Nutrients in New Zealand Rivers and Streams: An exploration and derivation of national nutrient criteria
Creators: name: Canning, Adam
Publisher: name: figshare
Year: 2020
Subjects: subject: 60202 Community Ecology (excl. Invasive Species Ecology)
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: 60204 Freshwater Ecology
subject: 50102 Ecosystem Function
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences
subject: Ecology
subject: 50204 Environmental Impact Assessment
subject: 50207 Environmental Rehabilitation (excl. Bioremediation)
subject: 50205 Environmental Management

description: Executive Summary<br>Many of New Zealand’s rivers are enriched with nutrients, primarily nitrogen and phosphorus,<br>which can reduce ecosystem health via eutrophication. According to Environment Aotearoa<br>(Ministry for the Environment and Statistics New Zealand, 2019), approximately 82% of rivers,<br>by length, in pastoral land do not meet the ANZG (2018) default guideline values for nitrate-nitrogen and 77% for dissolved reactive phosphorus (DRP). Eutrophication is the increased<br>production of organic matter. The most visible form of eutrophication is excessive algal growth,<br>i.e., where rivers become covered in green slime. However, eutrophication does not always<br>involve excessive algal growth, nor is algal growth a requirement. Nutrient enrichment can also<br>increase and alter microbial activity, which can: alter invertebrate and fish growth (via changes<br>in nutrient or macromolecule availability); increase hypoxia (which can suffocate invertebrates<br>and fish); disrupt food web cycling and stability; reduce greenhouse gas storage and abatement;<br>alter disease transmission; and alter the availability of nutrients and energy for downstream<br>systems. Nitrogen and phosphorus are critical components of ecosystems and the species that<br>inhabit them. Anthropogenic nutrient enrichment should be minimised to maximise an aquatic<br>ecosystem’s health.<br>New Zealand’s National Policy Statement for Freshwater Management (NPS-FM) contains<br>grades (or bands) for various measures (termed attributes) of aquatic ecosystem health. When<br>a measure receives a ‘D’ grade, or a grade lower than desired by the community (via a regional<br>planning process), then the environment must be managed in a way that, over time (decided by<br>the community), improves that measure. This report outlines the approach adopted by the<br>Science and Technical Advisory Group (STAG) to recommend dissolved inorganic nitrogen<br>(DIN) and DRP criteria for consideration during the Government’s Essential Freshwater reform.<br>Using the same format as the NPS-FM, DIN and DRP criteria are presented as attribute tables<br>with grades ranging from A-D band. In addition to the nutrient criteria derivation, the report<br>includes additional analyses that sense-check the criteria.<br>Nutrient criteria:<br> Nutrient criteria were developed by averaging multiple lines of evidence. Each line of<br>evidence represented a relationship between DIN and DRP concentrations and another<br>metric of ecosystem health. These metrics included periphyton, macroinvertebrates, fish<br>and ecosystem functioning. Nutrient criteria were not designed to guarantee a specific<br>ecological state; it is neither possible to summarise the health of an ecosystem into one,<br>all-encapsulating metric, nor to determine all ecosystem outcomes. Rather the nutrient<br>5<br>criteria are intended to be benchmarked to be similar stringency to other attributes.<br>Improvements in nutrient concentrations will increase the propensity or likelihood of a<br>healthy ecosystem.<br> DIN derived nutrient criteria grades are: A ≤ 0.24 mg/L, B ≤ 0.50 mg/L, C ≤ 1.0 mg/L<br>and D &gt; 1.0 mg/L.<br> DRP derived nutrient criteria grades are: A ≤ 0.006 mg/L, B ≤ 0.01 mg/L, C ≤ 0.018<br>mg/L and D &gt; 0.018 mg/L.<br> If there is strong evidence that a river reach would naturally exceed the D-band, then it<br>should be exempt from assessment against these criteria. Options for handling the<br>volcanic acidic geology class are included.<br>Additional supporting exploration:<br> Predicted river-specific change-points between nutrients and the macroinvertebrate<br>community index (MCI), suggest the large majority of DIN change-points occur between<br>0.8-1 mg/L. Whereas most DRP change-points occur between 0.017-0.023 mg/L. Rivers<br>with volcanic acidic (VA) geology have a tendency for change-points to occur at higher<br>concentrations (median = 0.021 mg/L) than other geologies (typically 0.017 mg/L),<br>though there is considerable spread within the VA geology class.<br> Boosted regression tree explorations consistently suggest that nitrogen is a highly<br>influential predictor of the macroinvertebrate metrics examined when a range of<br>temperature, vegetation, landscape, meso-habitat and hydrological factors are<br>accounted for.<br> Quantile regression was used to examine the nutrient concentrations that would, in the<br>worst case scenario, likely limit the proposed MCI, QMCI (quantitative MCI) and ASPM<br>(average score per metric) bottom-lines from being achieved, assuming no other factors<br>are interacting with nutrient limits. The proposed C/D thresholds for both DIN and DRP<br>were more protective than these levels.<br> Gradient forest modelling was used to examine the turnover of macroinvertebrate<br>assemblages. Out of the 30 environmental variables included DIN and DRP were the top<br>two most influential factors. Turnover of macroinvertebrate assemblages was largely<br>predicted to occur between approximately 0-1 mg/L of DIN and 0-0.05 mg/L of DRP.<br> An exploratory model of observed/expected (O/E) fish species presence suggests DIN<br>and DRP are strong predictors of O/E scores, and that scores decline until a plateau<br>between 1-1.3 mg/L before continuing to decline. For DRP a sharp decline occurs<br>6<br>between 0.015-0.02 mg/L, followed by a shallow decline between 0.02-0.025 mg/L and<br>then a plateau.<br> Binomial regressions were used to examine the proportion of the 60 most common<br>macroinvertebrate taxa that experience more than minor changes in their predicted<br>probability of occurrence (i.e., &gt;20% change from no enrichment levels) with increasing<br>DIN and DRP concentrations. The analysis suggests that for DIN, 5% of taxa experience<br>substantial impacts on their probability of occurrence at 0.20 mg/, 20% of taxa at 0.44<br>mg/L, 40% at 0.67 mg/L and 60% at 1.14 mg/L. Whereas for DRP, 5% are affected at<br>0.008 mg/L, 20% at 0.011 mg/L, 40% at 0.018 mg/L and 60% at 0.029 mg/L.<br> Negative correlations were observed in regions and river classes across the country<br>between nutrient concentrations and the macroinvertebrate metrics examined. No<br>positive correlations were observed. Where there were no statistically significant<br>correlations, these relationships usually failed to meet the basic assumptions of the<br>linear regression. Often they had too few sites, were highly non-random, did not cover<br>a substantial range of nutrient concentrations, or had influential outliers. Whilst simple<br>correlations may be noisy or fail to yield significance, this does not necessarily mean a<br>relationship does not exist (as lack of correlation does not mean lack of causation); they<br>are still useful for direction of change and can usually be improved by adding additional<br>and interacting factors.<br> Nutrient criteria were also compared against concentrations that are (1) predicted to<br>occur in reference condition and (2) measured in pristine habitat for different river<br>classes. Across both, the measured and estimated DIN reference conditions almost<br>always fell within the proposed A-band and never exceeded the proposed bottom-line.<br>Whereas with DRP, reference concentrations largely fell in the A-band, sometimes B<br>band, and some occasions in the D band. River classes with VA geology appear to have<br>considerably higher concentrations of DRP in reference state, with some sites or<br>estimates near or exceeding the proposed DRP bottom-line.<br>