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      Variation in nitrate sources and delivery in space and time within the 389 ha Lake Okaro catchment

      Eyberg, Claire Elizabeth
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      Eyberg, C. E. (2020). Variation in nitrate sources and delivery in space and time within the 389 ha Lake Okaro catchment (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13678
      Permanent Research Commons link: https://hdl.handle.net/10289/13678
      Abstract
      The eutrophication of surface water is a global issue, with excessive nitrate

      concentrations reducing water quality and affecting water supplies,

      ecosystem health and its recreational use. In New Zealand, the degradation

      of freshwater quality has largely been attributed to nitrate leaching from

      intensive landuse, in particular from intensively grazed pastoral systems.

      As agricultural activity in catchments can contribute a large proportion of the

      nutrient pollution in surface waters, an understanding of nitrogen dynamics

      is therefore vital in managing the downstream effects of diffuse nitrogen

      inputs. Nitrate isotopes (δ¹⁵N, δ¹⁸O) have been increasingly used for

      determining nitrate cycling and source identification. Together with water

      isotopes (δ²H, δ¹⁸O), these conservative tracers can provide the necessary

      tools for determining the transport mechanisms of nitrate.

      Lake Okaro has suffered from water quality degradation for several decades,

      and has been the focus of intense lake restoration projects focused on

      nutrient management. The 389 ha agriculture-dominated catchment

      exemplifies New Zealand’s complex physiographic landscape.

      Results from high-resolution monitoring of streamflow during storm events

      demonstrates the potential to capture dynamic shifts in distinct water

      sources, but can be limited by insufficient monitoring of ancillary parameters,

      or a lack of pre-event characterisation of streamflow. Spatial sampling

      indicated characteristic fractionation processes for sites of similar

      environments, likely due to enhanced plant and microbial processing of

      carbon and nitrogen. This spatial sampling demonstrates that even in small

      catchments, there may be a significant degree of heterogeneity in water and

      nitrate flows, in both space and time.

      Nitrate contributions were much lower in summer relative to autumn and

      winter during baseflow or non-storm flow. Storm events contributed a

      disproportionate amount of nitrate, but the effect was most notable in winter.

      During baseflow, or non-stormflow, in the main inflow stream in the Lake

      Okaro catchment, nitrate had δ¹⁵N and δ¹⁸O values indicative of a soil

      nitrogen origin (+5.8 ‰ to +7.3 ‰, and -0.5 ‰ to +1.7 ‰, respectively).

      The dominant nitrate sources shifted during rain events, with streamflow in

      the winter event having δ¹⁵N and δ¹⁸O values indicative of urine, whereas

      the summer event observed δ¹⁸O-enriched baseflow signatures. Water

      isotope ratios indicated the winter event was dominated by event water. The

      differing seasonal responses to rainfall suggest nitrate inputs during storms

      in this catchment are strongly linked to seasonal nitrate availability in water

      flow paths.

      Patterns observed in temporal and spatial data collected require more

      investigation around potential reasons or mechanisms of fractionation.

      Further refinement of the Okaro catchment flows and cycling of nitrogen will

      help create more catered management techniques.
      Date
      2020
      Type
      Thesis
      Degree Name
      Master of Science (Research) (MSc(Research))
      Supervisors
      Baisden, W. Troy
      Publisher
      The University of Waikato
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      All items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.
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