Case study 1 – Nutrient Load Reductions to Achieve Freshwater Objectives in the Rivers, Lakes and Estuaries of Southland

This report quantifies nutrient (nitrogen and phosphorus) load reductions required to achieve options for freshwater objectives (FWOs) in rivers, lakes and estuaries of Southland, excluding Fiordland and Islands. Southland Regional Council is required to set objectives and policies for achieving them under the National Policy Statement – Freshwater Management (NPS-FM).

The analysis does not consider how the nutrient load reductions would be achieved and only aims to inform the Southland Regional Council about the magnitude of the load reductions needed for each option. The load reductions required were assessed for all individual river segment, lake and estuary receiving environments in the study area. The results can be reported at several levels of detail including for individual receiving environments (river segments, and individual lakes and estuaries), individual ‘reporting catchments’, the catchments of eight individual estuaries, and the whole study area. The study also identified the ‘limiting environment’; i.e., whether it is an estuary, lake or river that has the most sensitive FWO and has therefore driven the load reduction required.

The study estimated the uncertainties associated with all assessments of the reductions in nutrient loads required to achieve nominated FWOs for rivers, lakes and estuaries. Uncertainty is unavoidable because the analyses are based on models that are simplifications of reality and because the models are informed by limited data. The uncertainties associated with two key components of the analyses: the estimated nutrient concentrations and loads were quantified and were combined in a Monte Carlo analyses. The Monte Carlo analyses simulated 100 ‘realisations’ of the load reduction calculations, which were used to define the probability distributions of all estimates. The probability distribution describes the range over which the true values of the load reductions are expected to lie and was represented by the 5th and 95th percentiles of the distribution (i.e., the limits of the 90% confidence interval).

The regional nitrogen load reduction required to achieve the national bottom lines (i.e., the least stringent acceptable FWO) for the study area was 47% (90% confidence interval 33% to 61%) and the regional phosphorus load reduction required was 21% (90% confidence interval 13% to 33%).

It is unlikely that the uncertainties associated with the assessments made by this study can be significantly reduced in the short to medium term (i.e., in less than 5 to 10 years). This is because, among other factors, the modelling is dependent on the collection of long-term water quality and ecosystem health data and reducing uncertainty would require data for considerably more sites than were available for the study.

This study can help inform the process for deciding on limits to resource use, by providing an assessment of the approximate magnitude of nutrient load reductions needed to achieve several options for FWOs, with a quantified level of confidence and risk associated with each option. However, this report does not consider what kinds of limits on resource might be used to achieve any load reductions, how such limits might be implemented, over what time frames and with what implications for other values. The NPS-FM requires regional councils to have regard to these and other things when making decisions on setting limits. This report shows that these decisions will ultimately need to be made in the face of uncertainty.

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