Lake Rotoroa

Lake Rotoroa is the largest of three lakes located within Hamilton city. The lake is used for a wide variety of recreational activities and is an important habitat for wildlife. Lake Rotoroa has high levels of phosphorus and nitrogen, which affect water clarity. However, water quality has improved over recent years as nutrient levels decline and aquatic plants re-establish.

Lake Rotoroa is located close to the centre of Hamilton city and has a surface area of about 54 hectares. It has a maximum depth of six metres and an average depth of two metres.

Lake Rotoroa is the largest of three lakes within Hamilton city. The lake catchment is urban, with a mix of residential housing and recreation reserve. Lake Rotoroa is used for a variety of recreational activities including:

• yachting
• dragon boating
• walking.

The National Institute of Water and Atmospheric Research (NIWA) monitors the water quality of Lake Rotoroa on behalf of the Hamilton City Council.

Water monitoring results show that Lake Rotoroa is eutrophic. It has:

• high levels of nutrients with phosphorus limiting plant growth
• moderate levels of microscopic algae (phytoplankton)
• moderate water clarity.

NIWA also monitor submerged aquatic plants (macrophytes). During the last vegetation survey in mid 2002 they found expanding areas of native aquatic plants called charophytes. However, they noted that the introduced invader, oxygen weed (Egeria densa), was also expanding in area.

Check out our Nutrient Enrichment of Shallow Lakes indicator and its data for Lake Rotoroa.

NIWA collected water quality samples monthly between January 1992 and December 2004. Lake Rotoroa’s water quality improved during this time.

Monitoring found:

• Secchi depth (an indicator of water clarity) improved as a result of less particulate matter in the water, water clarity is now about twice what it was a decade ago
• levels of Chlorophyll a and nutrients were reasonably stable.

The water quality of Lake Rotoroa declined significantly in 1989/90 following the die-off of the lake’s macrophytes. As the macrophytes decomposed they released large amounts of nutrients into the water. This resulted in the growth of microscopic algae (phytoplankton) and a decline in water clarity. The degraded water quality affected waterfowl and other wildlife.

Macrophytes began to grow back in Lake Rotoroa from mid 1998. Growing macrophytes absorb nutrients from the water, meaning less are available for phytoplankton. They also help to stabilise bottom sediments, reducing the amount getting stirred up by the wind.

Lake Rotoroa’s water quality may improve further as phytoplankton decrease and macrophytes increase. However, it’s important that invasive exotic plants like oxygen weed (Egeria densa) don’t replace the native plants present at the moment.