresholds, measured fluxes of NH4+ and Chlorophyll a across the sediment-water interface and characterised the sediment properties and macrofaunal communities. We anticipated complex feedbacks to occur where porewater N enrichment would increase microphytobenthic biofilm growth, stabilising the sediments (and reducing benthic-pelagic coupling), but at the same time, N enrichment might also alter the macrofaunal community, which graze the microphytobenthos and physically mix the sediment, with a potential destabilising effect (and increase in benthic-pelagic coupling). Although our enriched treatments doubled the biomass of microphytobenthos, altered the macrofauna communities and enhanced the fluxes of Chlorophyll a and NH4+ across the sediment-water interface, sediment erosion thresholds and rates were unaffected. Our results highlight a potential mismatch between the micro- and macro-biotic processes and the larger scale physical forces on dynamic intertidal sandflats, and this mismatch has implications for our ability to scale up small scale processes to entire ecosystems.

The links between entrance geometry, hypsometry and hydrodynamics in shallow tidally dominated basins

AbstractGeomorphological characteristics of tidal basins control hydrodynamics and sediment transport potential within such basins, for example, by adjusting the balance in tidal asymmetry. In this study we examine the effects of entrance geometry on tidal velocity asymmetry, slack water asymmetry, bed shear stress patterns and hypsometric profile shapes by comparison of six shallow meso‐tidal basins of Tauranga Harbour, New Zealand. Numerical model results show how tidal distortion increases with distance from a basin entrance. A simple ratio between basin width and entrance width defines levels of basin dilation. Sub‐basins with a constricted geometry and deep entrance channels are associated with small bed shear stress values and high rates of flood‐directed tidal velocity asymmetry in the sheltered basin centres, indicating a large potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins is weakly ebb‐directed, indicating a small potential for transport of fine sediments out of the basins. The constricted depositional basins are characterized by convex hypsometric profiles with elevated intertidal regions. Unconstricted geometries are associated with larger bed shear stress values and more ebb‐directed tidal velocity asymmetry within basin centres, suggesting limited potential for overall sediment deposition. The slack tide duration asymmetry is weakly flood‐dominant indicating that limited input of fine sediment into the basins is possible. The comparatively high‐energy conditions within these exposed basins are associated with a less convex hypsometric intertidal profile. The ability to estimate tidal asymmetries is advantageous when developing management strategies related to ecosystem functioning, navigability or coastal protection in specific geomorphic settings. © 2019 John Wiley & Sons, Ltd.

Light penetration in a temperate meso-tidal lagoon: Implications for seagrass growth and dredging in Tauranga Harbour, New Zealand

Benthic plants such as seagrasses rely on light availability, which is controlled by light attenuation in the water column. Elevated suspended sediment within estuaries strongly influences light attenuation and is governed by the recent history of natural events and human activities. To determine the constituents controlling light penetration (quantified by the irradiance attenuation coefficient, Kd(PAR)), we measured irradiance profiles, suspended particulate matter, chlorophyll-a and coloured dissolved organic matter (CDOM) in a barrier-enclosed estuary in New Zealand, subjected to dredging activity. These discrete measurements were then used to relate a continuous records of turbidity to potential light availability at multiple sites within the estuary. To do this, we use a regression model to enable turbidity measurements to be used as a proxy to estimate Kd(PAR). Measured Kd(PAR) ranged from 0.16 m-1 to 0.98 m-1 with overall average of 0.40 m-1, while Kd(PAR) inferred from continuous turbidity (which included many more events) reached higher values with average of 0.63 m-1. Continuous measurements of turbidity taken around the harbour dredging sites during dredging were slightly higher at some sites and slightly lower at others (∼2 mg l−1) compared to background measurements, indicating dredging had no consistent effect on conditions. Variations in Kd(PAR) were explained mostly by suspended particulate matter (accounting for 38%–50% of variance), chlorophyll-a (explaining 25%–33%) and to a lesser extent by coloured dissolved organic matter (CDOM explaining 10%–28%). Inferred benthic light availability in the intertidal zone when immersed ranged from 28% to 76% of surface irradiance, suggesting that light availability in the intertidal zone is not limiting seagrass photosynthesis but that growth in subtidal areas such as channels may be impacted. Results, inferred from continuous turbidity measurements, demonstrate the critical importance of including storm events in monitoring to develop robust limits on light attenuation for management.

