Marine Sciencehttp://hdl.handle.net/10523/952024-03-28T17:12:47Z2024-03-28T17:12:47ZDrivers of population dynamics and fishery status of blackfoot pāua (Haliotis iris)Ryder, Finn Joelhttp://hdl.handle.net/10523/166502024-03-18T14:40:31Z2024-03-17T20:06:43ZDrivers of population dynamics and fishery status of blackfoot pāua (Haliotis iris)
2024
Ryder, Finn Joel
Drivers of coastal fishery status and population dynamics are complex due to the broad range of stressors to which coastal species are subjected. For successful fisheries management it is essential to understand drivers of population variability so that management can be adjusted accordingly. The population dynamics, and individual traits of abalone (Haliotids) can vary on the scale of metres to kilometres. In New Zealand, the cultural keystone, pāua (Haliotis iris, blackfoot abalone) has undergone considerable stock declines, and stock enhancement efforts have had limited success. For fisheries managers, it is important to know how a population of interest has changed over time, what factors may influence a populations spatial distribution and if they can use this information to adjust management. Therefore, the aim of this thesis is to disentangle drivers of fishery status and population dynamics of H. iris. First, I examined how a H. iris population in Peraki Bay, Banks Peninsula changed after 45 years. Comprehensive historic population data from the 1970s provided a baseline of abundance, size structure, and population dynamics before exposure to a commercial fishery and a range of environmental stressors. Repeat monitoring of the H. iris population at a 45-year interval indicate that a large change in population size and structure had occurred. Since 1976, there has been an 84% decrease in estimated population size. The remaining population is restricted to where juveniles were found in the original survey. This chapter indicates that H. iris may be susceptible to multiple stressors including habitat loss, climate change, and low and inconsistent recruitment. I then examined spatial drivers of H. iris density and shell length using multiple regression modelling, and generalised linear modelling. Physical and biological habitat and fishing restrictions data were gathered across six Customary fishery Protection Areas (CPAs, Mātaitai Reserves and Taiāpure Local Fisheries) across the takiwā (tribal area) of the Te Waipounamu (South Island) iwi (tribe) Ngāi Tahu. Model averaging indicate that H. iris density and size are driven by different factors. Haliotis iris density was negatively correlated to time under fishing restrictions, and macroalgal density. Shell length of H. iris was larger in deep habitats and negatively related to the density of the gastropod, Cookia sulcata. Spatial drivers were then investigated further in the East Otago Taiāpure (EOT), a CPA in southern Te Waipounamu. In-situ and modelled wave data, and drift algae biomass were used to determine whether H. iris reach higher densities and larger shell lengths in more wave exposed environments. Principal component regression and bootstrapping indicate that H. iris density was positively related to wave bottom orbital velocity, but negatively related to days exposed to large wave events. No relationship was observed between drift algae biomass and density or mean shell length of H. iris. Lastly, I investigated the depth distribution of H. iris in the EOT in light of a proposal by the EOT Management Committee to harvest H. iris at wading depths. Wading-only harvest involves gathering in shallow waters (depths up to around knee deep) without the use of a mask and snorkel. The reasoning behind the proposal being that a wading-only fishery could keep the fishery open while providing protection for populations in deeper water. The abundance and shell length of H. iris were recorded at 0.1 m depth intervals using belt transects. Provided a wadable limit of 0.5 metres is set, 78% of the H. iris surveyed would be accessible to wading harvest at chart datum, although this percentage differed among sites from 55 – 99%. Shell length increased with depth, indicating that a depth refuge would provide protection to mature individuals. This study indicates that some sites may be suited to wading harvest while others may not, highlighting the need to consider the reef by reef implications of management decisions.
Overall, this thesis provides new insights on how H. iris populations can change on long timescales and small spatial scales. This information, can be used to make more informed management decisions for H. iris, on ecologically relevant scales, in New Zealand. The environmental drivers discussed throughout this thesis however, will likely apply to other abalone species, and may be useful to guide management decisions and restoration efforts in other abalone fisheries globally.
