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The ecological implications of reconstructed historic hydrodynamic conditions in Tauranga Harbour (1852-2006).

In order to manage, restore and mitigate the increasing anthropogenic impacts on estuarine systems in the future, it is useful to understand the magnitude and distribution of historic changes in environmental conditions that may have impacted on ecosystems through time. The general aim of this thesis is to hind cast historic environmental conditions in order to investigate the ecological implications of changes in geomorphology and hydrodynamics within the southern basin of Tauranga Harbour. This was achieved by compiling 11 bathymetries and developing a hydrodynamic numerical model. The bathymetries produced were from 1852, 1879, 1901, 1927, 1954 and 2006 (entrance adjusted bathymetries). An additional 5 bathymetries from 1852, 1879, 1901, 1927 and 1954 were compiled which had sediment removed from the intertidal areas to simulate infilling (intertidal adjusted bathymetries). The impact of the simulated infilling on depth Investigate the by comparing the entrance adjusted bathymetries with the intertidal adjusted bathymetries of each year. Using the numerical modelling environment Delft3D a numerical model of the southern basin of Tauranga harbour was developed to simulate historical hydrodynamic conditions for each of the 11 bathymetries. The impact of the simulated infilling on hydrodynamics was investigated by comparing the results of the entrance adjusted and intertidal adjusted numerical models of each year. The modelled hydrodynamic conditions of current speed, water level range and tracer decay from the intertidal adjusted bathymetries were used to investigate temporal changes in environmental conditions that may have influenced historic ecological health and distribution of species. The comparison of depth and hydrodynamics between the entrance and intertidal bathymetries for each year showed that the simulated infilling caused slight changes in the distribution of depth and a slight increase in the magnitude of current speed, water level range and rate of tracer decay. The temporal changes in the environmental conditions included large magnitude small scale changes in depth and current speed occurred in the harbour entrance channels. Water level had the most significant change prior to dredging across the Harbour potentially due to historic changes in the inlet cross-section. For depth, water level and current speed the 2006 conditions were most different to the earliest 1852 conditions. This is due to the accumulation of both large natural changes in geomorphology and hydrodynamics and changes from the anthropogenic dredging and development. The tracer decay had most difference through time in the regions with changes in the constricted entrance such as in the estuaries above Sulphur Point. Significant ecological implications include changes in depth and water level influencing functions such as bivalve feeding times, inundation of mangroves and light attenuation for seagrass. A significant change through time as a result of the intertidal infilling is the predicted expansion of mangroves. Differences in current speeds would alter the distribution of sediment and species as well as the settlement of larvae. Residence times have had largescale changes due to the anthropogenic changes around port development. These changes would result in an increase in the retention of sediments, contaminants and larvae and a shift from bivalve dominance to polychaete’s as well as altered community connectivity.
Type of thesis
Moffatt, A. (2021). The ecological implications of reconstructed historic hydrodynamic conditions in Tauranga Harbour (1852-2006). (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14138
The University of Waikato
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