EXECUTIVE SUMMARY

The purpose of this report is to identify the most dominant anthropogenic influences on water and habitat quality within the Maitai River catchment. The influence of the Maitai Reservoir is considered relative to other catchment pressures.

Observations of river health in the Maitai catchment approximately align with modelled pressures derived from various anthropogenic influences, where land-use type is the main influence on river health.

Plantation forestry and urban stormwater runoff appear to be the dominant pressures facing the Maitai catchment. Forestry is the main land use in the mid-catchment. Macroinvertebrate community health indicators are sensitive to changes in nutrient and deposited fine-sediment levels. High levels of both of these contaminants have been associated with tributaries in parts of the catchment dominated by Forestry. Therefore, the observed declines in macroinvertebrate community indicators throughout the mid-catchment suggest that forestry works are negatively impacting upon stream biota downstream through increased fine sediment and / or nutrient levels. Benthic cyanobacteria blooms may be an emerging issue due to increased input of nitrogen from forestry activities. Toxins produced by benthic cyanobacteria mats can restrict recreational activities.

The lower catchment, especially the lower tidal reach, is particularly degraded relative to the rest of the catchment in terms of macroinvertebrate community composition and contaminant loads in the sediments. Stormwater drains are the likely source of contaminants that bind to the Maitai River sediments. Concentrations of faecal indicator bacteria regularly exceed guideline values for swimming in parts of the lower Maitai. It seems likely that most of the faecal bacteria present in the water are derived from faults in the city's sewage system, though contamination from waterfowl is another potential source. There is evidence of a strong signal for human faecal markers in the Maitai River downstream from the Halifax Street footbridge.

Based on data assessed in this review, the impact of the Maitai Reservoir on the mid and lower Maitai River is likely to be comparatively minor when considered in the context of the magnitude and extent of other pressures facing the catchment. Habitat degradation in the lower catchment is largely attributable to forestry and urbanisation, more specifically, sediment and nutrient loading from Sharland and Groom Creeks, and nutrient and contaminant loading from various stormwater drains. Nutrients from the back-feed discharge may contribute to cumulative impacts within the catchment, but these are secondary to other land-use pressures.

However, three specific ecological issues that arise as a result of the Reservoir and may affect ecological values in the wider catchment are:

1. Concentrations of naturally occurring heavy metals (manganese, iron, nickel and chromium) are higher in the upper Maitai River than in the mid-catchment. There is a low to moderate risk that this issue may be exacerbated by the discharge of anoxic water from the Maitai Reservoir.
2. The Reservoir spillway is the most significant fish passage obstacle within the Maitai River, restricting access for native fish (particularly longfin eel and koaro) and trout to habitat in the Maitai Reservoir and North Branch.
3. Water chemistry is altered below the Reservoir's back-feed discharge, especially during periods when anoxic reservoir water is discharged. Subtle changes in water chemistry can alter algal communities, potentially providing favourable conditions for undesirable species (e.g. toxic cyanobacteria).