2024-03-17T20:06:43ZDistribution and habitat use of humpback whales (Megaptera novaeangliae) in the Fiordland Marine Area, New ZealandCorne, Chloehttp://hdl.handle.net/10523/165752024-03-28T15:33:28Z2024-02-13T21:25:44ZDistribution and habitat use of humpback whales (Megaptera novaeangliae) in the Fiordland Marine Area, New Zealand
2024
Corne, Chloe
There is limited information on the post-whaling spatiotemporal distribution and habitat preferences of humpback whales (Megaptera novaeangliae) migrating through the Fiordland Marine Area in southwest New Zealand. Data on humpback whale encounters (n=637) were collated from dedicated boat-based surveys for marine mammals in Fiordland between 2008-2022, from tourism trips between 2012-2022 and from a public sightings database between 2005-2022. There were few encounters prior to 2008. Ninety-five percent of visual encounters occurred during the southward migration in the austral spring (September to December). There were significantly higher sighting rates during spring compared to summer for boat-based surveys (p = 0.01) and tourism trips (p = 5.45×10-05). Annual encounter rates were variable between databases. Photo-identification revealed short-term residency and migration route fidelity in a small proportion of individuals. Apparent foraging behaviour was documented in 8.3% of photographed encounters and in 9.3% of encounters recorded during tourism trips. Spatial analyses showed that 61.2% of encounters were on the outer coast, with the remainder occurring within fjord boundaries. Kernel density estimation showed that the probability of encountering a humpback whale was highest off the entrance to Doubtful Sound/Patea, around the northwestern coastline of Taumoana/Five Fingers Peninsula and in the outer part of Tamatea/Dusky Sound. Analysis of passive acoustic data from a hydrophone deployed in Tamatea/Dusky Sound between 20th February – 31st December 2022 showed that humpback whale acoustic signals were detectable in all months bar February, indicating that hydrophones deployed inside a fjord entrance are likely able to detect humpback whales on the outer coast. Detection rates were highest during June (p = 3.59×10-4), coinciding with the northward migration period. Overall, acoustic detections were bimodal, with peaks in detections reflective of the north- and southward migration periods. Binomial generalised additive models were used to describe associations with abiotic habitat variables for humpback whales on the outer coast and inside the fjords; the models described 10-12.6% and 27.3-34.6% of deviance respectively. Humpback whale presence was significantly correlated with depth and mixed layer depth inside the fjords, and with slow tidal current on the outer coast. All of the significant habitat associations may be indicators of productivity, and thus of foraging, but may also be linked to resting. Future research should investigate the importance of the Fiordland Marine Area as a migratory stopover for humpback whales during the southbound migration period.
2024-02-13T21:25:44ZThe summer heat budget of Waitati Inlet, Ōtākou: Do coastal inlets act as heat incubators?Nelson, Katiehttp://hdl.handle.net/10523/165712024-03-28T15:33:19Z2024-02-11T22:34:10ZThe summer heat budget of Waitati Inlet, Ōtākou: Do coastal inlets act as heat incubators?
2024
Nelson, Katie
The coastal waters of Aotearoa New Zealand (ANZ) are experiencing increasingly frequent, strong and long-lasting marine heatwaves as a result of long-term ocean warming. Waitati Inlet, north of Otepoti Dunedin in south-eastern ANZ was chosen as a study site to examine how physical processes associated with the inlet might amplify the impacts of marine heatwaves over the inner-shelf. An improved understanding of the dynamics of extreme heating over the inner-shelf is important as coastal marine heatwaves can strongly impact ecosystem structure, such as kelp forests.
