MANGROVES AND ESTUARIES

Introduction

Mangrove ecosystems are made up of mangrove trees which have adapted to anaerobic soils and brackish, saltwater environments. They are found all around Mauritius, and in Bel Ombre in the river mouths.  They have developed lateral, prop and aerial roots which allow them anchor in waterlogged soil, expel excess salt and breathe oxygen. Mangroves also serve similar functions as wetlands by trapping sediments and nutrients from upland that would otherwise go into the lagoon and affect coastal marine ecosystems and coral reefs. There are two main species of mangroves in Mauritius: Rhizophora mucronata and Bruguiera gymnorrhiza. These provide a home to many crustaceans, bivalves, snails and juvenile fish who find shelter and food amongst the underwater parts of the roots while the upper parts of the tree are a habitat for coastal birds and reptiles.

Mangrove features

Mangrove trees have unique features that shape the coastal ecosystem in which they are found. Some mangrove species have the ability to filter as much as 90 percent of the salt found in seawater from their roots, while others excrete salt through glands in their leaves, or concentrate salt in older leaves or bark, ridding themselves of the salt when they shed them. In addition to excreting salt, mangroves hoard freshwater in thick succulent leaves, which have a waxy coating on some species that retains water and minimizes evaporation.

Functions and services

Mangrove habitats provide important breeding and nursery grounds for a number of commercially important marine species: these include fish such as barracuda, shrimp, crabs and other crustaceans. These species find cover in the maze of mangrove roots as juveniles, often forage in nearby seagrass beds and move to the reef and open ocean as adults. In addition to the roots which provide shelter, the leaves that fall from the tree decay to provide nutrients for invertebrates and algae, forming the basis of a very productive food web. The nutrients from decaying leaves are also circulated in other areas of the lagoon due to the tides. The invertebrates and algae are in turn eaten by small animals such as birds, sponges, worms, anemones, and young fish.

Mangroves are considered as wetland ecosystems, providing similar functions to coastal marshlands as they filter nutrients and other substances carried by rivers and streams before they reach the ocean. The trees also stabilize the coastlines against erosion and protect the shore against extreme weather events such as cyclones.

Threats and Management

Many mangrove tree stands have been removed to make way for coastal infrastructure and tourism developments, which has made adjacent coastal communities more vulnerable to the effects of storms and rising sea levels. Legal safeguards, however, provide support for protecting mangroves, as it is currently illegal to cut mangrove trees. Protected areas or reserves are another level of protection which can be provided. Finally, in areas where mangroves were cut down, they can be reintroduced through reforestation initiatives which involve replanting appropriate species. To support reforestation, mangrove propagules can be grown in nurseries and replanted along the coastline.

ROCKY SHORES

Introduction

Rocky shores are part of the intertidal zone and can be found all around Mauritius and in Bel Ombre. Although some are composed of limestone, it’s mostly  consist of basalt rocks. Animals and plants living on the rocky shores have developed unique characteristics to adapt to the ebb and flow of the tides; exposure to wave action, long hours under the sun and vulnerability to predators as well as a highly saline environment.    Rocky shores provide a habitat for most animal groups, including molluscs, crustaceans, echinoderms, worms and fish. Rocky shores are also a wonderful habitat for coastal birds to feed on small marine creatures.

Rocky shores features

Animals living on the rocky shores must be able to withstand extreme changes in temperature, salinity, moisture and wave action to survive. Rocky shores are divided by vertical zones depending on the amount of time the rocks are covered by the tide. The upper (landward) end of the intertidal zone are dominated by barnacles, limpets, chitons and other encrusting species. This zone is only flooded during high tides. Further downwards is the mid-intertidal zone which is uncovered twice a day depending on the tides. It is inhabited by algae and mussels, sponges, sea stars and crabs. Lastly, the low intertidal zone or the lower littoral zone is usually covered with water. The creatures found in this zone cannot withstand long periods of dryness or extreme temperatures. 

Functions and services

Sheltered rocky shores in Mauritian lagoons have contributed to shaping the beaches and delimitating bays, thereby creating micro-habitats along the coastline. On both sheltered and exposed rocky shores, the rocks, crevasses and tide pools provide micro-habitats for many marine species. Some rock pools provide spawning and nursery grounds for commercially important species.

Threats and management

Rocky shores have been disturbed or removed during coastal infrastructural works. Furthermore, rubbish dumped on beaches and rocky shores or waste washed up from the sea disturbs the marine life. Implementing measures to control pollution and the integration of rocky shores into protected areas are some of the tools available to help keep rocky shores healthy and productive environments.

 SANDY BEACHES AND DUNES

Introduction

Sand is formed as a result of the breakdown of rocks, the shells and exoskeletons of marine organisms and hard coral. Most beaches around Mauritius are made of coralline sand. Sandy shores are home to small creatures such as crabs and bivalves which have adapted to life in the intertidal zone. Sand dunes are part of the lagoon and beach ecosystem and are formed from the action of wind and waves which deposit the sand on the beach, which then accumulates and forms the crest of the beach. Dunes provide a natural store of beach sand and act as a buffer during extreme weather events such as cyclones. They provide a habitat for many specialized plants, which support the stability of the dune. Unfortunately, most of the beach dunes around Mauritius have been levelled for development. Beach and dune restoration projects help to re-establish the essential ecosystem functions served by dunes.

Sandy beaches and dunes features

Most beaches in Mauritius consist of coral sand that results from the bio-erosion of skeletons of marine creatures including; foraminifera, coralline algae, molluscs and crustaceans. As its name indicates, the sand is also composed of hard corals, which are often broken down by parrot fish which scrape and bite coral when feeding and excrete the coral pieces as small particles.

Sandy shores and dunes break wave action and protect the shoreline. Dunes are dynamic and constantly changing ecosystems formed from sand that has been blown by the wind and accumulates at the back of a natural beach, forming a crest parallel to the beach.

The types of animals found in sandy beaches must be able to adapt to incoming and receding tides and wave action. Due to the nature of the habitat, most of the animals found are either scavengers or filter-feeders.  Some animals may move vertically through the sand column, while others may move up and down the beach with the tides.

Functions and services

Sandy beaches and shores provide a number of important ecosystem services; they protect the coastline from strong wave action, and dunes act as a natural buffer between the sea and land. Sand dunes can either accumulate sand from the beach, growing the dunes and storing sand or can be a source of sand to the beach. Beaches and dunes are important habitats for native plant and animal species, as well as sea turtles which are dependent on beaches for laying their eggs. Sandy shores and dunes are of high recreational value for Mauritians and visitors alike and provide scenic beauty that is the cornerstone of the tourism industry.

Threats and management

The sandy beaches and dunes of Mauritius have been severely degraded throughout the history of the island. Coastal vegetation has been removed and dunes destroyed to make way for bungalows and hotels as well as coastal infrastructure. The very existence of sandy beaches around the island is now threatened by climate change and rising sea levels.  Extreme weather events cause erosion and sometimes the disappearance of beaches. Managing sandy shorelines through beach and dune restoration in conjunction with protection and restoration of lagoon ecosystems can help preserve beaches on the long term.