Dawn had already broken when our plane touched down on Mahé island, lighting up the cloudy shifting sky and the turquoise waters lapping at the runway’s edge. From my plane window I could already see sheer cliffs of pinky-grey granite rising up from the narrow coastal plain, covered in lush tropical vegetation. I was sleepy, but excited to be here. Mahé is the largest island of the small island nation of Seychelles – a tropical archipelago in the western Indian Ocean. I was visiting for three weeks to begin the field work component of my PhD research. My destination: the mangroves that fringe the western side of the island.

 

Mangrove trees

The mangrove forest at Barbarons, on the western side of Mahé island.

 

Wading

Wading in the mangrove forest.

Mangrove environments occupy a narrow coastal zone between high and low tide. These environments are dominated by mangrove trees – species that have evolved to tolerate frequent flooding by salty, marine waters as the tides wax and wane. Mangrove trees can be fussy – some species like to be flooded more often than others, and they tend to position themselves accordingly. As scientists, we can observe this in the vegetation patterns across a mangrove environment. Their fussy nature means mangroves are sensitive to sea-level changes over time, and sediments record these changes beneath the mangrove trees. My PhD project involves examining these mangrove sediments – to understand what parts of the sediment best inform us about past sea-level changes.

We drove the winding, cracked roads up over the mountainous centre of Mahé to a mangrove occupying the mouth of the Dauban River, near the village of Barbarons. Over the following weeks, the field team and I were squelching about in the mangrove mud and roots, collecting field observations and sediment samples. Our assumption is that as sea level rises and falls, mangrove species will shift up or down the environment in response. By sampling the modern surface sediments across the mangrove environment, we can better understand shifts we see in sediment cores, recording environmental changes over time.

 

Mangrove coring

Collecting sediment cores in the upper zone of the mangrove forest.

 

Mangrove survey

Surveying in the mangrove forest.

I soon discovered that working in a mangrove isn’t a glamorous as I thought it would be. While Seychelles has undoubtedly some of the most exquisite white-sand, coconut palm-fringed bays, the mangroves themselves are a different story. Even in the heat and humidity of the tropics, we kitted-up in full boiler suits and DEET to keep nasties and the mud at bay. In the mangrove canopy, golden potter wasps make their nests, and large palm spiders weave their webs across the branches, perfectly at head-height. Hungry mosquitoes thrive in the damp, shady mangroves, and stinging hairy caterpillar grubs drop down from the leaves above. We had to keep an eye on our watches, and make sure we had an escape out of the mangroves as high tide encroached and flooded our sampling areas.

While we didn’t enjoy the company of some our wildlife neighbours, there were others that were more exciting to come across. If you stayed still for a moment, the ground surface would erupt with movement of brightly-coloured crabs, all fighting for a leaf or crumb of our sandwiches to scurry away into their holes with. Bright green geckos scampered up tree trunks, and birdsong followed us through the forest. Guppies, hermit crabs, snails and juvenile reef fish could be seen swimming amongst the submerged mangrove roots.

 

Mangrove creatures

Red Claw Crab, Palm Spider (freaky but harmless), and the well-camouflaged Green Day Gecko.

 

Rising tide

Collecting surface sediment samples and racing the incoming tide!

As well as collecting sediment samples, traps were also deploys to collect pollen with the help of the Seychelles National Park Authority. This is to better understand how mangrove pollen is transported to the sediment surface, and these are left to collect material for about a year. After some successful sampling, the sediment material was all boxed up, and checked-in as our baggage home to the UK. Grateful for escaping the muddy mangroves but sad to leave tropical island paradise, we boarded our flight back to a drizzly and cold Newcastle… until next time!

 

Now, laboratory work begins. There are many different methods to gain an understanding of past environmental changes. An objective of my research is to explore brand new methods to reconstruct past sea level from mangrove sediments. The molecular remains of mangrove organic matter (called biomarkers) can be used to understand past salinity (salt concentrations), even though the leaves and roots the molecules come from have decomposed away. These molecules can be distilled and analysed in the laboratory at Durham University.

It will be many months before I have any further results, but I’m ready to get cracking. To be continued!

 

Below is a video of field work in the mangroves, with Juliet explaining how this research could help us understand sea level changes.