This thesis utilised oceanographic and meteorological data of Waitati Inlet from February 2022, together with 1D numerical models, to quantify a comprehensive heat budget of this estuarine system. Additionally, this thesis investigated whether the presence of the inlet increased exposure time of warm water in a nearby kelp forest. The hypotheses of the current study were that (1) vertical heat fluxes in the inlet were dominated by air-sea heat fluxes, with secondary contributions by sediment-sea heat fluxes; (2) that these vertical heat fluxes had a greater contribution to the heat budget than the horizontal heat fluxes; and (3) that a coastal inlet with the physical characteristics of Waitati Inlet acts as a heat incubator over summer months and therefore increases exposure time of a nearby kelp forest to warm water.
The contribution that vertical terms (air-sea and sediment-sea heat fluxes) played in the inlets heat budget were first investigated using in situ timeseries of water and sediment temperature, together with meteorological and sea level parameters. During the experiment, air-sea heat fluxes were found to warm the inlet’s main channel and cool the tidal flats. In comparison, sediment-sea heat fluxes acted to warm water within the inlet. The largest components of the air-sea heat exchange for both areas was the heating by shortwave radiation and cooling by longwave radiation and latent heat flux, with sensible having the weakest contribution. The vertical terms largely balanced over the experiment’s 28-day duration; however large residuals that had a periodicity of 12.3 hours were also evident, suggesting that advective heat fluxes linked to the semi-diurnal tide were also important for the inlets heat budget on shorter timescales.
A one-dimensional (1D), cross-flat, numerical model of Waitati Inlet was then setup to evaluate the relative importance of vertical and horizontal processes in governing the inlet’s heat budget, and explore interactions between them. The model incorporated air-sea and sediment sea heat fluxes, together with air-sediment heat fluxes through a 1D sediment module and horizontal heat fluxes from advection. The model was tuned through comparison with in situ temperature timeseries from the inlet. The model was then run for a 28-day period over February 2022. During this period, air-sea and advective heat fluxes were found to cool the main channel in the inlet, whilst sediment-sea heat fluxes acted to warm. In contrast, on the tidal flats, sediment-sea and advective heat fluxes added heat, whilst air-sea heat fluxes removed heat. In both locations, the vertical heat fluxes were found to play a greater role in controlling the local heat budget than advection, which largely balanced over a tidal cycle. The influence of solar and tidal phasing was also found to determine the relative contribution of vertical versus horizontal terms on a given day.
The influence that the presence of Waitati Inlet had on exposure time of warm water in a nearby kelp forest was then investigated. Exposure time, in the context of the current thesis, relates to time (hours) above a thermal threshold. Three short (5-day) case study periods during February 2022 were selected for this investigation. Timeseries of water temperature inside and outside the inlet, and within the kelp forest, demonstrated regular impact of warm inlet water on the neighbouring kelp forest. A similar pattern was evident in each case study with a noon peak in net air-sea heat flux leading to an afternoon peak in water temperature exiting the inlet on the ebb tide and the subsequent arrival of warm water in the kelp forest during the evening/night. Observed temperature variability within the kelp forest was then compared to temperatures predicted from a null model. The null model represents the water temperature variability in the kelp forest that can be accounted for by air-sea heat fluxes. Empirical cumulative distribution functions were subse2 quently constructed from the observed and predicted temperature timeseries to compare exposure time of the kelp forest to water temperatures above a threshold temperature of 16.25◦C. In all three case studies, the exposure time of warm water in the kelp forest was increased by ≈ 84, 73 and 19 hours due to presence of the coastal inlet, compared to the null model driven solely by air-sea heat fluxes.
Collectively, these results highlight the importance of having an improved understanding of dynamics of heating over the inner-shelf, and the ways in which inner-shelf temperatures can be amplified by the presence of coastal inlets that act as heat incubators. As warming over the inner-shelf continues, together with predicted increases in MHW conditions, negative impacts on coastal systems, including morbidity and mortality of kelp forests, are likely to be exacerbated. Future local studies should incorporate measurements from within the kelp forest to have greater confidence in the contribution of advective heat fluxes from the inlet. An improvement to the 1D inlet model could account for vertical stratification to incorporate more complex nuances of the physical system. Finally, a range of thermal threshold temperatures could be studied to assess the potential impact on a variation of kelp and benthic coastal species.
