Benthic Invertebrate Indices Show No Response to High Nitrate-Nitrogen in Lowland Agricultural Streams

Globally benthic invertebrate biotic indices are widely used to assess stream health. In New Zealand, the response of biotic indices to high nitrate-nitrogen (hereafter nitrate) concentrations has not been rigorously tested. We conducted a field survey of benthic invertebrates in 41 lowland intensively farmed Canterbury streams representing a wide nitrate gradient (0.4–11.3 mg/l) to determine if biotic indices respond in a predictable manner to increasing nitrate (e.g. show a subsidy stress response). Our results show commonly used biotic indices were not able to detect any effects of high nitrate. We found homogenous and tolerant benthic invertebrate communities dominated and suggest the lack of any detected effects may be because taxa sensitive to nutrients have already been lost from these systems. Therefore, we recommend caution is needed when using biotic indices in lowland agricultural streams where nutrients may be high.