Benthic invertebrates
Nitrate-nitrogen pollution is an important pathway in which agricultural land uses exert stress on stream ecosystems. Although nutrient impacts of eutrophication have been well documented, little is known how stream invertebrate communities are influenced by this stressor. Theory suggests a threshold may occur when basal resources (e.g., algae) no longer benefit from increased nutrients for growth, resulting in a more stable basal resource and stream community. To investigate this predicted threshold response, macroinvertebrate communities were surveyed from 39 lowland agricultural streams along a nitrate-nitrogen gradient (0.4 – 11.3 mg/l N-NO3-) on the Canterbury Plains. The response was assessed with traditional biotic indices and macroinvertebrate community presence/absence data. Almost all traditional invertebrate biotic indices (including the MCI) showed no relationship with nitrate-nitrogen; Taxa richness was the exception, where a weak quadratic model best described the relationship (F1,39=3.62, R2=0.12, P<0.05). In contrast, ordination of macroinvertebrate taxa presence/absence identified nitrate-nitrogen as a significant driver of community composition along with turbidity, Substrate Index and discharge were also associated. Indicator taxa most strongly correlated with the nitrate-nitrogen gradient were Elmidae larvae (positive) and Sphaeriidae (negative). These results suggest the relationship between nitrate-nitrogen and macroinvertebrate composition depends on how the biotic response is assessed. Furthermore, that the pollution-tolerant, generalist macroinvertebrate communities typical of Canterbury Plains streams either do not respond to nitrate-nitrogen, or respond weakly, which is difficult to detect. Overall, this study provides evidence to advise stream management, and highlights the tolerance of macroinvertebrates to nitrate-nitrogen pollution with implications for stream biodiversity, food-web structure, and ecosystem function.
Bioreactors
Tom Moore, Jon Harding, Angus McIntosh & Catherine Febria.
Overall, denitrifying bioreactors containing wood chip were selected to be the most suitable for use with tile drains on the Canterbury Plains. This research helped advise a large-scale study of the effectiveness of denitrifying bioreactors that is currently underway by the CAREX group.
Elevated nitrate-nitrogen pollution in the Canterbury Plains surface and ground waters has raised concerns for human and ecosystem health by environmental managers. Tile drains, which drain excess irrigation from agricultural land, are hotspots of nitrate-nitrogen pollution that can be targeted with carbon-based denitrifying bioreactors. These bioreactors remove nitrate-nitrogen at tile drain outlets to help improve downstream ecosystem health. The aim of the investigation was to determine which carbon-based media or combination of media in the denitrifying bioreactors would be most effective at removing nitrate-nitrogen pollution from Canterbury Plains tile drains.
To test this I designed and constructed a replicated small-scale denitrifying bioreactor experiment. This involved a pressurised system that delivered a constant flow of nitrate-nitrogen dosed water into bioreactors containing wood chip, coconut husk, a combination of coconut husk into wood chip, and wood chip into coconut husk. The experiment was run over 67 days in a temperature controlled room where routine maintenance, measurement of variables, and data analysis was required.
To test this I designed and constructed a replicated small-scale denitrifying bioreactor experiment. This involved a pressurised system that delivered a constant flow of nitrate-nitrogen dosed water into bioreactors containing wood chip, coconut husk, a combination of coconut husk into wood chip, and wood chip into coconut husk. The experiment was run over 67 days in a temperature controlled room where routine maintenance, measurement of variables, and data analysis was required.