2024-02-11T22:34:10ZOn the energetics, scales and boundary layer interactions of turbulent mixing in coastal seasValcarcel, Arnaud Françoishttp://hdl.handle.net/10523/165132024-03-18T14:40:19Z2024-01-16T23:00:04ZOn the energetics, scales and boundary layer interactions of turbulent mixing in coastal seas
2024
Valcarcel, Arnaud François
Turbulence-driven mixing is an essential mechanism that regulates energy transformations in the ocean and its role in the global climate. Acting as a pathway for the transfers of heat, mass and momentum in the ocean, turbulent mixing plays a vital role in maintaining the global overturning circulation, facilitating atmospheric carbon and heat uptake, and sustaining primary productivity. Understanding the energetics, scales and boundary layer interactions, in the fullest range of turbulence intensity conditions, is of fundamental importance to forecast ocean evolution in climate scenarios.
2024-01-16T23:00:04ZThe distribution, seasonality, and trends of phytoplankton groups in the Southern OceanHayward, Alexander Georgehttp://hdl.handle.net/10523/164872024-01-23T23:16:16Z2024-01-09T00:00:31ZThe distribution, seasonality, and trends of phytoplankton groups in the Southern Ocean
2023
Hayward, Alexander George
Phytoplankton are microscopic, photosynthetic organisms that dwell in the sunlit layers of our planet’s fresh water and oceans. Marine phytoplankton communities are made up of phylogenetic classes, with each of these groups having specific influences on biogeochemical processes such as carbon export and food-web dynamics. Phytoplankton groups are commonly identified by pigment biomarkers, coloured compounds that aid in light harvesting, photosynthesis or photoprotection. This thesis focuses on pigment-based methods to determine the biomass and distribution of phytoplankton groups in the Southern Ocean, and how these tools can be used to explore their spatial and temporal trends.
From the late 20th century many field campaigns have collected pigment samples in the Southern Ocean, generating an extensive dataset (~15, 000 samples) over broad spatial scales but mainly confined to summer months. To use this data in characterising the spatial and temporal variation of different Southern Ocean phytoplankton groups requires a high throughput (i.e., processing many datasets) method to convert the pigment data into estimates of biomass for different groups.
The initial phase of this thesis focuses on the development of a novel inversion method “phytoclass” to convert pigment data into chlorophyll a biomass for the different phytoplankton groups. This conversion involves matrix operations between pigment data and the ratios of pigments to chlorophyll a for different phytoplankton groups. These matrix operations calculate an estimate of the true pigment concentrations, thus providing a metric for error between the estimated and true pigment samples. An optimisation algorithm “simulated annealing” was then applied to approximate the global minimum error between the estimated and true phytoplankton pigments. The phytoclass program was tested on many synthetic datasets and also compared to the similar (and widely used) CHEMTAX program which is currently the “scientific consensus” inversion technique. The phytoclass program showed higher accuracy than CHEMTAX, and also provided additional benefits of simultaneous analysis of multiple datasets.
Patterns in the abundances of phytoplankton groups in the Southern Ocean were determined by applying the phytoclass methodology to a large pigment dataset (n = ~15,000). From this analysis, it was clear that the Antarctic Polar Front (APF) represents a major oceanographic divide in phytoplankton community composition, with higher diatom abundance to the south and greater haptophyte abundance north of the APF. Diverse oceanographic zones (latitudinal bands) south of the APF showed very similar community structures. Using Principal Component Analysis, temperature, sea ice, and nutrient concentrations (macro and micro) were shown to primarily explain the variation in phytoplankton community composition on both sides of the APF. Phytoplankton communities south of the APF showed a strong correspondence to iron and sea ice concentrations consistent with previous observations, whereas communities to the north showed an inverse relationship with iron, and a strong relationship with sea surface temperature.
In recent years, machine learning algorithms have become more accessible. To assess how phytoplankton groups have changed over the course of the Ocean Colour satellite record (1997 – present), random-forest (RF) models were built for different phytoplankton groups based on estimates of their biomass from phytoclass. Environmental and bio-optical characteristics determined from satellites and hydrodynamic models were offered as explanatory data to each model. The RF method proved to be robust at estimating phytoplankton groups with modelled output closely fit to in-situ samples. Based on the RF models, linear trend analysis was carried out for seven key groups: Diatoms, Haptophytes, Cryptophytes, Green algae, Dinoflagellates, Pelagophytes and Synechococcus. The results showed that the seasonalities of phytoplankton groups have changed over the course of the ocean colour satellite record, with winter biomass for many groups showing an increase north of the APF. Furthermore, diatoms showed the largest increase of all the phytoplankton groups, both north and south of the APF. This analysis also characterised the seasonal succession of phytoplankton groups either side of the APF and indicated earlier development of haptophyte blooms in December, south of the APF, followed by diatoms in January and February. Cryptophyte biomass was elevated throughout the time series along the west Antarctic coast, and trends in their biomass were heterogeneous, in contrast to other observations of their biomass increasing at the expense of diatoms.
This thesis details the development of new methodology for analysis of large datasets, and their interpretation to increase understanding of the biomass and distribution of key phytoplankton groups in the Southern Ocean. Application of the methodology, datasets and interpretation will advance assessment of the multifaceted impacts of a warming climate on Southern Ocean phytoplankton and inform forecasting of regional carbon export and ecosystem dynamics.
2024-01-09T00:00:31ZUnlocking the potential of Antarctic permafrost to constrain climate modelsAnderson, Jacob Thomas Herdhttp://hdl.handle.net/10523/163212023-11-13T13:02:08Z2023-11-13T00:16:31ZUnlocking the potential of Antarctic permafrost to constrain climate models
2023
Anderson, Jacob Thomas Herd
The response of the Antarctic ice sheets to global warming is an existential challenge. While past records show that melting from warming waters beneath the ice sheets can be quantified, recent research reveals that the East Antarctic Ice Sheet is susceptible to melting from atmospheric warming, a mechanism that is not well constrained by paleoclimate data. However, the information preserved within permafrost and glacial geomorphology has the potential to reveal greater detail of past terrestrial climate, landscape evolution and environmental conditions.
To test if permafrost and glacial deposits can be used to constrain past terrestrial climate, this thesis presents a novel data set that combines surface exposure dating and depth profiles of in-situ cosmogenic 10Be and 26Al, which provide age and erosion constraints for glacial deposits and permafrost strata from the McMurdo Dry Valleys of Antarctica. Alongside microbial community analyses, these data reveal a time-constrained record of permafrost paleoecology.
In the lower Wright Valley, the lack of attenuation in the 10Be and 26Al depth profiles show that the permafrost was deposited after the Last Glacial Maximum. The relatively recent permafrost conditions (including microbial diversity) at lower Wright Valley are used as a benchmark against which we can assess older permafrost environments further inland and at high elevations. At Pearse Valley, surface exposure ages and 10Be and 26Al depth profiles reveal that Taylor Glacier retreated from Pearse Valley ~65 – 74 ka, and that cold-based glaciers advanced in the McMurdo Dry Valleys during MIS 5. Beneath the surface drift, shallow permafrost at Pearse Valley was deposited ~180 ka, and permafrost deeper than 2.09 m is >180 ka. The 26Al/10Be ratios of permafrost sediments at both lower Wright Valley and Pearse Valley, have exposure-burial histories of at least 1.2 Ma, suggesting multiple recycling episodes of exposure, deposition, burial, and deflation prior to deposition at their current locations. In contrast, at the high elevation site, Table Mountain, surfaces of Sirius Group strata appear younger than their inferred depositional age derived from stratigraphic correlation. Thus, cosmogenic nuclide measurements at Table Mountain are best interpreted as erosion rates. Calculated erosion rates show the underlying Sirius Group sediments are eroding faster than the boulders above them, forming an erosional lag deposit.
