EXECUTIVE SUMMARY

The Maitai Reservoir has been operated by Nelson City Council (NCC) on the North Branch of the Maitai River just above the Forks to augment the Nelson municipal water supply since 1987. In anticipation of the re-consenting process in 2017, NCC has requested an assessment of the status of the Maitai Reservoir in terms of providing for the functioning and ecological health of the Maitai Catchment. This investigation conducted ecological surveys in the Maitai Reservoir and its major inflow, the North Branch tributary, in April 2013.

The purpose of these field investigations were to:

• assess if there were significant effects of North Branch drainage from mineral rich geological areas in the Dun mountain mineral belt area on the manganese (Mn) and iron (Fe) levels in the Maitai Reservoir, which have been reported to be high
• assess aquatic communities inhabiting the Maitai Reservoir and North Branch sincethe formation of the Maitai Dam in 1987. This included assessments of phytoplankton, zooplankton, periphyton, aquatic macrophytes, macrobenthic invertebrates, and fish
• use ecological metrics based on biological datasets to assess the ecological health ofthe Reservoir and the North Branch in comparison to the wider Maitai Catchment, as well as other systems in the region and on the South Island (for the reservoir).Evaluate reservoir habitats in the context of the Maitai Dam operations to better understand how these factors affect aquatic communities in the reservoir
• make recommendations on improving dam operations for aquatic community healthin the Reservoir and North Branch, and identify where further ecological survey work is required leading up to the re-consenting process in 2017.

Maitai North Branch

Manganese (Mn) and iron (Fe) concentrations in inflows did not appear to be influenced significantly by the Dun Mountain mineral belt area, which have been measured at high levels in the Maitai Reservoir during anoxic periods. However, more sampling in the North Branch would need to be undertaken to determine the natural background levels of metals, as this finding is based on only one sampling event.

Water quality (dissolved oxygen, conductivity, temperature and pH), periphyton and macroinvertebrate communities were similar to those at the upstream control biomonitoring site in the South Branch.

There was no indication that the degraded biological conditions downstream of the backfeed discharge were the result of follow through impacts of water quality in the North Branch inflow.

Fish passage may be an issue for longfin eels (not found in the North Branch) and koaro (a whitebait species found in low density in the North Branch). Koaro is the second most important species in the whitebait catch. Landlocked lake populations of this species can occur, but any koaro in the North Branch would historically have been from a sea-run population. The finding of only a few very large individual koaro in the North Branch suggests uncertainty regarding whether migrants could be bypassing the dam face to access the upper catchment.

Maitai Reservoir

Water levels in the Reservoir are operated over a reasonably narrow range by comparison to operations of most reservoirs with a mean (1994-2013) drawdown of 1.4 m, typically in late summer. The lowest lake level, being 3.06 m below the spillway height, occurred in 2006. These reservoir level operations are anticipated to maintain healthy ecological conditions in the lake-edge littoral community, which extends to the euphotic depth of around 6 m.

Water quality in the Reservoir can be characterised as a low productivity oligotrophicsystem, an overall Trophic Level Index (TLI) score of 2.0. Water clarity was relatively high (Secchi of 4.2) by comparison to other small South Island lakes, and is mediated mainly by humic tannin staining (DOC of 4.2 mg/l). The Reservoir is influenced by limestone geology of several rock formations (principally Dun Mountain, Stephen's argillite) in its catchment, and thus has high calcium cation concentrations and pH (8.2 in surface waters).

Thermal stratification in the Reservoir between December and April contribute todeoxygenaton in its hypolimnion, and was nearly anoxic between 10 and 25 m depth during April 2013 field survey. This is not entirely uncharacteristic of deep lakes in New Zealand, however is uncommon for low nutrient lakes such as the Maitai Reservoir. Dissolved oxygen (DO) declines are most likely related to dystrophic processes in the hypolimnion from the breakdown of organic materials that were either flooded or washed into the Reservoir basin. Dissolved oxygen declines present ecological issues associated with the backfeed of reservoir water to the South Branch of the Maitai River during stratified periods.

Phytoplankton communities present in the Reservoir are characteristic of low productivity systems, dominated by small celled cyanobacteria. Although one of the dominant species is known to produce cyanotoxins (Aphanocapsa sp.) further testing suggested there were no genetic markers for toxin producing strains of this species, and the overall biovolume concentrations for Aphanocapsa were low. The zooplankton community was entirely native species, dominated by the daphnids Daphnia carinata.

Surprisingly there were no submerged macrophyte species in the Reservoir, despite a suitable reservoir level operating regime. Possibly little colonisation has occurred due to the isolated nature of the Reservoir and lack of macrophytes occurring in upstream inflows which are steep and bouldery in nature. The lack of macrophytes in the Reservoir does mean poorer habitat quality for reservoir aquatic fauna, however this minimises use by waterfowl that could contribute to faecal bacteria loads that compromise human drinking water quality.

The littoral macroinvertebrate community in the Reservoir was of intermediate abundance and relatively low diversity by comparison to other South Island lakes, and Ceriodaphnia dubia. These species would be effective phytoplankton grazers and promote good water quality in the Reservoir should phytoplankton increase in spring following winter turn-over.

Surprisingly there were no submerged macrophyte species in the Reservoir, despite a suitable reservoir level operating regime. Possibly little colonisation has occurred due to the isolated nature of the Reservoir and lack of macrophytes occurring in upstream inflows which are steep and bouldery in nature. The lack of macrophytes in the Reservoir does mean poorer habitat quality for reservoir aquatic fauna, however this minimises use by waterfowl that could contribute to faecal bacteria loads that compromise human drinking water quality.

The littoral macroinvertebrate community in the Reservoir was of intermediate abundance and relatively low diversity by comparison to other South Island lakes, most likely related to the limited diversity of habitats and no submerged macrophytes being present.

Fish populations consisted of four species, numerically dominated by common bullies,followed by upland bullies and longfin eels. Koaro were found in the river and juveniles may inhabit the Reservoir, but were not detected. Brown trout are also present in the Reservoir but were not caught in the present survey principally due to survey methods used (fyke nets, gee-minnow traps).

Anoxic conditions in the hypolimnion limited fish to the shallow portions of the Reservoir, and no fish were caught below 5 m depth. The heavily skewed size class structure of longfin eels towards large (>600 mm) adult individuals indicates that limited or possibly no recruitment is occurring to the Reservoir from upstream juvenile migrants.