Stewart, S. D. (2008, May). A baseline description of nitrogen sources and food webs prior to large-scale restoration of Te Ihutai, the Avon-Heathcote Estuary, New Zealand. Avon-Heathcote Estuary Ihutai Trust.
Prepared in 2008, this report was a student project supported by the Avon-Heathcote Estuary Ihutai Trust, the Christchurch City Council (CCC), Environment Canterbury (ECan), and the National Institute of Water and Atmospheric Research (NIWA). The purpose of the study was to provide a basic isotopic description of major carbon and nitrogen sources in the Ihutai estuary. It also aimed to describe isotopic compositions and patterns in porewater and food webs. In 2008, treated sewage from Christchurch City was still discharged directly into the Ihutai estuary, causing high nutrient inputs and widespread environmental degradation, including hypoxia, loss of indigenous seagrasses, eutrophication, and mass growth of macroalgae species like Ulva lactuca. An ocean pipeline to divert the city effluent 3km offshore was proposed to be completed in 2009, removing 90% of the N input into Te Ihutai. This study was therefore crucial to gather quantitative data on anthropogenic influences on the estuary ecosystem, and how they would change after remediation.
Overall, it was concluded that U. lactuca primarily received N from wastewater, and therefore the pipeline diversion would likely result in drastic decreases in its biomass. Observed porewater concentrations were 10x higher than the surface water, likely due to biological N cycling in sediments. The study also concluded that the present food web was based on particulate organic matter (POM) due to eutrophication conditions favouring phytoplankton, however, future decreases in N availability may lead to a shift in the food web base to benthic microalgae (BMA). To conclude, the report recommended continued monitoring of the estuary to determine future impacts of the large-scale remediation project.