Expanding mangroves – burden or blessing?

12 June 2018

Dr Erik Horstman from the University of Waikato's Coastal Marine Group.

Dr Erik Horstman from the University of Waikato's Coastal Marine Group will explore how mangroves play an important role in helping to fight the impacts of climate change, sea-level rise and coastal inundation at Café Scientifique in Tauranga on Monday 18 June.

"Mangroves, or Mānawa, have gained a bad reputation, with people mainly focusing on the inconvenience they cause, due to their negative impacts on recreational, navigational and amenity values of estuaries," says Dr Horstman. “Communities often emphasise the positive effects of mangrove clearance on the restoration of these values. However, assessing such benefits should be balanced with the loss of other ecosystem services that benefit humans, such as flood protection and buffers against coastal erosion, but also other services related to carbon sequestration and nutrient storage.”

Dr Horstman says the removal of mangroves has been found to result in visible erosion of neighbouring saltmarsh vegetation at a number of mangrove removal sites. Primary production associated with mangrove vegetation, binding nutrients and building carbon stocks, is lost upon mangrove clearance and carbon emissions from the cleared soils increase, while nutrient concentrations in the water can proliferate.

“Mangroves’ contribution to coastal protection, especially on the longer term and in the face of a changing climate, is largely being ignored. The extent and impact of the negative effects of mangrove clearance are largely unaddressed and need much more attention for the development of integrated coastal zone management strategies that carefully weigh both the disadvantages and the opportunities related to mangrove expansion. Policy and actions of today will determine what our coasts look like in the future and hence our current decisions could limit the options to assure future coastal safety.”

At Café Scientifique Dr Horstman will explore why mangroves are flourishing in New Zealand despite their rapid decline worldwide. He will also address whether the current practices of mangrove management in New Zealand are solving or creating a problem.

“Mangroves generally provide a wide spectrum of ecosystem services. In the tropics they are an important habitat to many species; they are a source of wood and food, offer recreational and cultural values, and most of all, mangroves regulate the biophysical dynamics in the coastal zone. Regulating services are common for mangroves all around the world, as they are largely inherent to their location in intertidal parts of the coastal zone. These regulating services comprise flood protection, erosion mitigation, carbon sequestration and filtering sediments and nutrients out from the water.”

Dr Horstman, originally from the Netherlands, holds a PhD in civil engineering and has been studying the biophysical interactions in mangroves and their contribution to coastal safety and stability for over nine years. Before coming to New Zealand he researched mangroves in Singapore, Malaysia, Thailand and the Caribbean. He is currently working on a Royal Society Marsden-funded research project studying the dynamics of water and sediments in various mangroves around our coasts, trying to find the driving factors for their development and stability.

The lecture will take place at the Tauranga Yacht & Power Boat Club, 90 Keith Allen Drive, Tauranga on Monday 18 June at 7pm. There is a $5 cover charge. Register here.

Café Scientifique is a regular Tauranga-based seminar series where anyone can explore the latest scientific thinking and research from national and international speakers. Café Scientifique is organised by Saffron Consulting Ltd, with support from the University of Waikato. For event information contact: