Establishing riverine nutrient criteria using individual taxa thresholds (#684)
Adam D Canning
1
,
Russell G Death
2
- Centre for Tropical Water and Aquatic Ecosystem Research (TropWATER), James Cook University, Townsville, Queensland, Australia
- School of Natural Sciences, Massey University, Palmerston North, New Zealand
- Nutrient enrichment is one of the most pervasive impacts on aquatic ecosystems globally. While the range of impacts are well documented, approaches used to establish nutrient criteria that safeguard aquatic ecosystem health are varied and challenging. In many instances, criteria are derived from correlations between in-situ nutrient concentrations and biological indicators. However, summarising entire assemblages as a single biological indicator can result in a substantial loss of information and potentially weak relationships. In this study, we compare the derivation of nutrient criteria using biological indices with those from individual species.
- Random forests were used to model the change in three biological indices, and the probability of occurrence of individual macroinvertebrates, with increasing nutrient enrichment across all New Zealand river reaches to identify the nutrient concentrations for every river reach that did not cause more than 20% probability of change from baseline conditions.
- We observed substantially greater variability in nutrient impact cessation thresholds when modelling indices instead of individual species. The concentrations of DIN (Dissolved Inorganic Nitrogen) and DRP (Dissolved Reactive Phosphorus) were almost always the most influential factor predicting the distributions of individual species. Given that predictions were made for over 550,000 river reaches, we used classification trees to summarise nutrient thresholds by landscape position, climate and geology.
- Our analysis suggests that nutrient concentrations are consistently among the most influential factors predicting the distributions of New Zealand riverine macroinvertebrate assemblages and their biological indices. However, summarising an entire assemblage into a single indicator resulted in poor predictive performance.
- We consider that the derivation of nutrient criteria for waterways should not rely solely on relationships with ecological indicators, as is commonly practiced, but also involve examination of relationships with individual species.