Biomass-dependent seagrass resilience to sediment eutrophication

Seagrass beds provide a wealth of ecosystem services that benefit society (e.g., habitat and feeding ground for juvenile fisheries species), but the Anthropocene has seen to a global decline of these productive habitats. Many temperate estuaries are becoming eutrophic due to horticultural, agricultural, and urban nutrient run off, but the role of this enrichment in seagrass decline is not fully understood. In a multi-site manipulative field experiment (Tauranga Harbour, New Zealand; 37°S, 176°E), we elevated pore water nitrogen (N) concentrations (mimicking the consequences of long-term eutrophication) to examine effects on seagrass meadows. At six intertidal seagrass sites with differing sediment properties and macrofaunal communities, slow release urea fertiliser (200 g N m−2) was buried in 1 m2 plots at the start of the peak growing season (early summer). After 60 d, we measured several seagrass morphological variables (cover, leaf length and width, and above and below ground biomass), sediment properties, and macrofauna community structure. Results demonstrate that the resilience of seagrass meadows to N enrichment is highly site-dependent. Two of the six sites showed significant declines in a multivariate indicator of seagrass morphology, driven by marked reductions in seagrass cover and leaf length (of up to 78%). Whereas, other sites appeared resilient to N enrichment. It was expected that these site-specific responses would be correlated with changes in sediment properties that alter nutrient processing capacity (permeability and biogeochemistry). However, site-specific responses were instead correlated with the ambient seagrass biomass and macrofaunal diversity. Sites with low ambient seagrass biomass and macrofaunal diversity were less resilient to enrichment. These results highlight that seagrass biomass could be a good indicator of resilience to nutrient enrichment, and the biomass where resilience is lost may lie between 140 and 285 g DW m−2. This study contributes knowledge that is required for predicting and mitigating future impacts of estuarine eutrophication on seagrass ecosystems.

Recovery of the New Zealand muricid dogwhelk Haustrum scobina from TBT-induced imposex

Levels of imposex in the muricid dogwhelk Haustrum scobina (Quoy & Gaimard, 1833) were assessed in two major New Zealand ports between 2015 and 2017, 12+years after the banning of TBT-based antifouling paints. In the 1990s imposex was common adjacent to port facilities and marinas in Waitemata and Tauranga Harbours, and several populations were found to be largely sterile and in decline. By 2015–17 imposex was largely absent from Tauranga Harbour and considerably reduced in Waitemata Harbour. In Waitemata Harbour imposex remained in areas adjacent to hull cleaning facilities and marinas, but at low levels and frequencies. These data highlight the success of the 2003 banning of TBT-based antifouling paints in reducing levels of imposex in New Zealand ports. However, these reductions in antifouling chemicals and subsequent population recoveries may have had unforseen effects within these harbours including changes in community structure and the arrival of non-indigenous species.

Holocene evolution of the southern Tauranga Harbour

ABSTRACTTauranga Harbour is a major economic centre and home to New Zealand’s largest port. Despite the importance of the harbour basin, and especially of the shipping channel, the subsurface stratigraphy of this area has not been studied in detail. We use sediment cores retrieved in support of a recent dredging campaign to characterise the subsurface stratigraphy and interpret the sediments in terms of paleoenvironment. Four main facies (volcanic sand, lithic silt, coarse sand and pumiceous sand) were identified using textural, mineralogical and geochemical characteristics. They represent distinct depositional environments at different stages of the Holocene marine transgression (HMT). The oldest facies, deposited prior to the HMT (volcanic sand and lithic silt facies), represent the channel and overbank deposits of an aggrading river system with rapidly migrating channels. During the deposition of this facies, a change in sediment source is evident from geochemical data. This preceded a transition to a predominantly offshore sediment source (coarse sand facies), likely supplied by longshore drift from the Kaituna River, which was associated with the development of Mt Maunganui tombolo and Matakana barrier island. During the Holocene stillstand, the barrier island and tombolo restricted sediment input from offshore, resulting in the deposition of a predominantly locally derived unit (pumiceous sand facies), which still dominates sedimentation in the harbour today.

