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

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