Mangrove study

Our mangrove expert is Jamie Barclay, a Marine Biologist from Bangor University in Wales.

Aims of the project

Mangroves were not originally present on the island of Rodrigues, and planting has been carried out in some bays to try and reduce the amount of eroded sediment reaching the lagoon. The surveys aim to gather initial data on the biodiversity of organisms associated with these plantations. We should be able to see a variety of animals living at different levels of the bay. Over several years we should be able to see how the mangroves have changed the populations of animals living in the bay.

Equipment Required

50m measuring tape

Smaller measuring tape

Sediment sieves

Trowel

Spade

Collecting jars and bags

Waterproof notepad and pencil

25cmx25cm quadrats

Introduction to mangroves

Mangroves are important intertidal habitats found in tropical and sub tropical coasts worldwide. In parts of the Indian Ocean such as Seychelles, mangroves are naturally occurring and display a high biodiversity of plants and animals.

Tree planting in Rodrigues began in 1980 at certain bays in the north and south coast. The Rhizophora species are normally found mid way in the zonation of mangrove species.

Organic matter accumulates in the mangrove from both land and marine deposits, as well as from recycled leaf litter. Primary production exists in the form of micro algae on the sediment surface and roots.

Animals living within the mangroves include crabs such as Uca sp, Littorinid snails, burrowing polychaete worms and bivalve shells such as Tellina sp. These invertebrates feed on organic matter in the sediment. Sponges are also common amongst the roots of the trees and within the sediment. Juvenile fish such as goby and mullet are found in the shallow pools. Visitors to the mangroves include bees, which are important for fertilizing the flowers and many birds such as herons (Butorides sp.)

Importance of mangroves

They are important as stabilisers of sediment, in some areas creating new areas of land over long periods of time.

They are natural barriers against wind and wave disturbance on the shore

They provide a home for a variety of organisms, including many juvenile species of fish and prawns.

In the Seychelles, boardwalks have been put in place and the mangroves are a popular tourist attraction

The full potential of mangroves as a resource for chemicals used in drug manufacture is not known

Trees can be used in the manufacture of poles, building materials and fuel

Impacts on mangroves

Humans can cause destruction to the trees and the mangrove environment in numerous ways:

Entering the mangrove to dig for crabs and worms as bait

Cutting and boiling young roots to make la tisan, a herbal treatment for a variety of illnesses

Litter such as plastic bottles and car parts are evident in the bays, but since the trees are still not very closely packed, much of this is washed into the lagoon

Mangroves accumulate sediment and consequently many pollutants and toxins washed into the mangrove are retained there in the soil and plants

In some areas of the world, mangroves have been destroyed to make way for building, agriculture or intensive aquaculture projects

Carrying out the study

First of all to study the trees:

Run a tranect perpendicular to the shore from the lanward edge to the seaward edge.

Mark a 10m square plot at the high, mid and low shore along the transect. Choose the first corner of the plot by randomly throwing a quadrat. This corner should always be the bottom left one and the other measured out at right angles to ten metres.

Count all the adult trees within the plot. If there are two people, then both should count and make sure that the results are the same.

The density of seedlings which are densely packed under the adult trees can be counted by choosing three adult trees at random (by throwing a quadrat) and taking an average.

Ten adult trees are again chosen at random and the following variables are measured:

Girth at breast height. This means the trunk circumference at 1.3m height above the ground

The total number of stilt roots (those branching from the trunk) counted.

 

Mangrove survey
Marine Biologist Jamie Barclay digging up sediment

Secondly to study the animals:

Throw three 25cmx25cm quadrats around the roots of each of those ten trees. Count any burrows within the quadrat and dig up the sediment to a depth of 15cm. Sieve the sediment and identify and count and organisms. Don't forget to put the sediment back in the hole.

If you can't identify an animal in the field, you can take an individual back to the laboratory for expert identification. Use a sample bag or jar for this.

Use a dip net to sample the shallow pools around the roots.

Studying the animal burrows

Mangrove soils are more anoxic than terrestrial soils. (This means they have less oxygen and will be darker and stronger smelling). Mangrove root systems play an important role in aerating the soil, but burrowing organisms are also extremely important as bioturbators (Bringing organic rich deeper soil to the surface and allowing air to penetrate to deeper parts). It is extremely difficult to accurately assess infaunal numbers through catching organisms, but estimating burrow density for the mangrove may give an idea of organisms living in the plantation.

Throw 15 25cmx25cm quadrats at random either side of the transect line at high, mid and low tidal levels.

Count burrow holes in each quadrat. Try to grade the holes into three sizes: Small (less than 5mm), medium (6mm-2.5cm) and large (greater than 2.5cm).

Try and differentiate between crab and worm holes. Worm holes usually have a dark mound of excreted sediment around the burrow entrance.

Example data sheets

Baie Diamant - High Water level

Adult trees in plot: 45

Number of seedlings: Tree 1:32, Tree 2:27, Tree 3:54