Submarine Groundwater Discharge Estimates Using Radium Isotopes and Related Nutrient Inputs into Tauranga Harbour (New Zealand)

Land-based pollutants such as fertilizers and wastewater can infiltrate into aquifers and discharge into surrounding coastal water bodies as submarine groundwater discharge (SGD). Oceanic islands, with a large coast length to land area ratio, may be hot spots of SGD into the global ocean. Although SGD may be a major pathway of dissolved nutrients, carbon and metals to coastal waters, studies have been limited due to the difficulties in measuring this often diffuse process. This study used radium isotopes (223Ra, 224Ra, 226Ra) to investigate SGD and the associated fluxes of nutrients into Tauranga Harbour, New Zealand. We calculated the apparent water mass ages of the harbour to be between ~4.1 and 7.8 days, which was similar to a previous numerical model of ~2–8 days. A 226Ra mass balance was constructed to quantify SGD fluxes at the harbour scale. A minimum SGD flux rate of 0.53 cm day−1 was calculated by using the maximum groundwater end-member value from 22 sample sites. However, using the geometric mean from these samples as a representative end-member, a final value of 2.83 cm day−1 or a flux of 3.09 × 106 m3 day−1 was calculated. These values were between ~1 and 2.8 times greater than all the major river and creeks discharging into the harbour during the sampling period. Due to the higher observed nutrient concentrations in groundwater, the SGD-derived dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and total dissolved phosphorus (TDP) fluxes were calculated to be 1.07, 0.87 and 0.05 mmol m2 day−1, respectively. These SGD inputs were ~5 times (for nitrogen) and ~8 times (for phosphorus) greater than the input from surrounding rivers and streams. The average N:P ratio in groundwater samples was 36:1 (which was greatly in excess of the Redfield ratio of 16). The harbour water had a N:P ratio of ~17:1. A positive relationship between radium isotopes and N:P ratios in the harbour further supported the hypothesis that SGD can have major implications for primary production, including recurrent algal bloom events which occur in the harbour. We suggest SGD as a major driver of nutrient dynamics in Tauranga Harbour and potentially other similar coastal lagoon systems and estuaries on oceanic islands.

Sediment dynamics of an artificially deepened mesotidal coastal lagoon: An environmental magnetic investigation of Tauranga Harbour, New Zealand

Tauranga Harbour, New Zealand's largest barrier-enclosed coastal lagoon, comprises two sub-basins with separate catchments, inlets and tidal channel systems. This study sets out to assess and investigate the sediment dispersal system of Tauranga Harbour using standard environmental magnetic and sedimentological methods. Compilations of rock magnetic and grain size data of surficial sediments collected from inflowing rivers, various estuarine environments (estuaries, tidal flats and tidal channels) and the adjacent nearshore mirror the net and differential sediment fluxes into and through the two sub-basins of this lagoon. For all studied depositional environments, the magnetogranulometric ratios SIRM/κ and ARM/κ are positively correlated with magnetic mineral content (SIRM, ARM, χ) in the sense that larger magnetic particles are associated with higher magnetic enrichment. Grain-size analyses show that magnetic enrichment during particle transport and deposition can result from grain-size as well as from grain-density fractionation. The periodically changing accumulation/erosion conditions provide for a selective retention of specific grain sizes and an enhancement of the heavier magnetic mineral particles. Magnetic crystal size and clastic grain size correlate poorly over the whole study area, but group within similar depositional environments. Coarser magnetic and clastic grain sizes and higher magnetite enrichment in the southern sub-basin can be reconciled with episodic flood runoff of the Wairoa River and a much larger, artificially deepened southern tidal inlet which likely enables higher tidal current velocities. Our approach of combining magnetic and clastic grain-size could be successfully implemented to establish a conceptual model of sediment dynamics and gravitational sorting within Tauranga Harbour.

