Lake Rotoehu

Lake Rotoehu

Lake Rotoehu

Rotoehu means "turbid" or discoloured waters. In former times, Lake Rotoehu was heavily populated with many pa sites located around its shores. The lake is very shallow and hides an island underneath its waters.

The historical meaning of Rotoehu, "turbid or murky" water, implies that the lake may never have been clear. The water quality remained fairly stable until 1993 when the level of nutrients and algae rose dramatically. This was attributed to a drop in the water level and a warm summer. It is a shallow lake with geothermal inputs, and nutrient levels remain high.

At a glance  
Lake size: 800 ha
Catchment area: 4710 ha
Elevation: 295 m
Average depth: 8 m
Deepest point: 13 m
Formed: 8500 years ago
No surface outlet but drains via northwards groundwater flow to springs  

 

 

 

 

 

 

 

 

Water Quality Status

Water quality in Lake Rotoehu sitting above its target TLI. While actions over the last few years including phosphorus locking and weed harvesting have been taken there is still more work to be done.

Catchment Map

Click here to view image of map.

Target and Results

To meet community expectations of water quality for Lake Rotoehu nitrogen inputs need to reduce by a total of 8.8 Tonnes of Nitrogen per year and Phosphorous needs to reduce by 708 Kilograms per year.  The main long-term intervention is land management change (along with continued weed harvesting), two voluntary agreements with landowners in the catchment have been implemented to achieve the required reduction. However, it will take time for the effects of these land use change agreements to reach the lake. As a result, the Programme continues with short term solutions including alum dosing and weed harvesting to reduce nutrients in the lake and improve water quality until the benefits of land use change are seen in the lake.

Water Quality target 

3.9

2017 result

4.6 

2016 result

4.6

 

 

 

 

 

 

The 2015 TLI for Lake Rotoehu was well above the target TLI due to a long hot summer. However, lake water quality needs to be considered by a rolling average rather than annually to get a true picture of water quality conditions. This is because annual readings can be influenced by one off climatic conditions. The 3 year rolling average TLI for Lake Rotoehu at 2015 is 4.12.

 

Nutrient information

 

Nitrogen

Phosphorus 

Total load 

53 Tonnes 

2.4 Tonnes

Target

44 Tonnes

1.7 Tonnes

Actions and achievements

 

Action

Target

Achieved as at June 2016

Status

Land use and land management change

6.6 T N

460kg P

8.45 T N

810 kg P

Two land use change agreements have been implemented to deliver these.

Phosphorus Locking

Adjusted annually to manage target TLI.

3.1 T P

Phosphorus locking plant operational at Soda Springs.

Weed Harvesting

3.5 T N

1.47 T N

0.2 T P

Weed harvesting of hornwort is completed annually, based on weed availability, to improve water quality.

Floating wetland

132 kg N

21 kg P

45.35 kg N

9.07 kg P

Construction of the floating wetland has been completed and plants are established.

Aeration

0

0

Aeration has been trialled in Lake Rotoehu and has demonstrated improved water quality on a localised scale. Aerators are now removed.

Action Plan

Lake Action Plans direct what steps need to be taken to improve lake water quality to meet community expectations.

Read Lake Rotoehu Action Plan