Why aren't we managing water quality to protect ecological health?
Conference Contribution ResearchOnline@JCUAbstract
Eutrophication of waterbodies is a major stress on freshwater ecosystems globally and New Zealand is no exception. Expanding agricultural intensification is increasing nutrient levels in rivers throughout the country and as a response the New Zealand Government has established a policy of freshwater management (NPS-freshwater management) where waterbodies are managed within four states ranging from high to low ecosystem health (states A, B, C and D). However, the National Policy Statement for freshwater management does not currently have attributes to manage the two main stressors of lotic ecosystem health: deposited sediment and nutrients. It does have attribute states for nitrate (the dominant form of the nitrogen nutrient), but only at levels where it acts as a toxin. Levels at which nitrate acts as a toxin are however considerably higher than those where it can adversely impact on ecosystem health. There are currently no dissolved reactive phosphorus (DRP) attribute states for ecosystem health of rivers. We compiled a large range of data sources and used a weight-of-evidence approach to objectively determine nitrate and dissolved reactive phosphorus limits to manage rivers and streams into the four ecological states. This established that the critical nutrient concentrations differentiating rivers in each of the states are 0.11, 0.58 and 1.66 mg/l for nitrate and 0.006, 0.015 and 0.054 mg/l for DRP. While ecological health of rivers is affected by a range of interacting stressors we believe the evidence supports the view that managing to these nutrient thresholds will provide for better ecological condition in New Zealand’s rivers and streams. It seems strange to us that these nutrient attributes are currently present in the NPS-freshwater for lakes but not for rivers and streams, when the data for them is readily available. If we truly want to manage ecosystem health we must surely consider the most important determinates of its condition so that informed, objective decisions can be made on the implications of particular actions.
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Farm Environmental
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12
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Auckland, New Zealand
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Fertilizer and Lime Research Centre
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Auckland, New Zealand
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