Nutrient enrichment and alterations to flow regime can have far-reaching consequences on aquatic systems. Their impact on water quality, biotic response of plants and ecosystem metabolism were assessed in streams in the Pilbara region of Western Australia, affected by mine-derived groundwater drainage which was enriched with nitrate (NOx-N). Comparison of four streams, at upstream and downstream sites in flowing and pool habitats, investigated how increased flow and NOx-N affects naturally intermittent streams in a semi-arid climate.
Measurement of flow rate and nutrient concentrations determined that mine derived drainage increased nitrate load, and that load rather than NOx-N concentration of was the primary factor driving ecosystem response. Analysis of plant community, TN content, growth rates and density found that increased NOx-N load was higher than plant capacity for assimilation (no significant NOx-N reduction between upstream and downstream sites receiving mine-derived drainage) but increased plant nitrogen content (by up to 19x), growth rates (assessed using periphyton plates) and changed the plant community of downstream sites to one dominated by microbial material. Ecosystem metabolism was calculated from continuous recordings of dissolved oxygen at pool sites for each stream. These pools represented predictable conditions after cessation of mine-water discharge. High NOx-N loads contributed to increases and dominance of heterotrophic processes at mine-derived streams, and daily deoxygenation events.
Mine-derived drainage and associated NOx-N loads can significantly alter the ecology and ecosystem metabolism of Pilbara streams. When mine dewatering is terminated, reduced flow, increased nitrate and carbon load will promote a heterotrophic system with deoxygenation events. The duration of these impacts is controlled by the high variability of natural Pilbara stream flow. Management should therefore aim to increase NOx-N load (flow) and subsequent carbon loading as little as possible.