Meridian Energy Limited have been monitoring the physical and biological environment in Doubtful and Milford Sounds since 1997, as part of their consent to discharge freshwater from the Manapouri Power Station into Deep Cove.
For the past eight years scientists from the Cawthron Institute, NIWA and Otago University have been collecting and analysing environmental data to determine whether there are any changes to the environment beyond those associated with the original construction and operation of the station.
In 2010, Meridian was granted consent to increase the tailrace discharge, and in 2012 to 2013 this consent was implemented for a 32-day period. However, over the whole year the mean tailrace discharge was the lowest on record since the monitoring programme began, due to low lake levels.
Analysis of salinity and temperature data from eight moorings in Doubtful Sound and a reference mooring in Milford Sound showed that the behaviour of the low salinity layer (LSL) in Doubtful Sound was similar to previous monitoring years, and weather events and subsequent deepening of the LSL fell within the typical ranges observed in the fiords.
The short duration of the increased discharge means that a full assessment of the effects cannot be made at this stage, but will be investigated in more detail in 2014.
Most recently, in February 2013, staff from the Cawthron Institute and NIWA were in Doubtful and Milford Sounds to monitor rock wall habitats including seaweed and invertebrates such as sponges, mussels and black corals.
Established sites were visited and photographs were taken at different depths in the intertidal and subtidal habitats, and counts, measurements and depth ranges were recorded for a number of target species.
Several soft sediment sites in the inner sounds were also visited, and cockles and pipi were measured and counted at shallow depths.
“Over the course of the monitoring programme, we haven’t observed changes that suggest there are effects associated with ongoing operation of the tailrace,” Cawthron Institute marine scientist Robyn Dunmore says.
“We have seen some fluctuations in the abundances of some organisms, but these changes have also been observed at our reference sites and are therefore associated with wider natural variability.”
The increased discharge of the tailrace has been implemented for a longer period in 2013, and this will allow the research team to do a fuller assessment of the effects in 2014.
To assist with the monitoring programme, the research team has taken thousands of photographs of permanently marked underwater sites.
Over the past eight years, they have compiled what is believed to be New Zealand’s most extensive collection of photographs from permanently marked underwater sites, with more than 4000 photographs.
Having this long-term data set is valuable in monitoring changes in species, populations and habitats.
“Photographs taken at permanently marked places on the rock wall each year allow us to compare increases or decreases in the abundances of seaweeds and invertebrates,” Robyn says.
“It is interesting to see how long-lived some species are. For example, the same individual sponge, black coral, sea squirt or seaweed can be present every year (see photos below for an example).”
Counting important indicator species in the field also allows us to assess any changes in abundances, and recording depth limits enables us to determine any changes in species’ depth ranges.
Black coral, sponges, brachiopods, sea stars, urchins and sea lettuce are just some of the invertebrates and seaweed the scientists are monitoring data on.
“It is interesting to see how long-lived some species are; the same individual sponge, black coral, sea squirt or seaweed can be present every year,” Robyn says (see photos below for an example). Photographs taken in 2006 and 2013 at a site near First Arm at 10m depth, showing the same sponge.
Article and photographs provided by Andrew Feierabend, Meridian Energy.