Surface exposure dating, and depth profile modelling coupled with 16S rRNA gene amplicon sequencing reveals that surface soil and subsurface permafrost from lower Wright Valley (7,000 – 25,000-year-old) and Pearse Valley (>180,000-year-old) have diverse and distinct microbial communities. The deepest branching clade in both lower Wright Valley and Pearse Valley permafrost samples is Clostridiaceae, which is absent in the surface soil. Nitrospira, Gaiella, and Methyloceanibacter, also do not occur in the surface soil, and exhibit only a small number of branching points in both valleys, indicating their distant evolutionary relationships and isolation from the surface soil. At a high elevation site in the stable upland zone, the Friis Hills, DNA was below detection limits. The inability to identify DNA using amplicon sequencing in the Friis Hills is consistent with previous efforts to analyse high elevation soils and permafrost using 16S rRNA gene amplification sequencing, suggesting microbial habitability is severely restricted in persistent cold, arid habitats.
The study shows that surface sediments and subsurface permafrost can provide a temporally constrained record of environmental conditions and subsequently Antarctic ice sheet fluctuation. The integrated approach applied here can be extended to other permafrost occurrences and regions to expand our understanding of past environmental changes in Antarctica and other regions. Moreover, a complete Pleistocene paleoecological record could be used to develop a transfer function to reconstruct past climate by identifying microbial taxa that are indicative of specific environmental conditions, such as temperature, moisture, and nutrient availability.
2023-11-13T00:16:31ZMacroalgal species effects on organic matter uptake by subtidal and beach amphipods – a mechanism for food-web shiftsRodríguez-Jurado Pizano, Verónica Isabelhttp://hdl.handle.net/10523/162852023-11-02T05:56:07Z2023-11-02T05:55:13ZMacroalgal species effects on organic matter uptake by subtidal and beach amphipods – a mechanism for food-web shifts
2023
Rodríguez-Jurado Pizano, Verónica Isabel
To offer insight into the effects of macroalgal assemblage shifts under potential environmental stressors, the studies presented in the present thesis investigated the trophic connections between common native and invasive brown macroalgae and amphipods on subtidal rocky reefs and sandy beaches of Otago, Aotearoa New Zealand. I sampled macroalgae and their associated epifauna to reveal trophic relationships using stable isotope analysis (SIA). Mixing models revealed that epifaunal amphipods (Maeridae, Paradexamine and Aoridae) were heavily reliant on macroalgal organic matter (~80%) rather than phytoplankton, and that Sargassum sinclairii and Undaria pinnatifida were directly grazed on by epifauna, whereas Carpophyllum flexuosum was not consumed. Sandhopper (Bellorchestia quoyana) populations had similar trophic relationships with wrack material as epifauna did with host algae, suggesting a heavy reliance on organic matter derived from macroalgae in beach wracks. Analysis of populations sampled from different sites and wrack species showed Durvillaea sp. to attract the largest sandhoppers in a beach with multiple drift algae species, while also supporting more biomass at a beach with only Durvillaea sp. making up beach wrack. No-choice grazing experiments also revealed that U. pinnatifida and M. pyrifera were both readily consumed by sandhoppers, while C. flexuosum was not consumed. However, differences arose in the timing of consumption, as U. pinnatifida was eaten immediately and degraded within 48 hours, while M. pyrifera degraded within 144 hours and was consumed exponentially starting at 48 hours. Analyses of phlorotannin concentrations revealed U. pinnatifida and Durvillaea sp. to have the lowest (near zero) concentrations, followed by M. pyrifera (low concentrations), S. sinclairii (medium-low concentrations), and C. flexuosum (high concentrations). Additionally, experimental degradation of blades and analysis of phlorotannin concentrations over time demonstrated that concentrations in C. flexuosum decayed exponentially, while no difference was observed for M. pyrifera over seven days. However, field observations confirmed that phlorotannin concentrations were lower in Durvillaea sp. and M. pyrifera wracks than in fresh material. Consequently, I attribute differences in consumption of blades by both epifauna and sandhoppers to the initial (i.e., live blade tissue) phlorotannin concentration of macroalgae. I argue that my results and future work suggested herein could be used to understand the consequences of community shifts in kelp beds and to inform ecosystem-based management of macroalgal-based ecosystems of Otago.