Subseafloor Investigation of Sediments at Southern Tauranga Harbour, New Zealand, before Capital Dredging

ABSTRACT Jorat, M.E.; Moon, V.G.; Hepp, D.A.; Kreiter, S.; de Lange, W.P.; Feldmann, S., and Morz, T., 2017. Subseafloor investigation of sediments at southern Tauranga Harbour, New Zealand, before capital dredging. The Port of Tauranga plays a key role in New Zealand's export–import industry, and capital dredging commenced in October 2015 to extend the shipping channels to accommodate larger container vessels. This study investigated two-dimensional and three-dimensional subsurface estuarine sediment stratigraphy to predict the sedimentological conditions encountered during dredging operations to ensure that appropriate dredging methodologies were used to minimise the generation of turbidity. Eight cone penetration tests (CPTs), 14 core descriptions, and a high-resolution seismic investigation in Stella Passage (the main shipping channel of Tauranga Harbour) provided the basis for this research. Six major units comprise the stratigraphy; in ascending order they are lower pumiceous sand and silt (UNIT6), ...

Salinity and temperature tolerance of the invasive alga Undaria pinnatifida and native New Zealand kelps: Implications for competition

Invasive species are generally believed to be more tolerant to varying abiotic conditions than native species. Here, we report the combined effect of temperature (5, 15, 20 and 25 °C) and salinity (33, 24, 18, 12 and 6 SA) on the performance of the invasive kelp Undaria pinnatifida and two native kelp species (Lessonia variegata, Ecklonia radiata) from Tauranga Harbour, New Zealand (37°38′S, 176°10′E, 2014). Vegetative blade discs were exposed to temperature and salinity treatments in a 10-day laboratory experiment, and the physiological response was assessed employing photosynthetic (Fv/Fm, ETRmax) as well as biochemical parameters (chlorophyll a, xanthophylls and antioxidant pool size). U. pinnatifida sustained a high photosynthetic quantum yield in most treatments, with a negative synergistic effect on photosynthetic yield observed at 25 °C and low salinities (12, 6 SA). E. radiata died in salinities below 18 SA, except at 5 °C and L. variegata was highly susceptible to elevated temperatures (20, 25 °C). Antioxidant pool size showed species-specific responses to the experimental conditions, being most resilient in U. pinnatifida. Overall, U. pinnatifida displayed broader tolerance to the experimental salinity and temperature conditions than native kelps. The abilities to cope with a wide range in abiotic factors and to thrive in estuarine conditions might contribute to higher competitive strength compared to native kelps leading to its invasion success, especially with regard to ocean warming.

Validation and limitations of a cumulative impact model for an estuary

Comprehensive marine management approaches, such as ecosystem based management and marine spatial planning, would benefit from quantitative, spatially explicit estimates of the cumulative impact of human activities on marine ecosystems. In this study, a method to map and quantify cumulative impact was applied to estimate the combined impact of multiple stressors on Tauranga Harbour, a large estuary in New Zealand. The impact of eight stressors on seven ecosystems was assessed at a 100 m resolution, using New Zealand-specific expert judgement on the vulnerability of different ecosystems to each stressor. Estimated cumulative impact tended to be highest in the southern basin and inner estuaries, corresponding with sensitive ecosystems and multiple stressors and reflecting what is known about the distribution of pressures in Tauranga Harbour. Using benthic community data as an independent estimate of ecological condition, we had the novel opportunity to validate cumulative impact predicted from the model. Only a weak relationship was found between the estimated cumulative impact and measured ecological condition and several reasons for this are considered. Substitution of a more realistic sediment layer improved model outputs, highlighting the importance of accurate input data, particularly for stressors or ecosystems with high impact weights. Different standardisation methods did not greatly affect the spatial distribution of cumulative impact patterns in the harbour. The study highlights some fundamental issues for consideration when using this cumulative impact mapping approach, such as the importance of involving the expert panel throughout the course of the study and the availability and quality of the data used to construct the model.