Wellington lake in top notch condition

A NIWA survey has ranked a coastal lake in Greater Wellington’s East Harbour Regional Park as one of the most outstanding examples of a lowland lagoon found in New Zealand.

The 2011 LakeSPI aquatic vegetation survey, commissioned by Greater Wellington, ranks Lake Kohangatera in the Parangarahu lakes blocks as the 10th best lake in the country out of more than 200 New Zealand lakes assessed to date. NIWA found Lake Kohangatera to be in excellent ecological condition and considered it “a nationally outstanding example of a lowland lagoon system”.

The second Parangarahu lake – Lake Kohangapiripiri – was ranked 47th and its condition is rated as “high” while Lake Pounui in the Wairarapa – another lowland lake – was ranked 66th (also “high”). The findings for all three lakes are good news given that most lowland lakes in New Zealand are highly degraded from land use modification, with many affected by introduced plants and fish.

The survey found that both Parangarahu lakes contain several plant species considered naturally uncommon or endangered, highlighting the high indigenous biodiversity values these lakes support. However, the survey also identified a potential threat to these values in Lake Kohangatera – the presence of Canadian pondweed (Elodea canadensis), which may displace native vegetation and lower biodiversity values in the future. A follow-up survey found the more aggressive Brazilian Waterweed (Egeria densa) in the uppermost reaches of Gollans Swamp upstream of the lakes.

Greater Wellington Social and Cultural Wellbeing Committee Chair Nigel Wilson says the surveys will inform the review of Greater Wellington’s regional plans, in particular the development of policies to manage the lakes’ ecological values.

“The surveys reinforce the importance of these lakes and the high level of care needed to protect them. We’ve taken swift action to prevent the spread of weeds in Lake Kohangatera by creating exclusion zones around the infestations and organising further inspections in the Gollans Valley swamp area.” (Walking and cycling access along the tracks in the Parangarahu area isn’t affected.)

Port Nicholson Block Settlement Trust Chairman Professor Sir Ngatata Love says the lakes are a taonga for Taranaki Whanui and indeed the wider community. “The survey highlights the importance of this beautiful environment and reinforces the Trust’s belief that these lakes must be kept in the best possible condition for generations to come. We’re committed to working with Greater Wellington to make sure of this.” (The Port Nicholson Block Settlement Trust owns the lakebeds and surrounding esplanade strips and jointly manages the Parangarahu Lakes Area with Greater Wellington.)

The surveys form part of Greater Wellington’s Environmental Monitoring and Investigations department’s State of the Environment monitoring programme and were commissioned because little was known about the current ecological condition of the lakes.

About the surveys

The survey method was LakeSPI (Submerged Plant Index), a nationally recognised method for assessing lake “condition” using aquatic vegetation. This method involves scuba divers assessing a range of metrics, such as native plant diversity and the extent of invasive weeds, along several transects (from shore to the deepest part of the lake) representative of the lake.

The LakeSPI surveys are repeated every five years but for Lake Kohangatera, the next survey will be undertaken in two years time to reassess the status of exotic weed growth.