2023-11-02T05:55:13ZHydrodynamics and coastal dispersion from the Bay of Plenty, Aotearoa, New Zealand: A 25-year numerical modelling perspectiveMontano Orozco, Mireya Mercedeshttp://hdl.handle.net/10523/162432023-10-25T13:02:05Z2023-10-25T00:51:28ZHydrodynamics and coastal dispersion from the Bay of Plenty, Aotearoa, New Zealand: A 25-year numerical modelling perspective
2023
Montano Orozco, Mireya Mercedes
The Bay of Plenty (BoP), in the northeast of Aotearoa New Zealand, is a large focus of marine economic activity. The BoP possesses the largest natural harbour in Aotearoa, which is responsible for 41% of the country’s exports. Additionally, the Whakatōhea iwi (tribe) established the world’s first large open ocean green-lipped mussel farm in the eastern region of the BoP. Understanding coastal hydrodynamics such as coastal circulation and material dispersion is crucial. This dissertation aims to describe aspects of the Eulerian and Lagrangian hydrodynamics of the coastal region of the BoP.
We developed the Bay of Plenty Model (BoPM), a realistic coastal-resolving long-term BoP simulation, to achieve our objective. This 25+ year simulation was statistically evaluated against a set of multiple remote sensing and in situ observations. The BoPM had a good skill in reproducing ocean water temperature, salinity, sea level, and water column velocity over tidal and non-tidal timescales (Willmot skill >0.8 for most variables). The BoPM outperforms the nation-wide coarser resolution mode (Moana Ocean Hindcast, 5 km horizontal resolution), demonstrating the benefits of coastal-resolving simulations for coastal hydrodynamics research.
We explored BoP circulation and dispersion drivers using the BoPM. Up to 30% of the cross-shelf current variability in the central and western BoP comes from along-shore wind stress. The eastern region had no significant correlation to the along-shore winds. Lagrangian statistics suggest that the eastern region can act as a retention zone, constraining particles to the nearshore. Moreover, particles released further from the shore have a greater influence from the offshore mesoscale features and less influence from the local winds, resulting in the loss of particles from the coastal region.
The combination of climatological Eulerian and Lagrangian approaches identified persistent climatological flow structures throughout the Bay of Plenty. (1) A transport barrier north of the retention zone. (2) A seasonally-varying hook-like or barrier feature that disrupts the continuous eastward flow in the eastern region of the BoP. (3) Shadowed regions associated with island features incorporated into the study.
Three different virtual particle behavioural parametrizations were examined to assess their influence on coastal connectivity. Our study reveals that pelagic larvae duration (PLD) is the most important factor among the simulated behaviours in the quantification of coastal connectivity. Moreover, the eastern region displays higher self-recruitment, reinforcing the presence of a retention zone in the region.
Our findings suggest that the BoP is subject to high interannual variability. Therefore, we investigated the correlation between the Southern Oscillation Index (SOI) and the detrended 27-year time series of sea surface and atmospheric variables. Our findings imply that the SOI has a weak influence in the region, and composites of long-term trends must be considered to understand the BoP's interannual variability mechanisms.
The current thesis added to our understanding of oceanographic connectivity, average circulation, and the mechanism of interannual and event-scale variability of the coastal circulation of the BoP. In addition, we indicate the potential areas for future research in which the BoPM may prove to be a valuable tool.
2023-10-25T00:51:28Z