Lake Tomarata decline alarms ecologists

Lack of water clarity and declining health of aquatic plant life at Lake Tomarata has alarmed Auckland Council ecologists.

The finding came to light following analysis of recent monitoring data.

It’s thought the decline in plant life may reflect an upsurge in algal blooms.

When present in large numbers, microscopic algae ‘join hands’ and bloom; this turns a lake green and reduces the amount of light available to aquatic plants, to the point where the deepest-occurring ones begin to disappear.

Blooms can be caused by an increase in nutrient concentrations in a lake, but algae can also multiply when large numbers of pest fish feed on the zooplankton that ordinarily graze on the algae.
Charophyte, the deepest occurring of the native aquatic plants, have all but disappeared from Lake Tomarata.

Council freshwater ecologist Matt Bloxham says the aquatic plants in the lake are on “life support” and it’s essential to reduce the rate of decline and stabilise the lakes.

“We need to quickly get our heads around which of the nutrients or pest fish are causing the algae to become more dominant and whether the key to success is keeping pest fish at as low levels as possible in the lakes,” he says.

Council communications advisor Liz Kirschberg says Aucklanders’ support for an environmental targeted rate and support for the Council’s Regional Pest Management Plan mean resources are available to understand what is going on in the lake, reverse the trend, and bring it “back from the brink”.