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Fieldwork off the coast of Belize in Central America is revealing how coastal tropical forests composed of mangroves have kept up with sea-level rise over the Holocene Epoch.

A U.S. Geological Survey (USGS) project is being conducted in the Mesoamerican Barrier Reef system, which extends 220 km (140 mi) from the southern part of the Yucatan Peninsula to the Bay Islands of Honduras and contains the longest unbroken coral reef in the Western Hemisphere. Scattered throughout the barrier reef complex are hundreds of low-lying mangrove islands, which are the target of this research.

From September 2 to 17, 2009, USGS scientist Karen McKee conducted research on mangrove islands off Belize with her assistant, William Vervaeke, in partnership with the Smithsonian Institution and university colleagues. The information gathered during this recent trip will aid in understanding how U.S. coastal regions may be affected by future sea-level rise and climate change.

McKee has studied various aspects of these mangrove ecosystems for the past 25 years and knows that the logistics involved in carrying out fieldwork in such remote areas can be daunting. Mangrove forests are one of the most difficult environments in which a scientist can conduct field research.

Photo of the roots of a mangrove tree.
Conducting research in a mangrove forest is logistically challenging because of changing water levels, soft sediments, and tangled roots.

First, there are the tangled masses of aerial roots that make every step tedious and time consuming. Walking through mangroves is also difficult because the ground is soft and mucky. Mangroves occur in the intertidal zone, which means they are flooded part of the time, and research must be timed to coincide with the tides. To top it all off, the tropical setting is hot, humid, and full of biting insects. Fortunately, the Smithsonian Institution's field station on Carrie Bow Cay provides a base of operations from which several mangrove islands can be reached by a short boat ride. The station is equipped with laboratories, dormitories, and other facilities necessary to support studies of reef, seagrass, and mangrove ecosystems.

Why Study Mangroves and Sea-Level Change in Belize?

The mangrove islands in Belize are isolated from the influence of terrigenous (land derived) sediment and fresh water and thus are sensitive monitors of changes in sea level. Tectonically, the study area is considered to be relatively stable, without significant tectonic subsidence or uplift. Also, until very recently, the mangrove islands have been relatively unaffected by humans, unlike mangroves in other parts of the Caribbean or coastal wetlands in the United States.

These islands are underlain by deep deposits of peat (organic matter formed from plants) as much as 11 m (36 ft) thick, or as deep as a three-story building is high. These peat deposits have accumulated over the past 8,000 years as climate has warmed and sea level has risen. The peat is composed of the decaying parts of mangroves, mostly fine roots and other organic matter that are preserved in the flooded and anoxic (without oxygen) soil.

Because the roots and organic matter accumulated at or near sea level, the thick section of peat provides a record of rising sea level. The Belize site contains the longest continuous peat record of sea-level change currently known. The mangrove islands in Belize and other peat-forming mangroves in the Caribbean region are thus vital indicators of sea-level change and natural laboratories for examining the processes that allow coastal wetlands to keep up with rising seas.

How Are Elevation Change and Soil Accretion Measured in Mangrove Ecosystems?

Photo of a metal instrument being held by a hand in an area with lots of tree roots.
A Surface Elevation Table (SET) is used to track changes in soil elevations on mangrove islands. As peat forms, the soil surface expands upward, allowing mangroves to keep up with rising sea level.

High-resolution measurements of elevation change and soil accretion are made with Surface Elevation Tables (SETs) and sand marker horizons. We established 27 SETs at Twin Cays, Belize, by driving stainless steel rods 10 to 12 m (30 to 40 ft) into the ground to create a stable benchmark in the ancient limestone underlying the peat. A portable measuring arm is attached to the benchmark rod, and fiberglass pins are lowered to the soil surface.

The length of the pins relative to the arm is measured on each sampling date and plotted over time. As the soil surface changes, the height of the pins changes. Sand is used to mark the soil surface nearby, and over time, the depth of sediment deposited above the sand provides a measure of soil-accretion rate.

Are Belizean Mangrove Islands Sinking or Keeping Up with Sea-Level Rise?

Illustration to show a timeline below a mangrove forest, indicating how long ago historic events happened.
Chronology of selected events that occurred over the time period recorded in mangrove peat cores collected on Belizean islands. The peat record spans almost the entire Holocene Epoch (10,000 years) and shows how these mangrove islands have built vertically as sea level rose during this time period.

Measurements of elevation change at Twin Cays are showing that the surface in more productive, fringing mangroves (growing along the periphery of islands and along tidal creeks) is gaining elevation, whereas the interior of the islands is sinking. Adding nutrients to some experimental sites has altered the direction and rate of elevation change. Addition of phosphorus to interior mangroves, for example, caused peat expansion resulting from increased mangrove growth and created hummocks that are now higher than surrounding areas.

These findings are showing that vertical building of mangrove islands varies with the health and productivity of mangroves. Even though nutrient addition had a positive effect in some areas, there were negative effects in other areas. Thus, alteration of the nutrient regime could have unexpected and unwanted consequences for these mangrove ecosystems by disrupting the balance among processes controlling peat formation and soil elevations.

How Have Caribbean Mangroves Responded to Past Changes in Sea Level?

Two people collect a muddy sample in a tall skinny metal tube, from a mangrove forest.
Karen McKee (wearing cap) and assistant William Vervaeke collect a peat core in a mangrove forest in Belize (top panels). A section of mangrove peat that was formed 2,000 to 2,500 years ago (lower panel; scale in centimeters).

To answer this question, peat cores must be collected from the mangrove islands. A hand-driven corer is inserted into the peat, and segments are extracted one piece at a time. These peat sections are radiocarbon-dated to determine their age. The peat record at Twin Cays spans almost the entire Holocene Epoch (10,000 years) and the rise of human civilization. Botanical and chemical analyses show what plants were present and what the environmental conditions were like at various times. Watch a video about the project, shot at the field site in Belize.

The peat record indicates that these islands have been continuously dominated by mangrove vegetation throughout their Holocene history but have undergone natural cycles of building, dieback, and recolonization by mangroves. Additional cores from other areas show that mangroves became established on the Caribbean coasts of Honduras and Panama about 1,500 to 2,500 years ago.

These findings have been published in the journal Global Ecology and Biogeography, 2007, v. 16, no. 5, p. 545-556, doi: 10.1111/j.1466-8238.2007.00317.x

Future Work Will Focus on Sea-Level History and Sudden Climate Change

An accurate reconstruction of sea-level history since the last ice age is important to the development of models and the identification of rapid climatic events that may have occurred. Cores collected from Belize and other Caribbean locations are key to this reconstruction because these sites were outside the influence of glaciers and have a small tidal range, factors that reduce error and lead to more accurate models.

McKee will work with geologists from the Smithsonian Institution and universities to construct a sea-level curve for the Caribbean region by using new techniques that allow high-resolution measurements of peat components. The sea-level data produced from this project will be used by Earth-ice modelers to refine predictions of sea-level change and by researchers conducting national and regional hazard assessments along the U.S. Atlantic and Gulf coasts. This work will be carried out over the next 2 years through a grant from the National Science Foundation.

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