Rotting algae blamed for Waipu estuary lagoon eel, fish deaths

11 Mar 2021, 8:36 AM

The mass deaths of large numbers of eels, fish and other aquatic life in a Northland lagoon have been linked to unpleasant, but naturally-occurring, red algal blooms which have once again been plaguing the Waipu Cove and Bream Bay areas over summer.

Northland Regional Councillor Rick Stolwerk, who represents the council’s Coastal South Constituency, says the council was notified about dead eels and fish in the lagoon area of Waipu Estuary last weekend. 

Councillor Stolwerk says a staff member sent to investigate found dead fish and also recorded high water temperature and ‘incredibly low’ dissolved oxygen levels in the water on Sunday.

Council officers had continued to investigate the event this week and it appeared bacteria were hard at work breaking down a large quantity of dead and rotting red algae (seaweed) that had washed into the upper parts of the lagoon from Bream Bay and become trapped.

“In the upper area of the lagoon there’s very little circulation and exchange of water with the open coast and once again our staff recorded very low dissolved oxygen concentrations in the lagoon.”

The council’s Resource Scientist – Coastal, Richie Griffiths, says ironically, the warm and relatively shallow conditions in the lagoon were probably initially favourable for the red algae to continue to grow, but that had changed once the algae began to die with its resulting decomposition consuming much of the oxygen in the water.

“This can lead to a rapid decrease in dissolved oxygen levels that will cause stress and mortality to other marine plants and animals.  The dead and decaying plants and animals will also fairly rapidly cause an unpleasant smell.”

Councillor Stolwerk says red algae involved appears to be the same species that has been found at Waipu Cove in recent years and which had previously been sent to the National Institute of Water and Atmospheric Research (NIWA) for identification .

“As far as council’s aware, the algae itself isn’t toxic or harmful as such, but it obviously affects the amenity value of the area and can definitely generate unpleasant odour as a result of the generation of hydrogen sulphide gas, which smells like rotten eggs.”

Mr Griffiths says the regional council has advised the Northland District Health Board of the situation and the board will be assessing if the gas generation has any public health implications.

Given the area is a wildlife refuge the Department of Conservation had also been informed as had local tangata whenua Patuharakeke.

Mr Griffiths says quite a large section of the lagoon is affected and unfortunately given the scale of the problem and the relatively limited flushing of the water in the lagoon – even during a storm – the issue is likely to get worse before it gets better.

While unpleasant, the process was a natural one and was a typical cause of decreased dissolved oxygen levels in water.

Both Mr Griffiths and Cr Stolwerk say the problem is likely to persist until the algae has broken down or is flushed out in an autumn storm.