Report: TR 2008/35R
Author: Kerry Bodmin, Paul Champion, Fleur Matheson (NIWA)

Abstract

Environment Waikato engaged NIWA to carry out the following activities at Lake Mangahia: review the summer minimum water level, describe current vegetation including priorities for weed control, delimit aerial spray exclusion zones and set backs for fencing, recommend species and locations for planting, and present options for nutrient removal from inflow water, after reviewing water quality data. Subsequently NIWA was asked to provide indicative information on the use of an infiltration filter system at Lake Mangahia similar to the one proposed for Lake Serpentine.

A site visit was conducted on 11 February 2008 with landowners, NIWA staff and Environment Waikato staff to view the marginal vegetation, farm drains and discuss current management of the lake environs. NIWA staff made an additional site visit on 26 February 2008 and undertook a bathymetric survey using sonar equipment and a vegetation survey of the lake margins. Five transects were established from the lake edge to the fenceline with species presence, height and % cover recorded. Based on historic and recently recorded lake depths, the water level of Lake Mangahia appears to be lower than it was before the early 1990’s. The current lake depth set by the weir is estimated at 1.8 m, 0.5 m less than the surveyed depth of 2.3 m recorded in 1979 (Irwin, 1982) during a dry year. Therefore, an increase in weir outlet height of 0.5 m to 37.3 m a.s.l. (Moturiki Datum) is recommended. Re-measurement of water levels in relation to the bench mark used by Irwin would provide greater certainty to the conclusions based on historical data.

Six different vegetation types and 68 plant species were recorded from the margins of Lake Mangahia. Of note was the extent of peat influenced native vegetation that included parasol fern (Gleichenia microphylla), and regenerating kahikatea (Dacrycarpus dacrydioides) which contribute to the ecological values of the site. Extensive clearance of manuka (Leptospermum scoparium) and encroachment by grey willow (Salix cineria) is evident from comparisons of historic and current aerial photographs, whilst comparisons between the current survey and a previous survey in 1993 also showed a reduction in the shoreline occupation of bamboo spike sedge (Eleocharis sphacelata) and flax (Phormium tenax) / pukio (Carex secta). Pest plants recorded during the survey were prioritised for control with high risk species including grey willow, royal fern (Osmunda regalis), japanese honeysuckle (Lonicera japonica), arum lily (Zantedeschia aethiopica cv. green goddess) and Chinese privet (Ligustrum sinense). Guidance on treatment options was provided for high and medium priority species.

Four aerial spray exclusion zones were identified for willow control operations which incorporate areas of open willow canopy where spray penetration would kill non-target native plants in the understorey, areas with high value kahikatea vegetation and vegetation features the land owners wish to preserve. Recommended follow-up monitoring to assess the outcome of the spray operation is outlined. Recommended setback distances for fencing were identified based on the surveyed wetland vegetation patterns within the current grazed paddocks. Enhancement plantings of suitable species were suggested at priority sites including the wetland area at the north end of the lake, newly fenced pasture margins and the Waipa District Council reserve.

Analysis of available lake water quality data using the Trophic Level Index (TLI) method indicate an increasing trophic status (nutrient enriched) to a current status of hyper-eutrophic plus. However more data collected at least seasonally are recommended to confirm recent trends. A combination of a significant increasing trend in P concentrations and the low N:P ratios suggest inputs of both N and P nutrients to the lake should be minimised where possible.

Nutrient remediation options were considered and include the construction of an infiltration filter system to intercept and treat drainage water entering the lake from the extensive north-eastern catchment. The addition of a wood chip area of 0.13 ha in size would target nitrogen removal by denitrification. An estimated size for the complete infiltration filter system to treat the drainage inflow is approximately 0.83 ha. It is anticipated that the system could remove at least 80% nitrogen, 60-80% phosphorus, reduce suspended sediments and reduce E. coli from the main drain into the lake. Indicative costs for construction of the complete infiltration filter system, on commercial rates as if done by a contractor, are estimated at $200,000. Measurements of drainage nutrient inflows are recommended to confirm the assumptions made in the assessment.

Note: This report was originally published in July 2008 as Technical Report 2008/35. This new edition corrects a minor data error which appeared in Table 3 and Table 4 (pg 37). As a result of correcting this data a small change was required to be made to associated text on pages 40-41. These changes make no difference to the conclusions or recommendations in the report, but in the interests of data accuracy we are re-releasing this publication.