Nutrient concentration targets to achieve periphyton biomass objectives incorporating uncertainties

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Snelder, T. 2018 Nutrient concentration targets to achieve periphyton biomass objectives incorporating uncertainties. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/38. 41 p.; doi: 10.21420/AJSH-NW16


Nutrient concentration targets are an important component of efforts to manage the eutrophication of gravel-bed streams and rivers worldwide. In New Zealand, the periphyton attribute of the National Objectives Framework (NOF) National Policy Statement for Freshwater Management (NPS-FM) requires that nutrient concentration targets are set to achieve periphyton biomass objectives. Although guidance exists for setting nutrient concentration targets, there remains considerable uncertainty about what the appropriate nutrient concentration targets should be for any location. In addition, the uncertainty associated with the existing guidance is not quantified. Measures of the uncertainty associated with nutrient concentration targets is important because it enables the confidence associated any actions to achieve periphyton biomass objectives to be quantified. In addition, analysis of nutrient management actions generally depends on a chain of models. Quantification of all uncertainties in the modelling chain enables the identification of the most limiting (in terms of confidence) component in this chain. The relationship between periphyton biomass and site characteristics were represented by regression models fitted to a dataset characterizing 78 gravel-bed rivers in New Zealand. The regression models had large site scale uncertainties but identified broad-scale drivers of periphyton biomass. The models were used to derive concentration targets for nutrients TN and DRP for 21 river classes to achieve periphyton biomass thresholds of 50, 120 and 200 mg chlorophyll a m2. The targets incorporated a temporal exceedance criterion requiring that the specified biomass threshold will not be exceeded in more than 8% of samples (being the average of one sample occasion per year for monthly monitoring). The targets also incorporated a spatial exceedance criterion requiring that the biomass thresholds will not be exceeded at more than 10%, 20% or 50% of sites within the domain being managed. The allowance for a fixed level of spatial exceedance combines the models’ description of broad-scale variation with the assumption that, rather than requiring specific conditions at an individual site, the management objective is to restrict biomass to acceptable levels at a majority of locations within a domain of interest. The approach therefore uses the model in a manner that is appropriate to its precision and encourages management to define management objectives in terms that are analytically tractable. A Monte Carlo analysis was used to derive the uncertainty of the derived nutrient concentration targets for TN and DRP. The uncertainties reduce as the size of the spatial domain to which the spatial exceedance criteria applies is increased. Testing indicated the derived nutrient concentration targets are reasonably consistent with independent periphyton biomass data despite differences in the protocols used to measure biomass at the training and test sites. (auth)