Doctoral student looks at Tauranga sea lettuce blooms

23 April, 2012

Doctoral student looks for answers on sea lettuce blooms in Tauranga Harbour

It can be the scourge of harbour users in Tauranga, but sea lettuce is about to go under the microscope. PhD student Alex Port, from Germany, is spending three years in New Zealand researching why sea lettuce is so abundant in the harbour at certain times of year.

Alex is part of the Intercoast programme which is a joint initiative involving the University of Waikato and the University of Bremen in Germany. It involves PhD students working on coastal research projects of global significance in the two countries.

Sea lettuce (Ulva species) is a naturally-occurring green algae which becomes a nuisance to locals when wave action causes it to break up and wash ashore to rot... and smell!

Alex says a number of factors, such as water temperature, sunlight, sediment distribution and coastal ocean processes could be influencing when and why ‘blooms’ of sea lettuce build up in the harbour.

“My research will look into each of the potential influences that contribute to sea lettuce abundance and try to determine with more accuracy when we could expect to see a bloom occur and why they happen. At the moment it’s really difficult to say which factors will determine a bloom or not,” he says.

His research is being funded by the Bay of Plenty Regional Council which has monitored the abundance and nutrient status of sea lettuce in Tauranga Harbour since the early 1990s. Water Science and Support Manager Rob Donald says while monitoring has identified a number of factors influencing sea lettuce blooms, the key “trigger” factors remain unclear.

“We hope Alex’s research will clarify this and help us identify management interventions to minimise the problem,” says Mr Donald.

The Regional Council is supporting the project through a scholarship and by providing data, information and access to expertise which has been compiled over the past 20 years.

Developing research models

Alex has spent the last six months setting up his research framework, testing methodology, taking water nutrient samples in the harbour and trialing his custom-built collection cage, which allows him to measure the transport of sea lettuce in and out of Waikareao Estuary.

The next phase of the project will be based back in Germany for four months, where he will work with specialist mathematicians to develop the mathematical calculations and models on which to base his research.

“The models will allow me to process the information I get from my field work and run simulations. Luckily a lot of calculations into water nutrients and historical data on sea lettuce blooms are already available from ongoing research by the Bay of Plenty Regional Council and these will be helpful to my own research.”

Other calculations, such as the amount of sunlight and the depth to which it penetrates the water, still need to be worked out.

“Because there are a number of factors potentially influencing the growth of sea lettuce, there’s a lot of work to do on developing the mathematical models to enable me to conduct the research accurately,” Alex says.

Intensive sampling

His field campaign will focus on the 2012-2013 growth period from early spring until next winter, during which time he will complete six intensive sampling studies of Tauranga Harbour, each lasting around three days.

Each exercise will involve surveying the intertidal and subtidal distribution of sea lettuce at selected sites around the harbour, analysing water nutrient, oxygen and temperature levels and using an underwater camera to locate crops of young ulva plants.

Alex’s University of Waikato supervisor Dr Karin Bryan says it’s a challenging project.

“Most doctoral projects involve intensive studies into distinct aspects of the chosen research but Alex’s project is all interlinked. He needs to find answers in one part of the research before he can make progress on the next part,” she said.

Different species of sea lettuce are common all over the world, and part of Alex’s research will be to compile what has been learnt from sea lettuce incursions in other countries, and decide which findings can be transferred to the situation in the Bay of Plenty.

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