Cayman’s diminishing mangroves

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Longtime residents of Grand Cayman will already know how drastically the island has changed in recent decades, as construction and development have accelerated to keep pace with a growing population. Analysis from the Department of Environment can help quantify how drastic that change has been for the island’s wetlands.

In 1976, the western end of Grand Cayman was comprised largely of wetlands. From Prospect to West Bay, researchers recorded 5,330 acres of mangrove and sedge marsh wetlands that year. By 2013, that number had dropped 69% to around 1,633 acres.

But what does it mean to lose such habitat? In an age of rising sea levels and climate change, DoE Terrestrial Unit Manager Frederic Burton explained that the implications could be quite significant for Cayman.

“If sea level rise continues and the mangrove can’t keep pace, there is frankly very little we’d be able to do,” he said.

“We stand to lose a lot if the sea level comes up, and we need to be vocal about that internationally. We can’t stand up and speak about that if we aren’t doing anything about that ourselves.”

First, what are mangroves?

Mangroves are shrubs or small trees that grow along the coast. As salt-tolerant plants, they grow well in brackish waters and along Caribbean coastlines.

Their complex, tangled roots systems that seem to climb across the Earth are one of their most identifiable characteristics.

“In the Cayman Islands and elsewhere, mangroves are noted for their role in shoreline protection, carbon sequestration and storage, filtering of sediment and pollutants, and as habitat for threatened species,” Burton said.

In Cayman, there are four main species of mangrove: red, black, white and button.

Red mangroves, also known as Rhizophora mangle, have dark green leaves with a paler green underside.

Black mangroves, Avicennia germinans, have dark green leaves with a whitish-green to grey underside, and excrete salt crystals through their leaves.

White mangroves, Laguncularia racemosa, have thick rounded leaves that are light green on both sides and have notch at the tip.

Button mangroves, Conocarpus erectus, have leaves that are dark green on both sides.

Storm protection

When a storm surge hits a mangrove habitat, the web of roots and leaves help to dissipate the power of the impact. While a mangrove cannot fully stop the flow of water, it disperses the wave and diminishes its energy.

This effect was observed after Hurricane Ivan in 2004.

“What was amazing to me even in Ivan, we looked in the central mangrove wetland to see how far the storm surge had gone. It struck me that it didn’t get all the way in,” Burton said, noting that the inner depths of the central mangrove were largely safe from the storm.

“In West Bay, we don’t have that kind of protection any more. We’re just relying on the elevation of the land and keeping our fingers crossed.”

Weather and habitat regulation

Residents of Cayman may have noticed a common weather event on island: a rain cloud column seems to form in the middle area of the island and moves west, bringing showers with it.

Black mangroves excrete salt crystals through their leaves.

That common occurrence can be attributed to mangroves. Burton explained that as the island heats up during the day, the mangroves take up water in their roots and begin to transpire. That moisture accumulates and begins moving west.

“If we turn all the mangroves into tarmac and buildings, the island would get a lot drier,” Burton said. “The sea levels will move into dry land and the Cayman Islands will get a lot smaller.”

Mangroves also provide necessary support for farmers and others who use fresh water stores on island. The mangroves help keep the water table back and serve as a dam of sorts, particularly for the Lower Valley area.

Habitat

Tangled mangrove roots make great homes for schools of fish, turtles, lobsters and invertebrates such as oysters, crabs, snails and shrimp. The diversity of life in mangrove habitats is also what makes them ideal for snorkelling, fishing and kayaking.

“It’s a safe place for juvenile marine life to thrive because it’s entangled and they can hide,” Burton said.

Snapper, barracuda, tarpon, snook, grunts and a variety of other fish species call this ecosystem home.

Above water, Cayman’s parrots make use of hollowed trunks of dead black mangrove trees for nesting.

Red mangroves are well adapted to saltwater and so thrive where many other plants cannot grow. – Photo: Stephen Clarke

For ‘peat’ sake

As sea levels rise, so do mangrove communities. In fact, mangroves must keep up with sea level changes, or the plants risk drowning and death. To do this, mangroves lay down more and more peat as they creep inland to keep pace.

“First, imagine sea level creeping up on a coastline with a bit of sand and emerging bedrock. A few mangrove propagules get stranded, take root and a stand of red mangrove grows in the tidal zone,” Burton said.

“Hundreds of years later, the sea has come up more and the mangroves have dripped new propagules in the sheltered water inland, so the mangrove stand extends further and further inland as the sea level rises.”

If sea levels rise too rapidly, the mangroves cannot lay down peat at the same rate, resulting in drowning and die-off.

“Any increase in the frequency or intensity of hurricanes, especially when compounded by alteration or blocking of surface water outflow pathways by road construction and land filling, can also lead to general mangrove drowning and death,” Burton said.

Button mangroves, Conocarpus erectus, have leaves that are dark green on both sides.

Food and fertiliser

Dead leaves and debris, such as bird droppings and dead insects, that gather in mangrove habitats make the water there incredibly nutrient rich.

“When we get a tropical depression or some such rainmaker that dumps tons of rain on the wetland, the water table reaches a threshold where massive surface outflow pours the nutrient-laden mangrove water into North Sound. The tea-coloured water is fresher than seawater and so floats out over North Sound,” Burton said.

“This is fertiliser for all of North Sound, stimulating seagrass growth and feeding scavenging crustaceans and so on, fuelling the whole food web that ultimately includes the fish, conch, lobster and stingrays that are so important to many people.”