Sea lettuce impact revealed

By John Cousins of the Bay of Plenty Times -
Sea lettuce. Photo / APN
Sea lettuce. Photo / APN

An estimated 2.4 tonnes of drifting sea lettuce is entering Tauranga's Waikareao Estuary on each cycle of the tide during the worst affected times of the year.

The impact of sea lettuce on the estuary was revealed in studies carried out by Intercoast PHD student Alex Port.

It follows revelations earlier this week that the fumes from rotting sea lettuce in Matua had made two boys vomit, with an expert warning the gas could even kill.

Mr Port was a guest speaker at yesterday's inaugural Tauranga Harbour symposium organised by Environment Bay of Plenty. It drew nearly 200 people including politicians and representatives of iwi and environmental organisations.

He based his estimate of drifting sea lettuce on the nets set at the entrance to the estuary. Each net was catching 15kg to 20kg of wet sea lettuce per tidal cycle.

Mr Port said there was a huge amount of biomass floating around and there was always more on the incoming tide. He linked the growth of sea lettuce to nutrient concentrations in the harbour although there were still many questions around where it was coming from.

Mr Port was unable to comment on a question from Harbourwatch's Hylton Rhodes that algal blooms were being caused by discharges from the Ballance Agri-Nutrients fertiliser works at Mount Maunganui. Mr Port said it would have to be a very detailed survey.

Professor Chris Battershill of the University of Waikato's Chair of Coastal Science said specific "point source" discharges were being examined to track the sources of potential contaminants.

He said photographic surveys done off Tauranga's ocean beaches had revealed surprisingly extensive beds of sea lettuce. Mr Port said they were also surveying sea lettuce at the entrance to Tauranga Harbour.

Rob Donald from the regional council said the discharges from the fertiliser works were not in sufficient concentrations to be causing a sea lettuce bloom. The council monitored Ballance's resource consent.

Mr Donald said it was assumed that some of the nutrients came from oceanic upwellings entering the harbour although this had not been clearly demonstrated.

The Tauranga Bridge Marina complained about the "terrible smells" from where the rotting black "guts" of sea lettuce got trapped between marina piers.

Another Intercoast student Clarrise Neimand has set up a series of plots off Tuapiro Point for a month-long trial to measure the impact of sea lettuce mats on the habitat underneath.

She said the impact of sea lettuce on wading bird feeding grounds had global consequences, such as the migrations between Alaska and New Zealand of the bar-tailed godwit.

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