Establishing riverine nutrient criteria using individual taxa thresholds.

Area of Science:

• Environmental Science
• Ecology
• Water Quality Management

Background:

• Nutrient enrichment is a major global stressor on aquatic ecosystems.
• Current nutrient criteria often rely on correlations with biological indices, which can oversimplify ecosystem complexity.
• This simplification may lead to weaker relationships and loss of critical ecological information.

Purpose of the Study:

• To compare nutrient criteria derivation using biological indices versus individual taxa for New Zealand rivers and streams.
• To identify nutrient concentrations that protect a specific proportion (80%) of aquatic taxa.
• To assess the influence of nutrient concentrations on biological indices and individual taxa.

Main Methods:

• Utilized random forest models incorporating nutrient concentrations (Dissolved Inorganic Nitrogen - DIN, Dissolved Reactive Phosphorus - DRP) and biological data.
• Predicted responses of biological indices and individual taxa to nutrient levels across New Zealand's river network.
• Identified nutrient thresholds protecting 80% of taxa for different river types.

Main Results:

• Models predicting individual taxa were generally good, with nutrient concentrations being the most influential factor.
• Models for biological indices were poor, indicating information loss from summarizing entire assemblages.
• Estimated DIN concentrations below 0.57-1.32 mg/L and DRP below 0.019-0.033 mg/L are needed to protect 80% of taxa, with stricter criteria for high-order, low-slope rivers.

Conclusions:

• Deriving nutrient criteria based on individual taxa provides a more robust assessment than using aggregate biological indices.
• The relationship between nutrient concentrations and biological indices is weak, likely due to information loss.
• Future nutrient criteria development should incorporate analyses of individual taxa responses to ensure effective ecosystem protection.