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Investigating groundwater derived nutrient fluxes within Tauranga harbour, New Zealand

Submarine groundwater discharge (SGD) is one of the key pathways that connects land and ocean, delivering freshwater, nutrients, carbon, metals, and other chemical constituents to coastal waters. In New Zealand, local Māori have had a strong cultural connection to this natural process for centuries. Recently, it has been recognised that this process is important in global hydrological and biogeochemical cycles. Due to the large coastline to land area ratio, steep topography, permeable sediments and high rainfall, oceanic islands, may have the potential for disproportionally large SGD fluxes into the ocean compared to river inputs. Since SGD is driven under different timescales and processes (saline porewater through sediments and fresh groundwater from aquifers), it can provide a continual source of nutrients to coastal water bodies. This has previously received very little attention in New Zealand despite its potential importance, mainly due to the difficulty in measuring this diffuse process. Regional system scale studies among oceanic islands are poorly quantified and understood, especially in the South Pacific region. Population increases have placed added stresses on hydrological cycles and subsequent land use changes, particularly agriculture, have resulted in excess nutrient loading and nitrogen pollution in coastal systems. Radium isotopes were used to quantify SGD and related nutrient fluxes into Tauranga Harbour, a coastal lagoon system, on the largely volcanic North Island of New Zealand. A ²²⁶Ra mass balance quantified SGD fluxes at the harbour scale, resulting in a range of SGD flux rates between 0.53 cm d⁻¹ and 3.09 x10⁶ m³ d⁻¹. When compared to other freshwater discharges (rivers and streams) into the harbour this was between ~1 – 2.8 times greater in flow. SGD inputs were also calculated to be ~5 times (for nitrogen) and ~8 times (for phosphorus) greater than the input from surrounding rivers and streams. Over the past decades the harbour has suffered reduced water quality and has inter-annual macro-algal blooms of Ulva spp. (sea lettuce). Harbour waters had a N:P ratio of ~17:1 with a positive relationship between radium isotopes (assumed groundwater related) and N:P ratios. This demonstrated that SGD has major implications for primary production, including recurrent algal bloom events which commonly occur in the harbour. While higher loads of inorganic and organic nitrogen from surrounding water catchments are important for water quality, the physical processes that control estuarine flushing, such as tides, wind, rainfall, and freshwater discharge are equally as important. To better understand how future changes in climate over the coming century will impact coastal systems a numerical model was developed and combined with machine learning techniques to ‘emulate’ dilution rates based on time series of historical hydro-metrological data. A 22-year hind cast revealed the spatial variability of flushing times over short term ‘event’ and long range inter annual timescales. A significant relationship (r² 0.43 p =<0.001) between flushing rates and the southern oscillation index (SOI) was also established, highlighting direct and indirect implications of climate on water quality by altering flushing rates, nutrient availability, storage and transport across different regions. Using these data driven tools will be important into the future for monitoring water movement and ‘estuarine forecasting’. Finally, I present a study that assessed the effects of spring-neap tidal variations on estuarine connectivity over multiple spatial scales. Being a tidally dominated system, variations in tidal amplitudes can alter the movement of water and associated chemical exchanges between the harbours sub estuaries and coastal shelf. A combination of numerical modelling and artificial tracer experiments were used together with field-based measurements of naturally occurring radioactive tracers (radium). The modelled flushing times across all harbour regions (~4 – 20 days) agreed well with calculated apparent Ra ages. The average apparent radium water ages were revealed to be younger on spring tide at the harbour scale, suggesting tidal pumping of radium rich water from sub regions. Waters at the harbour scale were also well mixed from other sub regions. Nutrients and other chemical constituents varied substantially with the tide and were up to 4 times greater at the sub estuary scale on ebb tides. This demonstrated the important role of spring-neap tidal cycles in water transport, mixing and biogeochemical behaviour within sensitive coastal lagoon systems.
Type of thesis
Stewart, B. T. (2021). Investigating groundwater derived nutrient fluxes within Tauranga harbour, New Zealand (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14405
The University of Waikato
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